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 β (TGFβ)1 induced dephosphorylation of pRb at multiple serine and threonine residues including Ser249/Thr252, Thr373, Ser780, and Ser807/811 in MV4-11 cells. Likewise, TGFβ1 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. TGFβ1 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 TGFβ1 treatment, whereas p107 associated E2F-4 was dramatically decreased. In agreement with these results, p130-E2F-4 DNA binding activity was dominant in TGFβ1 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 TGFβ1-induced growth inhibition. E2F-4 is switched from p107 to p130 and pRb when cells are arrested in G1 phase by TGFβ1.