Evidence of a functional role for the cyclin-dependent kinase-inhibitor p21(WAF1/CIP1/MDA6) in promoting differentiation and preventing mitochondrialdysfunction and apoptosis induced by sodium butyrate in human myelomonocytic leukemia cells (U937)

The impact of dysregulation of the cyclin-dependent kinase inhibitor p21(WA F1/C1P1/MDA6) has been examined in U937 human monocytic leukemia cells in r elation to cell cycle arrest and differentiation following treatment with t he histone deacetylase inhibitor sodium butyrate (SB). Cells stably transfe cted with a p21(WAF1/CIP1/MDA6) antisense construct, in marked contrast to their wild-type counterparts, failed to up-regulate p21(WAF1/CIP1/MDA6), un dergo G, arrest, or express the maturation marker CD11b when exposed to 1 o r 3 mM SE. However, antisense-expressing cells were significantly more susc eptible to SE-mediated mitochondrial injury and apoptosis, manifested by in creased cytosolic translocation of cytochrome c, activation of pro-caspase 3, and degradation of PARP. Dysregulation of p21(WAF1/CIP1/MDA6) did not mo dify the extent of SE-induced histone acetylation, but did result in cleava ge 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 SE-mediated down-regulation of E2F-1 or c-Myc, but was associat ed with enhanced down-regulation of cyclins D, and E. Together, these findi ngs indicate that in U937 leukemia cells, p21(WAF1/CIP1/MDA6) plays a criti cal functional role in SE-mediated G, arrest and maturation, and suggest th at cells displaying dysregulation of this CDKI respond to SE by engaging a default apoptotic program.