2-AMINOPURINE UNRAVELS A ROLE FOR PRB IN THE REGULATION OF GENE-EXPRESSION BY TRANSFORMING GROWTH-FACTOR-BETA

Transforming growth factor type beta (TGF beta) is a pleiotropic facto r that regulates different cellular activities including cell growth, differentiation, and extracellular matrix deposition. All the known ef fects of TGF beta appear to be mediated by its interaction with cell s urface receptors that possess a serine/threonine kinase activity. Howe ver, the intracellular signals that follow receptor activation and lea d to the different cellular responses to TGF beta are still largely un known. On the basis of the different sensitivity to the protein kinase inhibitor 2-aminopurine and the phosphatase inhibitor okadaic acid, w e identified two distinct pathways through which TGF beta activates a genomic response. Consistently, a-aminopurine prevented and okadaic ac id potentiated the induction of JE by TGF beta. The induction of PAI-1 and junB was instead potentiated by 2-aminopurine, after a transient inhibition and was unaffected by okadaic acid. The superinducing effec t of 2-aminopurine required the presence of a functional RE protein si nce it was abolished in SV40 large T antigen-transfected cells, absent in the BT549 and Saos-2 RB-defective cell lines, and restored in BT54 9 and Saos-2 cells after reintroduction of pRB. The effects of 2-amino purine on the TGF beta inducible junB expression occur in all the cell lines examined suggesting that junB, and possibly other genes, can be regulated by TGF beta through a distinct pRB-dependent pathway.