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.