Selective Induction of Apoptosis by Histone Deacetylase Inhibitor SAHA in Cutaneous T-Cell Lymphoma Cells: Relevance to Mechanism of Therapeutic Action

Suberoylanilide hydroxamic acid (SAHA), an orally administered inhibitor of histone deacetylases, is currently in phase II clinical trials for cutaneous T cell lymphomas (CTCL), but the mechanism of SAHA action is unknown. In this study, we investigated the anti-tumor effects of SAHA in CTCL cell lines and freshly isolated peripheral blood lymphocytes (PBL) from CTCL patients with high percentage of circulating malignant T cells. Three cell lines (MJ, Hut78, and HH) and PBL from 11 patients and three healthy donors were treated with SAHA (1, 2.5, and 5 μM) for 24 and/or 48 h. Apoptosis was determined by flow cytometry analysis of sub-G₁ hypodiploid nuclei and/or annexin V binding populations. Acetylated histones and apoptosis-associated proteins were detected by Western blotting. SAHA at 1–5 μM for 24 and 48 h induced apoptosis in a concentration- and time-dependent manner in three cell lines: MJ (0%–7% and 1%–32%), Hut78 (4%–36% and 5%–54%), and HH (4%–67% and 8%–81%). SAHA at 1–5 μM for 48 h also induced more apoptosis of patients' PBL than healthy donors' (15%–32%versus 6%–13%, p<0.05). SAHA treatment caused an accumulation of acetylated histones (H2B, H3, and H4), an increase of p21^WAF1 and bax proteins, a decrease of Stat6 and phospho-Stat6 proteins, and activation of caspase-3 in CTCL cells. Our data suggest that selective induction of malignant T cell apoptosis and modulation of acetylated histones, p21^WAF1, bax, Stat6, and caspase-3 may underlie the therapeutic action of SAHA in CTCL patients.