- Email: [email protected]
More clues to the relationship between hepatic iron and steatosis: An association with insulin resistance?
November 1999
24.
25.
26.
27.
EDITORIALS 1241
and other gastrointestinal inflammation show defective CD2 pathway–induced apoptosis. Gastroenterology 1999;116:557– 565. Ina K, Ottaway CA, Musso A, Binion DG, West GA, Dobrea GM, Itoh J, Yamaguchi T, Sakai T, Kusugami K, Levine AD, Fiocchi C. Crohn’s disease mucosal T-cells are resistant to apoptosis. J Immunol 1999;163:1081–1090. van Deventer SJH. Oral presentation, Symposium on Trends in Inflammatory Bowel Disease Therapy 1999, Vancouver, Canada, 1999. Boirivant M, Fuss IJ, Chu A, Strober W. Oxazolone colitis: a murine model of T helper cell type 2 colitis treatable with antibodies to interleukin 4. J Exp Med 1998;188:1929–1939. Fiocchi C. Intestinal inflammation: a complex interplay of immune–
non immune cell interactions. Am J Physiol 1997;273:G769– G775.
Address requests for reprints to: Claudio Fiocchi, M.D., Division of Gastroenterology, University Hospitals of Cleveland, Case Western Reserve University School of Medicine (BRB 425), 10900 Euclid Avenue, Cleveland, Ohio 44106-4952. Fax: (216) 368-1674. Supported by grants DK30399 and DK50984 from the National Institutes of Health and by the Crohn’s & Colitis Foundation of America, Inc. r 1999 by the American Gastroenterological Association 0016-5085/99/$10.00
More Clues to the Relationship Between Hepatic Iron and Steatosis: An Association With Insulin Resistance? See article on page 1155.
he association between obesity, dyslipidemia, diabetes mellitus, and hepatic steatosis has been recognized for some time.1,2 Recently, interest has focused on the association of iron overload, albeit milder than in hemochromatosis, and hepatic steatosis.3,4 Most studies to date have identified patients with steatosis and then looked at the associated clinicopathologic features. Mendler et al.5 in this issue of GASTROENTEROLOGY now show an association between hepatic iron overload and steatosis by approaching this clinical issue from another perspective: identifying patients with primary iron overload who are not homozygous for the Cys282Tyr mutation of HFE and studying the clinicopathologic correlates of this iron overload. In their study group of 161 patients, 52% had steatosis and virtually all (94% of those with available data) had evidence of insulin resistance based on the presence of one or more components of the insulin resistance syndrome (IRS). Mendler et al. are not the first to propose an association between insulin resistance and hepatic iron overload.6–9 Nor are they the first to recognize the association of steatosis with components of IRS.10,11 However, previous studies of insulin resistance and increased iron stores either examined individuals with transfusional iron overload,7,8 or relied on serum iron markers.6,9 Mendler et al. measured hepatic iron stores in patients with primary iron overload, and are the first to note the association of this iron overload with steatosis in patients with components of the IRS.
T
In considering the implications of their observations, three issues need to be addressed: the features of IRS and their association with insulin resistance; the role of Cys282Tyr (C282Y) and His63Asp (H63D) mutations of HFE in hepatic iron accumulation; and the interactions between hepatic iron, steatosis, and IRS. Insulin resistance is associated with many clinical features including hypertension, obesity, dyslipidemia, and diabetes mellitus. These clinical features are components of the IRS and have been referred to as the ‘‘deadly quartet’’ because of their association with cardiovascular disease.12 However, insulin resistance is not synonymous with the IRS,13 and although the features of IRS are well described, there are no universally accepted criteria for diagnosis of IRS and little data on the coexistence of the various components of the IRS and the presence and severity of insulin resistance. The observation by Mendler et al. that virtually all patients in their study population with iron overload had IRS is partly a consequence of the criteria they used to define IRS. Their criteria for the presence of obesity were generous, defining any patient with a body mass index (BMI) of ⬎25 as obese. The BMI is calculated by dividing the weight in kilograms by 5height in meters62. The generally accepted criteria for obesity, endorsed by a recent National Institutes of Health Consensus meeting is a BMI ⬎30,14 although other investigators, using BMI from 26.1 to 27.8 to define obesity, have shown an association with insulin resistance.10,15,16 In the United States, the National Health and Nutrition Examination Surveys found that 59.4% of men and 50.7% of women older than 20 years had a BMI ⬎25.17 Using the criteria
1242 EDITORIALS
of a BMI ⬎25 to define IRS would mean that most adults in the United States have this syndrome. More recently it has become apparent that the distribution of adipose tissue is another important variable with respect to cardiovascular risk and the IRS.12,18 Body fat topography has also been previously identified as a risk factor for hepatic steatosis.19 In particular, central or upper body obesity carries the greatest risk for both the IRS and hepatic steatosis. Although the BMI is a good measure of overall adipose tissue, it does not distinguish central from global obesity. The easiest methods for clinically assessing central obesity include the waist to hip ratio and abdominal girth measurements. Future studies of the relationship between insulin resistance and liver disease should include an assessment of central obesity. Additionally, the dyslipidemia typically associated with IRS is hypertriglyceridemia, with decreased levels of high-density lipoproteins and increased small dense low-density lipoproteins.13 The criteria used by Mendler et al. to define dyslipidemia (hyperlipidemia of any type) may have included patients without these typical changes in the group designated as having IRS. Hypertension, a recognized feature of IRS, was not included in their criteria. Future studies should therefore also include an assessment of blood pressure, lipid profile, and possibly glucose tolerance in all patients to establish the prevalence of these components of the IRS in liver disease. Risk factors for the IRS do appear to cluster: 44% of the patients of Mendler et al. with IRS had one criteria, 46% had two criteria, and 10% had all three criteria they used to define IRS. Others have described similar clustering of components of the IRS.15,16,20 Typically, the more components of the IRS present, the higher the fasting plasma insulin level15,16,20 (a marker of insulin resistance12) and other measures of insulin resistance.15 This relationship persists after correction for BMI, waist to hip ratios, and abdominal girth,15,16,20 although insulin resistance is generally greater in obese than in lean individuals with the same number of other components of IRS.15 A similar clustering exists between hepatic steatosis and the presence of components of IRS10,11,21 and with measurements of insulin resistance.21,22 Marceau et al.11 found that the addition of each component of the IRS (elevated waist/hip ratio, impaired glucose tolerance, hypertension, and dyslipidemia) increased the risk of hepatic steatosis exponentially from 1- to 99-fold, with the risk of steatosis being greater for men than women. Refining the criteria for IRS, determining the effect of having multiple components of IRS present, and formally assessing insulin resistance seem to be logical steps in
GASTROENTEROLOGY Vol. 117, No. 5
further investigations of hepatic steatosis and hepatic iron overload. One of the conclusions Mendler et al. made is that most patients with primary iron overload not caused by hemochromatosis have insulin resistance. This led them to coin the term ‘‘insulin resistance–associated hepatic iron overload.’’ However, the finding of hepatic iron overload with mutant HFE genotypes other than homozygosity for C282Y is not unexpected. A small percentage of patients with a clinical diagnosis of hemochromatosis (on the basis of the histological appearance, hepatic iron stores, hepatic iron index, and phlebotomy requirements) have been found to be C282Y/H63D compound heterozygotes (3%–6%)23,24 or heterozygous for the C282Y mutation (1%).23 The penetrance of these genotypes with respect to iron overload is less than for C282Y homozygotes, as is the degree of iron loading. Nonetheless, in a study population selected on the basis of iron overload, it is to be expected that the C282Y/H63D compound heterozygotes would be more prevalent than in the control population. One of the most interesting questions now is the nature of the relationship between hepatic steatosis and iron overload. There are several possibilities: iron accumulation could result in steatosis, hepatic steatosis could lead to iron accumulation, both steatosis and iron overload may be distinct consequences of another factor (probably insulin resistance in this case), and finally, hepatic iron overload and steatosis may be unrelated but occurring together coincidentally. In this case, the combination may act synergistically to increase liver injury, whereas steatosis without iron overload may have been innocuous and escaped attention. Overall, this seems less likely because hepatic steatosis is known to be associated with insulin resistance, while iron accumulation also appears to be associated with insulin resistance6–9 that can be improved by phlebotomy.25,26 If iron accumulation led to insulin resistance and steatosis, then the prevalence and severity of steatosis would be expected to be greatest in individuals with the greatest iron overload, the HFE compound heterozygotes. Mendler et al. found that this group had significantly less steatosis and lower prevalence of nonalcoholic steatohepatitis (NASH) than the other genotypes. The most likely explanation seems to be that the same factors that led to hepatic steatosis also resulted in iron accumulation, although the rate of iron loading (as reflected by the hepatic iron index) is less than in HFE compound heterozygotes. Finally, the histological correlates of hepatic steatosis and iron overload are worthy of comment. Mendler et al. found that steatosis was associated with hepatic fibrosis, an observation we also made in patients with NASH.3
November 1999
This may be explained in part by their observation that there was a progressive increase in the prevalence of NASH with increasing severity of steatosis. The increase in inflammation and stellate cell activation associated with NASH is likely to be the cause of increased fibrosis in these patients. However, Mendler et al. did not confirm our previous observation of the association between hepatic iron stores and fibrosis in NASH,3 possibly because all their subjects had some degree of iron overload and regression analysis was not used. In summary, Mendler et al. demonstrate an association between hepatic iron overload and steatosis and provide evidence that the IRS may be the underlying thread that links these two conditions. Further work is required to confirm that insulin resistance is involved in the pathogenesis of insulin resistance–associated hepatic iron overload and to refine the diagnostic criteria for this condition. This should be obtained by assessing the role of truncal obesity and hypertension, by characterizing the dyslipidemia, and performing metabolic studies to assess the degree of insulin resistance present. Future studies should also aim to determine the nature of the relationship between insulin resistance, hepatic steatosis, and iron overload. Obesity is increasingly recognized as a risk factor for steatosis and fibrosis in various liver diseases including chronic hepatitis C infection27 and alcoholic liver disease.28 Together, these observations imply that steatosis is an independent risk factor for hepatic fibrosis but the same factors that lead to steatosis are also associated with iron accumulation. Given the postulated roles for iron and steatosis in a variety of liver diseases, understanding the interactions between insulin resistance, iron accumulation, and hepatic steatosis is relevant to the diagnosis and management of a number of common liver diseases. GRAEME A. MACDONALD Clinical Sciences Unit Queensland Institute of Medical Research and Department of Medicine University of Queensland LAWRIE W. POWELL Clinical Sciences Unit Queensland Institute of Medical Research and University of Queensland Brisbane, Queensland, Australia
References 1. Ludwig J, Viggiano TR, McGill DB, Oh BJ. Nonalcoholic steatohepatitis: Mayo Clinic experiences with a hitherto unnamed disease. Mayo Clin Proc 1980;5:434–438. 2. Powell EE, Cooksley WG, Hanson R, Searle J, Halliday JW, Powell LW. The natural history of nonalcoholic steatohepatitis: a fol-
EDITORIALS 1243
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7.
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low-up study of forty-two patients for up to 21 years. Hepatology 1990;11:74–80. George DK, Goldwurm S, Macdonald GA, Cowley LL, Walker NI, Ward PJ, Jazwinska EC, Powell LW. Increased hepatic iron stores in non-alcoholic steatohepatitis is associated with increased hepatic fibrosis. Gastroenterology 1998;114:311–318. Bacon BR, Farahvash MJ, Janney CG, Neuschwander Tetri BA. Nonalcoholic steatohepatitis: an expanded clinical entity. Gastroenterology 1994;107:1103–1109. Mendler M-H, Turlin B, Moirand R, Jouanolle A-M, Sapey T, Guyader D, Le Gall J-Y, Brissot P, David V, Deugnier Y. Insulin resistance–associated hepatic iron overload 1999;117:1155– 1163. Fernandez Real JM, Ricart Engel W, Arroyo E, Balanca R, Casamitjana Abella R, Cabrero D, Fernandez Castaner M, Soler J. Serum ferritin as a component of the insulin resistance syndrome. Diabetes Care 1998;21:62–68. Merkel PA, Simonson DC, Amiel SA, Plewe G, Sherwin RS, Pearson HA, Tamborlane WV. Insulin resistance and hyperinsulinemia in patients with thalassemia major treated by hypertransfusion. N Engl J Med 1988;318:809–814. Cavallo Perin P, Pacini G, Cerutti F, Bessone A, Condo C, Sacchetti L, Piga A, Pagano G. Insulin resistance and hyperinsulinemia in homozygous beta-thalassemia. Metabolism 1995;44:281–286. Tuomainen TP, Nyyssonen K, Salonen R, Tervahauta A, Korpela H, Lakka T, Kaplan GA, Salonen JT. Body iron stores are associated with serum insulin and blood glucose concentrations. Population study in 1,013 eastern Finnish men. Diabetes Care 1997;20:426– 428. Knobler H, Schattner A, Zhornicki T, Malnick SD, Keter D, Sokolovskaya N, Lurie Y, Bass DD. Fatty liver—an additional and treatable feature of the insulin resistance syndrome. QJM 1999; 92:73–79. Marceau P, Biron S, Hould FS, Marceau S, Simard S, Thung SN, Kral JG. Liver pathology and the metabolic syndrome X in severe obesity. J Clin Endocrinol Metab 1999;84:1513–1517. Kaplan NM. The deadly quartet. Upper-body obesity, glucose intolerance, hypertriglyceridemia, and hypertension. Arch Intern Med 1989;149:1514–1520. Consensus Development Conference on Insulin Resistance. November 5–6, 1997. American Diabetes Association. Diabetes Care 1998;21:310–314. Clinical guidelines on the identification, evaluation, and treatment of overweight and obesity in adults—the evidence report. National Institutes of Health. Obes Res 1998;6(suppl 2):51s– 209s. Mykkanen L, Haffner SM, Ronnemaa T, Bergman RN, Laakso M. Low insulin sensitivity is associated with clustering of cardiovascular disease risk factors. Am J Epidemiol 1997;146:315–321. Mau MK, Grandinetti A, Arakaki RF, Chang HK, Kinney EK, Curb JD. The insulin resistance syndrome in native Hawaiians. Native Hawaiian Health Research (NHHR) Project. Diabetes Care 1997; 20:1376–1380. Flegal KM, Carroll MD, Kuczmarski RJ, Johnson CL. Overweight and obesity in the United States: prevalence and trends, 1960– 1994. Int J Obes Relat Metab Disord 1998;22:39–47. Despres JP, Moorjani S, Lupien PJ, Tremblay A, Nadeau A, Bouchard C. Regional distribution of body fat, plasma lipoproteins, and cardiovascular disease. Arteriosclerosis 1990;10:497– 511. Kral JG, Schaffner F, Pierson RN Jr, Wang J. Body fat topography as an independent predictor of fatty liver. Metabolism 1993;42: 548–551. Greenlund KJ, Valdez R, Casper ML, Rith Najarian S, Croft JB. Prevalence and correlates of the insulin resistance syndrome among Native Americans. The Inter-Tribal Heart Project. Diabetes Care 1999; 22:441–447.
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21. Goto T, Onuma T, Takebe K, Kral JG. The influence of fatty liver on insulin clearance and insulin resistance in non-diabetic Japanese subjects. Int J Obes Relat Metab Disord 1995;19:841–855. 22. Banerji MA, Buckley MC, Chaiken RL, Gordon D, Lebovitz HE, Kral JG. Liver fat, serum triglycerides and visceral adipose tissue in insulin-sensitive and insulin-resistant black men with NIDDM. Int J Obes Relat Metab Disord 1995;19:846–850. 23. Crawford DH, Jazwinska EC, Cullen LM, Powell LW. Expression of HLA-linked hemochromatosis in subjects homozygous or heterozygous for the C282Y mutation. Gastroenterology 1998;114:1003– 1008. 24. Bacon BR, Olynyk JK, Brunt EM, Britton RS, Wolff RK. HFE genotype in patients with hemochromatosis and other liver diseases. Ann Intern Med 1999;130:953–962. 25. Facchini FS. Effect of phlebotomy on plasma glucose and insulin concentrations (letter). Diabetes Care 1998;21:2190. 26. Hramiak IM, Finegood DT, Adams PC. Factors affecting glucose
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tolerance in hereditary hemochromatosis. Clin Invest Med 1997; 20:110–118. 27. Hourigan LF, Macdonald GA, Purdie D, Whitehall VH, Shorthouse C, Clouston A, Powell EE. Fibrosis in chronic hepatitis C correlates significantly with body mass index and steatosis. Hepatology 1999;29:1215–1219. 28. Naveau S, Giraud V, Borotto E, Aubert A, Capron F, Chaput JC. Excess weight risk factor for alcoholic liver disease. Hepatology 1997;25:108–111.
Address requests for reprints to: Graeme A. Macdonald, M.D., C Floor, Clinical Sciences Building, Royal Brisbane Hospital, Brisbane, Queensland, 4029 Australia. e-mail:g.macdonald@medicine. herston.uq.edu.au; fax: (61) 7-3365-5462. r 1999 by the American Gastroenterological Association 0016-5085/99/$10.00
The Virtuosity of Hepatic Stellate Cells See articles on pages 1198 and 1205.
wenty years ago there was little music in hepatic fibrosis. In the intervening two decades, discrete melodies have emerged, none more so than through the voice of the hepatic stellate cell, a perisinusoidal mesenchymal cell type. Two articles in this issue of GASTROENTEROLOGY illustrate this progress, emphasizing the enlarging complexity of hepatic stellate cells on the one hand and their clear-cut emergence as a target for antifibrotic therapy on the other. Hepatic stellate cells are the principal fibrogenic cell type in liver, a conclusion made possible by the development of methods to isolate this cell type from rodents and then human liver. Because stellate cells in normal liver contain vitamin A droplets, which impart buoyancy, successful isolation methods used density gradients to recover a homogeneous population of these low-density cells. Growth of vitamin A–rich stellate cells on plastic led to a phenotypic response known as activation, which paralleled closely the response of stellate cells to injury in vivo. From this culture model grew a large body of information characterizing stellate cell activation, including the expression of cytoskeletal markers, secretion of cytokines, and production of extracellular matrix (ECM) molecules.1 Parallel efforts of this type explored the behavior of similar mesenchymal cells in other tissues, underscoring the heterogeneity of so-called myofibroblasts in normal tissue and wound-healing.2 Not long thereafter, the concept of heterogeneity was validated in stellate cells, demonstrating that this cell type may have
T
differing patterns of vitamin A storage, cytoskeletal protein expression, and/or matrix production.3–6 In vivo studies in animal models of liver injury have also emphasized the potential of hepatic mesenchymal cells to express multiple phenotypes.7 Stellate cell heterogeneity has now been further explored in the interesting study by Knittel et al.8 By use of an isolation method that apparently destroys vitamin A–rich stellate cells, a population of myofibroblasts lacking vitamin A, known as rat myofibroblasts, or rMFs, has been cultured and characterized. These cells have patterns of cytoskeletal protein expression distinct from typical vitamin A–rich stellate cells, including those of desmin, vimentin, smooth muscle actin, and glial fibrillary acidic protein. With progressive growth in culture, their cytoskeletal phenotype modulates further; the functional implications of this cytoskeletal plasticity are uncertain. More importantly, it seems that rMFs continue to grow after multiple passages in culture, whereas more typical vitamin A–containing stellate cells do not, implying a decreased propensity of rMFs to undergo apoptosis. Molecular comparison between rMFs and typical stellate cells using a differential display technique identified fibulin 2, a recently described ECM protein, and interleukin 6, a key regenerative cytokine, as being preferentially expressed in rMFs. A shortcoming of these efforts, however, is the failure to validate these culture-induced distinctions in phenotype between stellate cells and rMFs with behavior in vivo, although such in vivo correlation is forthcoming according to the authors. Without these data, it is uncertain whether cells with the rMF pheno-
24.
25.
26.
27.
EDITORIALS 1241
and other gastrointestinal inflammation show defective CD2 pathway–induced apoptosis. Gastroenterology 1999;116:557– 565. Ina K, Ottaway CA, Musso A, Binion DG, West GA, Dobrea GM, Itoh J, Yamaguchi T, Sakai T, Kusugami K, Levine AD, Fiocchi C. Crohn’s disease mucosal T-cells are resistant to apoptosis. J Immunol 1999;163:1081–1090. van Deventer SJH. Oral presentation, Symposium on Trends in Inflammatory Bowel Disease Therapy 1999, Vancouver, Canada, 1999. Boirivant M, Fuss IJ, Chu A, Strober W. Oxazolone colitis: a murine model of T helper cell type 2 colitis treatable with antibodies to interleukin 4. J Exp Med 1998;188:1929–1939. Fiocchi C. Intestinal inflammation: a complex interplay of immune–
non immune cell interactions. Am J Physiol 1997;273:G769– G775.
Address requests for reprints to: Claudio Fiocchi, M.D., Division of Gastroenterology, University Hospitals of Cleveland, Case Western Reserve University School of Medicine (BRB 425), 10900 Euclid Avenue, Cleveland, Ohio 44106-4952. Fax: (216) 368-1674. Supported by grants DK30399 and DK50984 from the National Institutes of Health and by the Crohn’s & Colitis Foundation of America, Inc. r 1999 by the American Gastroenterological Association 0016-5085/99/$10.00
More Clues to the Relationship Between Hepatic Iron and Steatosis: An Association With Insulin Resistance? See article on page 1155.
he association between obesity, dyslipidemia, diabetes mellitus, and hepatic steatosis has been recognized for some time.1,2 Recently, interest has focused on the association of iron overload, albeit milder than in hemochromatosis, and hepatic steatosis.3,4 Most studies to date have identified patients with steatosis and then looked at the associated clinicopathologic features. Mendler et al.5 in this issue of GASTROENTEROLOGY now show an association between hepatic iron overload and steatosis by approaching this clinical issue from another perspective: identifying patients with primary iron overload who are not homozygous for the Cys282Tyr mutation of HFE and studying the clinicopathologic correlates of this iron overload. In their study group of 161 patients, 52% had steatosis and virtually all (94% of those with available data) had evidence of insulin resistance based on the presence of one or more components of the insulin resistance syndrome (IRS). Mendler et al. are not the first to propose an association between insulin resistance and hepatic iron overload.6–9 Nor are they the first to recognize the association of steatosis with components of IRS.10,11 However, previous studies of insulin resistance and increased iron stores either examined individuals with transfusional iron overload,7,8 or relied on serum iron markers.6,9 Mendler et al. measured hepatic iron stores in patients with primary iron overload, and are the first to note the association of this iron overload with steatosis in patients with components of the IRS.
T
In considering the implications of their observations, three issues need to be addressed: the features of IRS and their association with insulin resistance; the role of Cys282Tyr (C282Y) and His63Asp (H63D) mutations of HFE in hepatic iron accumulation; and the interactions between hepatic iron, steatosis, and IRS. Insulin resistance is associated with many clinical features including hypertension, obesity, dyslipidemia, and diabetes mellitus. These clinical features are components of the IRS and have been referred to as the ‘‘deadly quartet’’ because of their association with cardiovascular disease.12 However, insulin resistance is not synonymous with the IRS,13 and although the features of IRS are well described, there are no universally accepted criteria for diagnosis of IRS and little data on the coexistence of the various components of the IRS and the presence and severity of insulin resistance. The observation by Mendler et al. that virtually all patients in their study population with iron overload had IRS is partly a consequence of the criteria they used to define IRS. Their criteria for the presence of obesity were generous, defining any patient with a body mass index (BMI) of ⬎25 as obese. The BMI is calculated by dividing the weight in kilograms by 5height in meters62. The generally accepted criteria for obesity, endorsed by a recent National Institutes of Health Consensus meeting is a BMI ⬎30,14 although other investigators, using BMI from 26.1 to 27.8 to define obesity, have shown an association with insulin resistance.10,15,16 In the United States, the National Health and Nutrition Examination Surveys found that 59.4% of men and 50.7% of women older than 20 years had a BMI ⬎25.17 Using the criteria
1242 EDITORIALS
of a BMI ⬎25 to define IRS would mean that most adults in the United States have this syndrome. More recently it has become apparent that the distribution of adipose tissue is another important variable with respect to cardiovascular risk and the IRS.12,18 Body fat topography has also been previously identified as a risk factor for hepatic steatosis.19 In particular, central or upper body obesity carries the greatest risk for both the IRS and hepatic steatosis. Although the BMI is a good measure of overall adipose tissue, it does not distinguish central from global obesity. The easiest methods for clinically assessing central obesity include the waist to hip ratio and abdominal girth measurements. Future studies of the relationship between insulin resistance and liver disease should include an assessment of central obesity. Additionally, the dyslipidemia typically associated with IRS is hypertriglyceridemia, with decreased levels of high-density lipoproteins and increased small dense low-density lipoproteins.13 The criteria used by Mendler et al. to define dyslipidemia (hyperlipidemia of any type) may have included patients without these typical changes in the group designated as having IRS. Hypertension, a recognized feature of IRS, was not included in their criteria. Future studies should therefore also include an assessment of blood pressure, lipid profile, and possibly glucose tolerance in all patients to establish the prevalence of these components of the IRS in liver disease. Risk factors for the IRS do appear to cluster: 44% of the patients of Mendler et al. with IRS had one criteria, 46% had two criteria, and 10% had all three criteria they used to define IRS. Others have described similar clustering of components of the IRS.15,16,20 Typically, the more components of the IRS present, the higher the fasting plasma insulin level15,16,20 (a marker of insulin resistance12) and other measures of insulin resistance.15 This relationship persists after correction for BMI, waist to hip ratios, and abdominal girth,15,16,20 although insulin resistance is generally greater in obese than in lean individuals with the same number of other components of IRS.15 A similar clustering exists between hepatic steatosis and the presence of components of IRS10,11,21 and with measurements of insulin resistance.21,22 Marceau et al.11 found that the addition of each component of the IRS (elevated waist/hip ratio, impaired glucose tolerance, hypertension, and dyslipidemia) increased the risk of hepatic steatosis exponentially from 1- to 99-fold, with the risk of steatosis being greater for men than women. Refining the criteria for IRS, determining the effect of having multiple components of IRS present, and formally assessing insulin resistance seem to be logical steps in
GASTROENTEROLOGY Vol. 117, No. 5
further investigations of hepatic steatosis and hepatic iron overload. One of the conclusions Mendler et al. made is that most patients with primary iron overload not caused by hemochromatosis have insulin resistance. This led them to coin the term ‘‘insulin resistance–associated hepatic iron overload.’’ However, the finding of hepatic iron overload with mutant HFE genotypes other than homozygosity for C282Y is not unexpected. A small percentage of patients with a clinical diagnosis of hemochromatosis (on the basis of the histological appearance, hepatic iron stores, hepatic iron index, and phlebotomy requirements) have been found to be C282Y/H63D compound heterozygotes (3%–6%)23,24 or heterozygous for the C282Y mutation (1%).23 The penetrance of these genotypes with respect to iron overload is less than for C282Y homozygotes, as is the degree of iron loading. Nonetheless, in a study population selected on the basis of iron overload, it is to be expected that the C282Y/H63D compound heterozygotes would be more prevalent than in the control population. One of the most interesting questions now is the nature of the relationship between hepatic steatosis and iron overload. There are several possibilities: iron accumulation could result in steatosis, hepatic steatosis could lead to iron accumulation, both steatosis and iron overload may be distinct consequences of another factor (probably insulin resistance in this case), and finally, hepatic iron overload and steatosis may be unrelated but occurring together coincidentally. In this case, the combination may act synergistically to increase liver injury, whereas steatosis without iron overload may have been innocuous and escaped attention. Overall, this seems less likely because hepatic steatosis is known to be associated with insulin resistance, while iron accumulation also appears to be associated with insulin resistance6–9 that can be improved by phlebotomy.25,26 If iron accumulation led to insulin resistance and steatosis, then the prevalence and severity of steatosis would be expected to be greatest in individuals with the greatest iron overload, the HFE compound heterozygotes. Mendler et al. found that this group had significantly less steatosis and lower prevalence of nonalcoholic steatohepatitis (NASH) than the other genotypes. The most likely explanation seems to be that the same factors that led to hepatic steatosis also resulted in iron accumulation, although the rate of iron loading (as reflected by the hepatic iron index) is less than in HFE compound heterozygotes. Finally, the histological correlates of hepatic steatosis and iron overload are worthy of comment. Mendler et al. found that steatosis was associated with hepatic fibrosis, an observation we also made in patients with NASH.3
November 1999
This may be explained in part by their observation that there was a progressive increase in the prevalence of NASH with increasing severity of steatosis. The increase in inflammation and stellate cell activation associated with NASH is likely to be the cause of increased fibrosis in these patients. However, Mendler et al. did not confirm our previous observation of the association between hepatic iron stores and fibrosis in NASH,3 possibly because all their subjects had some degree of iron overload and regression analysis was not used. In summary, Mendler et al. demonstrate an association between hepatic iron overload and steatosis and provide evidence that the IRS may be the underlying thread that links these two conditions. Further work is required to confirm that insulin resistance is involved in the pathogenesis of insulin resistance–associated hepatic iron overload and to refine the diagnostic criteria for this condition. This should be obtained by assessing the role of truncal obesity and hypertension, by characterizing the dyslipidemia, and performing metabolic studies to assess the degree of insulin resistance present. Future studies should also aim to determine the nature of the relationship between insulin resistance, hepatic steatosis, and iron overload. Obesity is increasingly recognized as a risk factor for steatosis and fibrosis in various liver diseases including chronic hepatitis C infection27 and alcoholic liver disease.28 Together, these observations imply that steatosis is an independent risk factor for hepatic fibrosis but the same factors that lead to steatosis are also associated with iron accumulation. Given the postulated roles for iron and steatosis in a variety of liver diseases, understanding the interactions between insulin resistance, iron accumulation, and hepatic steatosis is relevant to the diagnosis and management of a number of common liver diseases. GRAEME A. MACDONALD Clinical Sciences Unit Queensland Institute of Medical Research and Department of Medicine University of Queensland LAWRIE W. POWELL Clinical Sciences Unit Queensland Institute of Medical Research and University of Queensland Brisbane, Queensland, Australia
References 1. Ludwig J, Viggiano TR, McGill DB, Oh BJ. Nonalcoholic steatohepatitis: Mayo Clinic experiences with a hitherto unnamed disease. Mayo Clin Proc 1980;5:434–438. 2. Powell EE, Cooksley WG, Hanson R, Searle J, Halliday JW, Powell LW. The natural history of nonalcoholic steatohepatitis: a fol-
EDITORIALS 1243
3.
4.
5.
6.
7.
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19.
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low-up study of forty-two patients for up to 21 years. Hepatology 1990;11:74–80. George DK, Goldwurm S, Macdonald GA, Cowley LL, Walker NI, Ward PJ, Jazwinska EC, Powell LW. Increased hepatic iron stores in non-alcoholic steatohepatitis is associated with increased hepatic fibrosis. Gastroenterology 1998;114:311–318. Bacon BR, Farahvash MJ, Janney CG, Neuschwander Tetri BA. Nonalcoholic steatohepatitis: an expanded clinical entity. Gastroenterology 1994;107:1103–1109. Mendler M-H, Turlin B, Moirand R, Jouanolle A-M, Sapey T, Guyader D, Le Gall J-Y, Brissot P, David V, Deugnier Y. Insulin resistance–associated hepatic iron overload 1999;117:1155– 1163. Fernandez Real JM, Ricart Engel W, Arroyo E, Balanca R, Casamitjana Abella R, Cabrero D, Fernandez Castaner M, Soler J. Serum ferritin as a component of the insulin resistance syndrome. Diabetes Care 1998;21:62–68. Merkel PA, Simonson DC, Amiel SA, Plewe G, Sherwin RS, Pearson HA, Tamborlane WV. Insulin resistance and hyperinsulinemia in patients with thalassemia major treated by hypertransfusion. N Engl J Med 1988;318:809–814. Cavallo Perin P, Pacini G, Cerutti F, Bessone A, Condo C, Sacchetti L, Piga A, Pagano G. Insulin resistance and hyperinsulinemia in homozygous beta-thalassemia. Metabolism 1995;44:281–286. Tuomainen TP, Nyyssonen K, Salonen R, Tervahauta A, Korpela H, Lakka T, Kaplan GA, Salonen JT. Body iron stores are associated with serum insulin and blood glucose concentrations. Population study in 1,013 eastern Finnish men. Diabetes Care 1997;20:426– 428. Knobler H, Schattner A, Zhornicki T, Malnick SD, Keter D, Sokolovskaya N, Lurie Y, Bass DD. Fatty liver—an additional and treatable feature of the insulin resistance syndrome. QJM 1999; 92:73–79. Marceau P, Biron S, Hould FS, Marceau S, Simard S, Thung SN, Kral JG. Liver pathology and the metabolic syndrome X in severe obesity. J Clin Endocrinol Metab 1999;84:1513–1517. Kaplan NM. The deadly quartet. Upper-body obesity, glucose intolerance, hypertriglyceridemia, and hypertension. Arch Intern Med 1989;149:1514–1520. Consensus Development Conference on Insulin Resistance. November 5–6, 1997. American Diabetes Association. Diabetes Care 1998;21:310–314. Clinical guidelines on the identification, evaluation, and treatment of overweight and obesity in adults—the evidence report. National Institutes of Health. Obes Res 1998;6(suppl 2):51s– 209s. Mykkanen L, Haffner SM, Ronnemaa T, Bergman RN, Laakso M. Low insulin sensitivity is associated with clustering of cardiovascular disease risk factors. Am J Epidemiol 1997;146:315–321. Mau MK, Grandinetti A, Arakaki RF, Chang HK, Kinney EK, Curb JD. The insulin resistance syndrome in native Hawaiians. Native Hawaiian Health Research (NHHR) Project. Diabetes Care 1997; 20:1376–1380. Flegal KM, Carroll MD, Kuczmarski RJ, Johnson CL. Overweight and obesity in the United States: prevalence and trends, 1960– 1994. Int J Obes Relat Metab Disord 1998;22:39–47. Despres JP, Moorjani S, Lupien PJ, Tremblay A, Nadeau A, Bouchard C. Regional distribution of body fat, plasma lipoproteins, and cardiovascular disease. Arteriosclerosis 1990;10:497– 511. Kral JG, Schaffner F, Pierson RN Jr, Wang J. Body fat topography as an independent predictor of fatty liver. Metabolism 1993;42: 548–551. Greenlund KJ, Valdez R, Casper ML, Rith Najarian S, Croft JB. Prevalence and correlates of the insulin resistance syndrome among Native Americans. The Inter-Tribal Heart Project. Diabetes Care 1999; 22:441–447.
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21. Goto T, Onuma T, Takebe K, Kral JG. The influence of fatty liver on insulin clearance and insulin resistance in non-diabetic Japanese subjects. Int J Obes Relat Metab Disord 1995;19:841–855. 22. Banerji MA, Buckley MC, Chaiken RL, Gordon D, Lebovitz HE, Kral JG. Liver fat, serum triglycerides and visceral adipose tissue in insulin-sensitive and insulin-resistant black men with NIDDM. Int J Obes Relat Metab Disord 1995;19:846–850. 23. Crawford DH, Jazwinska EC, Cullen LM, Powell LW. Expression of HLA-linked hemochromatosis in subjects homozygous or heterozygous for the C282Y mutation. Gastroenterology 1998;114:1003– 1008. 24. Bacon BR, Olynyk JK, Brunt EM, Britton RS, Wolff RK. HFE genotype in patients with hemochromatosis and other liver diseases. Ann Intern Med 1999;130:953–962. 25. Facchini FS. Effect of phlebotomy on plasma glucose and insulin concentrations (letter). Diabetes Care 1998;21:2190. 26. Hramiak IM, Finegood DT, Adams PC. Factors affecting glucose
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tolerance in hereditary hemochromatosis. Clin Invest Med 1997; 20:110–118. 27. Hourigan LF, Macdonald GA, Purdie D, Whitehall VH, Shorthouse C, Clouston A, Powell EE. Fibrosis in chronic hepatitis C correlates significantly with body mass index and steatosis. Hepatology 1999;29:1215–1219. 28. Naveau S, Giraud V, Borotto E, Aubert A, Capron F, Chaput JC. Excess weight risk factor for alcoholic liver disease. Hepatology 1997;25:108–111.
Address requests for reprints to: Graeme A. Macdonald, M.D., C Floor, Clinical Sciences Building, Royal Brisbane Hospital, Brisbane, Queensland, 4029 Australia. e-mail:g.macdonald@medicine. herston.uq.edu.au; fax: (61) 7-3365-5462. r 1999 by the American Gastroenterological Association 0016-5085/99/$10.00
The Virtuosity of Hepatic Stellate Cells See articles on pages 1198 and 1205.
wenty years ago there was little music in hepatic fibrosis. In the intervening two decades, discrete melodies have emerged, none more so than through the voice of the hepatic stellate cell, a perisinusoidal mesenchymal cell type. Two articles in this issue of GASTROENTEROLOGY illustrate this progress, emphasizing the enlarging complexity of hepatic stellate cells on the one hand and their clear-cut emergence as a target for antifibrotic therapy on the other. Hepatic stellate cells are the principal fibrogenic cell type in liver, a conclusion made possible by the development of methods to isolate this cell type from rodents and then human liver. Because stellate cells in normal liver contain vitamin A droplets, which impart buoyancy, successful isolation methods used density gradients to recover a homogeneous population of these low-density cells. Growth of vitamin A–rich stellate cells on plastic led to a phenotypic response known as activation, which paralleled closely the response of stellate cells to injury in vivo. From this culture model grew a large body of information characterizing stellate cell activation, including the expression of cytoskeletal markers, secretion of cytokines, and production of extracellular matrix (ECM) molecules.1 Parallel efforts of this type explored the behavior of similar mesenchymal cells in other tissues, underscoring the heterogeneity of so-called myofibroblasts in normal tissue and wound-healing.2 Not long thereafter, the concept of heterogeneity was validated in stellate cells, demonstrating that this cell type may have
T
differing patterns of vitamin A storage, cytoskeletal protein expression, and/or matrix production.3–6 In vivo studies in animal models of liver injury have also emphasized the potential of hepatic mesenchymal cells to express multiple phenotypes.7 Stellate cell heterogeneity has now been further explored in the interesting study by Knittel et al.8 By use of an isolation method that apparently destroys vitamin A–rich stellate cells, a population of myofibroblasts lacking vitamin A, known as rat myofibroblasts, or rMFs, has been cultured and characterized. These cells have patterns of cytoskeletal protein expression distinct from typical vitamin A–rich stellate cells, including those of desmin, vimentin, smooth muscle actin, and glial fibrillary acidic protein. With progressive growth in culture, their cytoskeletal phenotype modulates further; the functional implications of this cytoskeletal plasticity are uncertain. More importantly, it seems that rMFs continue to grow after multiple passages in culture, whereas more typical vitamin A–containing stellate cells do not, implying a decreased propensity of rMFs to undergo apoptosis. Molecular comparison between rMFs and typical stellate cells using a differential display technique identified fibulin 2, a recently described ECM protein, and interleukin 6, a key regenerative cytokine, as being preferentially expressed in rMFs. A shortcoming of these efforts, however, is the failure to validate these culture-induced distinctions in phenotype between stellate cells and rMFs with behavior in vivo, although such in vivo correlation is forthcoming according to the authors. Without these data, it is uncertain whether cells with the rMF pheno-