Expression of the peripheral-type benzodiazepine receptor and apoptosis induction in hepatic stellate cells

Abstract

Hepatic stellate cell (HSC) transformation and proliferation play an important role in liver fibrogenesis, and HSC apoptosis may be involved in the termination of this response. Expression of the peripheral benzodiazepine receptor (PBR) and PBR-ligand-induced apoptosis were studied in cultured rat liver HSC. Transformation of HSC led to a transient expression of PBR at the messenger RNA and protein level, which was maximal after about 3 and 7 days of culture, respectively, and declined thereafter. Immunoreactive PBR showed a punctate staining and colocalized with mitochondrial manganese-dependent superoxide dismutase and adenine nucleotide translocator 1. The selective PBR ligands 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK11195) and 4' chlorodiazepam (Ro5-4864), but not the centrally acting benzodiazepine ligand clonazepam, induced dose-dependent apoptosis in HSC. The apoptotic potency of PK11195 paralleled the level of PBR expression. PK11195 induced dephosphorylation of protein kinase B/Akt and Bad and a downregulation of Bcl-2. Collapse of the mitochondrial membrane potential preceeded PBR-ligand-induced apoptosis. No apoptosis was induced by PK11195 in parenchymal cells, despite the presence of PBR, and PK11195 had no effect in these cells on Bad phosphorylation and Bcl-2 expression. Transformation of HSC leads to a transient expression of PBR and renders the cells sensitive to PBR-ligand-induced apoptosis, involving protein kinase B/Akt and Bad-dependent mechanisms.