Histone deacetylase inhibitors (HDIs) are emerging as potentially useful components of the anticancer armamentarium, and as useful tools to dissect mechanistic pathways. HDIs that globally inhibit histone deacetylases (HDACs) have radiosensitizing effects but the relative contribution of specific HDAC classes remain unclear. Newly characterized HDIs are now available that preferentially inhibit specific HDAC classes, including SK7041 (inhibits Class I HDACs) and splitomicin (inhibits Class III HDACs). We investigated in human cancer cells the relative radiosensitizations that result from blocking specific HDAC classes. We found that TSA (inhibitor of both Class I and II HDACs) was the most effective radiosensitizer, followed by the Class I inhibitor SK7041, while splitomicin (inhibitor of Class III) had least effect. Interestingly, radiosensitization by TSA in cell lines expressing p53 was more pronounced than in isogenic lines lacking p53. Radiosensitization of cells expressing p53 by TSA was reduced by pifithrin-α, a small-molecule inhibitor of p53. In contrast, the radiosensitization by TSA of cells expressing low levels of p53 was enhanced by transfection of wild type p53 expressing vector or pretreatment of leptomycin B (LMB), an inhibitor of nuclear export that results in nuclear accumulation and therefore increased intracellular levels of p53. These effects on radiosensitization were respectively muted or not seen in cells treated with SK7041 or splitomicin. To our knowledge, this may be among the first systematic investigations of the comparative anticancer effects of inhibiting specific classes of HDACs, with results suggesting differences in the degrees of radiosensitization, and which in some cell lines may be influenced by p53 expression.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]