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 c ell-surface receptors for activin A in cultured rat hepatocytes by mea suring I-125-activin A binding, Scatchard analysis of I-125-activin A binding indicated the existence of two classes of binding sites with a pparent Ma values of 3 x 10(-10) mol/L and 3.5 x 10(-9) mol/L. Pretrea tment of the cells with heparitinase reduced the number of low-affinit y binding sites, whereas pretreatment with excess exogenous follistati n increased the number of low-affinity binding sites. Affinity cross-l inking of I-125-activin A to hepatocytes-revealed distinct protein com plexes with molecular weights of approximately 48, 65, and 85 kd, whic h may represent cross-linked cell-bound follistatin, type I and type I I activin receptors, respectively, Another band with a molecular weigh t of 180 kd was also found, which may represent the type III activin r eceptor. 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 tha t two classes of I-125-activin A binding sites exist in cultured hepat ocytes: the high-affinity binding site may represent oligomeric comple x of the type I and type II receptors, and at least part of the low-af finity binding site may represent cell-bound follistatin. The number o f activin receptors in hepatocytes is increased after the stimulation with EGF.