Novel Mechanism by Which Histone Deacetylase Inhibitors Facilitate Topoisomerase IIα Degradation in Hepatocellular Carcinoma Cells

Histone deacetylase (HDAC) inhibitors exhibit a unique ability to degrade topoisomerase (topo)II alpha in hepatocellular carcinoma (HCC) cells, which contrasts with the effect of topoII-targeted drugs on topoII beta degradation. This selective degradation might foster novel strategies for HCC treatment in light of the correlation of topoII alpha overexpression with the aggressive tumor phenotype and chemoresistance. Here we report a novel pathway by which HDAC inhibitors mediate topoII alpha proteolysis in HCC cells. Our data indicate that HDAC inhibitors transcriptionally activated casein kinase (CK)2 alpha expression through increased association of acetylated histone H3 with the CK2 alpha gene promoter. In turn, CK2 facilitated the binding of topoII alpha to COP9 signalosome subunit (Csn)5 by way of topoII alpha phosphorylation. Furthermore, we identified Fbw7, a Csn5-interacting F-box protein, as the E3 ligase that targeted topoII alpha for degradation. Moreover, knockdown of CK2 alpha, Csn5, or Fbw7 reversed HDAC inhibitor-induced topoII alpha degradation. Mutational analysis indicates that the (SPKLSNKE1368)-S-1361 motif plays a crucial role in regulating topoII alpha protein stability. This motif contains the consensus recognition sites for CK2 (SXXE), glycogen synthase kinase (GSK)3 beta (SXXXS), and Fbw7 (SPXXS). This study also reports the novel finding that topoII alpha may be a target of GSK3 beta phosphorylation. Evidence suggests that CK2 serves as a priming kinase, through phosphorylation at Ser1365, for GSK3 beta-mediated phosphorylation at Ser1361. This double phosphorylation facilitated the recruitment of Fbw7 to the phospho-degron (1361)pSPKLpS(1365) of topoII alpha, leading to its ubiquitin-dependent degradation. Conclusion: This study shows a novel pathway by which HDAC inhibitors facilitate the selective degradation of topoII alpha, which underlies the complexity of the functional role of HDAC in regulating tumorigenesis and aggressive phenotype in HCC cells. (HEPATOLOGY 2011;53:148-159)