Characterization of the activin receptor in cultured rat hepatocytes

Activin A is an autocrine inhibitor of initiation of DNA synthesis in rat hepatocytes. The present study was conducted to characterize the cell- surface receptors for activin A in cultured rat hepatocytes by measuring ¹²⁵I-activin A binding. Scatchard analysis of ¹²⁵I-activin A binding indicated the existence of two classes of binding sites with apparent K(d) values of 3 x 10^-10 mol/L and 3.5 x 10^-9 mol/L. Pretreatment of the cells with heparitinase reduced the number of low-affinity binding sites, whereas pretreatment with excess exogenous follistatin increased the number of low-affinity binding sites. Affinity cross-linking of ¹²⁵I-activin A to hepatocytes revealed distinct protein complexes with molecular weights of approximately 48, 65, and 85 kd, which may represent cross-linked cell-bound follistatin, type I and type II activin receptors, respectively. Another band with a molecular weight of 180 kd was also found, which may represent the type III activin receptor. When hepatocytes were cultured with epidermal growth factor (EGF), both high- and low-affinity binding sites increased at 12 hours without altering their affinities. At 60 hours of the incubation with EGF, the high-affinity binding sites decreased while the number of low- affinity binding sites increased slightly. These results indicate that two classes of ¹²⁵I-activin A binding sites exist in cultured hepatocytes: the high-affinity binding site may represent oligomeric complex of the type I and type II receptors, and at least part of the low-affinity binding site may represent cell-bound follistatin. The number of activin receptors in hepatocytes is increased after the stimulation with EGF.