PMID: 21254166

Novel mechanism by which histone deacetylase inhibitors facilitate topoisomerase IIalpha degradation in hepatocellular carcinoma cells.

Abstract

Histone deacetylase (HDAC) inhibitors exhibit a unique ability to degrade topoisomerase (topo)IIalpha in hepatocellular carcinoma (HCC) cells, which contrasts with the effect of topoII-targeted drugs on topoIIbeta degradation. This selective degradation might foster novel strategies for HCC treatment in light of the correlation of topoIIalpha overexpression with the aggressive tumor phenotype and chemoresistance. Here we report a novel pathway by which HDAC inhibitors mediate topoIIalpha proteolysis in HCC cells. Our data indicate that HDAC inhibitors transcriptionally activated casein kinase (CK)2alpha expression through increased association of acetylated histone H3 with the CK2alpha gene promoter. In turn, CK2 facilitated the binding of topoIIalpha to COP9 signalosome subunit (Csn)5 by way of topoIIalpha phosphorylation. Furthermore, we identified Fbw7, a Csn5-interacting F-box protein, as the E3 ligase that targeted topoIIalpha for degradation. Moreover, knockdown of CK2alpha, Csn5, or Fbw7 reversed HDAC inhibitor-induced topoIIalpha degradation. Mutational analysis indicates that the (1361) SPKLSNKE(1368) motif plays a crucial role in regulating topoIIalpha protein stability. This motif contains the consensus recognition sites for CK2 (SXXE), glycogen synthase kinase (GSK)3beta (SXXXS), and Fbw7 (SPXXS). This study also reports the novel finding that topoIIalpha may be a target of GSK3beta phosphorylation. Evidence suggests that CK2 serves as a priming kinase, through phosphorylation at Ser1365, for GSK3beta-mediated phosphorylation at Ser1361. This double phosphorylation facilitated the recruitment of Fbw7 to the phospho-degron (1361) pSPKLpS(1365) of topoIIalpha, leading to its ubiquitin-dependent degradation. CONCLUSION: This study shows a novel pathway by which HDAC inhibitors facilitate the selective degradation of topoIIalpha, which underlies the complexity of the functional role of HDAC in regulating tumorigenesis and aggressive phenotype in HCC cells.