P53-DEPENDENT AND P53-INDEPENDENT DIFFERENTIATION OF LEUKEMIC U-937 CELLS - RELATIONSHIP TO CELL-CYCLE CONTROL

Observations based on overexpression of the suppressor gene p53 or int erference with endogenous p53 support a role for p53 in mediating not only growth inhibition and apoptosis but also differentiation. The aim of this study was to characterize the mechanisms of p53-dependent dif ferentiation in the monoblastic leukemia cell line U-937. These cells were transfected with a mutant of the p53 gene expressing Mild-type p5 3 at a permissive temperature. The results showed that wild-type p53 a nd interferon (IFN)-gamma were able to work synergistically to promote differentiation. This cooperative response was not associated with ea rly G(1) arrest of the cell cycle, indicating that p53 can mediate dif ferentiation by mechanisms other than those used for mediating G(1) ar rest. The differentiation response to transfected p53 with or without INF-gamma was inhibited by cyclic adenosine monophosphate (cAMP)-induc ing agents (dibutyryl cyclic adenosine 3':5'-monophosphate, forskolin, and 3-isobutyl-1-methylxanthine) in a dose-dependent manner. In contr ast, the differentiation response of p53-negative U-937 cells to 1,25- dihydroxycholecalciferol or all-trans retinoic acid was enhanced by cA MP-inducing agents at optimal concentrations and inhibited at higher c oncentrations. In addition, 1,25-dihydroxycholecalciferol-mediated dif ferentiation could be achieved in cells arrested in G(1) by concomitan t incubation with cAMP-inducing agents, indicating that differentiatio n can occur in the absence of proliferation. In conclusion, the result s of this study indicate that p53-dependent and -independent different iation can occur independently of cell cycle regulation.