NOREPINEPHRINE REVERSES THE EFFECTS OF ACTIVIN-A ON DNA-SYNTHESIS ANDAPOPTOSIS IN CULTURED RAT HEPATOCYTES

Activin A, an autocrine factor produced by hepatocytes, inhibits mitog en-stimulated DNA synthesis and induces apoptotic death of cultured ra t hepatocytes. Several lines of evidence indicate that norepinephrine (NE), as a comitogenic growth factor, alters the balance between growt h stimulation and inhibition and acts as a trigger for the initiation of hepatocyte proliferation. In the present study, we examined whether NE modulated the effects of activin A on rat hepatocytes in primary c ulture. Activin A, at a concentration of 10(-9) mol/L, blocked the eff ect of epidermal growth factor (EGF) on DNA synthesis, that was assess ed by measuring [H-3] thymidine incorporation and nuclear labeling, al most completely, and NE reversed the inhibitory effect of activin A on DNA synthesis. This effect of NE was dose-dependent, being significan t at concentrations of 10(-6) mol/L and above, but was overcome by hig her concentrations of activin A, and was attenuated by prazosin, but n ot by yohimbine or propranolol. NE exerted its effect during the first 24 hours of culture, but was ineffective when added after 24 hours. E GF augmented the release of follistatin, an activin-binding protein kn own to block the action of activin A, by hepatocytes and NE did not af fect the amount of follistatin they released In addition to inhibiting DNA synthesis by hepatocytes cultured with EGF, activin A induced dea th of hepatocytes cultured in the absence of EGF. The nuclear morpholo gy of cells cultured with activin A alone was strikingly changed compa red with untreated control cells and marked identation of the nuclear membranes and moderate chromatin condensation were observed. Fragmenta tion of DNA was also observed, suggesting that activin A induced apopt osis, and activin-mediated cell death was prevented significantly by N E. These results indicate that NE, acting on alpha(1)-adrenergic recep tors, attenuates the effects of activin A on DNA synthesis by and apop tosis of cultured rat hepatocytes.