Transforming Growth Factor β Inhibits the Phosphorylation of pRB at Multiple Serine/Threonine Sites and Differentially Regulates the Formation of pRB Family–E2F Complexes in Human Myeloid Leukemia Cells

Transforming growth factor beta (TGFbeta)1 induced dephosphorylation of pRb at multiple serine and threonine residues including Ser249/Thr252, Thr373, Ser780, and Ser807/811 in MV4-11 cells. Likewise, TGFbeta1 caused the dephosphorylation of p130, while inhibiting accumulation of p107 protein. Phosphorylated pRb was detected to bind E2F-1 and E2F-3, which appears to be a major form of pRb complexes in actively cycling cells. TGFbeta1 significantly downregulated pRb-E2F-1 and pRb-E2F-3 complexes as a result of inhibition of E2F-1 and E2F-3. In contrast, complexes of E2F-4 with pRb and with p130 were increased markedly upon TGFbeta1 treatment, whereas p107 associated E2F-4 was dramatically decreased. In agreement with these results, p130-E2F-4 DNA binding activity was dominant in TGFbeta1 treated cells, whereas p107-E2F-4 DNA binding activity was only found in proliferating cells. Our data strongly suggest that inhibition of E2Fs and differential regulation of pRb family-E2F-4 complexes are linked to TGFbeta1-induced growth inhibition. E2F-4 is switched from p107 to p130 and pRb when cells are arrested in G1 phase by TGFbeta1.