REGULATION OF THYMOSIN BETA-4 MESSENGER-RNA LEVELS DURING CELL-PROLIFERATION

The levels of thymosin beta 4 mRNA were studied throughout the cell cy cle of NIH 3T3 cells. In serum deprived, quiescent cells, the levels o f thymosin beta 4 were undetectable; after serum restoration, the cell s were induced to proliferate and we found a pronounced increase in th ymosin beta 4 mRNA levels at the G(1)/S transition. Thymosin beta 4 mR NA was induced even in the presence of cycloheximide. On the other han d, cycling cells that were synchronized at different stages of the cyc le by means of mitotic shake-off after nocodazole arrest or a double t hymidine block did not show any variation in the levels of thymosin be ta 4 mRNA when they progressed synchronously through the cycle. In con clusion, the present data indicate that the thymosin beta 4 gene is re gulated by cell proliferation but it is not a cell cycle-regulated gen e. Finally, we studied thymosin beta 4 mRNA stability by inhibiting th ymosin beta 4 gene transcription with actinomycin D. Our results sugge st that thymosin beta 4 mRNA has a pronounced stability, a fact that m ight be relevant to account for the presence of thymosin beta 4 in enu cleated cells like platelets.