Transforming growth factor beta (TGF‐β) inhibits hepatocyte DNA synthesis independently of EGF binding and egf receptor autophosphorylation

Abstract

Subpicomolar concentrations of human platelet-derived transforming growth factor beta (TGF-β) inhibited growth factor-stimulated DNA synthesis in primary cultures of adult rat hepatocytes. This inhibition was not the result of changes in the size of intracellular pools of ³H-thymidine and was not dependent on the state of confluence of the cells. A 24-hr exposure to TGF-β either before or after insulin/EGF stimulation was as inhibitory on DNA synthesis between 48 and 72 hr of culture as was TGF-β present throughout 72 hr of culture. From 12 hr in culture to 24 hr, hepatocyte EGF binding sites dropped from about 230,000 to 85,000 per cell with no significant change in K_d, but with a loss in capacity for EGF-induced receptor down-regulation. Maximally inhibitory concentrations of TGF-β did not compete with EGF for the EGF receptor, and a 4-to 24-hr exposure to TGF-β did not alter subsequent EGF binding. Coincubation of hepatocytes with TGF-β and EGF did not influence the 60% reduction in EGF binding sites produced by EGF alone. In addition, TGF-β did not prevent EGF-induced autophosphorylation of the 170,000 dalton EGF receptor in membranes form whole liver. Our studies suggest that TGF-β regulates hepatocyte growth independently of changes in EGF receptor number, ligand affinity, or postbinding autophosphorylation.