TRANSFORMING GROWTH-FACTOR-BETA-1 MODULATES P107 FUNCTION IN MYELOID CELLS - CORRELATION WITH CELL-CYCLE PROGRESSION

Transforming growth factor-beta 1 (TGF-beta 1) is a potent inhibitor o f hematopoietic cell growth. Here we report that TGF-beta 1 signals in hibition of IL-3-dependent 32D-123 murine myeloid cell growth by modul ating the activities of cyclin E and cyclin-dependent kinase 2 (cdk2) proteins and their complex formation in the G(1) phase of the cell cyc le. Whereas the cyclin E protein was hyperphosphorylated in TGF-beta 1 -treated cells, TGF-beta 1 decreased both the phosphorylation of cdk2 and the kinase activity of the cyclin E-cdk2 complex. Decreased cyclin E-cdk2 kinase activity correlated with decreased phosphorylation of t he retinoblastoma-related protein p107. In support of these observatio ns, transient overexpression of p107 inhibited the proliferation of th e myeloid cells, and expression of antisense oligo deoxynucleotides to p107 mRNA blocked TGF-beta 1 inhibition of myeloid cell growth. Furth ermore, as reported previously, in 32D-123 TGF-beta 1-treated cells, c -Myc protein expression was decreased. TGF-beta 1 increased the bindin g of p107 to the transcription factor E2F, leading to decreased c-Myc protein levels, p107 inhibited E2F transactivation activity and was al so found to bind the c-Myc protein, suggesting p107 negative regulatio n of c-Myc protein function. These studies demonstrate the modulation of p107 function by TGF-beta 1 and suggest a novel mechanism by which TGF-beta 1 blocks cell cycle progression in myeloid cells.