HDAC ‐inhibitor (S)‐8 disrupts HDAC 6‐ PP 1 complex prompting A375 melanoma cell growth arrest and apoptosis

Histone deacetylase inhibitors ( HDAC i) are agents capable of inducing growth arrest and apoptosis in different tumour cell types. Previously, we reported a series of novel HDAC i obtained by hybridizing SAHA or oxamflatin with 1,4‐benzodiazepines. Some of these hybrids proved effective against haematological and solid cancer cells and, above all, compound (S)‐8 has emerged for its activities in various biological systems. Here, we describe the effectiveness of (S)‐8 against highly metastatic human A375 melanoma cells by using normal PIG 1 melanocytes as control. (S)‐8 prompted: acetylation of histones H3/H4 and α‐tubulin; G 0 /G 1 and G 2 /M cell cycle arrest by rising p21 and hypophos‐phorylated RB levels; apoptosis involving the cleavage of PARP and caspase 9, BAD protein augmentation and cytochrome c release; decrease in cell motility, invasiveness and pro‐angiogenic potential as shown by results of wound‐healing assay, down‐regulation of MMP ‐2 and VEGF ‐A/ VEGF ‐R2, besides TIMP ‐1/ TIMP ‐2 up‐regulation; and also intracellular accumulation of melanin and neutral lipids. The pan‐caspase inhibitor Z‐ VAD ‐fmk, but not the antioxidant N‐acetyl‐cysteine, contrasted these events. Mechanistically, (S)‐8 allows the disruption of cytoplasmic HDAC 6‐protein phosphatase 1 ( PP 1) complex in A375 cells thus releasing the active PP 1 that dephosphorylates AKT and blocks its downstream pro‐survival signalling. This view is consistent with results obtained by: inhibiting PP 1 with Calyculin A; using PPP 1R2‐transfected cells with impaired PP 1 activity; monitoring drug‐induced HDAC 6‐ PP 1 complex re‐shuffling; and, abrogating HDAC 6 expression with specific si RNA . Altogether, (S)‐8 proved very effective against melanoma A375 cells, but not normal melanocytes, and safe to normal mice thus offering attractive clinical prospects for treating this aggressive malignancy.