Differential regulation of retinoblastoma tumor suppressor protein by G(1)cyclin-dependent kinase complexes in vivo

The retinoblastoma tumor suppressor protein (pRB) negatively regulates earl y-G(1) cell cycle progression, in part, by sequestering E2F transcription f actors and repressing E2F responsive genes. Although pRB is phosphorylated on up to 16 cyclin-dependent kinase (Cdk) sites by multiple G(1) cyclin-Cdk complexes, the active form(s) of pRB in vivo remains unknown. pRB is prese nt as an unphosphorylated protein in G(0) quiescent cells and becomes hypop hosphorylated (similar to2 mol of PO4 to 1 mol of pRB) in early G(1) and hy perphosphorylated (similar to 10 mol of PO4 to 1 mol of pRB) in late G(1) p hase. Here, we report that hypophosphorylated pRB, present in early G(1), r epresents the biologically active form of pRB in vivo that is assembled wit h E2Fs and E1A but that both unphosphorylated pRB in G(0) and hyperphosphor ylated PRE in late G(1) fail to become assembled with E2Fs and E1A, Further more, using transducible dominant-negative TAT fusion proteins that differe ntially target cyclin D-Cdk4 or cyclin D-Cdk6 (cyclin D-Cdk4/6) and cyclin E-Cdk2 complexes, namely, TAT-pl6 and TAT-dominant-negative Cdk2, respectiv ely, we found that, in vivo, cyclin D-Cdk4/6 complexes hypophosphorylate PR E in early G(1) and that cyclin E-Cdk2 complexes inactivate pRB by hyperpho sphorylation in late G(1). Moreover, we found that cycling human tumor cell s expressing deregulated cyclin D-Cdk4/6 complexes, due to deletion of the p16(INKa) gene, contained hypophosphorylated pRB that was bound to E2Fs in early G(1) and that E2F-responsive genes, including those for dihydrofolate reductase and cyclin E, were transcriptionally repressed. Thus, we conclud e that, physiologically, pRB is differentially regulated by G(1) cyclin-Cdk complexes.