Apoptotic Body Engulfment by a Human Stellate Cell Line Is Profibrogenic

Abstract

SUMMARY:Hepatocyte apoptosis and stellate cell activation are both features of chronic liver diseases, but a relationship between these events has not been explored. In macrophages, engulfment of apoptotic bodies induces expression of transforming growth factor-␤ (TGF-␤), a profibrogenic cytokine. We examined whether a similar response occurs in stellate cells. Fluorescently labeled hepatocyte apoptotic bodies were added to cultures of primary and immortalized human stellate cells. Stellate cells, but not hepatocytes, readily engulfed apoptotic bodies in a time-dependent manner as assessed by confocal microscopy. The activation of primary and immortalized human stellate cells after incubation with apoptotic bodies, as well as their fibrogenic activity, was indicated by an increase in ␣-smooth muscle actin (primary cells), TGF-␤1, and collagen ␣1(I) mRNA (primary and immortalized cells). The profibrogenic response was dependent upon apoptotic body engulfment, because nocodazole, a microtubule-inhibiting agent, blocked both the engulfment and the increase of TGF-␤1 and collagen ␣1(I) mRNA. As described in primary rodent stellate cells, up-regulation of collagen ␣1(I) mRNA was inhibited by a PI-3K inhibitor (LY294002) and a p38 mitogen-activated protein kinase inhibitor (SB203580) in LX-1 cells. In conclusion, these data support a model in which engulfment of hepatocyte apoptotic bodies by stellate cells leads to a fibrogenic response by eliciting a kinase-signaling pathway. (Lab Invest 2003, 83:655-663).L iver fibrosis is a cardinal feature of chronic liver injury. Indeed, despite the wide array of metabolic, genetic, viral, and cholestatic pathologies causing human liver injury, fibrosis is a common, if not generic, response of the liver to these injury processes. Extensive liver fibrosis results from the deposition and accumulation of type I collagen within the liver parenchyma and is recognized clinically as cirrhosis. The clinical complications of cirrhosis include portal hypertension and chronic liver failure. For many patients with advanced liver cirrhosis, the only viable treatment option is liver transplantation, highlighting the clinical severity of this fibrotic process. Insights into the mechanisms initiating and promoting liver fibrosis are, therefore, of broad scientific and clinical importance.Activated hepatic stellate cells have been established as the source of type I collagen in the liver (de Leeuw et al, 1984;Friedman et al, 1985;McGee and Patrick, 1972). Under basal conditions, hepatic stellate cells are in a quiescent state and function to store retinoids within the liver (Blomhoff et al, 1990;Geerts, 2001). However, upon activation, these cells undergo transformation, assuming a myofibroblast morphology and expressing smooth muscle actin (Ramadori et al, 1990;Schmitt-Graff et al, 1991). These activated stellate cells secrete collagen types I and III, the principle matrix protein responsible for the development of liver fibrosis and cirrhosis (Friedman, 2000;Kent et al, 1976). Over time, activ...

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“…22 Engulfment of apoptotic bodies by phagocytic cells leads to the production of transforming growth factor beta1 (TGF-␤1), which in turn stimulates activation of HSCs. 23,24 Once activated, HSCs transdifferentiate into myofibroblasts, which act as a key cell population in hepatic fibrogenesis. 25,26 ␣-SMA expression is seen in cell types such as portal fibroblasts and HSCs when they have been activated and begin to produce collagens.…”

In vivo assessment of liver cell apoptosis as a novel biomarker of disease severity in nonalcoholic fatty liver disease

“…21 Engulfment of apoptotic bodies by hepatic stellate cells stimulates the fibrogenic activity of these cells and may be one mechanism through which hepatocyte apoptosis promotes fibrosis. 22 In a recent study, Bantel et al 11 reported the first assessment of a blood biomarker for hepatocyte apoptosis. Many forms of apoptosis involve the activation of caspases, which are intracellular proteases that cleave aspartate residues.…”

Apoptotic hepatocyte DNA inhibits hepatic stellate cell chemotaxis via toll-like receptor 9

“…HSCs are known to phagocytose apoptotic hepatocyte bodies, with subsequent up-regulation of TGF-␤1 and collagen ␣1 mRNA. 25 Apoptotic bodies contain nuclear material, including DNA degradation products, and after phagocytosis by HSCs are likely to undergo fusion with endosomes containing TLR9. 26 B cells and plasmacytoid dendritic cells internalize DNA bound to immune complexes, via the IgM molecule on B cells and the Fc receptor on plasmacytoid dendritic cells.…”