Engagement of αvβ3 integrin regulates proliferation and apoptosis of hepatic stellate cells

Abstract

Hepatic stellate cells are the major source of the extracellular matrix that accumulates in fibrotic liver. During progressive liver fibrosis, hepatic stellate cells proliferate, but during resolution of fibrosis there is extensive stellate cell apoptosis that coincides with degradation of the liver scar. We have examined the possibility that the fate of stellate cells is influenced by the extracellular matrix through the intermediary of α vβ3 integrin. αvβ3 integrin was expressed by activated, myofibroblastic rat and human stellate cells in culture. Antagonism of this integrin using neutralizing antibodies, echistatin, or small inhibitory RNA to silence αv subunit expression inhibited stellate cell proliferation and their expression of proliferating cell nuclear antigen and activated forms of p44 and p42 MAPK. These αvβ3 antagonists also increased apoptosis of cultured stellate cells, and this was associated with an increase in the BAX/BCL-2 protein ratio, induction of nuclear DNA fragmentation, and activation of intracellular caspase-3. Expression of tissue inhibitor of metalloproteinases-1 by activated stellate cells was reduced by the α vβ3 antagonists, while matrix metalloproteinase-9 synthesis was enhanced. Stellate cells incubated with active recombinant matrix metalloproteinase-9 showed enhanced apoptosis, while cells treated with a synthetic inhibitor of this protease showed increased survival. Our studies suggest that αvβ3 integrin regulates the fate of hepatic stellate cells. Degradation of αvβ3 ligands surrounding activated stellate cells during resolution of liver fibrosis might decrease αvβ3 integrin ligation, suppressing stellate cell proliferation and inducing a fibrolytic, matrix metalloproteinase-secreting phenotype that may prime stellate cells for apoptosis.