An Intact NF-kappaB Pathway is Required for Histone Deacetylase Inhibitor Induced G1 Arrest and Maturation in U937 Human Myeloid Leukemia Cells

Abstract

The role of NF-kB in regulating G1 arrest in maturation induced by the histone deacetylase inhibitor sodium butyrate (NaB) was examined in human myelomonocytic leukemia cells (U937). Cells stably transfected with an IkB" "super-repressor" lacking phosphorylation sites necessary for proteasomal degradation exhibited diminished IkB" phosphorylation and NF-kB DNA binding upon exposure to TNF-a. When exposed to NaB (1 mM; 48 hr) or 5 nM PMA (24 hr), IkBaM cells displayed a marked reduction in G1 arrest compared to Neo controls. In each case, this was accompanied by a significant reduction in the percentage of cells expressing the differentiation markers CD11a, CD11b, and CD18. The impairment in NaB-induced maturation in mutant cells was associated with a reciprocal increase in apoptosis. In contrast to impairment in NaB- or PMA-induced NF-kB DNA binding, stable expression of the IkBaM did not modify DNA binding of SP1 or AP2 transcription factors. IkBaM mutant cells also displayed impairment in NaB- and PMA-mediated induction of p21CIP1 and phosphorylation (inactivation) of p34cdc2, as well as diminished levels of pRb-bound E2F1. Finally, the NF-kB inhibitor CAPE antagonized NaB- and PMA-related NF-kB DNA binding as well as induction of p21CIP1. Together, these findings suggest that NF-kB plays an important functional role in mediating NaB-induced p21CIP1 induction, G1 arrest, and maturation in human myelomonocytic leukemia cells, and that disruption of the NF-kB pathway causes cells to engage an alternative, apoptotic program.