Tumor suppressor p53 status does not determine the differentiation-associated G(1) cell cycle arrest induced in leukemia cells by 1,25-dihydroxyvitamin D-3 and antioxidants

Vitamin D derivatives can induce differentiation of human acute myeloid leukemia (AML) cells. Here, we investigated if the G1 cell cycle block associated with monocytic differentiation is modulated by the p53 status of the cells treated with 1,25D, alone or with plant antioxidants carnosic acid or silibinin, and a p38 MAPK inhibitor SB202190 (SB), a combination (D-C/S-SB) previously shown to enhance differentiation of AML p53null cells. D-C/S-SB enhanced differentiation of OCI-AML3 (p53wt) and as expected HL60 (p53null) cells, but not of MOLM-13 (p53wt) cells. Conversely, MOLM-13 (p53wt) cells treated with 1,25D and/or D-C/S-SB, resembled HL60 (p53 null) cells in rapid G1 block, while OCI-AML3 (p53wt) cells showed a delayed G1 block when treated in a similar way, indicating that there is no relationship between the p53 status and G1 block. Western blot analysis revealed that 1,25D and D-C/S-SB increased the inhibitory phosphorylation levels MEK-1 (P-Thr286), but decreased the levels of activated ERK1/2 (Thr202/Tyr204;Thr185/Tyr187), again without any apparent relationship to the p53 status. Interestingly, the increased levels of p21Waf1/Cip1 were insufficient to promote a G1 block in this system, as only cell lines with increased levels of p27Kip1 and p35Nck5a, an activator of Cdk5, showed a rapid G1 block. Overall, our data show that the p53-p21 axis is unlikely to have a role in differentiation-associated G1 block in AML cells with p53, and that this block is achieved by several, possibly co-operating but redundant pathways, that include inhibition of MEK-1 by p35Nck5a-activated Cdk5.