Retinoic acid-induced RB (retinoblastoma) hypophosphorylation enhanced by CGP 52411 (4,5-dianilinophthalimide), an EGF family tyrosine kinase receptor inhibitor.

Retinoic acid is a known morphogen which can regulate cell proliferation and differentiation and also induces the hypophosphorylation of the RB (retinoblastoma tumor suppressor gene) protein, a known cell cycle regulatory protein. The mechanism by which these processes occur is unclear. We find that these processes can be regulated by CGP 52411, 4,5-dianilinophthalimide, an inhibitor of tyrosine protein kinases of the EGF receptor subfamily. Retinoic acid causes the largely phosphorylated RB protein expressed in proliferating HL-60 human promyelocytic leukemia cells to shift to the unphosphorylated form, as well as causing the cells to G0 arrest and differentiate. Addition of CGP 52411 accelerated the redistribution of the RB protein expressed in HL-60 cells to the unphosphorylated form, enhancing the effects of the retinoic acid. By itself CGP 52411 had no apparent effect on the RB protein expressed in HL-60 cells. CGP 52411 also accelerated the retinoic acid-induced accumulation of cells in G1/0 and the phenotypic conversion of cells to the mature myeloid phenotype, suggesting that its target is common to the regulation of both RB phosphorylation and cell proliferation and differentiation. CGP 52411 had a similar effect on the RB phosphorylation shift induced by 1,25-dihydroxy vitamin D3, a ligand for a receptor in the same steroid thyroid hormone superfamily as retinoic acid. Increasing the concentration of CGP 52411 enhanced the acceleration of RB hypophosphorylation in the case of both retinoic acid and 1,25-dihydroxy vitamin D3. The data are consistent with the negative regulation of retinoic acid induced RB protein dephosphorylation coupled to cell cycle arrest and differentiation by a receptor tyrosine kinase sensitive to CGP 52411.