TRANSFORMING GROWTH-FACTOR RECEPTORS IN LIVER-REGENERATION FOLLOWING PARTIAL-HEPATECTOMY IN THE RAT

Abstract

Transforming growth factors .alpha. and .beta. (TGF-.alpha. and TGF-.beta.) are produced in the liver and appear to play an important role in the regulation of hepatic growth. We investigated changes in receptors for these polypeptide growth factors in regenerating liver after partial hepatectomy in the rat with comparisons to livers from normal and sham-operated animals. In normal rats, binding of 125I-epidermal growth factor (EGF) to liver membranes was fully and competitively displaced displaced by TGF-.alpha., indicating that these two growth factors share similar sites on the same hepatic receptor. Scatchard analyses revealed that EGF receptors bound EGF with 4- to 8-fold higher affinity than TGF-.alpha.. Following partial hepatectomy or sham operation, EGF/TGF-.alpha. receptor number decreased by 25, 40, and 55% at 12, 24, and 72 h, respectively. In all cases, Scatchard analysis obtained with EGF yielded a linear plot indicating a single population of binding sites with a dissociation constant (Kd) of approximately 0.9 nM. Scatchard analysis of TGF-.beta. binding showed that liver membranes from sham-operated and normal rats express binding sites with a Kd of approximately 35 pM. In contrast, membranes obtained from 12-, 24-, and 72-h regenerating livers were altered in a manner consistent with uncovering or appearance of a higher affinity site. Affinity labeling of liver plasma membranes with 125I-TGF-.beta. revealed two predominant proteins with Mr 85,000 and 66,00. In unfractionated membrane preparations, two other proteins with Mr 105,000 and .apprx. 150,000 were seen. Following partial hepatectomy the major change in affinity-labeled proteins was a small (10-25%) but consistent decrease in the Mr 85,000 species. These results show that receptors for TGF-.alpha. and TGF-.beta. are modulated after partial hepatectomy, a further indication that these polypeptides may have an important role in liver regeneration.