Received for publication April 29, 2004.
Revised July 2, 2004.
Accepted for publication July 2, 2004.

CONTRIBUTION OF DISRUPTION OF THE NF-B PATHWAY TO INDUCTION OF APOPTOSIS IN HUMAN LEUKEMIA CELLS BY HISTONE DEACETYLASE INHIBITORS AND FLAVOPIRIDOL

Abstract

Interactions between the cyclin-dependent kinase inhibitor (CDKI) flavopiridol and the histone deacetylase inhibitors (HDACIs) sodium butyrate (NaB) and SAHA have been examined in human leukemia cells in relation to effects on NF-B activation. Exposure (24 hr) of U937 human leukemia cells to NaB (1 mM) or SAHA (1.5 µM) resulted in a marked increase in NF-B DNA binding, effects that were essentially abrogated by co-administration of flavopiridol (100 nM). These events were accompanied by a marked increase in mitochondrial injury, caspase activation, and apoptosis. Mutant cells expressing an IB super-repressor exhibited impairment of NF-B DNA binding in response to HDACIs and a significant although modest increase in apoptosis. However, disruption of the NF-B pathway also increased mitochondrial injury and caspase activation in response to flavopiridol and to an even greater extent to the combination of flavopiridol and HDACIs. Co-administration of flavopiridol with HDACIs down-regulated XIAP, Mcl-1, and p21^CIP1/WAF1 and activated JNK; moreover, these effects were considerably more pronounced in IB& mutants. Similar responses were observed in U937 mutant cells stably expressing RelA/p65 siRNA. In all cases, flavopiridol was significantly more potent than genetic interruption of the NF-B cascade in promoting HDACI-mediated lethality. Together, these findings are consistent with the notion that while inhibition of NF-B activation by flavopiridol contributes to antileukemic interactions with HDACIs, other NF-B-independent flavopiridol actions (e.g., down-regulation of Mcl-1, XIAP, and p21^CIP1/WAF1) play particularly critical roles in this phenomenon.

Key words: NFkappaB, Apoptosis