Evidence of a functional role for the cyclin-dependent kinase-inhibitor p21WAF1/CIP1/MDA6 in promoting differentiation and preventing mitochondrial dysfunction and apoptosis induced by sodium butyrate in human myelomonocytic leukemia cells (U937)

The impact of dysregulation of the cyclin-dependent kinase inhibitor p21 WAF1/CIP1/MDA6 has been examined in U937 human monocytic leukemia cells in relation to cell cycle arrest and differentiation following treatment with the histone deacetylase inhibitor sodium butyrate (SB). Cells stably transfected with a p21 WAF1/CIP1/MDA6 antisense construct, in marked contrast to their wild-type counterparts, failed to up-regulate p21 WAF1/CIP1/MDA6 , undergo G 1 arrest, or express the maturation marker CD11b when exposed to 1 or 3 mM SB. However, antisense-expressing cells were significantly more susceptible to SB-mediated mitochondrial injury and apoptosis, manifested by increased cytosolic translocation of cytochrome c, activation of pro-caspase 3, and degradation of PARP. Dysregulation of p21 WAF1/CIP1/MDA6 did not modify the extent of SB-induced histone acetylation, but did result in cleavage of p27 KIP1 , Bcl-2 and pRb, as well as diminished levels of full-length underphosphorylated pRb. Finally, dysregulation of p21 WAF1/CIP1/MDA6 did not modify SB-mediated down-regulation of E2F-1 or c-Myc, but was associated with enhanced down-regulation of cyclins D 1 and E. Together, these findings indicate that in U937 leukemia cells, p21 WAF1/CIP1/MDA6 plays a critical functional role in SB-mediated G 1 arrest and maturation, and suggest that cells displaying dysregulation of this CDKI respond to SB by engaging a default apoptotic program.