P120-V-ABL EXPRESSION OVERCOMES TGF-BETA-1 NEGATIVE REGULATION OF C-MYC TRANSCRIPTION BUT NOT CELL-GROWTH

Transformation of interleukin-3 dependent (IL-3) 32D-123 myeloid cells by p120-v-Abl produced the factor-independent 32D-abl cell Line, In 3 2D-abl cells, myc expression was found to be significantly higher than in the parental cells and was correlated with increased E2F-1 protein expression and DNA binding ability, Surprisingly, in 32D-abl cells, T GF-beta 1, a potent G(1)/S inhibitor of 32D-123 and 32D-abl cell growt h, increased E2F transactivation as shown by increased c-myc promoter- CAT and GAL4-E2F-1 activity, In addition, TGF-beta 1 was also found to increase E2F-1 protein levels but had no effect on steady-state retin oblastoma (RE) protein levels or phosphorylation state, In the absence of TGF-beta 1, transient expression of RE in v-Abl expressing cells r esulted in decreased c-myc transcription, inhibition of GAL4-E2F-1 dri ven transactivation and inhibition of cellular proliferation, RE and v -Abl were found to physically asssociate in vivo and in vitro via v-Ab l's ATP binding region, In summary, these studies established that in myeloid cells: (1) v-Abl binds RE resulting in increased E2F-1-driven c-myc transcription, and (2) an alternative pathway exists for TGF-bet a 1-mediated growth inhibition of v-Abl-transformed cells, in which in creased rather than decreased E2F-mediated c-myc transcription is obse rved.