Platelet-derived growth factor is a principal inductive factor modulating mannose 6-phosphate/insulin-like growth factor-II receptor gene expression via a distal E-box in activated hepatic stellate cells

Abstract

Hepatic stellate cells (HSCs) 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 produce the hepatic nodule. Platelet-derived growth factor (PDGF), a potent HSC mitogen, is released early in hepatic injury and activates several signalling pathways in HSCs. In this study we examined the role of PDGF-BB in HSC regulation of M6P/IGFIIR gene expression. Several promoter elements were found and characterized that modulate M6P/IGFIIR expression in activated stellate cells. The presence of a distal CACGTG E-box at -2695 was required for M6P/IGFIIR expression in transfected stellate cells. When the distal E-box was removed there was no significant M6P/IGFIIR promoter activity. The distal E-box-binding protein responded specifically to PDGF-BB with increased binding. This coincided with PDGF-BB up-regulation of M6P/IGFIIR mRNA transcript levels. Downstream elements include two proximal (-2 to -48) CACGTG E-boxes that bind a different protein to the distal (-2695) E-box. The proximal E-boxes respond moderately to PDGF-BB. The promoter segment encompassing -144 to +109 is able to respond dramatically to serum but is refractory to PDGF-BB. However, a constitutively bound protein binding to the -611/-716 fragment appears to be a repressor that suppresses inductive changes in protein binding occurring downstream of -611. These results indicate that the M6P/IGFIIR promoter responds primarily and specifically to PDGF-BB through a distal E-box element and possibly through two proximal E-box elements.