The liver X receptor in hepatic stellate cells: A novel antifibrogenic target?
Authors: Ariane Mallat, Sophie Lotersztajn
Journal: Journal of Hepatology (2011)

COMMENTARY ON: Liver X receptor signaling is a determinant of stellate cell activation and susceptibility to fibrotic liver disease. Beaven SW, Wroblewski K, Wang J, Hong C, Bensinger S, Tsukamoto H, Tontonoz P. Gastroenterology. 2011 Mar;140(3):1052–1062. Copyright (2011). Abstract reprinted with permission from the American Gastroenterological Association. http://www.ncbi.nlm.nih.gov/pubmed/21134374 Abstract. Background & Aims: Liver X receptors (LXRs) are lipid-activated nuclear receptors with important roles in cholesterol transport, lipogenesis, and anti-inflammatory signaling. Hepatic stellate cells activate during chronic liver injury and mediate the fibrotic response. These cells are also major repositories for lipids, but the role of lipid metabolism during stellate cell activation remains unclear. We investigated the role of LXR signaling stellate cell activation and susceptibility to fibrotic liver disease. Methods: Immortalized and primary stellate cells purified from mice were treated with highly specific LXR ligands. Carbon tetrachloride and methionine/choline deficiency were used as chronic liver injury models. Reciprocal bone marrow transplants were performed to test the importance of hematopoietically derived cells to the fibrotic response. Results: LXR ligands suppressed markers of fibrosis and stellate cell activation in primary mouse stellate cells. Lxrαβ(−/−) stellate cells produce increased levels of inflammatory mediators, and conditioned media from Lxrαβ(−/−) cells increases the fibrogenic program of wild-type cells. Furthermore, Lxrαβ(−/−) stellate cells exhibit altered lipid morphology and increased expression of fibrogenic genes, suggesting they are primed for activation. In vivo, Lxrαβ(−/−) mice have marked susceptibility to fibrosis in two injury models. Bone marrow transplants point to altered stellate cell function, rather than hematopoietic cell inflammation, as the primary basis for the Lxrαβ(−/−) phenotype. Conclusions: These results reveal an unexpected role for LXR signaling and lipid metabolism in the modulation of hepatic stellate cell function. Liver fibrosis is driven by a heterogeneous population of myofibroblasts, among which hepatic stellate cells play an important role [[1]Lotersztajn S. Julien B. Teixeira-Clerc F. Grenard P. Mallat A. Hepatic fibrosis: molecular mechanisms and drug targets.Ann Rev Pharmacol Toxicol. 2005; 45: 605-628Crossref PubMed Scopus (279) Google Scholar]. In the normal liver, these cells are in a quiescent phenotype, characterized by the expression of a large panel of adipogenic transcription factors and adipocyte genes, and their main function is to store vitamin A into lipid droplets. In response to a variety of fibrogenic stimuli, and via complex interactions between parenchymal and immune cells, quiescent hepatic stellate cells undergo a transdifferentiation process to a fibrogenic myofibroblastic phenotype [[1]Lotersztajn S. Julien B. Teixeira-Clerc F. Grenard P. Mallat A. Hepatic fibrosis: molecular mechanisms and drug targets.Ann Rev Pharmacol Toxicol. 2005; 45: 605-628Crossref PubMed Scopus (279) Google Scholar]. Activated hepatic stellate cells express smooth muscle alpha actin, show high proliferative activity, are motile and contractile, and produce proinflammatory and profibrogenic mediators, extracellular matrix proteins, and inhibitors of matrix degradation [[1]Lotersztajn S. Julien B. Teixeira-Clerc F. Grenard P. Mallat A. Hepatic fibrosis: molecular mechanisms and drug targets.Ann Rev Pharmacol Toxicol. 2005; 45: 605-628Crossref PubMed Scopus (279) Google Scholar]. Acquisition of the myofibroblastic phenotype is associated with a parallel loss in retinoids and lipid droplets, and a strong reduction in the expression of several adipogenic transcription factors, including the liver X receptor α [[2]She H. Xiong S. Hazra S. Tsukamoto H. Adipogenic transcriptional regulation of hepatic stellate cells.J Biol Chem. 2005; 280: 4959-4967Abstract Full Text Full Text PDF PubMed Scopus (249) Google Scholar]. Liver X receptors (LXR) are nuclear receptors that have recently attracted interest as therapeutic targets due to their beneficial effects on cholesterol homeostasis, atherosclerosis, and diabetes [[3]Viennois E. Pommier A.J. Mouzat K. Oumeddour A. El Hajjaji F.Z. Dufour J. et al.Targeting liver X receptors in human health: deadlock or promising trail?.Expert Opin Ther Targets. 2011; 15: 219-232Crossref PubMed Scopus (67) Google Scholar]. These receptors occur as two isoforms, LXRα and LXRβ, and function as ligand-activated transcription factors that form obligatory heterodimers with retinoid X receptors (RXR) [[3]Viennois E. Pommier A.J. Mouzat K. Oumeddour A. El Hajjaji F.Z. Dufour J. et al.Targeting liver X receptors in human health: deadlock or promising trail?.Expert Opin Ther Targets. 2011; 15: 219-232Crossref PubMed Scopus (67) Google Scholar]. LXRα and LXRβ show a distinct distribution, ubiquitous for LXRβ, and more restricted for LXRα predominantly expressed in the liver and to a lower extent in the small intestine, kidney or adipose tissue. Endogenous ligands for LXR are oxidized forms of cholesterol known as oxysterols and pharmacological agonists include T0901317 and GW3965. LXR act as “cholesterol sensors” that maintain cholesterol homeostasis and display potent anti-inflammatory properties in macrophages, by reducing the expression of a large panel of pro-inflammatory cytokines and chemokines [[3]Viennois E. Pommier A.J. Mouzat K. Oumeddour A. El Hajjaji F.Z. Dufour J. et al.Targeting liver X receptors in human health: deadlock or promising trail?.Expert Opin Ther Targets. 2011; 15: 219-232Crossref PubMed Scopus (67) Google Scholar]. The cholesterol-lowering and anti-inflammatory effects of LXR confer significant anti-atherosclerotic properties to LXR receptors, as shown in experimental models [[4]Calkin A.C. Tontonoz P. Liver X receptor signaling pathways and atherosclerosis.Arterioscler Thromb Vasc Biol. 2010; 30: 1513-1518Crossref PubMed Scopus (224) Google Scholar]. In addition, LXR activation also improves glucose tolerance in rodent models of type 2 diabetes [[3]Viennois E. Pommier A.J. Mouzat K. Oumeddour A. El Hajjaji F.Z. Dufour J. et al.Targeting liver X receptors in human health: deadlock or promising trail?.Expert Opin Ther Targets. 2011; 15: 219-232Crossref PubMed Scopus (67) Google Scholar]. However, activation of LXR is also associated with enhanced hepatic triglyceride synthesis following induction of hepatic SREBP-1c and its target genes, acetyl CoA carboxylase and fatty acid synthase [3Viennois E. Pommier A.J. Mouzat K. Oumeddour A. El Hajjaji F.Z. Dufour J. et al.Targeting liver X receptors in human health: deadlock or promising trail?.Expert Opin Ther Targets. 2011; 15: 219-232Crossref PubMed Scopus (67) Google Scholar, 5Uppal H. Saini S.P. Moschetta A. Mu Y. Zhou J. Gong H. et al.Activation of LXRs prevents bile acid toxicity and cholestasis in female mice.Hepatology (Baltimore, Md). 2007; 45: 422-432Crossref PubMed Scopus (104) Google Scholar]. The resulting liver steatosis and hypertriglyceridemia have therefore hampered any therapeutic development relying on LXR agonism. The specificity of the two isoforms is unclear, since LXRα and LXRβ regulate an overlapping set of genes, and generally compensate for one another in a variety of pathological conditions. However, several lines of evidence suggest that enhancement of triglyceride synthesis is specifically mediated by LXRα as LXRα-deficient mice are resistant to steatosis when exposed to a non selective LXRαβ agonist [[6]Lund E.G. Peterson L.B. Adams A.D. Lam M.H. Burton C.A. Chin J. et al.Different roles of liver X receptor alpha and beta in lipid metabolism: effects of an alpha-selective and a dual agonist in mice deficient in each subtype.Biochem Pharmacol. 2006; 71: 453-463Crossref PubMed Scopus (124) Google Scholar]. If confirmed, this specificity would allow potential therapeutic development of LXRβ-selective molecules. In the liver, LXRα and LXRβ are expressed both in parenchymal and non parenchymal cells, including Kupffer cells and endothelial cells (Fig. 1). In addition to enhancing fatty acid accumulation, activation of LXR in hepatocytes increases bile acid export [5Uppal H. Saini S.P. Moschetta A. Mu Y. Zhou J. Gong H. et al.Activation of LXRs prevents bile acid toxicity and cholestasis in female mice.Hepatology (Baltimore, Md). 2007; 45: 422-432Crossref PubMed Scopus (104) Google Scholar, 7Joseph S.B. Laffitte B.A. Patel P.H. Watson M.A. Matsukuma K.E. Walczak R. et al.Direct and indirect mechanisms for regulation of fatty acid synthase gene expression by liver X receptors.J Biol Chem. 2002; 277: 11019-11025Abstract Full Text Full Text PDF PubMed Scopus (614) Google Scholar]. As in other tissues, Kupffer cell LXR receptors suppress the release of inflammatory mediators in experimental models of liver injury [[8]Wang Y.Y. Dahle M.K. Agren J. Myhre A.E. Reinholt F.P. Foster S.J. et al.Activation of the liver X receptor protects against hepatic injury in endotoxemia by suppressing Kupffer cell activation.Shock (Augusta, Ga). 2006; 25: 141-146Crossref PubMed Scopus (66) Google Scholar]. Expression of LXRα has also been reported in quiescent hepatic stellate cell, and that of LXRβ detected in human activated hepatic stellate cells although their functions remained unknown [2She H. Xiong S. Hazra S. Tsukamoto H. Adipogenic transcriptional regulation of hepatic stellate cells.J Biol Chem. 2005; 280: 4959-4967Abstract Full Text Full Text PDF PubMed Scopus (249) Google Scholar, 9Fickert P. Fuchsbichler A. Moustafa T. Wagner M. Zollner G. Halilbasic E. et al.Farnesoid X receptor critically determines the fibrotic response in mice but is expressed to a low extent in human hepatic stellate cells and periductal myofibroblasts.Am J Pathol. 2009; 175: 2392-2405Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar]. Given the central role of hepatic stellate cells in liver fibrogenesis, Beaven et al. [[10]Beaven S.W. Wroblewski K. Wang J. Hong C. Bensinger S. Tsukamoto H. et al.Liver X receptor signaling is a determinant of stellate cell activation and susceptibility to fibrotic liver disease.Gastroenterology. 2011; 140: 1052-1062Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar] recently investigated the impact of LXR on the regulation of hepatic stellate cell function and liver fibrogenesis. Analysis of the pattern of nuclear receptor expression in myofibroblastic stellate cells indicated that LXRβ and RXRα are predominant, contrasting with a faint expression of LXRα. Combined results obtained from cultures of hepatic stellate cells stimulated with various LXR agonists or cells from LXRα/β deficient mice, showed that LXR down-regulates the fibrogenic properties of myofibroblastic stellate cells, as reflected by reduced expression of collagen(1)α1 and proinflammatory genes (Fig. 1). Interestingly, LXR activation also induces stellate cell features reminiscent of a reversion from the fibrogenic/myofibroblastic to a quiescent phenotype, including loss in smooth muscle alpha actin expression and parallel induction of SREBP-1 c and accumulation of neutral lipids. In keeping with in vitro data, Beaven et al. further show that LXRα/β-deficient mice are more susceptible to fibrosis, as induced by chronic carbon tetrachloride administration or by a methionine-choline deficient diet. Altogether, these findings obtained in two models indicate that progression of liver fibrogenesis is limited by endogenous activation of LXR in hepatic myofibroblasts. Considering the known anti-inflammatory properties of LXR in macrophages [3Viennois E. Pommier A.J. Mouzat K. Oumeddour A. El Hajjaji F.Z. Dufour J. et al.Targeting liver X receptors in human health: deadlock or promising trail?.Expert Opin Ther Targets. 2011; 15: 219-232Crossref PubMed Scopus (67) Google Scholar, 8Wang Y.Y. Dahle M.K. Agren J. Myhre A.E. Reinholt F.P. Foster S.J. et al.Activation of the liver X receptor protects against hepatic injury in endotoxemia by suppressing Kupffer cell activation.Shock (Augusta, Ga). 2006; 25: 141-146Crossref PubMed Scopus (66) Google Scholar], and the recognized contribution of Kupffer cells to fibrogenesis [[1]Lotersztajn S. Julien B. Teixeira-Clerc F. Grenard P. Mallat A. Hepatic fibrosis: molecular mechanisms and drug targets.Ann Rev Pharmacol Toxicol. 2005; 45: 605-628Crossref PubMed Scopus (279) Google Scholar], Beaven et al. further explored the possibility that Kupffer cell LXR may initiate the antifibrogenic response. To address this issue, they performed reciprocal bone marrow transplants between wild type and LXRα/β-deficient mice and, surprisingly, found no effect on fibrosis severity in animals exposed to carbon tetrachloride. From these results, they concluded that macrophage LXR do not contribute to the regulation of liver fibrogenesis. However, additional data should be provided in order to allow this interpretation. In particular, in the light of negative results on liver fibrosis progression, one would have expected the authors to validate their bone marrow transplant experiments by providing data on hepatic inflammation. Alternatively, the use of LXRαβ mice specifically deleted in macrophages would have even been more convincing. In the absence of these informations, a role of Kupffer cell LXR cannot be ruled out. The results of Beaven et al. convincingly demonstrate that LXR display antifibrogenic properties, via a direct LXRβ-dependent effect on activated hepatic stellate cells. In addition, they open the intriguing possibility that LXRβ may trigger reversal of activated stellate cells into a quiescent phenotype. Whether LXR may also display antifibrogenic properties in other fibrogenic cells such as portal myofibroblasts is an important issue, considering the major contribution of these cells to the fibrogenic process. Finally, further experiments are needed to address the specific contribution of LXRβ and, most importantly, to exclude that of LXRα expressed by other liver cell types, including Kupffer cells and hepatocytes. This is a prerequisite to further consider the potential therapeutic value of LXR agonists, in the light of potential steatogenic side effects of non selective LXRαβ-targeting molecules. The authors declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.