- Email: [email protected]
GIF biology and fat metabolism
ELSEVIER
PII s0024-3205(!I9)00314-8
LifeSciences,Vol.66, No. 2, pp. 91-103, 2000 Copyright8 1999ElsevierScienceInc. Printedin the USA.All rights resewed 0024-3205/00/$--see front matter
MINIREXIEW GIP BIOLOGY AND FAT METABOLISM Rupert G. C. Yip and M. Michael Wolfe Section of Gastroenterology, Department of Medicine, Boston University School of Medicine, Boston Medical Center, 88 East Newton Street, Evans 201, Boston, MA 02118, U.S.A. (Received in final form July 30, 1999)
summary The gastrointestinal hormone, gastric inhibitory polypeptide (GIP), is synthesized and released from the duodenum and proximal jejunum postprandially. Its release depends upon several factors including meal content and pre-existing health status (ie. obesity, diabetes, age, etc.). It was initially discovered and named for its gastric acid inhibitory properties. However, its more physiologically relevant role appears to be as an insulinotropic agent with a stimulatory effect on insulin release and synthesis. Accordingly, it was later renamed glucosedependent insulinotropic polypeptide because its action on insulin release depends upon an increase in circulating levels of glucose. GIP is considered to be one of the principle incretin factors of the enteroinsular axis. The GIP receptor is a G-protein-coupled receptor belonging to the family of secretinMP receptors. GIP receptor mRNA is widely distributed in peripheral organs, including the pancreas, gut, adipose tissue, heart, adrenal cortex, and brain, suggesting it may have other functions in addition to the ones mentioned above. An overactive enteroinsular axis has been suggested to play a role in the pathogenesis of diabetes and obesity. In addition to stimulating insulin release, GIP has been shown to amplify the effect of insulin on target tissues. In adipose tissue, GIP has been reported to (1) stimulate fatty acid synthesis, (2) enhance insulin-stimulated incorporation of fatty acids into triglycerides, (3) increase insulin receptor a&r&y, and (4) increase sensitivity of insulinstimulated glucose transport. In addition, although controversial, lipolytic properties of GIP have been proposed. The mechanism of action of GIP-induced effects on adipocytes is unknown, and it is unclear whether these effects of GIP on adipocytes are direct or indirect. However. there is now evidence that GIP receptors are expressed on adipocytes and that these receptors respond to GIP stimulation. Given the location of its release and the timing of its release, GIP is an ideal anabolic agent and expanding our understanding of its physiology will be needed to determine its exact role in the etiology of diabetes mellitus and obesity. Key Words: GIP, fat, adipocytes, metabolism In 1886, Ewald and Boas (1) showed that olive oil mixed with a meal inhibited both gastric emptying and acid secretion. Kosaka and Lim (2,3) theorized that this mixture liberated a chemical from the small intestine and named this chemical “‘enterogastrone”. During the ensuing decades, the search was
92
GlPNKiRilMaabdism
Vol. 66, No. 2,2MJO
on for a gastrointestinal hormone exerting this effect. It was found that acids, fats, and hypertonic solutions all inhibited acid secretion, which consequently led to the suspicion that several enterogastrones might in fact exist. In searching for a novel gut hormone, Brown and Pederson (4) observed that a crude preparation of porcine cholecystokinin (CCK) produced less stomach acid secretion than a purer preparation of CCK from the duodenum. They concluded that during the purification process, a factor must have been removed that inhibited acid secretion. Later, Brown and colleagues (5-7) isolated this factor, which was found to be localized in the duodenum and jejunum (8.9) in specific endocrine cells designated K-cells (10). Due to its ability to inhibit gastric acid secretion, the hormone was named “gastric inhibitory polypeptide” (GIP) (11). However, after its isolation, it was found that GIP possessed another important biological property, the ability to potentiate insulin secretion (12). Numerous studies have shown that GIP is released from the duodenum and jejunum in response to the ingestion of a meal containing glucose or fat (13). The released GIP, in combination with postprandial hyperglycemia, induced insulin release from pancreatic islet &cells. This “incretin” property attributed to GIP was considered more physiologically relevant than its inhibitory effect on acid secretion, and GIP was accordingly renamed “glucose-dependent insulinotropic polypeptide”, thus retaining the original acronym. GIP secretion is very sensitive to acute and chronic changes in diet, especially changes in dietary lipid content. The degree to which fat stimulates GIP secretion is also species-dependent. In humans, who normally consume a diet relatively high in lipid content (40% energy from fat in a typical Western diet), fat is a more potent stimulator of GIP release than carbohydrates. In contrast, in rats and pigs, whose diets usually contain ~10% energy as fat, carbohydrates are more potent than fat in stimulating GIP release (14). Moreover, the postprandial level of circulating GIP is dependent on meal size (15,16), and the contribution of the enteroinsular axis is proportionately greater after a large meal. Acute exercise training has no effect on GIP secretion. Chronic exercise, however, has been shown to increase GIP secretion in pre- and early adolescent children (17), but has no effect in obese adult women (18). There is also an apparent correlation between age and GIP plasma levels. In general, the older the subject, the higher their GIP levels, although, it is unknown whether this correlation is due to age or due to the fact that older subjects tend to have slightly higher body mass indices (BMI) compared to younger adults. There is also a significant correlation between gender and GIP levels, with males having higher circulating levels of the peptide (19). Although only the rat GIP is shown in Figure 1A the amino acid sequence for human, porcine, bovine, and rat are remarkably similar, with greater than 90% homology. In these species, GIP is a 42 amino acid polypeptide that includes a consensus tryptic sequence (KGKK) at positions 30-33. This sequence has led to the possibility of truncated forms of GIP in the small intestinal mucosa and in the circulation. Indeed, the GIP hormone itself is derived from the proteolytic processing of its preprohormone precursor (Figure 1B). Although the mature polypeptide is 42 amino acids in length, only the first 30 amino acids are required to produce its biological effects and are sufficient for ligand binding and receptor activation (20), consistent with the existence of a biologically active GIP in its truncated form in viva Sequence analysis of the human GIP cDNA clones has demonstrated a 459-bp open reading frame encoding a 153~amino acid polypepti& and a 432&p open reading frame encoding a 14kmino acid polypeptide in the rat (21-23) (Fig. 2). The predicted amino acid sequences indicate that both the human and rat GIP are derived by proteolytic processing of a preprohormone. The human preproGIP has a 5 1-amino acid N-terminal segment and a 60-amino acid C-terminal segment flanking the 42amino acid GIP moiety. The N-terminal segment contains a potential signal peptide with a possible cleavage site at glycine 21. The rat preproGIP is smaller in length than the human precursor by nine
Vol.66. No. 2.2000
GIPandFatMetabolism
93
FIG.1. The ratglucose-dependent insulinotropic polypeptide or gastric inhibitory polypeptide is a 42 amino acid polypeptide (A). Although there is species differences. the degree of homology is greater then 90%. suggesting conservation through mammalian evolution. The first 2 amino acid residues YA are cleaved by dipeptidyl peptidase IV rendering the polypeptide biologically inactive. At position 30. a tryptic cleavage site (I$GKK) exists suggesting the possibility of furthertruncated forms of GIF’in vivo.
residues. The N-terminal segment of the rat preproGIP consists of 43 amino acids while the C-terminal segment is 59 amino acids in length. The rat precursor contains two potential cleavage sites for signal peptidase. The signal peptide of preproGIP is cleaved following either glycine 19 or glycine 21. It appears that in both human and rat, mature GIP( l-42) is generated by proteolytic processing at arginine residues that immediately flank the region corresponding to the GIP moiety (2 1,22,24).
Recently, GIP(7-42) was puritied from the upper part of the porcine intestine and was shown to have antibacterial activity (25). Although it has been presumed that the peptide is also derived by proteolytic cleavage of the preproGIP, it is unclear whether this cleavage is a physiological phenomenon or rather an extraction artifact. In addition, the concentrations needed for antibacterial action are high, implying a non-physiological effect. Although no biological properties have yet been attributed to either the N-terminal or the C-terminal peptides of preproGIP, the C-terminal segment does share homology with regions of both pancreastatin and chromagranin A. The function of chromagranin A is unknown, while pancreastatin appears to inhibit insulin release from pancreatic gcells (24). Computer analysis of the predicted amino acid sequence of GIP has demonstrated structural homology between GIP and other members of the secretin family of gastrointestinal regulatory polypeptides, which include secretin, glucagon, glucagon-like peptide (GLP-1 and GLP-2), vasoactive intestinal polypeptide (VIP), peptide histidine isoleucine (PHI), growth hormone releasing hormone (GHRH), and pituitary adenylyl cyclase-activating polypeptide (PACAP) (21). Like GIP, the biosynthesis of other members of the secretin family involves proteolytic processing of their respective preprohormone (26). Although the structural arrangements of these precursors are the same, the sequence homology observed is restricted only to the biologically active portion of the precursor polypeptides, with little similarity in the other domains of the molecule (21).
The half-life of immunoreactive GIP in humans has been estimated at roughly 20 min as measured by radioimmunoassay (RIA) (27-30). The mature GIP polypeptide, GIP(1-42), is susceptible to proteolytic digestion by dipeptidyl peptidase IV present in serum (31,32). Cleavage of GIP(1-42) at
94
GIP and Fat Maabolism
SIGNAL
Vol. 66, No. 2,uKx)
PEPTIDE:
Met-Val-Ala-Thr-Lys-Thr-Phe-Ala-Leu-Leu-Leu-Leu-Ser-Leu-Phe-Leu-Ala-Val-Gly-LeuGly
N-TERMINAL
PEPTIDE:
Glu-Lys-Lys-Glu-Gly-His-Phe-Ser-Ala-Leu-Pro-Ser-Leu-Pro-Val-Gly-Ser-His-Ala-LysVal-Ser-Ser-Pro-Gln-Pro-Arg-Gly-Pro-Arg
MATURE
GIP:
Tyr-Ala-Glu-Gly-Thr-Phe-lle-Ser-Asp-Tyr-Ser-lle-Ala-Met-Asp-Lys-lle-His-Gln-Gln-AspPhe-Val-Asn-Trp-teu-Leu-Ala-Gln-Lys-Gly-Lys-Lys-Asn-Asp-Trp-Lys-His-Asn-lle-ThrGln
C-TERMINAL
PEPTIDE:
Arg-Glu-Ala-Arg-Ala-Leu-Glu-Leu-Ala-Ser-Gin-Ala-Asn-Ar~Lys-Glu-Glu-Glu-Ala-Val-Glu-Pr~Gln~ Ser-Ser-Pro-Ala-Lys-Asn-Pro-Ser-Asp-Glu-Asp-Leu-Leu-Arg-As~Leu-Leu-lle-Gln-Glu-Leu-LeuAla-Cys-Leu-Leu-Asp-Gln-Thr-Asn-Leu-Cys-Arg-Leu-Arg-Ser-Arg FIG. 2.
The human preprohonnone (B) consist of a 153 amino acid polypeptide. Proteolytic processing of the preprohormoneyields a signal peptide, N-km&al peptidc the mature peptide. and a C-terminal’peptide. the N-terminal second position results in GIP(3-42), which is biologically inactive (3 1,32). Although there may be many diierent proteases in serum, the predominant GIP-degrading protease appears to be dipeptidyl peptidase IV (33). When using immunological methods to examine honuone metabolism, one should be aware of the fact that immunoreactivity may not necessarily correlate with biological activity. For example, it is possible for GIP to be proteolytically degraded and rendered biologically inactive while still retaining immunoreactivity as measured by RIA. Thus, the true biological half-life of GIP may actually be less than 20 min.
The GIP receptor was first cloned in 1993 from rat (34) and subsequently cloned in hamster (35) and human (36). The processed rat GIP receptor has a predicted molecular weight of -50 kDa, but covalent cross-linking studies have shown the GIP receptor migrating with an apparent molecular weight of 64 kDa (37,38). The receptor is a glycoprotein belonging to the secretin/VIP family of receptors, a seven transmembrane G-protein-coupled receptor family that includes receptors for secretin, VIP, glucagon, GLP-1, growth hormone releasing hormone (GHRH), and PACAP. The Nterminus contains a consensus sequence for N-glycosylation (N-X-W) (37), and the third cytoplasmic loop and the C-terminus are rich in threonine and serine as potential phosphorylation sites (Figs. 3 & 4). The GIP receptor, as with other receptors in the secretinMP family of receptors, has a large Nterminal domain. Unlike adrenergic receptors, however, in which the ligand binds to a pocket formed by the seven transmembrane domains, receptors belonging to the secretinMP family bind ligands via
Vol. 66. No. 2 2000
a~ and Fat Me&aWism
FIG. 3. T%e rat GIP receptor is a seven trammembrane-spanning receptor. It belongs to the secretin/VP family of receptors. The extensive N-terminal extracellular damains of this family of receptors are usually glycosylati and form the recognition and binding domain of the receptor.
GIF sad Fat Metabolism
%
I
I
I
Vol. 66.No.2.2000
I
I
1
MPLRLLLLLLWLWGLSLQRTDSEGQTTGELYQRWERYGWECQNTLEATEPPSG~C~
61
SFDMYACWNYTAANTTARVSCPWYLPWYRQVAAGFVFRQCGSDGQWGSWRDHTQCENPEK m =L
121
NGAFQDQKLILEsALHCTRNYIqk
I
I
I
I
I
I
I
I
181
241
LLVVVRRSEK&+&YLYEN;QCWERNEV&
301
I
I
I
QAEGBA
421
RAVPLSSAPQEAAIRNALPSGMLHVPGDEVLESYC
---
_--A
I
I
I
I
EVQSEIRRLRLSLQEQCPRPHLGQAP -
361
I
Potential Casein Kinase 2 sites Potential N-Glycosylation sites _...-._ 1 Potential Protein Kinase C sites Poldfacel represents putative transmembrane domains FIG.4. The primary sequence of the rat GIP receptor. The GIP receptor is also speculated to be glycosylated as there are several consensus glycosylation sites. In addition, there are putative sites for phosphorylation by cssein ldnase 2 and protein kinase C for posttranslational modification. their large N-terminal domain (3945). Chimeric GWGLP-1 receptor studies have shown that the first 132~amino acids of the N-terminal domain is necessary for high-affinity binding of GIP, but receptor coupling to CAMP production requires the first 222 N-terminal amino acids (46). It is generally assumed that GIP binding to its receptor activates a heterotrimeric stimulatory Gprotein, which in turn activates adenylyl cyclase. The resulting increase in intracellular CAMP and a subsequent increase in Ca*’ influx via voltage-gated Ca” channels are thought to represent the major signalling pathways by which GIP exerts its insulinotropic effects (47,48). Recently, an additional but distinct signal transduction pathway for GIP was described. As stated above, GIP and its receptor are closely related to VIP and PACAP and their respective receptors. Straub and Sharp (49) have shown that while VIP and PACAP increase intracellular CAMP and Ca”
Vol. 66. No. 2,200O
GIP and Fat Metabolism
97
only during the frost 20 mitt of stimulation, the release of insulin from HIT cells continues for greater than 50 min. long after CAMP and Ca” levels have returned to baseline levels. It was thus shown that the prolonged stimulatory mechanism might be wortmannin sensitive (49). It was later shown that wortmannin, a potent inhibitor of phosphatidylinositol3 kinase (PI 3kinase). was able to inhibit GIPstimulated insulin release, suggesting that GIP does utilize a wortmannin-sensitive mechanism for signal transduction (50). Additionally, Kubota et al. (5 1) demonstrated that GIP receptor activation with 0.59 nM GIP was sufficient to activate mitogen-activated protein (MAP) kinase. The study went on to show that MAP kinase activation by GIP was via wortmannin-sensitive and -insensitive pathways. Thus there is sufficient evidence to indicate that GIP recruits intracellular signalling mechanisms other than cyclic AMP. Northern blot, RT-PCR, and in situ hybridization studies have shown GIP receptor mRNA present in the pancreas, stomach, intestine, adrenal cortex, heart, lung, brain, endothelium of major blood vessels, and adipose tissue. There is also ample GIP receptor mRNA and protein expression in the rat testes (unpublished data). Most of these regions have not been previously considered major targets for GIP, and although receptor mRNA may be present in these tissues, no Western or immunocytochemical data have as yet been reported. The locations of GIP receptor transcripts may suggest novel actions of GIP or possibly novel ligand(s) for the GIP receptor. Several recent reports of ectopically expressed GIP receptors on adrenal gland have been published (52-54). These patients exhibit food-dependent Cushing’s syndrome. GIP receptors have been found in the rat adrenal cortex, but until recently, no information has been available on the presence or absence of GIP receptors in human adrenals. In situ hybridization studies have demonstrated the expression of GIP receptor transcripts in adrenocortical adenoma samples taken from patients with food-dependent Cushing’s syndrome, while in situ hybridization failed to detect the GIP receptor in adrenal adenoma patients without food-induced Cushing’s syndrome (54). Kaplan and Vigna (55) have studied GIP binding sites in the rat brain. Using [‘2SI]GIP,displaceable GIP binding sites were localized to cortical areas of the telencephalon, and forebrain regions. Interestingly, no [‘zsI]GIP binding was detected in the anterior pituitary despite a report that GIP altered growth hormone (GH) and follicle stimulatory hormone (FSH) secretion (56). However, in this latter study, pharmacological doses of GIP were administered, and it is possible that other related receptors may have been activated by GIP and thus responsible for the changes in GH and FSH secretion. Although the notion that GIP receptors are present in the brain is interesting, there is, at present, no evidence for GIP synthesis in the brain, and the only source of GIP in brain may thus be systemic. Alternatively, novel GIP-like ligand(s) may be binding to GIP receptors in the brain.
tw_en
.
.
The relationship between GIP and obesity/diabetes has long been controversial. GIP concentrations may be influenced by obesity, diabetes, and glucagon deficiency and may be under negative feedback by insulin. Obesity is characterized with fasting hyperinsulinemia, and the ingestion of carbohydrates results in an exaggerated insulin response (5760). GIP has been proposed as a major mediator of the enterically induced component of insulin secretion (61) and one potential explanation for the increase in insulin levels has been that of an overactive enteroinsular axis (62). The timely release of GIP postprandially likewise make it an ideal anabolic agent in the etiology of obesity. Currently, it is unknown whether the alterations in GIP physiology are a cause or consequence of obesity/diabetes. In one case, alterations in GIP secretion were observed before the onset of diabetes (63). Additionally, ob/ob mice chronically fed a high fat diet have been shown to have increased concentrations of GIP in plasma and in the intestinal mucosa as well as increased density of GIP-
98
GIP and Fat Metabolism
Vol. 66, No. 2,2UOO
secreting K-cells in the upper jejunum. In contrast, a high carbohydrate diet had no such effect (64). Moreover, the extent of hyperglycemia and hyperinsulinemia present in ob/ob mice was not significantly altered by high fat or high carbohydrate diets (64), while orally administered fat, glucose, and amino acids increased GIP concentrations (65). In humans, fat constitutes the most potent stimuhis for GIP secretion (66), and plasma GIP levels are closely correlated with plasma triglyceride levels (67). In obese human subjects, oral triglyceride elicited a greater increase in serum GIP levels when compared to normal weight subjects (68). In the same study, obese subjects exhibited elevated basal insulin levels and higher levels following an oral triglyceride load. However, it was unknown whether GIP was responsible for the increased levels of insulin. In the obese Zucker rat model, there appears to be hypersensitivity of adipose tissue to Gil? GIP-stimulated fatty acid incorporation into fat tissues was greater for all concentrations tested when compared to lean Zucker rats (69). However, in the Zucker rat model, plasma GIP concentrations, in response to an oral glucose challenge, remained similar to levels in lean rats. It is possible that in this model the increased fatty acid incorporation may be the result of increased affinity for GIP by its receptor or increased potency of GIP. With that in mind, the following hypothesis may be proposed: individuals with a condition of increased GIP responsiveness may be more prone to obesity and hyperinsulinemia. Increased GIP responsiveness may manifest itself in the form of either increased affinity for GIP by GIP receptors, increased GIP receptor number, or increased synthesis and secretion of GIP following normal meal stimulation. The increased GIP responsiveness would likely increase insulin release resulting in greater and prolonged hyperinsulinemia. To compound this abnormality, GIP can also increase the affinity of the insulin receptor for insulin as well as increase the insulin-like effects in fat cells, all of which may serve to increase efftciency in the accumulation of triglycerides in fat cells. Thus, anecdotal accounts of individuals accumulating fat tissue mass at greater rates under normal meal conditions may be a consequence of increased GIP responsiveness. Such individuals have greater efficiency in fat storage as a result of increased GIP and insulin responses when compared with normal persons. Although insulin resistance is a constant feature of noninsulin dependent diabetes mellitus (NIDDM) the insulin response to different stimuli can be exaggerated, normal, or impaired (70). The reasons for these differences in insulin secretion have not been elucidated, although a disturbance in the enteroinsular axis has been proposed (7 1,72). Published reports have shown great variability in both basal and stimulated GIP levels in obesity and NIDDM. In NIDDM patients, the GIP response to glucose and other stimuli has been increased (27,73), normal (74), or decreased (75). In obese subjects, GIP concentrations have been either normal (76) or increased (77). While advanced age has been shown to be associated with a normal GIP response to oral glucose, p-cell sensitivity to GIP was decreased (78), and plasma GIP levels appear to be elevated (19). A recent study demonstrated that the insulinotropic effect of GIP was selectively reduced in diabetic patients, whereas that of GLP-1 was preserved (79). suggesting that abnormalities in GIP release and function may be involved in the pathogenesis of diabetes mellitus. The GIP receptor gene may be involved in the pathogenesis and development of NIDDM. Kubota et al. (80) have recently identified two missense mutations in the GIP receptor gene: Glyigs-Cys and G1u354*Gln.Both Glylg8 and Glu3” are conserved among human, rat, and hamster GIP receptors (34,35,8 1). The substitutions of a thiolcontaining Cys residue for Gly and that of uncharged Gln residue for the acidic Glu were expected to affect protein structure and function. In fact, it was found that GIP-stimulated CAMP formation was considerably impaired with the Gly’g*-Cys mutation, but was normal for the Glu3”-Gln mutation. Examination of the allelic frequencies of Gly’*e Cys and Glu3M-Gln in NIDDM and control subjects, however, showed no differences in frequency, suggesting that these mutations do not contribute significantly to the loss of GIP induced secretion in NIDDM patients. However, because NIDDM is polygenic, the possibility that these or other unidentified mutations of the GIP receptor may represent a risk factor in a small subset of individuals with NIDDM cannot be excluded.
Vol. 66, No. 2,200O
GE and Fat Metabolism
99
There is some evidence to indicate that GIP may affect insulin sensitivity of adipocytes. Incubation of isolated adipocytes with GIP was shown to increase receptor a&&y and insulin-stimulated glucose transport (82). However, the concentrations used were supraphysiological and the observed changes minimal, with an IC, change from 8.6 rig/ml to 6.6 ng/ml. A more likely effect of GIP is to modulate the effects of other hormones on adipocyte metabolism. GIP has been shown to stimulate glucose transport in isolated rat adipocytes and the incorporation of glucose into extractable lipids in one study (83), but not in another (84). A study by Beck and Max has shown that GIP alone was capable of inducing a slight decrease in fatty acid incorporation into adipose tissue, while in the presence of insulin, GIP significantly enhanced insulin-induced fatty acid incorporation into adipose tissue (85). In that study, the authors speculated that a hyperactive enteroinsular axis in obese individuals might be a factor in the development of obesity (83). However, in contrast to the findings in rodents, Baba and Butterlly showed that GIP stimulated lipogenesis, but reduced insulin-stimulated lipogenesis (86). GIP and GLP- 1, in common with insulin, but not GLP-2 and glucagon, stimulate fatty acid synthesis in explants of rat adipose tissue, as measured by the incorporation of [i4C]acetate into saponifiable fat (87). GIP and GLP-1 may thus contribute in vivo to a more effective postprandial uptake of glucose and may enhance the effect of insulin on fatty acid synthesis from glucose as precursor. Consistent with this anabolic role, GIP has been shown to inhibit the lipolytic action of glucagon in adipocytes (88), which may occur as a result of competitive inhibition of glucagon binding to its receptor. In humans, the accumulation of adipose-tissue triacylglycerol from dietary fat is quantitatively more important than de nova lipogenesis. Adipose lipoprotein lipase (LPL) plays a key role in the hydrolysis of circulating triacylglycerol, liberating non-esterilied fatty acids for uptake and storage within the adipocyte. In animal studies, exogenous GIP infusion has been shown to promote the clearance of chylomicron triglycerides (89) and to lower postprandial circulating triacylglycerol levels (90). GIP has been shown to stimulate the synthesis and release of LPL in cultured mouse 3T3-Ll preadipocytes (91). GIP, in common with insulin, stimulates LPL activity in explants of rat adipose tissue, although GLP- 1 over the same concentration range has no effect (92). In rats, a high-fat diet increases GIP and insulin secretion and elevates both basal and insulin- and GIP-stimulated LPL activity compared with control animals (87). Under conditions of high fat and energy feeding, higher circulating levels of GIP and insulin, together with an enhanced sensitivity of LPL to these hormones, may therefore facilitate the uptake of circulating triacylglycerol and contribute to increased adiposity in some circumstances. At present, the signaling mechanisms mediating the effects of GIP on fat cells are unknown. It was speculated that GIP may function via inhibition of glucagon action by competitively binding to glucagon receptors (88) Although the GIP receptor cDNA has been shown to be present in adipose tissue by polymerase chain reaction (PCR) (34) no direct evidence for GIP receptors on adipocytes had been reported. Recently, we have found evidence of GIP receptor expression in rat adipocytes and in the adipocyte cell model, the differentiated 3T3-Ll mouse fibroblast (93). As stated previously, the GIP receptor is a G-protein coupled receptor that is presumably activated via a cyclic AMP-mediated mechanism. In our study, but not others (88,94), an increase in cyclic AMP in rat adipose tissue and in differentiated 3T3-Ll adipocytes in response to GIP stimulation was detected. These differences may reflect the use of a 3T3-Ll adipocyte model in the presence of isobutylmethlyxanthine (IBMXa potent cyclic AMP phosphodiesterase inhibitor) versus rat adipocytes in the absence of IBMX.
Presently, the two principle incretins are GIP and GLP-1. GIP is ideally located in the duodenum and proximal jejunum, in contrast to GLP-1, which is found in highest concentration in the distal jejunum
100
GlPandFatMetabelism
Vol. 66, No. 22000
and ileum. GIP may thus play a greater physiological role as an insulinotropic and anabolic agent. Moreover, there is evidence that GIP regulates the release of GLP-1(95), at least in the dog. The finding that GIP receptors are found in other organs, such as pancreas, stomach, intestine, adrenal cortex, heart, lung, brain, and endothelium of major blood vessels, suggests other possible functions for this hormone. As with other “permissive” hormones, GIP may not produce a direct stimulator-y effect on its target cells, but rather a modulatory effect on actions of other hormones. Further, work will be needed to clarify the precise role of GIP in the pathogenesis of diabetes and obesity. While there is one report of elevated serum GIP levels prior to the onset of diabetes mellitus (63), the GIP response to glucose and other stimuli has been reported to be increased (27,73), normal (74). or decreased (75). There is even speculation of a mutant form of the GIP receptor in certain subtypes of NIDDM (80). It is also possible that increased GIP responsiveness will result in increased efficiency in fat accumulation and hyperinsulinemia. A greater understanding of the biology of GIP and its receptor will help to resolve the importance of the enteroinsular axis in health and disease.
1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
C.A. EWALD and J. BOAS, Virchows Archives m 271-305 (1880). T. KOSAKA and R.K.S.LIM. Chinese Journal of Physiology 4 213-220 (1930). T. KOSAKA and R.K.S. LIM, Proceeding of the Society for Experimental Biology and Medicine 22 890-891 (1930). J.C. BROWN and R.A. PEDERSON, Scandinavian Journal of Gastroenterology 2 537~541(1970). J.C. BROWN, V. MUTT and J.R. DRYBURGH, Canadian Journal of Physiology & Pharmacology & 399-405 (1971). J.C. BROWN, V. MUTT and R.A. PEDERSON, Journal of Physiology ZQ$!57-64 (1970). J.C. BROWN and J.R. DRYBURGH, Canadian Journal of Biochemistry& 867-872 (1971). J.M. POLAK. S.R. BLOOM, M. KUZIO, J.C. BROWN and A.G. PEARSE. Gut 14 284-288 (1973). R. BUFFA, J.M. POLAK, A.G. PEARSE. E. SOLCIA. L. GRIMELIUS and C. CAPELLA, Histochemistry Q 249-255 (1975). A.M. BUCHAN, J.M. POLAK, C. CAPELLA, E. SOLCIA and A.G. PEARSE, Histochemistry ti 3744 (1978). J.C. BROWN, R.A. PEDERSON, E. JORPES and V. MUTT, Canadian Journal of Physiology & PharmacologyQ 113-114(1969). J. DUPRE, S.A. ROSS, D. WATSON and J.C. BROWN, Journal of Clinical Endocrinology & Metabolism u 826-828 (1973). L.M. MORGAN, P.R. FLATT and V. MARKS, Nutrition Research Reviews 179-97 (1988). L.M. MORGAN. J.M. KNAPPER, J.M. FLEICHBR and V. MARKS, Proceedings of the Society of Nutrition Physiology 4 1l-20 (1995). S.M. HAMPTON, L.M. MORGAN, J.A. ‘IREDGER, R. CRAMB and V. MARKS, Diabetes X 612616 (1986). MC. MURPHY, S.G. ISHERWOOD, S. SEIHI, B.J. GOULD, J.W. WRIGHT, J.A. KNAPPER and CM. WILLIAMS, European Journal of Clinical Nutrition a 578-588 (1995). E.B. KAHLE, O.D. TM, R.B. WALKER, P.A. EISENMAN. S. REISER. S. CATALAND and W.B. ZIPF, Diabetes s 579-582 (1986). M. KRCYTKIEWSKI.P. BJGRNTORP, G. HOLM, V. MARKS, L. MORGAN, U. SMITH and G.E. FEURLE, International Journal of Obesity 8 193- 199 (1984). J.G. SCHWARTZ, C.A. MCMAHAN, G.M. GREEN and W.T. PHILLIPS, Digestive Diseases & Sciences fi 624-630 (1995). B. GAJLWllZ, M. WI’IT, C. MORYS-WORTMANN, U.R. FGLSCH and W.E. SCHMIDT, Regulatory Peptides fi 17-22 ( 1996). C.C. TSBNG, L.A. JARBOE, S.B. LANDAU, E.K. WILLIAMS and M.M. WOLFE, Proceedings of the National Academy of Sciences of the United States of America a 1992-1996 (1993).
Vol.66. No. 2.2000
22.
23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42.
43. 44. 45. 46. 47. 48. 49. 50. 51.
GlP and Fat Metabolism
101
J. TAKEDA, Y. SEINO. K. TANAKA, H. FUKUMOTO, T. KAYANO, H. TAKAHASHI, T. MITANI, M. KURONO, T. SUZUKI, T. ‘IGBE and H. IMURA. Proceedings of the National Academy of Sciences of the United States of America 84 7005-7008 (1987). Y. HIGASHIMO’I0, E.C. OPARA and R.A. LIDDLE, Comparative Biochemistry & Physiology. Part C Pharmacology, Toxicology, Endocrinology Ilp 207-214 (1995). Y. HIGASHIMO’IO, J. SIMCHOCK and R.A. LIDDLE, Biochirnica et Biophysics Acta lK2 72-74 (1992). B. AGERBERTH. A. BOMAN, M. ANDERSSON, H. JORNVALL, V. MU’lT and H.G. BOMAN, European Journal of Biochemistry u 623-629 (1993). G.I. BELL, Peptides 2 27-36 (1986). J.C. BROWN, J.R. DRYBURGH, S.A. ROSS and J. DUPRE, Recent Progress in Hormone Research 11487-532 (1975). D. ELAHI, D.K. ANDERSEN, J.C. BROWN, H.T. DEBAS, R.J. HERSHCOPF, G.S. RAIZES, J.D. TOBIN and R. ANDRES, American Journal of Physiology m El 85- 19 1 (1979). M. NAUCK. W.E. SCHMIDT, R. EBERT, J. STRIE’IZEL, P. CANTOR, G. HOFFMANN and W. CREU’IZFELDT, Journal of Clinical Endocrinology & Metabolism he 654-662 (1989). D.L. SARSON, R.C. HAYTER and S.R. BLOOM, European Journal of Clinical Investigation 12 457461 (1982). T.J. KIEFFER, C.H. MCINTOSH and R.A. PEDERSON, Endocrinology m 3585-3596 (1995). R. MENTLBIN, B. GALLWI’IZ and W.E. SCHMIDT, European Journal of Biochemistry 214 829-835 (1993). R.A. PEDERSON, T.J. KIEFFBR, R. PAULY, H. KOFOD, J. KWONG and C.H. MCINTOSH, Metabolism: Clinical &Experimental & 1335-1341 (1996). T.B. USDIN, E. MEZEY, D.C. BUTTON, M.J. BROWNSTEIN and T.I. BONNER, Endocrinology m 2861-2870 (1993). K. YASUDA, N. INAGAKI, Y. YAMADA, A. KUBOTA, S. SEINO and Y. SEINO, Biochemical & Biophysical Research Communications m 1556-1562 (1994). A. VOLZ, R. GiiKE, B. LANKAT-BUTTGEREIT, H.C. FEHMANN, H.P. BODE and B. GOKE, FBBS Letters m 23-29 (1995). B. AMIRANOFF, A. COUVINEAU, N. VAUCLIN-JACQUES and M. LABURTHE, European Journal of Biochemistry u 353-358 (1986). A. COUVINEAU, B. AMIRANOFF, N. VAUCLIN-JACQUES and M. LABURTHE, Biochemical and Biophysical Research Communications 122 282-288 (1984). J.J. BUGGY, J.N. LIVINGSIGN, D.U. RABIN and H. YOO-WARREN, Journal of Biological Chemistry 21a 7474-7478 (1995). A. COUVINBAU, P. GAUDIN, J.J. MAORET, C. ROUYER-FESSARD, P. NICOLE and M. LABURTHE, Biochemical & Biophysical Research Communications 246 246-252 (1995). P. GAUDIN, A. COWINEAU, J.J. MAORET, C. ROUYER-FESSARD and M. LABURTHE, Biochemical & Biophysical Research Communications 2.l.I 901-908 (1995). H. JUPPNER, E. SCHIPANI, F.R. BRINGHURST, I. MCCLURE, H.T. KEUTMANN, J.T. POTTS, JR, H.M. KRONENBERG, A.B. ABOU-SAMRA, G.V. SEGRE and T.J. GARDELLA, Endocrinology m 879-884 (1994). B. VAN EYLL, B. Gt%E, A. WILMEN and R. GGKE, Peptides n 565-570 (1996). A. WILMEN. B. VAN EYLL, B. GiiKE and R. G&E, Peptides .Ej 301-305 ( 1997). J.P. VILARDAGA, P. DE NEEF, E. DI PAOLO, A. BOLLEN, M. WAELBROECK and P. ROBBERECHT, Biochemical & Biophysical Research Communications u 885-891 (1995). R.W. GELLING, M.B. WHEELER, J. XUE, S. GYOMOREY, C. NIAN. R.A. PEDERSON and C.H. MCINTOSH, Endocrinology m 2640-2643 (1997). M. LU. M.B. WHEELER, X.H. LBNG and A.E.D. BOYD, Endocrinologym 94-100 (1993). Y. MIURA, M. KATO, K. OGINO and H. MATSUI, Endocrinology m 2769-2775 (1997). S.G. STRAUB and G.W.G. SHARP, Journal of Biological Chemistry a 1660- 1668 ( 1996). S.G. STRAUB and G.W. SHARP, Biochemical & Biophysical Research Communications 224 369-374 (1996). A. KUBOTA, Y. YAMADA, K. YASUDA, Y. SOMEYA, Y. IHARA, S. KAGIMOTO, R. WATANABE, A. KUROE, H. ISHIDA and Y. SEINO, Biochemical & Biophysical Research Conununications~ 171-175 (1997).
102
52.
53.
54.
55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85.
GlP andFat Metabolism
Vol.66, No. 2,2ooO
A. LACROIX, E. BOLTE, J. TREMBLAY, J. DUPRE. P. POITRAS, H. FOURNIER, J. GARON, D. GARREL, F. BAYARD, R. TAILLEFER, R.J. FLANAGAN and P. HAMET, New England Journal of Medicine m 974-980 (1992). Y. REZNIK, V. ALLALI-ZERAH, J.A. CHAYVIALLE. R. LEROYER. P. LEYMARIE, G. TRAVQRT. M.C. LEBRETHON, I. BUDI, A.M. BALLIERE and J. MAHOUDEAU, New England Journal of Medicine =981-986 (1992). W.W. DE HERDER, L.J. HOFLAND, T.B. USDIN, F.H. DE JONG, P. UI-lTERLINDEN, P. VAN KOETSVELD, E. MEZEY, T.I. BONNER, H.J. BONJER and S.W. LAMBERTS, Journal of Clinical Endocrinology & Metabolism BL 3 168-3 172 (1996). A.M. KAPLAN and S.R. VIGNA, Peptides fi 297-302 (1994). A. O’ITLECZ, W.K. SAMSON and S.M. MCCANN, Peptides 4 115-l 19 (1985). J.H. KARAM, G.M. GRODSKY and P.H. FORSHAM. Diabetes 12 197-205 (1963). J.D. BAGDADE, E.L. BIERMAN and D. PORTE, JR., Journal of Clinical Investigation 46 1549-1557 (1967). R.A. KREISBERG, B.R. BOSHELL, J. DIPLACIDO and R.F. RODDAM, New England Journal of Medicine 226 314-319 (1967). R.S. YALOW, S.M. GLICK, J. ROTH and S.A. BERSON, Annals of the New York Academy of Sciences 11L 357-373 (1965). W. CREUTZFELDT, Gastroenterology fl748-750 (1974). W. CREUTZFELDT, R. EBERT, B. WILLMS, H. FRERICHS and J.C. BROWN, Diabetologia 14 15-24 (1978). M. ELSALHY, S. ZACHRISSON and A. SPANGEUS. J Diabetes Complications 12 215-223 (1998). C.J. BAILEY, P.R. FLATr, P. KWASOWSKI, C.J. POWELL and V. MARKS. Acta Endocrinologica m 224-229 (1986). P.R. FLATr, C.J. BAILEY, P. KWASOWSKI, T. PAGE and V. MARKS, Journal of Endocrinologv. m 249-256 (1984). E. PENMAN, J.A. WASS, S. MEDBAK, L. MORGAN, J.M. LEWIS, G.M. BESSER and L.H. REES. Gastmenteroiogy fl692-699 (1981). R.M. ELLIOIT, L.M. MORGAN, J.A. TREDGER, S. DEACON, J. WRIGHT and V. MARKS, Journal of Endocrinology m 159- 166 ( 1993). R. EBERT, H. FRBRICHS and W. CRE UIZFELDT, European Joumal of Clinical Investigation 2 129135 (1979). B. BECK and J.P. MAX, Cellular & Molecular Biology a 555-562 (1987). R.A. tiEFRONZ0, E. FERRANNINI and V. KOIVISTO, American Journal of Medicine 24 52-81 (1983). E. CERASI, S. EPENDIC and R. LUF’I’, Lancet 1794-797 (1973). M.J. PERLEY and D.M. KIPNIS, Journal of Clinical Investigation 46 1954-1962 (1967). SE. CROCKETT, E.L. MAZZAFERRI and S. CATALAND, Diabetes 2 931-935 (1976). W. CRE-, M. TALAULICAR, R. EBERT and B. WILL&IS. Diabetes 2e 140-145 (1980). F.J. .&RVICE. R.A. RIZZA. R.E. WESTLAND, L.D. HALL, J.E. GERICH and V.L. GO, Journal of Clinical Endocrinology & Metabolism a 1133-l 140 (1984). D.L. SARSON, P.G. KOPELMAN, H.S. BES-IERMAN, T.R. PILKINGTON and S.R. BLOOM. Diabetologia z 386-391 (1983). W. CREUTZFELDT, Diabetologia 16 75-85 (1979). D. ELAH&D.K. ANDERSEN,D.C. MULLER, J.D. TOBIN, J.C. BROWN audR. ANDRI%S,Diabetes 2 950-957 (1984). M.A. NAUCK, M.M. HEIMESAAT. C. ORSKOV, J.J. HOLST, R. EBERT and W. CRB-, Journal of Cl&xl Investigation fi 301-307 (1993). A. KUBOTA, Y. YAMADA,T. HAYAMI, K. YASUDA, Y. SOMEYA, Y. IHARA, S. KAGIMCYIQ R. WATANABQ, T. TAMINATO, K. TSUDA and Y. SEINO, Diabetes fi 1701-1705 (1996). Y. YAMADA,T. HAYAMI, K. NAKAMURA, P.J. KAISAKI. Y. SOMEYA, C.Z. WANG, S. SEINO and Y. SEINO, Genomics 24.773-776 (1995). American Joumal of Physiology 2& E603-E607 G.H. STARICH, R.S. BAR and E.L. Mw, (1985). H. HAUNER, G. GLAT’I’ING, D. KAMINSKA and E.F. PFEIFFER, Annals of Nutrition 8c Metabolism ;12 282-288 (1988). B. BECK and J.P. MAX, Diabetologiaa 68 (1986). B. BECK and J.P. MAX, Regulatory Peptides 2 3-8 (1983).
Vol.66,No. 2.2000
86. 87. 88. 89. 90.
91. 92. 93. 94. 95.
GIPand Fat Metabolism
103
A.S.H. BABA and P.J. BUTTERFLY, Biochemical Society Transactions 1p 309s (1991). J. OBEN, L. MORGAN, J. FLETCHER and V. MARKS, Journal of Endocrinology m 267-272 (1991). J. DUPRE. N. GREENIDGE, T.J. MCDONALD, S.A. ROSS and D. RUBINSTEIN, Metabolism: Clinical & Experimental a 1197-l 199 (1976). T. WASADA, K. MCCORKLE. V. HARRIS, K. KAWAI, B. HOWARD and R.H. UNGER, Journal of Clinical Investigation 68 1106-l 107 (1981). R. EBERT. M. NAUCK and W. CREUTZFELDT, Hormone & Metabolic Research Z 517-521 (1991). R.H. ECKEL, W.Y. FUJIMOTO and J.D. BRUNZELL. Diabetes 28 1141-2 (1979). J.M. KNAPPER, SM. PUDDICOMBE, L.M. MORGAN and J.M. FLETCHER, Journal of Nutrition u 183-188 (1995). R.G.C. YIP. M.O. BOYLAN, T.J. KIEFFER and M.M. WOLFE, Endocrinology m 4004-4007 (1998). B. BECK and J.P. MAX, Hormone & Metabolic Research 2Q 24-27 (1988). A.B. DAMHOLT, A.M. BUCHAN and H. KOFOD. Endocrinology l3$! 2085-2091(1998).
PII s0024-3205(!I9)00314-8
LifeSciences,Vol.66, No. 2, pp. 91-103, 2000 Copyright8 1999ElsevierScienceInc. Printedin the USA.All rights resewed 0024-3205/00/$--see front matter
MINIREXIEW GIP BIOLOGY AND FAT METABOLISM Rupert G. C. Yip and M. Michael Wolfe Section of Gastroenterology, Department of Medicine, Boston University School of Medicine, Boston Medical Center, 88 East Newton Street, Evans 201, Boston, MA 02118, U.S.A. (Received in final form July 30, 1999)
summary The gastrointestinal hormone, gastric inhibitory polypeptide (GIP), is synthesized and released from the duodenum and proximal jejunum postprandially. Its release depends upon several factors including meal content and pre-existing health status (ie. obesity, diabetes, age, etc.). It was initially discovered and named for its gastric acid inhibitory properties. However, its more physiologically relevant role appears to be as an insulinotropic agent with a stimulatory effect on insulin release and synthesis. Accordingly, it was later renamed glucosedependent insulinotropic polypeptide because its action on insulin release depends upon an increase in circulating levels of glucose. GIP is considered to be one of the principle incretin factors of the enteroinsular axis. The GIP receptor is a G-protein-coupled receptor belonging to the family of secretinMP receptors. GIP receptor mRNA is widely distributed in peripheral organs, including the pancreas, gut, adipose tissue, heart, adrenal cortex, and brain, suggesting it may have other functions in addition to the ones mentioned above. An overactive enteroinsular axis has been suggested to play a role in the pathogenesis of diabetes and obesity. In addition to stimulating insulin release, GIP has been shown to amplify the effect of insulin on target tissues. In adipose tissue, GIP has been reported to (1) stimulate fatty acid synthesis, (2) enhance insulin-stimulated incorporation of fatty acids into triglycerides, (3) increase insulin receptor a&r&y, and (4) increase sensitivity of insulinstimulated glucose transport. In addition, although controversial, lipolytic properties of GIP have been proposed. The mechanism of action of GIP-induced effects on adipocytes is unknown, and it is unclear whether these effects of GIP on adipocytes are direct or indirect. However. there is now evidence that GIP receptors are expressed on adipocytes and that these receptors respond to GIP stimulation. Given the location of its release and the timing of its release, GIP is an ideal anabolic agent and expanding our understanding of its physiology will be needed to determine its exact role in the etiology of diabetes mellitus and obesity. Key Words: GIP, fat, adipocytes, metabolism In 1886, Ewald and Boas (1) showed that olive oil mixed with a meal inhibited both gastric emptying and acid secretion. Kosaka and Lim (2,3) theorized that this mixture liberated a chemical from the small intestine and named this chemical “‘enterogastrone”. During the ensuing decades, the search was
92
GlPNKiRilMaabdism
Vol. 66, No. 2,2MJO
on for a gastrointestinal hormone exerting this effect. It was found that acids, fats, and hypertonic solutions all inhibited acid secretion, which consequently led to the suspicion that several enterogastrones might in fact exist. In searching for a novel gut hormone, Brown and Pederson (4) observed that a crude preparation of porcine cholecystokinin (CCK) produced less stomach acid secretion than a purer preparation of CCK from the duodenum. They concluded that during the purification process, a factor must have been removed that inhibited acid secretion. Later, Brown and colleagues (5-7) isolated this factor, which was found to be localized in the duodenum and jejunum (8.9) in specific endocrine cells designated K-cells (10). Due to its ability to inhibit gastric acid secretion, the hormone was named “gastric inhibitory polypeptide” (GIP) (11). However, after its isolation, it was found that GIP possessed another important biological property, the ability to potentiate insulin secretion (12). Numerous studies have shown that GIP is released from the duodenum and jejunum in response to the ingestion of a meal containing glucose or fat (13). The released GIP, in combination with postprandial hyperglycemia, induced insulin release from pancreatic islet &cells. This “incretin” property attributed to GIP was considered more physiologically relevant than its inhibitory effect on acid secretion, and GIP was accordingly renamed “glucose-dependent insulinotropic polypeptide”, thus retaining the original acronym. GIP secretion is very sensitive to acute and chronic changes in diet, especially changes in dietary lipid content. The degree to which fat stimulates GIP secretion is also species-dependent. In humans, who normally consume a diet relatively high in lipid content (40% energy from fat in a typical Western diet), fat is a more potent stimulator of GIP release than carbohydrates. In contrast, in rats and pigs, whose diets usually contain ~10% energy as fat, carbohydrates are more potent than fat in stimulating GIP release (14). Moreover, the postprandial level of circulating GIP is dependent on meal size (15,16), and the contribution of the enteroinsular axis is proportionately greater after a large meal. Acute exercise training has no effect on GIP secretion. Chronic exercise, however, has been shown to increase GIP secretion in pre- and early adolescent children (17), but has no effect in obese adult women (18). There is also an apparent correlation between age and GIP plasma levels. In general, the older the subject, the higher their GIP levels, although, it is unknown whether this correlation is due to age or due to the fact that older subjects tend to have slightly higher body mass indices (BMI) compared to younger adults. There is also a significant correlation between gender and GIP levels, with males having higher circulating levels of the peptide (19). Although only the rat GIP is shown in Figure 1A the amino acid sequence for human, porcine, bovine, and rat are remarkably similar, with greater than 90% homology. In these species, GIP is a 42 amino acid polypeptide that includes a consensus tryptic sequence (KGKK) at positions 30-33. This sequence has led to the possibility of truncated forms of GIP in the small intestinal mucosa and in the circulation. Indeed, the GIP hormone itself is derived from the proteolytic processing of its preprohormone precursor (Figure 1B). Although the mature polypeptide is 42 amino acids in length, only the first 30 amino acids are required to produce its biological effects and are sufficient for ligand binding and receptor activation (20), consistent with the existence of a biologically active GIP in its truncated form in viva Sequence analysis of the human GIP cDNA clones has demonstrated a 459-bp open reading frame encoding a 153~amino acid polypepti& and a 432&p open reading frame encoding a 14kmino acid polypeptide in the rat (21-23) (Fig. 2). The predicted amino acid sequences indicate that both the human and rat GIP are derived by proteolytic processing of a preprohormone. The human preproGIP has a 5 1-amino acid N-terminal segment and a 60-amino acid C-terminal segment flanking the 42amino acid GIP moiety. The N-terminal segment contains a potential signal peptide with a possible cleavage site at glycine 21. The rat preproGIP is smaller in length than the human precursor by nine
Vol.66. No. 2.2000
GIPandFatMetabolism
93
FIG.1. The ratglucose-dependent insulinotropic polypeptide or gastric inhibitory polypeptide is a 42 amino acid polypeptide (A). Although there is species differences. the degree of homology is greater then 90%. suggesting conservation through mammalian evolution. The first 2 amino acid residues YA are cleaved by dipeptidyl peptidase IV rendering the polypeptide biologically inactive. At position 30. a tryptic cleavage site (I$GKK) exists suggesting the possibility of furthertruncated forms of GIF’in vivo.
residues. The N-terminal segment of the rat preproGIP consists of 43 amino acids while the C-terminal segment is 59 amino acids in length. The rat precursor contains two potential cleavage sites for signal peptidase. The signal peptide of preproGIP is cleaved following either glycine 19 or glycine 21. It appears that in both human and rat, mature GIP( l-42) is generated by proteolytic processing at arginine residues that immediately flank the region corresponding to the GIP moiety (2 1,22,24).
Recently, GIP(7-42) was puritied from the upper part of the porcine intestine and was shown to have antibacterial activity (25). Although it has been presumed that the peptide is also derived by proteolytic cleavage of the preproGIP, it is unclear whether this cleavage is a physiological phenomenon or rather an extraction artifact. In addition, the concentrations needed for antibacterial action are high, implying a non-physiological effect. Although no biological properties have yet been attributed to either the N-terminal or the C-terminal peptides of preproGIP, the C-terminal segment does share homology with regions of both pancreastatin and chromagranin A. The function of chromagranin A is unknown, while pancreastatin appears to inhibit insulin release from pancreatic gcells (24). Computer analysis of the predicted amino acid sequence of GIP has demonstrated structural homology between GIP and other members of the secretin family of gastrointestinal regulatory polypeptides, which include secretin, glucagon, glucagon-like peptide (GLP-1 and GLP-2), vasoactive intestinal polypeptide (VIP), peptide histidine isoleucine (PHI), growth hormone releasing hormone (GHRH), and pituitary adenylyl cyclase-activating polypeptide (PACAP) (21). Like GIP, the biosynthesis of other members of the secretin family involves proteolytic processing of their respective preprohormone (26). Although the structural arrangements of these precursors are the same, the sequence homology observed is restricted only to the biologically active portion of the precursor polypeptides, with little similarity in the other domains of the molecule (21).
The half-life of immunoreactive GIP in humans has been estimated at roughly 20 min as measured by radioimmunoassay (RIA) (27-30). The mature GIP polypeptide, GIP(1-42), is susceptible to proteolytic digestion by dipeptidyl peptidase IV present in serum (31,32). Cleavage of GIP(1-42) at
94
GIP and Fat Maabolism
SIGNAL
Vol. 66, No. 2,uKx)
PEPTIDE:
Met-Val-Ala-Thr-Lys-Thr-Phe-Ala-Leu-Leu-Leu-Leu-Ser-Leu-Phe-Leu-Ala-Val-Gly-LeuGly
N-TERMINAL
PEPTIDE:
Glu-Lys-Lys-Glu-Gly-His-Phe-Ser-Ala-Leu-Pro-Ser-Leu-Pro-Val-Gly-Ser-His-Ala-LysVal-Ser-Ser-Pro-Gln-Pro-Arg-Gly-Pro-Arg
MATURE
GIP:
Tyr-Ala-Glu-Gly-Thr-Phe-lle-Ser-Asp-Tyr-Ser-lle-Ala-Met-Asp-Lys-lle-His-Gln-Gln-AspPhe-Val-Asn-Trp-teu-Leu-Ala-Gln-Lys-Gly-Lys-Lys-Asn-Asp-Trp-Lys-His-Asn-lle-ThrGln
C-TERMINAL
PEPTIDE:
Arg-Glu-Ala-Arg-Ala-Leu-Glu-Leu-Ala-Ser-Gin-Ala-Asn-Ar~Lys-Glu-Glu-Glu-Ala-Val-Glu-Pr~Gln~ Ser-Ser-Pro-Ala-Lys-Asn-Pro-Ser-Asp-Glu-Asp-Leu-Leu-Arg-As~Leu-Leu-lle-Gln-Glu-Leu-LeuAla-Cys-Leu-Leu-Asp-Gln-Thr-Asn-Leu-Cys-Arg-Leu-Arg-Ser-Arg FIG. 2.
The human preprohonnone (B) consist of a 153 amino acid polypeptide. Proteolytic processing of the preprohormoneyields a signal peptide, N-km&al peptidc the mature peptide. and a C-terminal’peptide. the N-terminal second position results in GIP(3-42), which is biologically inactive (3 1,32). Although there may be many diierent proteases in serum, the predominant GIP-degrading protease appears to be dipeptidyl peptidase IV (33). When using immunological methods to examine honuone metabolism, one should be aware of the fact that immunoreactivity may not necessarily correlate with biological activity. For example, it is possible for GIP to be proteolytically degraded and rendered biologically inactive while still retaining immunoreactivity as measured by RIA. Thus, the true biological half-life of GIP may actually be less than 20 min.
The GIP receptor was first cloned in 1993 from rat (34) and subsequently cloned in hamster (35) and human (36). The processed rat GIP receptor has a predicted molecular weight of -50 kDa, but covalent cross-linking studies have shown the GIP receptor migrating with an apparent molecular weight of 64 kDa (37,38). The receptor is a glycoprotein belonging to the secretin/VIP family of receptors, a seven transmembrane G-protein-coupled receptor family that includes receptors for secretin, VIP, glucagon, GLP-1, growth hormone releasing hormone (GHRH), and PACAP. The Nterminus contains a consensus sequence for N-glycosylation (N-X-W) (37), and the third cytoplasmic loop and the C-terminus are rich in threonine and serine as potential phosphorylation sites (Figs. 3 & 4). The GIP receptor, as with other receptors in the secretinMP family of receptors, has a large Nterminal domain. Unlike adrenergic receptors, however, in which the ligand binds to a pocket formed by the seven transmembrane domains, receptors belonging to the secretinMP family bind ligands via
Vol. 66. No. 2 2000
a~ and Fat Me&aWism
FIG. 3. T%e rat GIP receptor is a seven trammembrane-spanning receptor. It belongs to the secretin/VP family of receptors. The extensive N-terminal extracellular damains of this family of receptors are usually glycosylati and form the recognition and binding domain of the receptor.
GIF sad Fat Metabolism
%
I
I
I
Vol. 66.No.2.2000
I
I
1
MPLRLLLLLLWLWGLSLQRTDSEGQTTGELYQRWERYGWECQNTLEATEPPSG~C~
61
SFDMYACWNYTAANTTARVSCPWYLPWYRQVAAGFVFRQCGSDGQWGSWRDHTQCENPEK m =L
121
NGAFQDQKLILEsALHCTRNYIqk
I
I
I
I
I
I
I
I
181
241
LLVVVRRSEK&+&YLYEN;QCWERNEV&
301
I
I
I
QAEGBA
421
RAVPLSSAPQEAAIRNALPSGMLHVPGDEVLESYC
---
_--A
I
I
I
I
EVQSEIRRLRLSLQEQCPRPHLGQAP -
361
I
Potential Casein Kinase 2 sites Potential N-Glycosylation sites _...-._ 1 Potential Protein Kinase C sites Poldfacel represents putative transmembrane domains FIG.4. The primary sequence of the rat GIP receptor. The GIP receptor is also speculated to be glycosylated as there are several consensus glycosylation sites. In addition, there are putative sites for phosphorylation by cssein ldnase 2 and protein kinase C for posttranslational modification. their large N-terminal domain (3945). Chimeric GWGLP-1 receptor studies have shown that the first 132~amino acids of the N-terminal domain is necessary for high-affinity binding of GIP, but receptor coupling to CAMP production requires the first 222 N-terminal amino acids (46). It is generally assumed that GIP binding to its receptor activates a heterotrimeric stimulatory Gprotein, which in turn activates adenylyl cyclase. The resulting increase in intracellular CAMP and a subsequent increase in Ca*’ influx via voltage-gated Ca” channels are thought to represent the major signalling pathways by which GIP exerts its insulinotropic effects (47,48). Recently, an additional but distinct signal transduction pathway for GIP was described. As stated above, GIP and its receptor are closely related to VIP and PACAP and their respective receptors. Straub and Sharp (49) have shown that while VIP and PACAP increase intracellular CAMP and Ca”
Vol. 66. No. 2,200O
GIP and Fat Metabolism
97
only during the frost 20 mitt of stimulation, the release of insulin from HIT cells continues for greater than 50 min. long after CAMP and Ca” levels have returned to baseline levels. It was thus shown that the prolonged stimulatory mechanism might be wortmannin sensitive (49). It was later shown that wortmannin, a potent inhibitor of phosphatidylinositol3 kinase (PI 3kinase). was able to inhibit GIPstimulated insulin release, suggesting that GIP does utilize a wortmannin-sensitive mechanism for signal transduction (50). Additionally, Kubota et al. (5 1) demonstrated that GIP receptor activation with 0.59 nM GIP was sufficient to activate mitogen-activated protein (MAP) kinase. The study went on to show that MAP kinase activation by GIP was via wortmannin-sensitive and -insensitive pathways. Thus there is sufficient evidence to indicate that GIP recruits intracellular signalling mechanisms other than cyclic AMP. Northern blot, RT-PCR, and in situ hybridization studies have shown GIP receptor mRNA present in the pancreas, stomach, intestine, adrenal cortex, heart, lung, brain, endothelium of major blood vessels, and adipose tissue. There is also ample GIP receptor mRNA and protein expression in the rat testes (unpublished data). Most of these regions have not been previously considered major targets for GIP, and although receptor mRNA may be present in these tissues, no Western or immunocytochemical data have as yet been reported. The locations of GIP receptor transcripts may suggest novel actions of GIP or possibly novel ligand(s) for the GIP receptor. Several recent reports of ectopically expressed GIP receptors on adrenal gland have been published (52-54). These patients exhibit food-dependent Cushing’s syndrome. GIP receptors have been found in the rat adrenal cortex, but until recently, no information has been available on the presence or absence of GIP receptors in human adrenals. In situ hybridization studies have demonstrated the expression of GIP receptor transcripts in adrenocortical adenoma samples taken from patients with food-dependent Cushing’s syndrome, while in situ hybridization failed to detect the GIP receptor in adrenal adenoma patients without food-induced Cushing’s syndrome (54). Kaplan and Vigna (55) have studied GIP binding sites in the rat brain. Using [‘2SI]GIP,displaceable GIP binding sites were localized to cortical areas of the telencephalon, and forebrain regions. Interestingly, no [‘zsI]GIP binding was detected in the anterior pituitary despite a report that GIP altered growth hormone (GH) and follicle stimulatory hormone (FSH) secretion (56). However, in this latter study, pharmacological doses of GIP were administered, and it is possible that other related receptors may have been activated by GIP and thus responsible for the changes in GH and FSH secretion. Although the notion that GIP receptors are present in the brain is interesting, there is, at present, no evidence for GIP synthesis in the brain, and the only source of GIP in brain may thus be systemic. Alternatively, novel GIP-like ligand(s) may be binding to GIP receptors in the brain.
tw_en
.
.
The relationship between GIP and obesity/diabetes has long been controversial. GIP concentrations may be influenced by obesity, diabetes, and glucagon deficiency and may be under negative feedback by insulin. Obesity is characterized with fasting hyperinsulinemia, and the ingestion of carbohydrates results in an exaggerated insulin response (5760). GIP has been proposed as a major mediator of the enterically induced component of insulin secretion (61) and one potential explanation for the increase in insulin levels has been that of an overactive enteroinsular axis (62). The timely release of GIP postprandially likewise make it an ideal anabolic agent in the etiology of obesity. Currently, it is unknown whether the alterations in GIP physiology are a cause or consequence of obesity/diabetes. In one case, alterations in GIP secretion were observed before the onset of diabetes (63). Additionally, ob/ob mice chronically fed a high fat diet have been shown to have increased concentrations of GIP in plasma and in the intestinal mucosa as well as increased density of GIP-
98
GIP and Fat Metabolism
Vol. 66, No. 2,2UOO
secreting K-cells in the upper jejunum. In contrast, a high carbohydrate diet had no such effect (64). Moreover, the extent of hyperglycemia and hyperinsulinemia present in ob/ob mice was not significantly altered by high fat or high carbohydrate diets (64), while orally administered fat, glucose, and amino acids increased GIP concentrations (65). In humans, fat constitutes the most potent stimuhis for GIP secretion (66), and plasma GIP levels are closely correlated with plasma triglyceride levels (67). In obese human subjects, oral triglyceride elicited a greater increase in serum GIP levels when compared to normal weight subjects (68). In the same study, obese subjects exhibited elevated basal insulin levels and higher levels following an oral triglyceride load. However, it was unknown whether GIP was responsible for the increased levels of insulin. In the obese Zucker rat model, there appears to be hypersensitivity of adipose tissue to Gil? GIP-stimulated fatty acid incorporation into fat tissues was greater for all concentrations tested when compared to lean Zucker rats (69). However, in the Zucker rat model, plasma GIP concentrations, in response to an oral glucose challenge, remained similar to levels in lean rats. It is possible that in this model the increased fatty acid incorporation may be the result of increased affinity for GIP by its receptor or increased potency of GIP. With that in mind, the following hypothesis may be proposed: individuals with a condition of increased GIP responsiveness may be more prone to obesity and hyperinsulinemia. Increased GIP responsiveness may manifest itself in the form of either increased affinity for GIP by GIP receptors, increased GIP receptor number, or increased synthesis and secretion of GIP following normal meal stimulation. The increased GIP responsiveness would likely increase insulin release resulting in greater and prolonged hyperinsulinemia. To compound this abnormality, GIP can also increase the affinity of the insulin receptor for insulin as well as increase the insulin-like effects in fat cells, all of which may serve to increase efftciency in the accumulation of triglycerides in fat cells. Thus, anecdotal accounts of individuals accumulating fat tissue mass at greater rates under normal meal conditions may be a consequence of increased GIP responsiveness. Such individuals have greater efficiency in fat storage as a result of increased GIP and insulin responses when compared with normal persons. Although insulin resistance is a constant feature of noninsulin dependent diabetes mellitus (NIDDM) the insulin response to different stimuli can be exaggerated, normal, or impaired (70). The reasons for these differences in insulin secretion have not been elucidated, although a disturbance in the enteroinsular axis has been proposed (7 1,72). Published reports have shown great variability in both basal and stimulated GIP levels in obesity and NIDDM. In NIDDM patients, the GIP response to glucose and other stimuli has been increased (27,73), normal (74), or decreased (75). In obese subjects, GIP concentrations have been either normal (76) or increased (77). While advanced age has been shown to be associated with a normal GIP response to oral glucose, p-cell sensitivity to GIP was decreased (78), and plasma GIP levels appear to be elevated (19). A recent study demonstrated that the insulinotropic effect of GIP was selectively reduced in diabetic patients, whereas that of GLP-1 was preserved (79). suggesting that abnormalities in GIP release and function may be involved in the pathogenesis of diabetes mellitus. The GIP receptor gene may be involved in the pathogenesis and development of NIDDM. Kubota et al. (80) have recently identified two missense mutations in the GIP receptor gene: Glyigs-Cys and G1u354*Gln.Both Glylg8 and Glu3” are conserved among human, rat, and hamster GIP receptors (34,35,8 1). The substitutions of a thiolcontaining Cys residue for Gly and that of uncharged Gln residue for the acidic Glu were expected to affect protein structure and function. In fact, it was found that GIP-stimulated CAMP formation was considerably impaired with the Gly’g*-Cys mutation, but was normal for the Glu3”-Gln mutation. Examination of the allelic frequencies of Gly’*e Cys and Glu3M-Gln in NIDDM and control subjects, however, showed no differences in frequency, suggesting that these mutations do not contribute significantly to the loss of GIP induced secretion in NIDDM patients. However, because NIDDM is polygenic, the possibility that these or other unidentified mutations of the GIP receptor may represent a risk factor in a small subset of individuals with NIDDM cannot be excluded.
Vol. 66, No. 2,200O
GE and Fat Metabolism
99
There is some evidence to indicate that GIP may affect insulin sensitivity of adipocytes. Incubation of isolated adipocytes with GIP was shown to increase receptor a&&y and insulin-stimulated glucose transport (82). However, the concentrations used were supraphysiological and the observed changes minimal, with an IC, change from 8.6 rig/ml to 6.6 ng/ml. A more likely effect of GIP is to modulate the effects of other hormones on adipocyte metabolism. GIP has been shown to stimulate glucose transport in isolated rat adipocytes and the incorporation of glucose into extractable lipids in one study (83), but not in another (84). A study by Beck and Max has shown that GIP alone was capable of inducing a slight decrease in fatty acid incorporation into adipose tissue, while in the presence of insulin, GIP significantly enhanced insulin-induced fatty acid incorporation into adipose tissue (85). In that study, the authors speculated that a hyperactive enteroinsular axis in obese individuals might be a factor in the development of obesity (83). However, in contrast to the findings in rodents, Baba and Butterlly showed that GIP stimulated lipogenesis, but reduced insulin-stimulated lipogenesis (86). GIP and GLP- 1, in common with insulin, but not GLP-2 and glucagon, stimulate fatty acid synthesis in explants of rat adipose tissue, as measured by the incorporation of [i4C]acetate into saponifiable fat (87). GIP and GLP-1 may thus contribute in vivo to a more effective postprandial uptake of glucose and may enhance the effect of insulin on fatty acid synthesis from glucose as precursor. Consistent with this anabolic role, GIP has been shown to inhibit the lipolytic action of glucagon in adipocytes (88), which may occur as a result of competitive inhibition of glucagon binding to its receptor. In humans, the accumulation of adipose-tissue triacylglycerol from dietary fat is quantitatively more important than de nova lipogenesis. Adipose lipoprotein lipase (LPL) plays a key role in the hydrolysis of circulating triacylglycerol, liberating non-esterilied fatty acids for uptake and storage within the adipocyte. In animal studies, exogenous GIP infusion has been shown to promote the clearance of chylomicron triglycerides (89) and to lower postprandial circulating triacylglycerol levels (90). GIP has been shown to stimulate the synthesis and release of LPL in cultured mouse 3T3-Ll preadipocytes (91). GIP, in common with insulin, stimulates LPL activity in explants of rat adipose tissue, although GLP- 1 over the same concentration range has no effect (92). In rats, a high-fat diet increases GIP and insulin secretion and elevates both basal and insulin- and GIP-stimulated LPL activity compared with control animals (87). Under conditions of high fat and energy feeding, higher circulating levels of GIP and insulin, together with an enhanced sensitivity of LPL to these hormones, may therefore facilitate the uptake of circulating triacylglycerol and contribute to increased adiposity in some circumstances. At present, the signaling mechanisms mediating the effects of GIP on fat cells are unknown. It was speculated that GIP may function via inhibition of glucagon action by competitively binding to glucagon receptors (88) Although the GIP receptor cDNA has been shown to be present in adipose tissue by polymerase chain reaction (PCR) (34) no direct evidence for GIP receptors on adipocytes had been reported. Recently, we have found evidence of GIP receptor expression in rat adipocytes and in the adipocyte cell model, the differentiated 3T3-Ll mouse fibroblast (93). As stated previously, the GIP receptor is a G-protein coupled receptor that is presumably activated via a cyclic AMP-mediated mechanism. In our study, but not others (88,94), an increase in cyclic AMP in rat adipose tissue and in differentiated 3T3-Ll adipocytes in response to GIP stimulation was detected. These differences may reflect the use of a 3T3-Ll adipocyte model in the presence of isobutylmethlyxanthine (IBMXa potent cyclic AMP phosphodiesterase inhibitor) versus rat adipocytes in the absence of IBMX.
Presently, the two principle incretins are GIP and GLP-1. GIP is ideally located in the duodenum and proximal jejunum, in contrast to GLP-1, which is found in highest concentration in the distal jejunum
100
GlPandFatMetabelism
Vol. 66, No. 22000
and ileum. GIP may thus play a greater physiological role as an insulinotropic and anabolic agent. Moreover, there is evidence that GIP regulates the release of GLP-1(95), at least in the dog. The finding that GIP receptors are found in other organs, such as pancreas, stomach, intestine, adrenal cortex, heart, lung, brain, and endothelium of major blood vessels, suggests other possible functions for this hormone. As with other “permissive” hormones, GIP may not produce a direct stimulator-y effect on its target cells, but rather a modulatory effect on actions of other hormones. Further, work will be needed to clarify the precise role of GIP in the pathogenesis of diabetes and obesity. While there is one report of elevated serum GIP levels prior to the onset of diabetes mellitus (63), the GIP response to glucose and other stimuli has been reported to be increased (27,73), normal (74). or decreased (75). There is even speculation of a mutant form of the GIP receptor in certain subtypes of NIDDM (80). It is also possible that increased GIP responsiveness will result in increased efficiency in fat accumulation and hyperinsulinemia. A greater understanding of the biology of GIP and its receptor will help to resolve the importance of the enteroinsular axis in health and disease.
1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
C.A. EWALD and J. BOAS, Virchows Archives m 271-305 (1880). T. KOSAKA and R.K.S.LIM. Chinese Journal of Physiology 4 213-220 (1930). T. KOSAKA and R.K.S. LIM, Proceeding of the Society for Experimental Biology and Medicine 22 890-891 (1930). J.C. BROWN and R.A. PEDERSON, Scandinavian Journal of Gastroenterology 2 537~541(1970). J.C. BROWN, V. MUTT and J.R. DRYBURGH, Canadian Journal of Physiology & Pharmacology & 399-405 (1971). J.C. BROWN, V. MUTT and R.A. PEDERSON, Journal of Physiology ZQ$!57-64 (1970). J.C. BROWN and J.R. DRYBURGH, Canadian Journal of Biochemistry& 867-872 (1971). J.M. POLAK. S.R. BLOOM, M. KUZIO, J.C. BROWN and A.G. PEARSE. Gut 14 284-288 (1973). R. BUFFA, J.M. POLAK, A.G. PEARSE. E. SOLCIA. L. GRIMELIUS and C. CAPELLA, Histochemistry Q 249-255 (1975). A.M. BUCHAN, J.M. POLAK, C. CAPELLA, E. SOLCIA and A.G. PEARSE, Histochemistry ti 3744 (1978). J.C. BROWN, R.A. PEDERSON, E. JORPES and V. MUTT, Canadian Journal of Physiology & PharmacologyQ 113-114(1969). J. DUPRE, S.A. ROSS, D. WATSON and J.C. BROWN, Journal of Clinical Endocrinology & Metabolism u 826-828 (1973). L.M. MORGAN, P.R. FLATT and V. MARKS, Nutrition Research Reviews 179-97 (1988). L.M. MORGAN. J.M. KNAPPER, J.M. FLEICHBR and V. MARKS, Proceedings of the Society of Nutrition Physiology 4 1l-20 (1995). S.M. HAMPTON, L.M. MORGAN, J.A. ‘IREDGER, R. CRAMB and V. MARKS, Diabetes X 612616 (1986). MC. MURPHY, S.G. ISHERWOOD, S. SEIHI, B.J. GOULD, J.W. WRIGHT, J.A. KNAPPER and CM. WILLIAMS, European Journal of Clinical Nutrition a 578-588 (1995). E.B. KAHLE, O.D. TM, R.B. WALKER, P.A. EISENMAN. S. REISER. S. CATALAND and W.B. ZIPF, Diabetes s 579-582 (1986). M. KRCYTKIEWSKI.P. BJGRNTORP, G. HOLM, V. MARKS, L. MORGAN, U. SMITH and G.E. FEURLE, International Journal of Obesity 8 193- 199 (1984). J.G. SCHWARTZ, C.A. MCMAHAN, G.M. GREEN and W.T. PHILLIPS, Digestive Diseases & Sciences fi 624-630 (1995). B. GAJLWllZ, M. WI’IT, C. MORYS-WORTMANN, U.R. FGLSCH and W.E. SCHMIDT, Regulatory Peptides fi 17-22 ( 1996). C.C. TSBNG, L.A. JARBOE, S.B. LANDAU, E.K. WILLIAMS and M.M. WOLFE, Proceedings of the National Academy of Sciences of the United States of America a 1992-1996 (1993).
Vol.66. No. 2.2000
22.
23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42.
43. 44. 45. 46. 47. 48. 49. 50. 51.
GlP and Fat Metabolism
101
J. TAKEDA, Y. SEINO. K. TANAKA, H. FUKUMOTO, T. KAYANO, H. TAKAHASHI, T. MITANI, M. KURONO, T. SUZUKI, T. ‘IGBE and H. IMURA. Proceedings of the National Academy of Sciences of the United States of America 84 7005-7008 (1987). Y. HIGASHIMO’I0, E.C. OPARA and R.A. LIDDLE, Comparative Biochemistry & Physiology. Part C Pharmacology, Toxicology, Endocrinology Ilp 207-214 (1995). Y. HIGASHIMO’IO, J. SIMCHOCK and R.A. LIDDLE, Biochirnica et Biophysics Acta lK2 72-74 (1992). B. AGERBERTH. A. BOMAN, M. ANDERSSON, H. JORNVALL, V. MU’lT and H.G. BOMAN, European Journal of Biochemistry u 623-629 (1993). G.I. BELL, Peptides 2 27-36 (1986). J.C. BROWN, J.R. DRYBURGH, S.A. ROSS and J. DUPRE, Recent Progress in Hormone Research 11487-532 (1975). D. ELAHI, D.K. ANDERSEN, J.C. BROWN, H.T. DEBAS, R.J. HERSHCOPF, G.S. RAIZES, J.D. TOBIN and R. ANDRES, American Journal of Physiology m El 85- 19 1 (1979). M. NAUCK. W.E. SCHMIDT, R. EBERT, J. STRIE’IZEL, P. CANTOR, G. HOFFMANN and W. CREU’IZFELDT, Journal of Clinical Endocrinology & Metabolism he 654-662 (1989). D.L. SARSON, R.C. HAYTER and S.R. BLOOM, European Journal of Clinical Investigation 12 457461 (1982). T.J. KIEFFER, C.H. MCINTOSH and R.A. PEDERSON, Endocrinology m 3585-3596 (1995). R. MENTLBIN, B. GALLWI’IZ and W.E. SCHMIDT, European Journal of Biochemistry 214 829-835 (1993). R.A. PEDERSON, T.J. KIEFFBR, R. PAULY, H. KOFOD, J. KWONG and C.H. MCINTOSH, Metabolism: Clinical &Experimental & 1335-1341 (1996). T.B. USDIN, E. MEZEY, D.C. BUTTON, M.J. BROWNSTEIN and T.I. BONNER, Endocrinology m 2861-2870 (1993). K. YASUDA, N. INAGAKI, Y. YAMADA, A. KUBOTA, S. SEINO and Y. SEINO, Biochemical & Biophysical Research Communications m 1556-1562 (1994). A. VOLZ, R. GiiKE, B. LANKAT-BUTTGEREIT, H.C. FEHMANN, H.P. BODE and B. GOKE, FBBS Letters m 23-29 (1995). B. AMIRANOFF, A. COUVINEAU, N. VAUCLIN-JACQUES and M. LABURTHE, European Journal of Biochemistry u 353-358 (1986). A. COUVINEAU, B. AMIRANOFF, N. VAUCLIN-JACQUES and M. LABURTHE, Biochemical and Biophysical Research Communications 122 282-288 (1984). J.J. BUGGY, J.N. LIVINGSIGN, D.U. RABIN and H. YOO-WARREN, Journal of Biological Chemistry 21a 7474-7478 (1995). A. COUVINBAU, P. GAUDIN, J.J. MAORET, C. ROUYER-FESSARD, P. NICOLE and M. LABURTHE, Biochemical & Biophysical Research Communications 246 246-252 (1995). P. GAUDIN, A. COWINEAU, J.J. MAORET, C. ROUYER-FESSARD and M. LABURTHE, Biochemical & Biophysical Research Communications 2.l.I 901-908 (1995). H. JUPPNER, E. SCHIPANI, F.R. BRINGHURST, I. MCCLURE, H.T. KEUTMANN, J.T. POTTS, JR, H.M. KRONENBERG, A.B. ABOU-SAMRA, G.V. SEGRE and T.J. GARDELLA, Endocrinology m 879-884 (1994). B. VAN EYLL, B. Gt%E, A. WILMEN and R. GGKE, Peptides n 565-570 (1996). A. WILMEN. B. VAN EYLL, B. GiiKE and R. G&E, Peptides .Ej 301-305 ( 1997). J.P. VILARDAGA, P. DE NEEF, E. DI PAOLO, A. BOLLEN, M. WAELBROECK and P. ROBBERECHT, Biochemical & Biophysical Research Communications u 885-891 (1995). R.W. GELLING, M.B. WHEELER, J. XUE, S. GYOMOREY, C. NIAN. R.A. PEDERSON and C.H. MCINTOSH, Endocrinology m 2640-2643 (1997). M. LU. M.B. WHEELER, X.H. LBNG and A.E.D. BOYD, Endocrinologym 94-100 (1993). Y. MIURA, M. KATO, K. OGINO and H. MATSUI, Endocrinology m 2769-2775 (1997). S.G. STRAUB and G.W.G. SHARP, Journal of Biological Chemistry a 1660- 1668 ( 1996). S.G. STRAUB and G.W. SHARP, Biochemical & Biophysical Research Communications 224 369-374 (1996). A. KUBOTA, Y. YAMADA, K. YASUDA, Y. SOMEYA, Y. IHARA, S. KAGIMOTO, R. WATANABE, A. KUROE, H. ISHIDA and Y. SEINO, Biochemical & Biophysical Research Conununications~ 171-175 (1997).
102
52.
53.
54.
55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85.
GlP andFat Metabolism
Vol.66, No. 2,2ooO
A. LACROIX, E. BOLTE, J. TREMBLAY, J. DUPRE. P. POITRAS, H. FOURNIER, J. GARON, D. GARREL, F. BAYARD, R. TAILLEFER, R.J. FLANAGAN and P. HAMET, New England Journal of Medicine m 974-980 (1992). Y. REZNIK, V. ALLALI-ZERAH, J.A. CHAYVIALLE. R. LEROYER. P. LEYMARIE, G. TRAVQRT. M.C. LEBRETHON, I. BUDI, A.M. BALLIERE and J. MAHOUDEAU, New England Journal of Medicine =981-986 (1992). W.W. DE HERDER, L.J. HOFLAND, T.B. USDIN, F.H. DE JONG, P. UI-lTERLINDEN, P. VAN KOETSVELD, E. MEZEY, T.I. BONNER, H.J. BONJER and S.W. LAMBERTS, Journal of Clinical Endocrinology & Metabolism BL 3 168-3 172 (1996). A.M. KAPLAN and S.R. VIGNA, Peptides fi 297-302 (1994). A. O’ITLECZ, W.K. SAMSON and S.M. MCCANN, Peptides 4 115-l 19 (1985). J.H. KARAM, G.M. GRODSKY and P.H. FORSHAM. Diabetes 12 197-205 (1963). J.D. BAGDADE, E.L. BIERMAN and D. PORTE, JR., Journal of Clinical Investigation 46 1549-1557 (1967). R.A. KREISBERG, B.R. BOSHELL, J. DIPLACIDO and R.F. RODDAM, New England Journal of Medicine 226 314-319 (1967). R.S. YALOW, S.M. GLICK, J. ROTH and S.A. BERSON, Annals of the New York Academy of Sciences 11L 357-373 (1965). W. CREUTZFELDT, Gastroenterology fl748-750 (1974). W. CREUTZFELDT, R. EBERT, B. WILLMS, H. FRERICHS and J.C. BROWN, Diabetologia 14 15-24 (1978). M. ELSALHY, S. ZACHRISSON and A. SPANGEUS. J Diabetes Complications 12 215-223 (1998). C.J. BAILEY, P.R. FLATr, P. KWASOWSKI, C.J. POWELL and V. MARKS. Acta Endocrinologica m 224-229 (1986). P.R. FLATr, C.J. BAILEY, P. KWASOWSKI, T. PAGE and V. MARKS, Journal of Endocrinologv. m 249-256 (1984). E. PENMAN, J.A. WASS, S. MEDBAK, L. MORGAN, J.M. LEWIS, G.M. BESSER and L.H. REES. Gastmenteroiogy fl692-699 (1981). R.M. ELLIOIT, L.M. MORGAN, J.A. TREDGER, S. DEACON, J. WRIGHT and V. MARKS, Journal of Endocrinology m 159- 166 ( 1993). R. EBERT, H. FRBRICHS and W. CRE UIZFELDT, European Joumal of Clinical Investigation 2 129135 (1979). B. BECK and J.P. MAX, Cellular & Molecular Biology a 555-562 (1987). R.A. tiEFRONZ0, E. FERRANNINI and V. KOIVISTO, American Journal of Medicine 24 52-81 (1983). E. CERASI, S. EPENDIC and R. LUF’I’, Lancet 1794-797 (1973). M.J. PERLEY and D.M. KIPNIS, Journal of Clinical Investigation 46 1954-1962 (1967). SE. CROCKETT, E.L. MAZZAFERRI and S. CATALAND, Diabetes 2 931-935 (1976). W. CRE-, M. TALAULICAR, R. EBERT and B. WILL&IS. Diabetes 2e 140-145 (1980). F.J. .&RVICE. R.A. RIZZA. R.E. WESTLAND, L.D. HALL, J.E. GERICH and V.L. GO, Journal of Clinical Endocrinology & Metabolism a 1133-l 140 (1984). D.L. SARSON, P.G. KOPELMAN, H.S. BES-IERMAN, T.R. PILKINGTON and S.R. BLOOM. Diabetologia z 386-391 (1983). W. CREUTZFELDT, Diabetologia 16 75-85 (1979). D. ELAH&D.K. ANDERSEN,D.C. MULLER, J.D. TOBIN, J.C. BROWN audR. ANDRI%S,Diabetes 2 950-957 (1984). M.A. NAUCK, M.M. HEIMESAAT. C. ORSKOV, J.J. HOLST, R. EBERT and W. CRB-, Journal of Cl&xl Investigation fi 301-307 (1993). A. KUBOTA, Y. YAMADA,T. HAYAMI, K. YASUDA, Y. SOMEYA, Y. IHARA, S. KAGIMCYIQ R. WATANABQ, T. TAMINATO, K. TSUDA and Y. SEINO, Diabetes fi 1701-1705 (1996). Y. YAMADA,T. HAYAMI, K. NAKAMURA, P.J. KAISAKI. Y. SOMEYA, C.Z. WANG, S. SEINO and Y. SEINO, Genomics 24.773-776 (1995). American Joumal of Physiology 2& E603-E607 G.H. STARICH, R.S. BAR and E.L. Mw, (1985). H. HAUNER, G. GLAT’I’ING, D. KAMINSKA and E.F. PFEIFFER, Annals of Nutrition 8c Metabolism ;12 282-288 (1988). B. BECK and J.P. MAX, Diabetologiaa 68 (1986). B. BECK and J.P. MAX, Regulatory Peptides 2 3-8 (1983).
Vol.66,No. 2.2000
86. 87. 88. 89. 90.
91. 92. 93. 94. 95.
GIPand Fat Metabolism
103
A.S.H. BABA and P.J. BUTTERFLY, Biochemical Society Transactions 1p 309s (1991). J. OBEN, L. MORGAN, J. FLETCHER and V. MARKS, Journal of Endocrinology m 267-272 (1991). J. DUPRE. N. GREENIDGE, T.J. MCDONALD, S.A. ROSS and D. RUBINSTEIN, Metabolism: Clinical & Experimental a 1197-l 199 (1976). T. WASADA, K. MCCORKLE. V. HARRIS, K. KAWAI, B. HOWARD and R.H. UNGER, Journal of Clinical Investigation 68 1106-l 107 (1981). R. EBERT. M. NAUCK and W. CREUTZFELDT, Hormone & Metabolic Research Z 517-521 (1991). R.H. ECKEL, W.Y. FUJIMOTO and J.D. BRUNZELL. Diabetes 28 1141-2 (1979). J.M. KNAPPER, SM. PUDDICOMBE, L.M. MORGAN and J.M. FLETCHER, Journal of Nutrition u 183-188 (1995). R.G.C. YIP. M.O. BOYLAN, T.J. KIEFFER and M.M. WOLFE, Endocrinology m 4004-4007 (1998). B. BECK and J.P. MAX, Hormone & Metabolic Research 2Q 24-27 (1988). A.B. DAMHOLT, A.M. BUCHAN and H. KOFOD. Endocrinology l3$! 2085-2091(1998).