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Fibrosis is a multicellular wound healing process, where myofibroblasts that express extracellular matrix components extensively cross-talk with other cells resident in the liver or recruited from the bloodstream. Macrophages and infiltrating monocytes participate in the development of fibrosis via several mechanisms, including secretion of cytokines and generation of oxidative stress-related products. However, macrophages are also pivotal in the process of fibrosis resolution, where they contribute to matrix degradation. T lymphocytes modulate the fibrogenic process by direct interaction with myofibroblasts and secreting cytokines. In general, Th2 polarized responses promote fibrosis, while Th1 cytokines may be antifibrogenic. NK cells limit the development of fibrosis and favor its resolution, at least in part via killing of fibrogenic cells. The possible role of NKT cells and B cells is emerging in recent studies. Thus, mononuclear cells represent a critical regulatory system during fibrogenesis and may become an appealing target for therapy.

Obesity and insulin resistance accelerate the progression of fibrosis during chronic liver disease. Resistin antagonizes insulin action in rodents, but its role in humans is still controversial. The aims of this study were to investigate resistin expression in human liver and to evaluate whether resistin may affect the biology of activated human hepatic stellate cells (HSCs), key modulators of hepatic fibrogenesis. Resistin gene expression was low in normal human liver but was increased in conditions of severe fibrosis. Up-regulation of resistin during chronic liver damage was confirmed by immunohistochemistry. In a group of patients with alcoholic hepatitis, resistin expression correlated with inflammation and fibrosis, suggesting a possible action on HSCs. Exposure of cultured HSCs to recombinant resistin resulted in increased expression of the proinflammatory chemokines monocyte chemoattractant protein-1 and interleukin-8, through activation of nuclear factor (NF)-B. Resistin induced a rapid increase in intracellular calcium concentration , mainly through calcium release from intracellular inositol triphosphate-sensitive pools. The intracellular calcium chelator BAPTA-AM blocked resistin-induced NF-B activation and monocyte chemoattractant protein-1 expression. In conclusion, this study shows a role for resistin as an intrahepatic cytokine exerting proinflammatory actions in HSCs, via a Ca 2؉ /NF-B-dependent pathway and suggests involvement of this adipokine in the pathophysiology of liver fibrosis.

Leptin modulates the angiogenic properties of hepatic stellate cells (HSC), but the molecular mechanisms involved are poorly understood. We investigated the pathways regulating hypoxia-inducible factor 1α (HIF-1α) and vascular endothelial growth factor (VEGF) in leptin-stimulated myofibroblastic HSC. Exposure to leptin enhanced the phosphorylation of TSC2 on T1462 residues and of p70 S6 kinase and the translational inhibitor 4E-binding protein-1, indicating the ability of leptin to activate the mammalian target of rapamycin (mTOR) pathway. Similar findings were observed when HSC were exposed to PDGF. Both leptin and PDGF increased the expression of HIF-1α and VEGF in HSC. In the presence of rapamycin, a specific mTOR inhibitor, leptin and PDGF were no longer able to activate mTOR, and expression of VEGF was reduced, whereas HIF-1α abundance was not affected. Moreover, knockdown of Raptor, a component of the mTORC1 complex, reduced the ability of leptin to increase VEGF. mTOR was also necessary for leptin- and PDGF-dependent increase in HSC migration. Leptin increased the generation of reactive oxygen species in HSC, which was reduced by NADP(H) oxidase inhibitors. Both N-acetyl cysteine and diphenylene iodonium, a NADP(H) inhibitor, inhibited the expression of HIF-1α and VEGF stimulated by leptin or PDGF. Finally, conditioned media from HSC treated with leptin or PDGF induced tube formation in cultured human umbilical vein endothelial cells. In conclusion, in HSC exposed to leptin or PDGF, increased expression of VEGF requires both activation of mTOR and generation of reactive oxygen species via NADPH-oxidase. Induction of HIF-1α requires NADP(H) oxidase but not mTOR activation.

. Differential requirement of members of the MAPK family for CCL2 expression by hepatic stellate cells. Am J Physiol Gastrointest Liver Physiol 287: G18 -G26, 2004. First published March 11, 2004 10.1152/ajpgi.00336. 2003.-Hepatic stellate cells (HSC) coordinate the liver wound-healing response through secretion of several cytokines and chemokines, including CCL2 (formerly known as monocyte chemoattractant protein-1). In this study, we evaluated the role of different proteins of the MAPK family (ERK, p38 MAPK , and JNK) in the regulation of CCL2 expression by HSC, as an index of their proinflammatory activity. Several mediators activated all three MAPK, including TNF, IL-1, and PDGF. To assess the relative role of the different MAPKs, specific pharmacological inhibitors were used; namely, SB203580 (p38 MAPK ), SP600125 (JNK), and PD98059 (MEK/ERK). The efficacy and specificity of the different inhibitors in our cellular system were verified analyzing the enzymatic activity of the different MAPKs using in vitro kinase assays and/or testing the inhibition of phosphorylation of downstream substrates. SB203580 and SP600125 dose-dependently inhibited CCL2 secretion and gene expression induced by IL-1 or TNF. In contrast, inhibition of ERK did not affect the upregulation of CCL2 induced by the two cytokines. Finally, activin A was also found to stimulate CCL2 expression and to activate ERK, JNK, p38, and their downstream targets. Unlike in cells exposed to proinflammatory cytokines, all three MAPKs were required to induce CCL2 secretion in response to activin. We conclude that members of the MAPK family differentially regulate cytokine-induced chemokine expression in human HSC. activin; chemokines; fibrosis; platelet-derived growth factor STUDIES CONDUCTED IN DIFFERENT laboratories have highlighted the importance of hepatic stellate cells (HSC) in the pathophysiology of the liver response to injury (30). HSC are not only the major matrix-producing cells during chronic liver injury, but they contribute to the modulation of the liver "wound-healing" response through several biological actions. A critical aspect of acute and chronic tissue damage is represented by the recruitment of inflammatory cells, and HSC have been shown to modulate the inflammatory response via secretion of several soluble mediators that regulate the recruitment and activation of leukocytes (reviewed in Ref. 20). In general, low expression of these mediators in quiescent HSC becomes dramatically upregulated on activation, suggesting that the modulation of inflammation occurs in conditions associated with tissue injury and the transition of HSC to a myofibroblastlike phenotype.The chemokine family of cytokines is a large group of proteins capable of regulating migration of target cells via activation of specific membrane receptors (1). Chemokine receptors were initially identified on leukocytes, and several studies (25) have investigated the role of chemokines in the regulation of inflammatory cell recruitment in conditions of liver injury. However...

SUMMARYNon-alcoholic steatohepatitis has been recognized as a significant cause of end-stage liver disease and hepatic decompensation. Despite the growing interest in this condition, the molecular mechanisms underlying the development of fibrosis in this setting are only partially understood. In this article, the cellular and molecular basis of fibrosis in chronic liver disease are briefly outlined. In addition, mechanisms specifically operating in the context of fatty liver and steatohepatitis are examined, including: insulin resistance, oxidative stress, and inflammation. Finally, recent developments indicating the possible contribution of cytokines derived from adipose tissue (adipokines) to liver fibrosis is discussed.