Phosphorylation of the Retinoblastoma Protein in Childhood Acute Lymphoblastic Leukemia

The cell cycle distribution of childhood acute lymphoblastic leukemia (ALL) cells shares many common features with that of normal CD34(+) progenitor cells. The majority of ALL and CD34(+) cells reside in G1 phase, although their cell cycle distribution within the G1 phase may be different. Traverse of the restriction point in late G1 phase is controlled by phosphorylation of the retinoblastoma protein (pRb). Different phosphoforms of pRb were recognized by their slower mobility on SDS-PAGE in ALL cells, while CD34(+) cells only contained the fast migrating p110Rb. Phosphorylated residues of pRb were detected only in ALL cells by immunoblotting with phosphospecific antibodies. Phosphorylation of pRb was due to cyclinD-Cdk4/6 protein kinase at Ser-608, Ser-780 and Ser-795. In contrast, phosphorylation at Ser-249/Thr-252, Thr-373, Ser-807/811 varied between ALL samples. No phosphorylation of pRb was detected at these residues in CD34(+) cells by immunoprecipitation. We concluded that the G1 phase distribution of ALL and CD34(+) was different, ALL cells progressed to mid G1 phase while unstimulated CD34(+) resided in early G1 phase.