Prostaglandin E suppression of platelet-derived-growth-factor-induced Ito cell mitogenesis occurs independent of raf perinuclear translocation and nuclear proto-oncogene expression

Abstract

Ito cell mitogenesis occurs during liver injury and fibrogenesis in vivo coincident with the de novo expression of Ito cell PDGFβ receptor messenger RNA, PDGF-induced mitogenesis was studied in cultured rat hepatic Ito cells which resemble the myofibroblast associated with liver injury. Pretreatment with prostaglandin E markedly suppressed the PDGF response in a dose-dependent fashion. The PDGF-induced cascade was studied with or without PGE to determine the level of regulation which induced the observed suppression. PGE caused no apparent diminution in the abundance of the surface PDGF β receptor nor its subsequent activation and tyrosine phosphorylation following PDGF stimulation. The cytoplasmic ‘secondary messengers’ mitogen-activated protein kinase pp42–44 and raf kinase, appeared to be comparably induced and therefore unaffected by PGE. Raf perinuclear translocation was also intact and comparable degrees of nuclear egr, fos, and jun expression occurred. Since other studies have suggested that many of these features of the PDGF cascade may be causally and sequentially linked, the data collectively suggests that the dominant PGE mitogenic suppressive effect resides at a raf-MAP parallel pathway or at a nuclear level distal to the induction of these early growth response genes.