ACTIVIN-A - AN AUTOCRINE INHIBITOR OF INITIATION OF DNA-SYNTHESIS IN RAT HEPATOCYTES

The present study was conducted to examine the effect of activin A on growth of rat hepatocytes. EGF induced a 10-fold increase in DNA synth esis as assessed by [H-3] thymidine incorporation in cultured hepatocy tes. When activin A was added together with EGF, DNA synthesis induced by EGF was markedly inhibited. Inhibition was detected at a concentra tion of 10(-10) M, and 5 x 10(-9) M activin A almost completely blocke d EGF-mediated DNA synthesis. Similarly, activin A completely blocked DNA synthesis induced by hepatocyte growth factor/scatter factor. Acti vin A was capable of inhibiting EGF-mediated DNA synthesis, even when added 36 h after the addition of EGF. With the same time interval, TGF -beta also blocked EGF-induced DNA synthesis. Although both activin A and TGF-beta inhibited growth of hepatocytes in a similar manner, eith er activin A or TGF-beta did not compete with each other in their bind ing when assessed by competitive binding using an iodinated ligand. Wh en hepatocytes were incubated with EGF, release of bioactivity of acti vin A into culture medium was detected after 48 h or later. Activity o f activin A was released from parenchymal cells but not from nonparenc hymal cells. mRNA for beta(A), subunit of activin was detected only sl ightly in unstimulated hepatocytes, but markedly increased at 48 h aft er the addition of EGF. To determine whether endogenously produced act ivin A affects DNA synthesis, we examined the effect of follistatin, a n activin-binding protein that blocks the action of activin A. An addi tion of follistatin significantly enhanced EGF-induced DNA synthesis. Finally, in partial hepatectomized rat, expression of mRNA for beta(A) subunit in liver was markedly increased 24 h after the partial hepate ctomy. These results indicate that activin A inhibits initiation of DN A synthesis in hepatocytes by acting on its own receptor and that acti vin A acts as an autocrine inhibitor of DNA synthesis in rat hepatocyt es.