E-box-binding repressor is down-regulated in hepatic stellate cells during up-regulation of mannose 6-phosphate/insulin-like growth factor-II receptor expression in early hepatic fibrogenesis

Abstract

Hepatic stellate cells become activated during the early stages of hepatic injury associated with fibrogenesis. The mannose 6-phosphate/insulin- like growth factor-II receptor (M6P/IGFIIR) plays an important role in early fibrogenesis by participating in the activation of latent transforming growth factor-β, a potent inducer of the matrix proteins in activated stellate cells that define the fibrotic phenotype. In this study we examined hepatic stellate cell regulation of M6P/IGFIIR expression and found that M6P/IGFIIR mRNA transcript levels increased in stellate cells from rats exposed to carbon tetrachloride (CCl4), a potent fibrogenic stimulant. Two E-boxes residing in the proximal promoter of M6P/IGFIIR were found to each bind a novel 75-kDa transcription factor (P75) in quiescent stellate cells of normal livers. This E-box binding was down-regulated as an early response in stellate cells exposed to CCl4, coinciding with increased M6P/IGFIIR transcript levels. Mutagenized E-boxes in M6P/IGFIIR promoter- chloramphenicol acetyltransferase (CAT) reporter constructs produced a substantial increase in reporter expression when compared with the corresponding native promoter-CAT construct when transfected in culture- activated stellate cells, suggesting P75's role as a repressor. The results indicate P75's participation in the regulation of M6P/IGFIIR transcription in hepatic stellate cells during fibrogenesis.