Endogenous inhibition of histone deacetylase 1 by tumor-suppressive maspin

Maspin, a noninhibitory serine protease inhibitor, exerts multifaceted tumor-suppressive effects. Maspin expression is associated with better differentiated phenotypes, better cancer prognosis, and better drug sensitivity. Consistently, maspin also correlates with increased expression of Bax and p21WAF1/CIP1. Interestingly, histone deacetylase 1 (HDAC1), a major HDAC responsible for histone deacetylation, was shown to interact with maspin in a yeast two-hybrid screening. In this study, we confirmed the maspin/HDAC1 interaction in human prostate tissues, in prostate cancer cell lines, and with purified maspin. We produced several lines of evidence that support an inhibitory effect of maspin on HDAC1 through direct molecular interaction, which was detected in both the nucleus and the cytoplasm. Both endogenously expressed maspin and purified maspin inhibited HDAC1. In contrast, small interfering RNA (siRNA) silencing of maspin in PC3 cells increased HDAC activity. Accordingly, maspin-transfected DU145 cells exhibited increased expression of HDAC1 target genes Bax, cytokeratin 18 (CK18), and p21(WAF1/CIP1), whereas maspin siRNA decreased CK18 expression in PC3 cells. The maspin effect on HDAC1 correlated with an increased sensitivity to cytotoxic HDAC inhibitor M344. Interestingly, glutathione S-transferase (GST, another maspin partner) was detected in the maspin/HDAC1 complex. Furthermore, a COOH-terminally truncated maspin mutant, which bound to HDAC1 but not GST, did not increase histone acetylation. Although HDACs, especially the highly expressed HDAC1, are promising therapeutic targets in cancer intervention, our data raise a novel hypothesis that the endogenous inhibitory effect of maspin on HDAC1 is coupled with glutathione-based protein modification, and provide new leads toward future developments of specific HDAC1-targeting strategies.