Disruption of IkappaB kinase (IKK)-mediated RelA serine 536 phosphorylation sensitizes human multiple myeloma cells to histone deacetylase (HDAC) inhibitors.

ABSTRACT: Post-translational modifications of RelA play an important role in regulation of NF-?B activation. We previously demonstrated that in malignant hematopoietic cells, histone deacetylase inhibitors (HDACIs) induced RelA hyperacetylation and NF-?B activation, attenuating lethality. We now present evidence that I?B kinase (IKK) ?-mediated RelA Ser-536 phosphorylation plays a significant functional role in promoting RelA acetylation, inducing NF-?B activation, and limiting HDACI lethality in human multiple myeloma (MM) cells. Immunoblot profiling revealed that although basal RelA phosphorylation varied in MM cells, Ser-536 phosphorylation correlated with IKK activity. Exposure to the pan-HDACIs vorinostat or LBH-589 induced phosphorylation of IKK?/? (Ser-180/Ser-181) and RelA (Ser-536) in MM cells, including cells expressing an I?B? "super-repressor," accompanied by increased RelA nuclear translocation, acetylation, DNA binding, and transactivation activity. These events were substantially blocked by either pan-IKK or IKK?-selective inhibitors, resulting in marked apoptosis. Consistent with these events, inhibitory peptides targeting either the NF-?B essential modulator (NEMO) binding domain for IKK complex formation or RelA phosphorylation sites also significantly increased HDACI lethality. Moreover, IKK? knockdown by shRNA prevented Ser-536 phosphorylation and significantly enhanced HDACI susceptibility. Finally, introduction of a nonphosphorylatable RelA mutant S536A, which failed to undergo acetylation in response to HDACIs, impaired NF-?B activation and increased cell death. These findings indicate that HDACIs induce Ser-536 phosphorylation of the NF-?B subunit RelA through an IKK?-dependent mechanism, an action that is functionally involved in activation of the cytoprotective NF-?B signaling cascade primarily through facilitation of RelA acetylation rather than nuclear translocation.

SUBMITTER: Dai Y 

PROVIDER: S-EPMC3190767 | biostudies-literature | 2011 Sep

REPOSITORIES: biostudies-literature