OSU-HDAC42, a Novel Histone Deacetylase Inhibitor, Induces Apoptosis in a Caspase-Dependent Manner and Induces p^21WAF1/CIP1 and p16 Expression in Multiple Myeloma Cell Lines.

Multiple myeloma (MM) is a neoplastic disorder characterized by accumulation of slowly-proliferating clonal plasma cells. OSU-HDAC42 [a.k.a. (S)-HDAC-42] is a novel histone deacetylase inhibitor that induces apoptosis in various types of cancer cells and is being developed as an anti-cancer therapy in the NCI Rapid Access to Intervention Therapy (RAID) program. In this study, we tested the in vitro activity of OSU-HDAC42 against human MM cells.

OSU-HDAC42 induced myeloma cell death, with an LC⁵⁰ of less than 1.6μM after 48 hours in the four cell lines tested - U266, IM-9, RPMI 8226 and ARH-77 using the MTT assay. OSU-HDAC42 induced cleavage of caspases 3, 8 and 9, as well as polyADP-ribose polymerase (PARP). Addition of the pan-caspase inhibitor Q-VD-OPH before exposure to the drug prevented apoptosis at 48 hours, as determined by Annexin V/propidium iodide staining. These results indicate that OSU-HDAC42 induced apoptosis by a mainly caspase-dependent manner. Bax expression was up-regulated at 24 and 48 hours, while Bcl-2 remains relatively constant. Mcl-1 showed increasing cleavage at increasing doses of OSU-HDAC42. These findings support a mitochondrial pathway of apoptosis. Cell cycle suppressor proteins p21WAF1/CIP1 and p16 were also significantly induced after treatment with the drug, suggesting that OSU-HDAC42 may also acts on pathways to halt cell cycle progression.

In addition, the gp130 (signal-transducing) subunit of the IL-6 receptor was down-regulated by OSU-HDAC42 exposure. The tyrosine-phosphorylated form of STAT3, which is phosphorylated by dimerized gp130, was also dramatically reduced following incubation with OSU-HDAC42, supporting the finding that gp130 expression is diminished. As IL-6 is an important growth and survival factor for MM cells, down-regulation of gp130 may be an important mechanism for the activity of OSU-HDAC42 against MM cells. TRAIL, FasL, XIAP, and p53 expression were not affected by OSU-HDAC42. While other HDAC inhibitors have been shown to activate the death receptor pathway or down-regulate XIAP, this was not observed with OSU-HDAC42 in myeloma cells.

In conclusion, OSU-HDAC42 has in vitro activity against myeloma cells and acts via activation of caspases, inducing the cell cycle suppressors p21^WAF1/CIP1 and p16, as well as interfering with the IL-6 signal transduction pathway.