Histone Acetylation-independent Effect of Histone Deacetylase Inhibitors on Akt through the Reshuffling of Protein Phosphatase 1 Complexes

Abstract

Despite advances in understanding the role of histone deacetylases (HDACs) in tumorigenesis, the mechanism by which HDAC inhibitors mediate antineoplastic effects remains elusive. Modifications of the histone code alone are not sufficient to account for the antitumor effect of HDAC inhibitors. The present study demonstrates a novel histone acetylation-independent mechanism by which HDAC inhibitors cause Akt dephosphorylation in U87MG glioblastoma and PC-3 prostate cancer cells by disrupting HDACprotein phosphatase 1 (PP1) complexes. Of four HDAC inhibitors examined, trichostatin A (TSA) and HDAC42 exhibit the highest activity in down-regulating phospho-Akt, followed by suberoylanilide hydroxamic acid, whereas MS-275 shows only a marginal effect at 5 M. This differential potency parallels the respective activities in inducing tubulin acetylation, a non-histone substrate for HDAC6. Evidence indicates that this Akt dephosphorylation is not mediated through deactivation of upstream kinases or activation of downstream phosphatases. However, the effect of TSA on phosphoAkt can be rescued by PP1 inhibition but not that of protein phosphatase 2A. Immunochemical analyses reveal that TSA blocks specific interactions of PP1 with HDACs 1 and 6, resulting in increased PP1-Akt association. Moreover, we used isozyme-specific small interfering RNAs to confirm the role of HDACs 1 and 6 as key mediators in facilitating Akt dephosphorylation. The selective action of HDAC inhibitors on HDAC-PP1 complexes represents the first example of modulating specific PP1 interactions by small molecule agents. From a clinical perspective, identification of this PP1-facilitated dephosphorylation mechanism underscores the potential use of HDAC inhibitors in lowering the apoptosis threshold for other therapeutic agents through Akt down-regulation. Histone deacetylase (HDAC)2 is recognized as one of the promising targets for cancer treatment because many HDAC inhibitors have entered clinical trials in both solid and liquid tumors (1-3). Nevertheless, the mechanisms underlying the antiproliferative effects of HDAC inhibitors remain elusive. Although they have been shown to activate the transcription of a defined set of genes through chromatin remodeling (4), increasing evidence suggests that modifications of the epigenetic histone code may not represent the primary cause for HDAC inhibitormediated growth inhibition and apoptosis in cancer cells (5, 6). To date, at least two distinct histone acetylation-independent mechanisms have been described for the action of HDAC inhibitors on cellular targets. First, because certain HDAC members can mediate the deacetylation of non-histone proteins, their inhibition interferes with signaling processes in which these proteins are involved independently of the activity of HDAC inhibitors in transcriptional activation. For example, HDAC3 regulates NFB signaling in the nucleus by deacetylation the Rel-A subunit (7), and HDAC6 modulates microtubule acetylation and chemotactic cell motility by acting as a tubulin dea...

MS-275 synergistically enhances the growth inhibitory effects of RAMBA VN/66-1 in hormone-insensitive PC-3 prostate cancer cells and tumours

“…This concentration of VN/66-1 is biologically relevant, because it is well below the maximum plasma concentration (26.9 mM), which was achieved in mice following a single subcutaneous administration of 10 mg kg À1 of VN/66-1 (Patel et al, 2007a). It should also be noted that these concentrations of MS-275 have been used previously for a similar study using PC-3 cells (Chen et al, 2005). Furthermore, it has been reported that MS-275 has an IC 50 value of 4.8 mM in inhibiting HDAC activity in nuclear extracts (Miller et al, 2003).…”

Blockade of mTOR signaling potentiates the ability of histone deacetylase inhibitor to induce growth arrest and differentiation of acute myelogenous leukemia cells

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Honokiol, a natural biphenyl, inhibits in vitro and in vivo growth of breast cancer through induction of apoptosis and cell cycle arrest

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