Hepatic stellate cells (HSC) undergo transdifferentiation (activation) from lipid-storing pericytes to myofibroblastic cells to participate in liver fibrogenesis. Our recent work demonstrates that depletion of peroxisome proliferator-activated receptor ␥ (PPAR␥) constitutes one of the key molecular events for HSC activation and that ectopic expression of this nuclear receptor achieves the phenotypic reversal of activated HSC to the quiescent cells. The present study extends these findings to test a novel hypothesis that adipogenic transcriptional regulation is required for the maintenance of HSC quiescence. Transdifferentiation of vitamin A-storing hepatic stellate cells (HSC)1 to vitamin A-depleted myofibroblastic cells represents a key cellular event in the genesis of cirrhosis, for which no effective medial treatments are currently available except for liver transplantation. Transdifferentiated (activated) HSC are proliferative, proinflammatory, and fibrogenic with induced ability to synthesize and deposit extracellular matrices (1). Thus, better understanding of the mechanism underlying HSC transdifferentiation is the pivotal step toward identification of molecular targets for new and effective treatments for the disease. The most fundamental prerequisite for the understanding of HSC transdifferentiation is defining the cell type of differentiated HSC. This question relates to the origin of HSC that continues to puzzle the field. HSC are believed to serve as pericytes for hepatic capillaries called sinusoids. They represent 5-8% of total liver cells and 15-23% of nonparenchymal cells in the normal liver (2). HSC are positive for a mesenchymal marker such as vimentin. Rodent HSC express desmin (3) and glial fibrillary acidic protein (4), suggesting smooth muscle cell and glial cell lineage, respectively. Upon activation, both rodent and human HSC lose vitamin A and begin to express ␣-smooth muscle actin (5, 6). Interestingly, undifferentiated HSC in fetal livers that do not yet exhibit vitamin A storage also express ␣-smooth muscle actin (7), supporting a smooth muscle cell lineage. Synaptophysin, which controls exocytosis and the release of neurotransmitters in neurons and neuroendocrine cells, is also expressed in both rodent and human HSC (8). Neurotrophins such as nerve growth factor, brain-derived neurotrophic factor (BDNF), neutrophin NT-3, and NT-4/5 are also expressed (9), and so are their receptors, Trk-A, B, and C (9, 10), further supporting the neural and glial lineage.Peroxisome proliferator-activated receptor ␥ (PPAR␥) has been proposed as a potential molecular target for inhibition of HSC transdifferentiation (11-13). PPAR␥ level and activity are reduced in activated HSC, and the treatment of HSC with synthetic ligands for PPAR␥ such as thiazolidinediones effectively suppresses fibrogenic activity of and in vivo in experimental animals (13). However, these ligands are known to have PPAR␥-independent effects (14), and it was yet to be tested whether PPAR␥ per se had a direct effect to suppress ...

Activation of hepatic stellate cells (HSC), a key event in liver fibrosis, is caused by diminished adipogenic transcription. This study investigated whether Wnt signaling contributes to "antiadipogenic" activation of HSC and liver fibrogenesis. Culture-activated HSC from normal rats and HSC from cholestatic rat livers were examined for expression of Wnt, Frizzled (Fz) receptors, and coreceptors by quantitative PCR. Wnt signaling was assessed by nuclear beta-catenin and T cell factor (TCF) promoter activity. Dickkopf-1 (Dkk-1), a Wnt coreceptor antagonist, was transduced by an adenoviral vector to assess the effects of Wnt antagonism on culture activation of HSC and cholestatic liver fibrosis in mice. Messenger RNA for canonical (Wnt3a and 10b) and noncanonical (Wnt4 and 5a) Wnt genes, Fz-1 and 2, and coreceptors [low-density lipoprotein-receptor-related protein (LRP)6 and Ryk] are increased approximately 3-12-fold in culture-activated HSC compared with quiescent HSC. The nuclear beta-catenin level and TCF DNA binding are markedly increased in activated HSC. TCF promoter activity is stimulated with Wnt1 but inhibited by Chibby, a protein that blocks beta-catenin interaction with TCF, and by Dkk-1. Dkk-1 enhances peroxisome proliferator-activated receptor-gamma (PPARgamma)-driven PPAR response element (PPRE) promoter activity, a key adipogenic transcriptional parameter, abrogates agonist-stimulated contraction, and restores HSC quiescence in culture. High expression of Dkk-1 increases apoptosis of cultured HSC. Expression of Wnt and Fz genes is also induced in HSC isolated from experimental cholestatic liver fibrosis, and Dkk-1 expression ameliorates this form of liver fibrosis in mice. These results demonstrate antiadipogenic Wnt signaling in HSC activation and therapeutic potential of Wnt antagonism for liver fibrosis.

Nonalcoholic steatohepatitis is prevalent among obese individuals with excessive caloric intake, insulin resistance, and type II diabetes. However, no animal models exist that recapitulate this important association. This study produced and characterized steatohepatitis (SH) caused by intragastric overfeeding in mice. C57BL/6, tumor necrosis factor (TNF) type I receptor-deficient, and genetically matched wild type mice were fed via an implanted gastrostomy tube a high-fat diet for 9 weeks in the increasing amount up to 85% in excess of the standard intake. Animals were examined for weight gain, insulin sensitivity, and histology and biochemistry of liver and white adipose tissue (WAT). Overfed C57BL/6 mice progressively became obese, with 71% larger final body weights. They had increased visceral WAT, hyperglycemia, hyperinsulinemia, hyperleptinemia, glucose intolerance, and insulin resistance. Of these mice, 46% developed SH with increased plasma alanine aminotransferase (121 ؎ 27 vs. 13 ؎ 1 U/L), neutrophilic infiltration, and sinusoidal and pericellular fibrosis. Obese WAT showed increased TNF␣ and leptin expression and reciprocally reduced adiponectin expression. The expression of lipogenic transcription factors (SREBP-1c, PPAR␥, LXR␣) was increased, whereas that of a lipolytic nuclear factor PPAR␣ was reduced in SH. SH was associated with reduced cytochrome P450 (Cyp)2e1 but increased Cyp4a.

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