Plasma high-density lipoprotein concentration and subfraction distribution in relation to triglyceride metabolism

Plasma high-density lipoprotein concentration and subfraction distribution in relation to triglyceride metabolism Esko A. Nikkihi, M.D., Marja-Riitta Timo Sane, M.D. Helsinki, FinZand

Taskinen,

M.D.,*

In the classical series of studies of plasma lipoproteins, Gofman et al1 showed that in normal human subjects an inverse correlation was present between the levels of high-density lipoprotein (HDL) subfraction 2 (HDL& and triglyceride-rich lipoproteins (particles with Sf values of 20 to 100 and 100 to 400, corresponding to very lowdensity lipoprotein [VLDL]). They also found that in many disease conditions associated with increased levels of VLDL, HDLt concentration was low. This relationship was not reflected in HDL3 or it was much weaker so that HDL3 concentration was decreased only in cases with marked elevation of VLDL. These early &idings have been confirmed repeatedly in a number of population studies and in various intervention experiments in which changes in VLDL levels are frequently accompanied by reciprocal alterations in HDL (HDL& concentration.zp3 This association between HDL and triglyceride transport may appear to be a central issue in the pathogenesie of atherosclerosis, since both high VLDL and low HDL (particularly HDLz) are well recognized lipid risk factors of arterial disease.’ In multivariate analyses of epidemiologic data the HDL has been revealed as a more independent risk factor than triglycerides (the latter primarily reflecting the concentration of VLDL), but these results do not exclude the possibility that the primary disturbance responsible for the low HDL may ultimately lie in triglyceride metabolism. If this is the case, the HDL could be a mediator between the basic metabolic defect and the vascular lesion or, alternatively, it could be just an ‘%mocent bystander,” a marker of abnormal lipoprotein metabolism From the Second* and Third Department of Medicine, Helsinki. Reprint requestsz Marja-Riitta Taskinen, Second Department University Hospital, SF-00290 Helsinki, Finland.

University of Msdicine,

of

and

without position in the pathogenic chain of events. Whichever of these possibilities is true, it seems important to recognixe the close mechanisms by which the total concentration of HDL and the distribution of various particles within the HDL density spectrum are controlled by the transport and catabolism of triglyceride-rich lipoproteins, that is, chylomicrons and VLDL. On the basis of our current knowledge, there are at least three separate metabolic processes that link HDL to triglyceride-rich lipoproteins. First, HDL acts as an acceptor of material released from chylomicrons and VLDL during their lipolysis. Second, there is an exchange of lipids between HDL and VLDL mediated by lipid transfer protein. Third, the intravascular metabolism of the major HDL apoproteins AI and AH is related to VLDL transport by some mechanism that is so far poorly understood. REGULATION OF HDL CONCENTRATlON AND SlJf3FRACTlON DlSTRlBUllON BY TRlGLYCERlDE LtPOLYSlS

Rarly studies showed that HDL levels increased during heparin-induced in viva or in vitro lipolysirP and after oral fat intake,’ suggesting that HDL was involved in the cataboliim of triglyceride-rich particles in plasma. In both instances the rise occurs mainly in the HDL* subfraction,kg whereas the HDL3 levels either remain unchanged or show a reciprocal fall.*,lo, *I It is now well recognixed that this phenomenon is based on a transfer of phoepholipids, free cholesterol, and C apoproteins from VLDL and chylomicrons to HDL following the breakdown of the triglycerides of the former particles by lipoprotein lipase. On removal of triglyceride the particles shrink and the surplus of the surface material is released and incorporated into HDL either directly or through an intermediate vesicular structure.2 It is likely that both HDL2 and HDL3 are !543

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potential acceptors of this material, but the major acceptor particles are probably HDL3, which can incorporate substantial amounts of free cholesterol, phospholipids, and apoproteins.lz Because the lipid/ protein ratio of the surface components of triglyceride-rich lipoproteins is higher than that of the acceptor HDL particles, the average density of HDL3 decreases on the lipolytic process and may reach the HDL2 density range and particle size. In a closed in vitro system in which VLDL is incubated with lipoprotein lipase, the increase of the material within HDL2 is proportional to the amount of VLDL added to the mixture.13 With higher substrate concentrations the HDL3 decreases13 and may be completely transformed to particles with the density of HDL2.12 Formation of true genuine HDL2 particles in vitro needs the presence of lecithin-cholesterol acyltransferase (LCAT),14 but in vivo this is apparently a continuous process. Tall et all5 analyzed the redistribution of HDL particles after a fatty meal and found that the greatest increment occurred in a density range of 1.10 to 1.14 gm/ml, that is, at the border of HDL2 and HDL* In addition, there was an increase of a particle fraction within HDL2 density. On the basis of the in vitro experiments, it seems that the rate of HDL2 formation is dependent on the rate of flux of the surface components from triglyceride-rich lipoproteins, whose rate in turn is determined by the activities of lipoprotein lipase and LCAT, as well as by the substrate availability, the VLDL providing more material per particle than chylomicrons. The lipoprotein lipase activity appears to have a particularly important role in the regulation of plasma HDL2 and total HDL levels. It may be that most or all of HDL2 and part of HDL8 (the least dense end of the HDL3 spectrum) are formed in the intravascular lipolysis. This view is based on a number of findings. First, there is a relatively high positive correlation between HDL2 (and total HDL) concentration and lipoprotein lipase activity measured either from postheparin plasma or adipose tissue.16+17This correlation applies to all HDL2 constituents except trig1ycerides.l’ Second, patients lacking lipoprotein lipase or apoprotein CII do not have any HDL2 in their plasma, and the concentration of HDL3 is remarkably 10w.l~ Third, inactivation of lipoprotein lipase in vivo by a specific antiserum is followed by a rapid fall of HDL cholesterol, phospholipid, and protein concentrations and by a simultaneous increase of the average density of the remaining HDL particles.ls Moreover, the magnitude of the rise of HDL2 phospholipids and HDL2

mass concentration after a standard fatty meal is positively correlated with the lipoprotein lipase activity measured from adipose tissue (Taskinen and Nikkihi, unpublished observation). HDL in relation to efiux and influx rates of pZasma trigzycerides. Because the lipoprotein lipase activity is the major determinant of the rate of catabolism (lipolysis) of triglyceride-rich lipoproteins, it is expected that HDL (HDL2) concentration is related to the efficiency of triglyceride removal. This is indeed the case. A number of studies= have shown that both total HDL and HDL2 but not HDL3 concentrations are positively correlated to the fractional removal rate of intravenous fat. Moreover, there is an inverse relationship between the magnitude of alimentary lipemia (reciprocal of chylomicron removal rate) and HDL2 or HDLJHDL3 ratio.21r22 In a longitudinal follow-up of single subjectsZ2 the degree of postprandial triglyceridemia and HDL2 levels vary in closely reciprocal fashion. These intercorrelations are best explained by the precursor-product relationship between chylomicron surface components (other than apoprotein B) and HDL2. The higher the rate of chylomicron degradation, the greater is the formation of HDL2. In addition to the removal efficiency of exogenous fat, the HDL and HDL2 levels are positively correlated to the quantity of dietary fat intake.23,24 The HDL concentration is related to the fractional catabolic rate of VLDL triglycerides, as shown in Fig. 1. In contrast, there is no correlation between the HDL levels and the production rate of VLDL, or this relationship may even be negative. Thus increased absolute transport rate of VLDL through plasma compartment does not result in an increase of HDL or HDL2, as would be expected, but on the contrary, may be associated with a reduction of HDL2 (not only HDL2 cholesterol but also the HDL2 particle concentration tends to decrease). This suggesm that the “overproduced” VLDL particles are not catabolized in a similar manner to normal VLDL. The former might contain less surface constituents in relation to core triglycerides, or their hydrolysis could remain incomplete, ending up in an intermediate-density lipoprotein (IDL) particle, which is taken up by the liver. Consistent with the latter possibility is the fact that in cases with marked overproduction of VLDL (triglycerides or apoprotein B or both), not only HDL but also the LDL and LDL apoprotein B levels are lower than expected on the basis of the known precursorproduct relationship between VLDL apoprotein B and LDL. Kinetic studies have shown that in

vohml. 113 Number 2, Part 2

HDL subfractions and triglyceride metabolism

!XS

patients with increasedVLDL production rate, part of VLDL apoprotein B is removed from the circulation without being convertedto LDL apoprotein B, as is the casein normal subjects.%In support of the incomplete hydrolysis hypothesis is also the finding that the fractional catabolic rate of IDL is positively correlatedwith lipoprotein lipase activity in subjects with normal VLDL levels but not in patients with elevated VLDL levels.z6 Concordant variations of HDL (HDL& and lipoprotein lipase uctiuity. The influence of many phys-

iologic factors and diseasestates on plasma HDL (most of which is only on HDLz) is well recognized. Concordant variations in HDL (HDLJ levels and lipoprotein lipase activity suggestthe presenceof a causalrelationship betweenthe two variables.Thus higher activity of lipoprotein lipase in female versus male subjects,in alcohol usersversusnonusers,and in physically active versusinactive individuals could account for the respective differences present in HDL and HDLs,concentrations?Also, the low HDL levels commonly observedin insulin-deficient diabetic patients and in insulin-resistant type 2 diabetic patients could be explained at least partly by a decrease of lipoprotein lipase activity invariably present in these conditions.3 To get further insight into this relationship we haverecently studied the responsesof HDL subfraction and lipoprotein lipase activity in three separate acuteintervention experiments,that is, after intake of alcohol, following administration of prednisone, and during insulin treatment of type 2 diabetic patients. In all these situations both HDLz concentration and lipoprotein lipase activity increase significantly over the baseline value, but it is not yet known as to whether the change in the enzyme activity precedesthe increaseof the HDLz, In the acute alcohol experiment, which is describedin detail in the paper by Taskinen et al.z7 the HDLt phospholipids and HDLz apoproteins increased in parallel with VLDL, but the HDL2 cholesterol level remained unchanged during the first 2 days of alcohol consumption. This suggests that the initial increaseof HDLz following alcohol intake is causedby the acute stimulation of VLDL production and concomitant increasein the flux of surface components to HDL. The reason for the failure of HDLz cholesterolto increasecould be the alcohol-inducedstimulation of triglyceride synthesis and initial production of triglyceride-rich, but cholesterol-depleted VLDL particles. On continued intake, alcohol markedly increasesthe lipoprotein lipaseactivity, and this further stimulates the trans-

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Fig.

fer of lipids and apoproteins,to HDL, resulting in redistribution of HDL subpopulations toward HDLz. However, there is no simultaneous decrease in the HDLa concentration, suggesting that new HDL3 particles are being,formed either by increased secretion of nascent HDL or by recombination of plasma apoprotein Al-phospholipid complexeswith cholesterol, Our recent studies have shown that administration of prednisoneto healthy human subjectsresults in an increaseof HDLz concentrationin only 2 days. This changeis accompaniedby pidecreasein HbL3 levels (Fig. 2), indicating a shift of particles from HDL3 density class to HDLz. However, already at this time point the increaseof HDLz is greaterthan the respectivedecreaseof HDL (total HDL concentration is increased),suggestingthat lipid material is being directly incorporated into HDLt particles. After a few days of prednisonetreatment, the HDLs levels return to the baseline (Fig. 2). Prednisone administration is also accompanied by a rise of lipoprotein lipase activity (Fig. 3), and it is possible that this mediates the changesin HDL. In a seriesof patients with type 2 diabetes resistant to oral antidiabetic drugs, we have monitored the responseof plasma lipoproteins and lipoprotein lipase activity to insulm treatment. Insulin administration wasfollowed by a reduction of diurnal blood glucoseto near normal levels. Simultaneously, the

February

546

NikMi,

Taskinen, and Sane

Amarican

1-i

I /

HDL2

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t I 2 4 DAYS ON PREDNISONE

I 7

2. Response of HDL and HDL3 mass concentrations to prednisone (30 mg/day) in four healthy male volunteers.

Fig.

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3. Change of adipose tissue lipoprotein lipase activity during 1 week of prednisone treatment of healthy male volunteers.

ml

4. Tonal ultracentrifugation profile of HDL in a patient with type 2 diabetes before insulin treatment and after 1 month of immlin administration.

Fig.

elevated VLDL concentrations were markedly reduced, attaining in most cases a normal range. Lipoprotein lipase activity of adipose tissue increased threefold during the treatment. In HDL a redistribution of particles from HDL3 to HDL2 was observed in the majority of patients, but in contrast to the situation following alcohol consumption or prednisone administration, the decrease of HDL3 and the increase of HDL2 fully comp8nsated each other so that there was no increase in total HDL concentration. A typical HDL response to insulin in a single patient is shown in Fig. 4. The reason for this difference is not immediately evident. METABOLISM OF HDL APOLIPOPROTEINS RELATION TO TRIGLYCERIDE TRANSPORT

Fig.

1987

Heart Journal

IN

It is important to realize that the synthesis and catabolism of HDL apoproteins are not equivalent to the metabolism of whole HDL particles, which undergo a continuous modeling by uptake and removal of their lipid components before being irreversibly removed as particles. It has even been suggested that circulating HDL may represent an “eternal” lipoprotein particle, which can renew all its components in various processes, taking place with variabl8 rates but without disapp8arance of the whole par&id8 as a unit. z* However, the metabolism of the HDL apoproteins may be influenced by the other reactions going on within plasma HDL particles, and conversely, the apoprotein kinetics may regulate the conc8ntration and subspecies distribution of HDL particles. This is particularly the case if

volwne 113 Number 2, Part 2

the transformation of HDL3 to HDLz in vivo needs incorporation of one apo AI molecule, as suggested by Eisenberg.2 Several studies have shown that the metabolism of apoprotein AI or AI1 is in some way connected to VLDL transport. Most consistent is the finding that the fractional catabolic rate (FCR) of apoproteins AI and AI1 is accelerated in patients with hypertriglyceridemia.2g-31 The FCR of apo AI is inversely related to the FCR of VLDL apoprotein B31; that is, a rapid removal of VLDL and corresponding increase in the incorporation of the surface components to HDL are accompanied by a retardation of the cataboliim of AI, whereas on impaired removal of VLDL the apoprotein AI catabolic rate is increased. Because the concentration of AI and AI1 is inversely related to the concentration of VLDL, it is possible that the FCR reflects only variations in the pool sixe of the A apoproteins, but it is more likely that a rapid catabolism of AI and AI1 in hypertriglyceridemia is the primary cause of the reduced apoprotein A concentrations. The mechanism by which VLDL concentration or VLDL catabolism regulates the catabolic rate of A apoproteins is far from clear. One possible explanation is schematically presented in Fig. 5. It is based on the assumption that apoprotein AI (and probably also AII) is present in plasma as an AI-phospholipid complex with a density greater than 1.210 gm/ml (“VHDL”) and that on rapid flux of VLDL surface components to HDL, the AI molecule needed for conversion of HDL3 to HDL22 is derived from this complex, whereas with slow VLDL catabolism the requirement for apoprotein AI decreases as a result of less formation of HDL% If the catabolism of apoprotein AI present in the complex form is significantly more rapid than that of the AI residing in HDL particles, an increased transfer of AI to HDL (high FCR of triglyceride-rich lipoproteins) would be associated with a slower catabolism of whole plasma AI (or AII). Unfortunately, this hypothesis is difficult to prove by any direct measurements of apoprotein A kinetics because of the continuous exchange of apoproteins between different HDL particles and between HDL and the assumed complex. CONCLUSIONS

The concentrations of HDLs and total HDL but not of HDL3 are inversely correlated to the levels of VLDL. This association is based on a precursorproduct relationship between surface phospholipids and free cholesterol of triglyceride-rich lipoproteins and those of HDLP During intravascular lipolysis of

HDL subfractions and triglyceride metabolism

LOW

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547

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“FREE”

t APO-AI

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I CATABOLISM

Fig. 5. Scheme on the redistribution of HDL particles according to variation of lipoprotein lipase activity from low to high. In the presence of low lipoprotein lipase activity the lipolytic rate is also low, and HDL3 will

incorporate the released surface components, the particle density shifting to lower range but still remaining as HDL3. With increasing lipoprotein lipase activity the HDL% and then also HDL2,, particles are formed and additional apoprotein AI is incorporated into HDL from a “free” apoprotein AI pool, probably present as “very high-density lipoprotein” AI phospholipid complexes.

VLDL and chylomicron triglycerides by lipoprotein lipase, the redundant surface compounds of these lipoprotein particles are transferred to HDL3 and partly to HDL, causing a decrease in the average density of HDL particles and a shift toward HDL from HDL* The HDL2 concentration is positively correlated to lipoprotein lipase activity aud to the fractional disappearance rate of intravenous fat and of VLDL triglycerides. When the activity of lipoprotein lipase is high, both chylomicrons and VLDL are cleared rapidly from the blood, and the HDLz (as well as total HDL and all HDL constituents except triglycerides) levels are raised. The HDL2 level is also related to the amout of dietary fat, but in contrast does not show any correlation to the VLDL production rate (or is even inversely correlated). The difference in HDLz levels observed between female versus male subjects, physically active versus inactive people, and those who consume alcohol versus those who do not are partly explained by respective differences in lipoprotein lipase activity. Also, prednisone administration to normal subjects and insulin treatment of type 2 diabetic patients cause an increase of HDLz and of lipoprotein lipase activity.

548

Nikkilti, Taskinen, and Sane

The well-recognized association between atherosclerosis and decreased HDL2 concentration may be based either on a diiect inhibitory actioti of HDL2 on atherogeneBis or on a more basic disturbance of lipoprotein metabolism, in which HDL is only a marker without a direct role in the process itself. REFERENCES

1. Gofman JW, DeLalla 0, Glasier F, et al. Serum lipoprotein transport system in health, metabolic disorders, atherosclerosis and coronary heart disease. Plasma 195%2z413. 2. Eisenherg S. High density lipoprotein metabolism. J Lipid Res 1984;25zlOl7. 3. Nikkihi EA. HDL in relation to the metabolism of triglyceride-rich lipoproteins. In: Miller NE, Miller GJ, eds. Clinical and metabolic aspecta of high-density lipoproteins. Amsterdam: Elsevier Science Publishers, 1984:217-45 4. Miller NE. Associations of HDL subclasses and anolinonroteins with ischemic heart disease and coronary athero&rosis. Proceedings of the 11th Sigrid Juselius Symposium on Lipoprotein Metabolism in Relation to Coronary Heart Disease. Haikko, Fiiend: June r-5, 1986. 5. NikkiB EA. The effect of heparin on serum lipoproteins. Stand J Clin Lab Invest 195%4:369. 6. Lindgren FT, Nichols AV, Freeman NK. Physical and chemical composition studies on the lipoproteins of fasting and heparinixed human sera. J Phys Chem 195$5%930. 7. Have1 R-J. Transport and metabolism of chylomicra. Am J Clin Nutr 195&6:662. 8. Forte TM, Krauss RM, Lindgren FT, Nichols AV. Changes in plasma lipoprotein distribution and formation of two unusual articles after heparin-induced lipolysis in hypertriglyceridemic subject& Proc Nat1 Acad Sci USA 197%7&5934. 9. Have1 RJ, Kane JP, Kashyap ML. Interchange of apolipoproteins between chylomicrons and high density lipoproteins during alimentary lipemia in man. J Clin Invest 197$52:32. 10. Baggio G, Fellin R, Baiocchi MR, et al. Relationship between triglyceride-rich lipoprotein (chylomicrons and VLDL) and HDLz and HDL3 in the post-prandial phase in humans. Atherosclerosis 19m,37:271. 11. Taskinen M-R, Nikkila EA, Kuusi T, Tulikoura I. Changes of high density lipoprotein subfraction concentration and composition by Intralipid in vivo and by hpolysis of Intralipid in vitro. Arteriosclerosis 1983;3:607. 12. Patach JR, Gotto Jr AM, Olivecrona T, Eisenberg S. Formation of high density lipoprot&-like particles during lipolysis of very low density lipoproteins in vitro. Proc Nat1 Acad Sci USA 197&754519. 13. Taskinen M-R, Kashyap ML, Srivastava LS, et al. In vitro catabolism of human plasma very low density lipoproteins. Effecte of VLDL concentration on the interconversion of high density lipoprotein subfractions. Atherosclerosis 1982; 41:381. 14. Dieplinger H, Zechner R, Kostner GM. The in vitro formation of HDLz during the action of LCAT the role of triglyceride-rich lipoproteins. J Lipid Res 1985;26:273.

American

February 1987 Hem Journal

15. Tall AR, Bhnn CB, Forester GP, Nelson CA. Changes in the distribution and composition of plasma high density lipoproteins after ingestion of fat. J Biol Chem 198%257:198. 16. Nikkihi EA, Taskinen M-R, Kekki M. Relation of plasma high-density lipoprotein cholesterol to lipoprotein-lipase activity in adipose tissue and skeletal muscle of man. Atherosclerosis 197X$29:497. 17. Taskinen M-R, Nikkila EA. High density lipoprotein subfractions in relation to lipoprotein lipase activity of tissues in man-evidence for reciprocal regulation of HDLz and HDLa levels by lipoprotein hpase. Chn Chim Acta 1981;112:32532. 18. Breckenridge WC. Deficiencies of plasma lipolytic activities. Proceedings of the 11th Sigrid~Juselius Symposium on Lipoprotein Metabolism in Relation to Coronary Heart Disease. Haikko, Finland June l-5, 1986. 19. Behr SR, Patach JR, Forte T, Bensadoun A. Plasma lipoprotein changes resulting from immunologically blocked hpolysis. J Lipid Res 198li22z443. 20. Rtissner S. Kirstein P. Relationahin between linonroteins including HDL subfractions and the intravenoui fat tolerance test. Atherosclerosis 1984;52z287. 21. Mattock M, Salter A, Omer T, Keen H. Changes in high density lipoprotein subfractions during alimentary lipaemia. Experientia 1981;37:945. 22. Pa&h JR, Karlin JB, Scott LW, Smith LC, Gotto Jr AM. Inverse relationship between blood -levels of high density lipoprotein subfraction 2 and magnitude of postprandial lipemia. Proc Nat1 Acad Sci USA 1983;8&1449. 23. Blum CB, Levy RI, Eisenberg S, Hall M III, Goebel RH, Berman M. High density lipoprotein metabolism in man. J Clin Invest 1977$%795. 24. McNerney CA, Kashyap ML, Barnhart RL, Jackson RL. -. Comparison of gradient gel electrophoresis and tonal ultracentrifugation for quantitation of high density lipoprotein. J Lipid Res 1985;26:1363. 25. Reardon MF, Fidge NH, Nestel PJ. Catabolism of very low density lipoprotein B apoprotein in man. J Clin Invest 197& 61:850. 26. Reardon MF, Sakai H, Steiner G. Roles of lipoprotein lipase and hepatic triglyceride lipase in the catabolism in vivo of triglyceride-rich lipoproteins. Arteriosclerosis 198&2:396. 27. Taskinen M-R, et al. Alcohol-induced changes in serum lipoproteins and in their metabolism. Proceedings of the 11th Sigrid Jus~lius Symposium on Lipoprotein Metabolism in Relation to Coronary Heart Disease, Haikko, Finland: June l-5, 1986. 28. Nikkila EA. Metabolic regulation of plasma high density linonrotein concentration. Eur J Clin Invest 1978:8:111. 29. F&nan RH, Ranbar SS, Alaupovic P, Brandford’RH, Howard RP. Studies of the metabolism of radioiodinated human serum alpha lipoprotein in normal and hyperlipidemic subiects. J Lab Clin Med 19a63:193. 30. Schaefer EJ, Zech LA, Jenkins LL, et al. Human apolipoprotein A-I and A-II metabolism. J Lipid Res 198%23:856. 31. Ma&l1 P. Rao SN. Miller NE. et al. Relationshins between themetabohsm of high-density and very-low-density lipopro-

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