Sulfated oxysterol 25HC3S as a therapeutic target of non-alcoholic fatty liver disease

In mammals, triglycerides are predominantly synthesized in the liver by de novo lipogenesis and in turn, exported in the form of very low density lipoprotein (VLDL) particles to adipose and other tissues [[1]Hillgartner F.B. Salati L.M. Goodridge A.G. Physiological and molecular mechanisms involved in nutritional regulation of fatty acid synthesis.Physiol Rev. 1995; 75: 47-76Crossref PubMed Scopus (392) Google Scholar]. Although triglycerides serve as an important source of energy during fasting, excessive accumulation of triglycerides in the liver resulted in hepatic steatosis (fatty liver), a major element of liver diseases such as non-alcoholic fatty liver disease (NAFLD) [[2]Farrell G.C. Larter C.Z. Nonalcoholic fatty liver disease: from steatosis to cirrhosis.Hepatology. 2006; 43: S99-S112Crossref PubMed Scopus (1941) Google Scholar]. NAFLD affects one-third of the adult population and is found in more than two thirds of obese people in the United States [3Wieckowska A. McCullough A.J. Feldstein A.E. Noninvasive diagnosis and monitoring of nonalcoholic steatohepatitis: present and future.Hepatology. 2007; 46: 582-589Crossref PubMed Scopus (350) Google Scholar, 4Browning J.D. Szczepaniak L.S. Dobbins R. et al.Prevalence of hepatic steatosis in an urban population in the United States: impact of ethnicity.Hepatology. 2004; 40: 1387-1395Crossref PubMed Scopus (2880) Google Scholar]. It is clear that NAFLD is strongly associated with obesity and insulin resistance but its pathogenesis is not fully understood and therapeutic options are limited [[2]Farrell G.C. Larter C.Z. Nonalcoholic fatty liver disease: from steatosis to cirrhosis.Hepatology. 2006; 43: S99-S112Crossref PubMed Scopus (1941) Google Scholar]. Hepatic steatosis is often developed when fatty-acid metabolism is unbalanced in several ways, including uncontrolled delivery of free fatty acids to the liver from adipose tissue and inadequate de novo synthesis of triglycerides [5Zhu L. Baker S.S. Liu W. et al.Lipid in the livers of adolescents with nonalcoholic steatohepatitis: combined effects of pathways on steatosis.Metabolism. 2011; 60: 1001-1011Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar, 6Wang S. Kamat A. Pergola P. et al.Metabolic factors in the development of hepatic steatosis and altered mitochondrial gene expression in vivo.Metabolism. 2011; 60: 1090-1099Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar, 7Xie Z. Li H. Wang K. et al.Analysis of transcriptome and metabolome profiles alterations in fatty liver induced by high-fat diet in rat.Metabolism. 2010; 59: 554-560Abstract Full Text Full Text PDF PubMed Scopus (66) Google Scholar]. Lipogenesis is thought to be regulated via the transcription factors sterol regulatory element binding protein-1c (SREBP-1c) and carbohydrate response element binding protein (ChREBP), both of which are increased by insulin and glucose [8Shimomura I. Bashmakov Y. Ikemoto S. et al.Insulin selectively increases SREBP-1c mRNA in the livers of rats with streptozotocin-induced diabetes.Proc Natl Acad Sci USA. 1999; 96: 13656-13661Crossref PubMed Scopus (619) Google Scholar, 9Uyeda K. Yamashita H. Kawaguchi T. Carbohydrate responsive element-binding protein (ChREBP): a key regulator of glucose metabolism and fat storage.Biochem Pharmacol. 2002; 63: 2075-2080Crossref PubMed Scopus (158) Google Scholar]. Evidence also suggests that the sterol and glucose sensor, liver X receptors (LXRs) play important roles in regulating the lipogenic pathway [10Schultz J.R. Tu H. Luk A. et al.Role of LXRs in control of lipogenesis.Genes Dev. 2000; 14: 2831-2838Crossref PubMed Scopus (1380) Google Scholar, 11Mitro N. Mak P.A. Vargas L. et al.The nuclear receptor LXR is a glucose sensor.Nature. 2007; 445: 219-223Crossref PubMed Scopus (432) Google Scholar].LXRs, LXRα and LXRβ, are members of the nuclear receptor superfamily of ligand-activated transcription factors that are involved in the control of cholesterol, lipid and carbohydrate metabolism, as well as inflammatory processes [[12]Torocsik D. Szanto A. Nagy L. Oxysterol signaling links cholesterol metabolism and inflammation via the liver X receptor in macrophages.Mol Aspects Med. 2009; 30: 134-152Crossref PubMed Scopus (63) Google Scholar]. LXRs regulate cholesterol metabolism by modulating genes involved in cholesterol efflux [[13]Joseph S.B. Castrillo A. Laffitte B.A. et al.Reciprocal regulation of inflammation and lipid metabolism by liver X receptors.Nat Med. 2003; 9: 213-219Crossref PubMed Scopus (1000) Google Scholar], cholesterol conversion to bile acids [14Alberti S. Schuster G. Parini P. et al.Hepatic cholesterol metabolism and resistance to dietary cholesterol in LXRbeta-deficient mice.J Clin Invest. 2001; 107: 565-573Crossref PubMed Scopus (318) Google Scholar, 15Peet D.J. Turley S.D. Ma W. et al.Cholesterol and bile acid metabolism are impaired in mice lacking the nuclear oxysterol receptor LXR alpha.Cell. 1998; 93: 693-704Abstract Full Text Full Text PDF PubMed Scopus (1230) Google Scholar], and cholesterol secretion into bile for excretion [16Repa J.J. Berge K.E. Pomajzl C. et al.Regulation of ATP-binding cassette sterol transporters ABCG5 and ABCG8 by the liver X receptors alpha and beta.J Biol Chem. 2002; 277: 18793-18800Crossref PubMed Scopus (675) Google Scholar, 17Plosch T. Kok T. Bloks V.W. et al.Increased hepatobiliary and fecal cholesterol excretion upon activation of the liver X receptor is independent of ABCA1.J Biol Chem. 2002; 277: 33870-33877Crossref PubMed Scopus (176) Google Scholar], showing a beneficial role for LXRs in metabolic disorders characterized by cholesterol accumulation. Administration of LXR agonists to mice induces hepatic steatosis and hypertriglyceridemia by enhancing hepatic fatty-acid synthesis [18Grefhorst A. Elzinga B.M. Voshol P.J. et al.Stimulation of lipogenesis by pharmacological activation of the liver X receptor leads to production of large, triglyceride-rich very low density lipoprotein particles.J Biol Chem. 2002; 277: 34182-34190Crossref PubMed Scopus (405) Google Scholar, 19Chen G. Liang G. Ou J. et al.Central role for liver X receptor in insulin-mediated activation of Srebp-1c transcription and stimulation of fatty acid synthesis in liver.Proc Natl Acad Sci USA. 2004; 101: 11245-11250Crossref PubMed Scopus (429) Google Scholar]. The molecular mechanism responsible for this is most likely due to the significant increase in expression of SREBP-1c, ChREBP and lipogenic genes such as FAS and SCD1 [20Repa J.J. Liang G. Ou J. et al.Regulation of mouse sterol regulatory element-binding protein-1c gene (SREBP-1c) by oxysterol receptors, LXRalpha and LXRbeta.Genes Dev. 2000; 14: 2819-2830Crossref PubMed Scopus (1401) Google Scholar, 21Cha J.Y. Repa J.J. The liver X receptor (LXR) and hepatic lipogenesis. The carbohydrate-response element-binding protein is a target gene of LXR.J Biol Chem. 2007; 282: 743-751Crossref PubMed Scopus (362) Google Scholar, 22Joseph S.B. McKilligin E. Pei L. et al.Synthetic LXR ligand inhibits the development of atherosclerosis in mice.Proc Natl Acad Sci USA. 2002; 99: 7604-7609Crossref PubMed Scopus (770) Google Scholar]. Collectively, these data suggest that modulation of LXR activity could be an attractive strategy for the development of lipid-lowering therapies in NAFLD.Oxysterols, such as 24(S) hydroxycholesterol (HC), 25HC, 27HC, and cholestenoic acid, are generally believed to be the most important physiological activators for LXRs [23Song C. Liao S. Cholestenoic acid is a naturally occurring ligand for liver X receptor alpha.Endocrinology. 2000; 141: 4180-4184Crossref PubMed Google Scholar, 24Lehmann J.M. Kliewer S.A. Moore L.B. et al.Activation of the nuclear receptor LXR by oxysterols defines a new hormone response pathway.J Biol Chem. 1997; 272: 3137-3140Crossref PubMed Scopus (1029) Google Scholar, 25Janowski B.A. Willy P.J. Devi T.R. et al.An oxysterol signalling pathway mediated by the nuclear receptor LXR alpha.Nature. 1996; 383: 728-731Crossref PubMed Scopus (1441) Google Scholar]. Chen et al [[26]Chen W. Chen G. Head D.L. et al.Enzymatic reduction of oxysterols impairs LXR signaling in cultured cells and the livers of mice.Cell Metab. 2007; 5: 73-79Abstract Full Text Full Text PDF PubMed Scopus (242) Google Scholar] reported that overexpression of cholesterol sulfotransferase SULT2B1b, an enzyme that metabolizes oxysterol ligands into the 3-sulfated forms, led to inactivation of LXR signaling in vitro and in vivo, suggesting that oxysterols are likely in vivo ligands for LXR and sulfation of oxysterols inactivates the LXR signaling cascade. Recently, a novel sulfated oxysterol, 5-cholesten-3β, 25-diol 3-sulfate (sulfated 25-hydroxycholesterol, 25HC3S) has been identified in hepatocytes overexpressing the mitochondrial cholesterol delivery protein, StarD1 [27Ren S. Hylemon P. Zhang Z.P. et al.Identification of a novel sulfonated oxysterol, 5-cholesten-3beta,25-diol 3-sulfonate, in hepatocyte nuclei and mitochondria.J Lipid Res. 2006; 47: 1081-1090Crossref PubMed Scopus (42) Google Scholar, 28Rodriguez-Agudo D. Ren S. Hylemon P.B. et al.Human StarD5, a cytosolic StAR-related lipid binding protein.J Lipid Res. 2005; 46: 1615-1623Crossref PubMed Scopus (67) Google Scholar]. As shown in Fig. 1, the 25HC3S biosynthetic pathway is comprised of two enzymes, CYP27A1 and SULT2B1b, in liver hepatocytes. Cholesterol is hydroxylated by CYP27A1 in the mitochondria to 25HC and subsequently sulfated at the 3β-position by SULT2B1b in the cytosol to form 25HC3S [[29]Li X. Pandak W.M. Erickson S.K. et al.Biosynthesis of the regulatory oxysterol, 5-cholesten-3beta,25-diol 3-sulfate, in hepatocytes.J Lipid Res. 2007; 48: 2587-2596Crossref PubMed Scopus (69) Google Scholar]. Studies with primary human aortic endothelial cells indicate that SULT2B1b overexpression with 25HC supplementation decreases cholesterol and fatty-acid biosynthesis by inhibiting SREBP-1/2 expression and inactivating SREBP-1c [[30]Bai Q. Xu L. Kakiyama G. et al.Sulfation of 25-hydroxycholesterol by SULT2B1b decreases cellular lipids via the LXR/SREBP-1c signaling pathway in human aortic endothelial cells.Atherosclerosis. 2011; 214: 350-356Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar]. Similar findings are also observed in primary human hepatocytes treated with 25HC3S [31Ren S. Li X. Rodriguez-Agudo D. et al.Sulfated oxysterol, 25HC3S, is a potent regulator of lipid metabolism in human hepatocytes.Biochem Biophys Res Commun. 2007; 360: 802-808Crossref PubMed Scopus (48) Google Scholar, 32Ma Y. Xu L. Rodriguez-Agudo D. et al.25-Hydroxycholesterol-3-sulfate regulates macrophage lipid metabolism via the LXR/SREBP-1 signaling pathway.Am J Physiol Endocrinol Metab. 2008; 295: E1369-E1379Crossref PubMed Scopus (64) Google Scholar, 33Xu L. Bai Q. Rodriguez-Agudo D. et al.Regulation of hepatocyte lipid metabolism and inflammatory response by 25-hydroxycholesterol and 25-hydroxycholesterol-3-sulfate.Lipids. 2010; 45: 821-832Crossref PubMed Scopus (56) Google Scholar]. These results demonstrate that 25HC3S may act as an LXR antagonist rather than only an inactive form of 25HC in lipid metabolism.In this issue, Bai and colleagues [[34]Bai Q. Zhang X. Xu L. Oxysterol sulfation by cytosolic sulfotransferase suppresses liver X receptor/sterol regulatory element binding protein-1c signaling pathway and reduces serum and hepatic lipids in mouse models of nonalcoholic fatty liver disease.Metabolism. 2012; 61: 836-845Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar] demonstrate that adenovirus-mediated overexpression of SULT2B1b in animal models of NAFLD in the presence of 25HC causes a significant increase in 25HC3S level in the liver. These effects are accompanied by decreases in serum and hepatic triglycerides, total cholesterol, free cholesterol, and free fatty acids. The lipid-lowering effects of SULT2B1b are mediated by the reduction of key regulators and enzymes in lipid metabolism, including SREBP-1/2, ACC1, and FAS. In addition, the expression of genes involved in cholesterol efflux (ABCA1 and ABCG1) and clearance of lipid from circulation (low-density-lipoprotein receptor, LDLR) was decreased when SULT2B1b expression was increased, indicating that SULT2B1b-mediated reduction of intracellular lipid levels is due to decreased uptake and synthesis of lipids. Using LDLR-deficient mice, Bai et al further demonstrated that the main cause of decreases in serum and hepatic lipid levels mainly occurs during the suppression of lipid biosynthesis rather than clearance. Notably, overexpression of SULT2B1b in the LDLR knockout mice not only reduces neutral lipid levels in the liver but also changes the lipid profiles in the sera, revealing that SULT2B1b overexpression is sufficient to decrease VLDL and LDL synthesis and secretion from the liver. Thus, the current data provide important evidence that 25HC3S, the sulfated form of 25HC by SULT2B1b, is a key regulator that links cholesterol metabolism to fatty-acid biosynthesis and appears to be an excellent drug target for hepatic lipid dysfunction.Of note, hepatic steatosis can ultimately progress to nonalcoholic steatohepatitis (NASH), which is distinguished from steatosis by the presence of oxidative stress, inflammation, cell death, and fibrosis [2Farrell G.C. Larter C.Z. Nonalcoholic fatty liver disease: from steatosis to cirrhosis.Hepatology. 2006; 43: S99-S112Crossref PubMed Scopus (1941) Google Scholar, 35Nakayama H. Otabe S. Ueno T. et al.Transgenic mice expressing nuclear sterol regulatory element-binding protein 1c in adipose tissue exhibit liver histology similar to nonalcoholic steatohepatitis.Metabolism. 2007; 56: 470-475Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar]. It has been reported that, besides inducing hepatic fatty-acid biosynthesis, LXRs actively participate in inflammatory responses. The expression of proinflammatory cytokines interleukin-1β, IL-6, and tumor necrosis factor α (TNFα) is enhanced in LXR-null murine macrophages in response to lipopolysaccharide, suggesting that LXRs are required for anti-inflammatory effects [[13]Joseph S.B. Castrillo A. Laffitte B.A. et al.Reciprocal regulation of inflammation and lipid metabolism by liver X receptors.Nat Med. 2003; 9: 213-219Crossref PubMed Scopus (1000) Google Scholar]. In contrast, LXRs are also involved in proinflammatory responses, as evidenced by the fact that treatment of LPS-stimulated human monocytes with LXR agonists potentiated the expression of TNFα [[36]Walcher D. Kummel A. Kehrle B. et al.LXR activation reduces proinflammatory cytokine expression in human CD4-positive lymphocytes.Arterioscler Thromb Vasc Biol. 2006; 26: 1022-1028Crossref PubMed Scopus (83) Google Scholar]. Although the role of LXRs in inflammation is complex and controversial, evidence demonstrates that 25HC3S reduces nuclear-factor kappa B-mediated inflammatory response by decreasing LXR activity whereas 25HC elicits opposite effects by increasing LXR activity in hepatocytes [[33]Xu L. Bai Q. Rodriguez-Agudo D. et al.Regulation of hepatocyte lipid metabolism and inflammatory response by 25-hydroxycholesterol and 25-hydroxycholesterol-3-sulfate.Lipids. 2010; 45: 821-832Crossref PubMed Scopus (56) Google Scholar]. However, the molecular mechanism by which 25HC3S regulates inflammatory response in the NASH model remains to be elucidated, with particular emphasis on LXR-mediated hepatic steatosis and inflammation.The study by Bai et al raises the possibility that normal cholesterol and lipid metabolism are linked by the physiologic balance between 25HC and 25HC3S in the liver. It is therefore important to determine what degree of changes in 25HC and 25HC3S cellular levels can contribute to LXR activity in the physiological milieu, in conjunction with SULT2B1b expression and activity. Understanding the physiological mechanisms for specificity of SULT2B1b and the effects of 25HC3S on cholesterol/lipid metabolism and anti-inflammation could lead to new therapeutic targets for NAFLD and NASH.FundingThis study was supported by a grant from the Korean Health Technology R&D Project, Ministry of Health & Welfare, Republic of Korea ( A100899 ).Conflict of interestThe authors declare that they have no conflict interest. In mammals, triglycerides are predominantly synthesized in the liver by de novo lipogenesis and in turn, exported in the form of very low density lipoprotein (VLDL) particles to adipose and other tissues [[1]Hillgartner F.B. Salati L.M. Goodridge A.G. Physiological and molecular mechanisms involved in nutritional regulation of fatty acid synthesis.Physiol Rev. 1995; 75: 47-76Crossref PubMed Scopus (392) Google Scholar]. Although triglycerides serve as an important source of energy during fasting, excessive accumulation of triglycerides in the liver resulted in hepatic steatosis (fatty liver), a major element of liver diseases such as non-alcoholic fatty liver disease (NAFLD) [[2]Farrell G.C. Larter C.Z. Nonalcoholic fatty liver disease: from steatosis to cirrhosis.Hepatology. 2006; 43: S99-S112Crossref PubMed Scopus (1941) Google Scholar]. NAFLD affects one-third of the adult population and is found in more than two thirds of obese people in the United States [3Wieckowska A. McCullough A.J. Feldstein A.E. Noninvasive diagnosis and monitoring of nonalcoholic steatohepatitis: present and future.Hepatology. 2007; 46: 582-589Crossref PubMed Scopus (350) Google Scholar, 4Browning J.D. Szczepaniak L.S. Dobbins R. et al.Prevalence of hepatic steatosis in an urban population in the United States: impact of ethnicity.Hepatology. 2004; 40: 1387-1395Crossref PubMed Scopus (2880) Google Scholar]. It is clear that NAFLD is strongly associated with obesity and insulin resistance but its pathogenesis is not fully understood and therapeutic options are limited [[2]Farrell G.C. Larter C.Z. Nonalcoholic fatty liver disease: from steatosis to cirrhosis.Hepatology. 2006; 43: S99-S112Crossref PubMed Scopus (1941) Google Scholar]. Hepatic steatosis is often developed when fatty-acid metabolism is unbalanced in several ways, including uncontrolled delivery of free fatty acids to the liver from adipose tissue and inadequate de novo synthesis of triglycerides [5Zhu L. Baker S.S. Liu W. et al.Lipid in the livers of adolescents with nonalcoholic steatohepatitis: combined effects of pathways on steatosis.Metabolism. 2011; 60: 1001-1011Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar, 6Wang S. Kamat A. Pergola P. et al.Metabolic factors in the development of hepatic steatosis and altered mitochondrial gene expression in vivo.Metabolism. 2011; 60: 1090-1099Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar, 7Xie Z. Li H. Wang K. et al.Analysis of transcriptome and metabolome profiles alterations in fatty liver induced by high-fat diet in rat.Metabolism. 2010; 59: 554-560Abstract Full Text Full Text PDF PubMed Scopus (66) Google Scholar]. Lipogenesis is thought to be regulated via the transcription factors sterol regulatory element binding protein-1c (SREBP-1c) and carbohydrate response element binding protein (ChREBP), both of which are increased by insulin and glucose [8Shimomura I. Bashmakov Y. Ikemoto S. et al.Insulin selectively increases SREBP-1c mRNA in the livers of rats with streptozotocin-induced diabetes.Proc Natl Acad Sci USA. 1999; 96: 13656-13661Crossref PubMed Scopus (619) Google Scholar, 9Uyeda K. Yamashita H. Kawaguchi T. Carbohydrate responsive element-binding protein (ChREBP): a key regulator of glucose metabolism and fat storage.Biochem Pharmacol. 2002; 63: 2075-2080Crossref PubMed Scopus (158) Google Scholar]. Evidence also suggests that the sterol and glucose sensor, liver X receptors (LXRs) play important roles in regulating the lipogenic pathway [10Schultz J.R. Tu H. Luk A. et al.Role of LXRs in control of lipogenesis.Genes Dev. 2000; 14: 2831-2838Crossref PubMed Scopus (1380) Google Scholar, 11Mitro N. Mak P.A. Vargas L. et al.The nuclear receptor LXR is a glucose sensor.Nature. 2007; 445: 219-223Crossref PubMed Scopus (432) Google Scholar]. LXRs, LXRα and LXRβ, are members of the nuclear receptor superfamily of ligand-activated transcription factors that are involved in the control of cholesterol, lipid and carbohydrate metabolism, as well as inflammatory processes [[12]Torocsik D. Szanto A. Nagy L. Oxysterol signaling links cholesterol metabolism and inflammation via the liver X receptor in macrophages.Mol Aspects Med. 2009; 30: 134-152Crossref PubMed Scopus (63) Google Scholar]. LXRs regulate cholesterol metabolism by modulating genes involved in cholesterol efflux [[13]Joseph S.B. Castrillo A. Laffitte B.A. et al.Reciprocal regulation of inflammation and lipid metabolism by liver X receptors.Nat Med. 2003; 9: 213-219Crossref PubMed Scopus (1000) Google Scholar], cholesterol conversion to bile acids [14Alberti S. Schuster G. Parini P. et al.Hepatic cholesterol metabolism and resistance to dietary cholesterol in LXRbeta-deficient mice.J Clin Invest. 2001; 107: 565-573Crossref PubMed Scopus (318) Google Scholar, 15Peet D.J. Turley S.D. Ma W. et al.Cholesterol and bile acid metabolism are impaired in mice lacking the nuclear oxysterol receptor LXR alpha.Cell. 1998; 93: 693-704Abstract Full Text Full Text PDF PubMed Scopus (1230) Google Scholar], and cholesterol secretion into bile for excretion [16Repa J.J. Berge K.E. Pomajzl C. et al.Regulation of ATP-binding cassette sterol transporters ABCG5 and ABCG8 by the liver X receptors alpha and beta.J Biol Chem. 2002; 277: 18793-18800Crossref PubMed Scopus (675) Google Scholar, 17Plosch T. Kok T. Bloks V.W. et al.Increased hepatobiliary and fecal cholesterol excretion upon activation of the liver X receptor is independent of ABCA1.J Biol Chem. 2002; 277: 33870-33877Crossref PubMed Scopus (176) Google Scholar], showing a beneficial role for LXRs in metabolic disorders characterized by cholesterol accumulation. Administration of LXR agonists to mice induces hepatic steatosis and hypertriglyceridemia by enhancing hepatic fatty-acid synthesis [18Grefhorst A. Elzinga B.M. Voshol P.J. et al.Stimulation of lipogenesis by pharmacological activation of the liver X receptor leads to production of large, triglyceride-rich very low density lipoprotein particles.J Biol Chem. 2002; 277: 34182-34190Crossref PubMed Scopus (405) Google Scholar, 19Chen G. Liang G. Ou J. et al.Central role for liver X receptor in insulin-mediated activation of Srebp-1c transcription and stimulation of fatty acid synthesis in liver.Proc Natl Acad Sci USA. 2004; 101: 11245-11250Crossref PubMed Scopus (429) Google Scholar]. The molecular mechanism responsible for this is most likely due to the significant increase in expression of SREBP-1c, ChREBP and lipogenic genes such as FAS and SCD1 [20Repa J.J. Liang G. Ou J. et al.Regulation of mouse sterol regulatory element-binding protein-1c gene (SREBP-1c) by oxysterol receptors, LXRalpha and LXRbeta.Genes Dev. 2000; 14: 2819-2830Crossref PubMed Scopus (1401) Google Scholar, 21Cha J.Y. Repa J.J. The liver X receptor (LXR) and hepatic lipogenesis. The carbohydrate-response element-binding protein is a target gene of LXR.J Biol Chem. 2007; 282: 743-751Crossref PubMed Scopus (362) Google Scholar, 22Joseph S.B. McKilligin E. Pei L. et al.Synthetic LXR ligand inhibits the development of atherosclerosis in mice.Proc Natl Acad Sci USA. 2002; 99: 7604-7609Crossref PubMed Scopus (770) Google Scholar]. Collectively, these data suggest that modulation of LXR activity could be an attractive strategy for the development of lipid-lowering therapies in NAFLD. Oxysterols, such as 24(S) hydroxycholesterol (HC), 25HC, 27HC, and cholestenoic acid, are generally believed to be the most important physiological activators for LXRs [23Song C. Liao S. Cholestenoic acid is a naturally occurring ligand for liver X receptor alpha.Endocrinology. 2000; 141: 4180-4184Crossref PubMed Google Scholar, 24Lehmann J.M. Kliewer S.A. Moore L.B. et al.Activation of the nuclear receptor LXR by oxysterols defines a new hormone response pathway.J Biol Chem. 1997; 272: 3137-3140Crossref PubMed Scopus (1029) Google Scholar, 25Janowski B.A. Willy P.J. Devi T.R. et al.An oxysterol signalling pathway mediated by the nuclear receptor LXR alpha.Nature. 1996; 383: 728-731Crossref PubMed Scopus (1441) Google Scholar]. Chen et al [[26]Chen W. Chen G. Head D.L. et al.Enzymatic reduction of oxysterols impairs LXR signaling in cultured cells and the livers of mice.Cell Metab. 2007; 5: 73-79Abstract Full Text Full Text PDF PubMed Scopus (242) Google Scholar] reported that overexpression of cholesterol sulfotransferase SULT2B1b, an enzyme that metabolizes oxysterol ligands into the 3-sulfated forms, led to inactivation of LXR signaling in vitro and in vivo, suggesting that oxysterols are likely in vivo ligands for LXR and sulfation of oxysterols inactivates the LXR signaling cascade. Recently, a novel sulfated oxysterol, 5-cholesten-3β, 25-diol 3-sulfate (sulfated 25-hydroxycholesterol, 25HC3S) has been identified in hepatocytes overexpressing the mitochondrial cholesterol delivery protein, StarD1 [27Ren S. Hylemon P. Zhang Z.P. et al.Identification of a novel sulfonated oxysterol, 5-cholesten-3beta,25-diol 3-sulfonate, in hepatocyte nuclei and mitochondria.J Lipid Res. 2006; 47: 1081-1090Crossref PubMed Scopus (42) Google Scholar, 28Rodriguez-Agudo D. Ren S. Hylemon P.B. et al.Human StarD5, a cytosolic StAR-related lipid binding protein.J Lipid Res. 2005; 46: 1615-1623Crossref PubMed Scopus (67) Google Scholar]. As shown in Fig. 1, the 25HC3S biosynthetic pathway is comprised of two enzymes, CYP27A1 and SULT2B1b, in liver hepatocytes. Cholesterol is hydroxylated by CYP27A1 in the mitochondria to 25HC and subsequently sulfated at the 3β-position by SULT2B1b in the cytosol to form 25HC3S [[29]Li X. Pandak W.M. Erickson S.K. et al.Biosynthesis of the regulatory oxysterol, 5-cholesten-3beta,25-diol 3-sulfate, in hepatocytes.J Lipid Res. 2007; 48: 2587-2596Crossref PubMed Scopus (69) Google Scholar]. Studies with primary human aortic endothelial cells indicate that SULT2B1b overexpression with 25HC supplementation decreases cholesterol and fatty-acid biosynthesis by inhibiting SREBP-1/2 expression and inactivating SREBP-1c [[30]Bai Q. Xu L. Kakiyama G. et al.Sulfation of 25-hydroxycholesterol by SULT2B1b decreases cellular lipids via the LXR/SREBP-1c signaling pathway in human aortic endothelial cells.Atherosclerosis. 2011; 214: 350-356Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar]. Similar findings are also observed in primary human hepatocytes treated with 25HC3S [31Ren S. Li X. Rodriguez-Agudo D. et al.Sulfated oxysterol, 25HC3S, is a potent regulator of lipid metabolism in human hepatocytes.Biochem Biophys Res Commun. 2007; 360: 802-808Crossref PubMed Scopus (48) Google Scholar, 32Ma Y. Xu L. Rodriguez-Agudo D. et al.25-Hydroxycholesterol-3-sulfate regulates macrophage lipid metabolism via the LXR/SREBP-1 signaling pathway.Am J Physiol Endocrinol Metab. 2008; 295: E1369-E1379Crossref PubMed Scopus (64) Google Scholar, 33Xu L. Bai Q. Rodriguez-Agudo D. et al.Regulation of hepatocyte lipid metabolism and inflammatory response by 25-hydroxycholesterol and 25-hydroxycholesterol-3-sulfate.Lipids. 2010; 45: 821-832Crossref PubMed Scopus (56) Google Scholar]. These results demonstrate that 25HC3S may act as an LXR antagonist rather than only an inactive form of 25HC in lipid metabolism. In this issue, Bai and colleagues [[34]Bai Q. Zhang X. Xu L. Oxysterol sulfation by cytosolic sulfotransferase suppresses liver X receptor/sterol regulatory element binding protein-1c signaling pathway and reduces serum and hepatic lipids in mouse models of nonalcoholic fatty liver disease.Metabolism. 2012; 61: 836-845Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar] demonstrate that adenovirus-mediated overexpression of SULT2B1b in animal models of NAFLD in the presence of 25HC causes a significant increase in 25HC3S level in the liver. These effects are accompanied by decreases in serum and hepatic triglycerides, total cholesterol, free cholesterol, and free fatty acids. The lipid-lowering effects of SULT2B1b are mediated by the reduction of key regulators and enzymes in lipid metabolism, including SREBP-1/2, ACC1, and FAS. In addition, the expression of genes involved in cholesterol efflux (ABCA1 and ABCG1) and clearance of lipid from circulation (low-density-lipoprotein receptor, LDLR) was decreased when SULT2B1b expression was increased, indicating that SULT2B1b-mediated reduction of intracellular lipid levels is due to decreased uptake and synthesis of lipids. Using LDLR-deficient mice, Bai et al further demonstrated that the main cause of decreases in serum and hepatic lipid levels mainly occurs during the suppression of lipid biosynthesis rather than clearance. Notably, overexpression of SULT2B1b in the LDLR knockout mice not only reduces neutral lipid levels in the liver but also changes the lipid profiles in the sera, revealing that SULT2B1b overexpression is sufficient to decrease VLDL and LDL synthesis and secretion from the liver. Thus, the current data provide important evidence that 25HC3S, the sulfated form of 25HC by SULT2B1b, is a key regulator that links cholesterol metabolism to fatty-acid biosynthesis and appears to be an excellent drug target for hepatic lipid dysfunction. Of note, hepatic steatosis can ultimately progress to nonalcoholic steatohepatitis (NASH), which is distinguished from steatosis by the presence of oxidative stress, inflammation, cell death, and fibrosis [2Farrell G.C. Larter C.Z. Nonalcoholic fatty liver disease: from steatosis to cirrhosis.Hepatology. 2006; 43: S99-S112Crossref PubMed Scopus (1941) Google Scholar, 35Nakayama H. Otabe S. Ueno T. et al.Transgenic mice expressing nuclear sterol regulatory element-binding protein 1c in adipose tissue exhibit liver histology similar to nonalcoholic steatohepatitis.Metabolism. 2007; 56: 470-475Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar]. It has been reported that, besides inducing hepatic fatty-acid biosynthesis, LXRs actively participate in inflammatory responses. The expression of proinflammatory cytokines interleukin-1β, IL-6, and tumor necrosis factor α (TNFα) is enhanced in LXR-null murine macrophages in response to lipopolysaccharide, suggesting that LXRs are required for anti-inflammatory effects [[13]Joseph S.B. Castrillo A. Laffitte B.A. et al.Reciprocal regulation of inflammation and lipid metabolism by liver X receptors.Nat Med. 2003; 9: 213-219Crossref PubMed Scopus (1000) Google Scholar]. In contrast, LXRs are also involved in proinflammatory responses, as evidenced by the fact that treatment of LPS-stimulated human monocytes with LXR agonists potentiated the expression of TNFα [[36]Walcher D. Kummel A. Kehrle B. et al.LXR activation reduces proinflammatory cytokine expression in human CD4-positive lymphocytes.Arterioscler Thromb Vasc Biol. 2006; 26: 1022-1028Crossref PubMed Scopus (83) Google Scholar]. Although the role of LXRs in inflammation is complex and controversial, evidence demonstrates that 25HC3S reduces nuclear-factor kappa B-mediated inflammatory response by decreasing LXR activity whereas 25HC elicits opposite effects by increasing LXR activity in hepatocytes [[33]Xu L. Bai Q. Rodriguez-Agudo D. et al.Regulation of hepatocyte lipid metabolism and inflammatory response by 25-hydroxycholesterol and 25-hydroxycholesterol-3-sulfate.Lipids. 2010; 45: 821-832Crossref PubMed Scopus (56) Google Scholar]. However, the molecular mechanism by which 25HC3S regulates inflammatory response in the NASH model remains to be elucidated, with particular emphasis on LXR-mediated hepatic steatosis and inflammation. The study by Bai et al raises the possibility that normal cholesterol and lipid metabolism are linked by the physiologic balance between 25HC and 25HC3S in the liver. It is therefore important to determine what degree of changes in 25HC and 25HC3S cellular levels can contribute to LXR activity in the physiological milieu, in conjunction with SULT2B1b expression and activity. Understanding the physiological mechanisms for specificity of SULT2B1b and the effects of 25HC3S on cholesterol/lipid metabolism and anti-inflammation could lead to new therapeutic targets for NAFLD and NASH. FundingThis study was supported by a grant from the Korean Health Technology R&D Project, Ministry of Health & Welfare, Republic of Korea ( A100899 ). This study was supported by a grant from the Korean Health Technology R&D Project, Ministry of Health & Welfare, Republic of Korea ( A100899 ).