[Hepatocytes in heterokaryons do not retard the nuclei of stimulated fibroblasts entering the S period].

Serum-deprived (0.2%) resting and serum-stimulated (10%) proliferating NIH 3T3 mouse fibroblasts were fused with hepatocytes from intact, regenerating and embryonic mouse livers to elucidate mechanisms of liver cell proliferation, DNA synthesis being investigated in nuclei of heterokaryons and non-fused cells using radioautography. Hepatocytes in heterokaryons were found to have no inhibitory effect on the entry of stimulated fibroblast nuclei into the S-period, but on the contrary they were involved in DNA synthesis. In addition, the nuclei in heterokaryons mutually stimulated each other to enter the S-period. In their turn, the resting fibroblasts did not prevent the proliferating hepatocytes from the regenerating and embryonic livers to enter the S-period. Possible reasons of the absence of inhibitory effect of differentiated cells in heterokaryons are discussed. The data obtained enable us to conclude that the mechanism of proliferative process control in resting immortalized cells differs from that in differentiated cells where proliferation seems to be stopped without affecting the endogenous inhibitor postulated for the resting and ageing fibroblasts.