Abstract
Histone deacetylase inhibitors (HDACi) show promise as a novel class of antitumoral agents and have shown the ability to induce apoptosis of tumor cells. To gain a better understanding of the action of HDACi, we conducted a functional gene screen approach named suppression of mortality by antisense rescue technique to identify the key genes responsible for the tumor-selective killing trichostatin A. Over 20 genes associated with HDACi-induced mortality were identified. One of the confirmed positive hits is LIV1, a putative zinc transporter. LIV1 is significantly induced by treatment with HDACi in a number of tumor cells, but not in normal cells. Knockdown of LIV1 suppressed apoptosis induced by HDACi in tumor cells. Although HDACi induced a slight increase in the free intracellular zinc concentration, knockdown of LIV1 significantly enhanced the intracellular zinc level, which was associated with resistance to apoptosis. On the other hand, pretreatment of the cells with a specific zinc chelator TPEN reversed the apoptosis resistance conferred by knockdown of LIV1. However, the biological effects of TPEN were abolished by addition of physiologic concentrations of zinc. Taken together, the present study identifies LIV1 as a critical mediator responsible for HDACi-induced apoptosis. The effect of LIV1 is, at least in part, mediated by affecting intracellular zinc homeostasis, which may be related to alteration of the catalytic activity of the Caspase 3 and expression of some BCL-2 family genes. As such, these findings highlight a novel mechanism underlying the action of HDACi that could be potentially useful in the clinical setting.
Highlights
Histone deacetylase inhibitors (HDACi) are potent apoptosis inducers in a variety of tumor cells and have been promoted as a promising novel class of antitumoral agents in over 100 clinical trials [1, 2] to date
We identified 24 genes including Smad4, LIV1, or Ubiquitin B, which were essential for the trichostatin A (TSA)-induced apoptosis, of which LIV1, as a novel zinc transporter, was isolated by its remarkable ability to confer a resistance against HDACi-induced apoptosis when expressed in an antisense orientation
Selective Induction of Cell Death by TSA HDACis have been shown to selectively induce tumor cell death when compared with normal cells [16]
Summary
Histone deacetylase inhibitors (HDACi) are potent apoptosis inducers in a variety of tumor cells and have been promoted as a promising novel class of antitumoral agents in over 100 clinical trials [1, 2] to date. The mechanism of action for HDACi was believed to be mediated by inhibiting HDACs, increasing the acetylation state of the nucleosomal histones and thereby influencing gene transcription [4]. In contrast to conventional chemotherapeutic agents, HDACi show strong tumor selectivity and cause less toxicity in normal tissues [5]. These findings suggest that the mechanisms of HDACi activities are far more complex than previously thought. Despite the rapid clinical progress achieved, the mechanisms of action of HDACi are not yet well understood, which significantly limits the optimal application of this class of therapeutic drugs
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