Despite the understanding of the importance of
phosphoinositide 3-kinase (PI 3-K) signaling pathway in the regulation of cellular proliferation, little is known about its role during
phorbol 12-myristate 13-acetate (PMA)-induced differentiation in human
leukemia cells. Here, we report a novel finding that PI 3-K inhibition by
LY294002 significantly increases p21WAF1/Cip1 expression in PMA-stimulated human
leukemia cells NB4 and THP1.
LY294002 potentiated expression of p21WAF1/Cip1 via a p53-independent mechanism and did not affect
mitogen activated protein kinase (MAPK) activation. Electrophoretic mobility shift (EMSA) experiments revealed that blocking of PI 3-K was associated with increased binding of
transcription factor Sp1 to the PMA-responsive sites on the p21WAF1/Cip1 promoter. Pretreatment with
rapamycin, an inhibitor of mTOR
kinase, decreased the expression of p21WAF1/Cip1
protein in PMA-stimulated NB4 cells. The level of PMA-induced p21WAF1/Cip1
protein expression was lower in NB4 cells overexpressing wild type
protein kinase C zeta (PKC zeta) compared to those transfected with empty vector or with
kinase inactive PKC zeta. Sp1 binding to the p21WAF1/Cip1 promoter was completely lost in a wild type PKC zeta overexpressing and PMA-stimulated NB4 cells. We demonstrate that PI 3-K signaling pathway suppresses PMA-induced expression of p21WAF1/Cip1 in human
leukemia cells, and that this effect is partly mediated by PKC zeta.