Enhancing the anti-angiogenic action of histone deacetylase inhibitors

Background: Histone deacetylase inhibitors (HDACIs) have many effects on cancer cells, such as growth inhibition, induction of cell death, differentiation, and anti-angiogenesis, all with a wide therapeutic index. However, clinical trials demonstrate that HDACIs are more likely to be effective when used in combination with other anticancer agents. Moreover, the molecular basis for the anticancer action of HDACIs is still unknown. In this study, we compared different combinations of HDACIs and anti-cancer agents with anti-angiogenic effects, and analysed their mechanism of action. Results: Trichostatin A (TSA) and alpha-interferon (IFN alpha) were the most effective combination across a range of different cancer cell lines, while normal non-malignant cells did not respond in the same manner to the combination therapy. There was a close correlation between absence of basal p21(WAFI) expression and response to TSA and IFN alpha treatment. Moreover, inhibition of p21(WAFI) expression in a p21(WAFI)-expressing breast cancer cell line by a specific siRNA increased the cytotoxic effects of TSA and IFN alpha. In vitro assays of endothelial cell function showed that TSA and IFN alpha decreased endothelial cell migration, invasion, and capillary tubule formation, without affecting endothelial cell viability. TSA and IFN alpha co-operatively inhibited gene expression of some pro-angiogenic factors: vascular endothelial growth factor, hypoxia-inducible factor 1 alpha and matrix metalloproteinase 9, in neuroblastoma cells under hypoxic conditions. Combination TSA and IFN alpha therapy markedly reduced tumour angiogenesis in neuroblastoma-bearing transgenic mice. Conclusion: Our results indicate that combination TSA and IFN alpha therapy has potent cooperative cytotoxic and anti-angiogenic activity. High basal p21(WAFI) expression appears to be acting as a resistance factor to the combination therapy.