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 anti-cancer 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 (IFNalpha) 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 p21WAF1 expression and response to TSA and IFNalpha treatment. Moreover, inhibition of p21WAF1 expression in a p21WAF1-expressing breast cancer cell line by a specific siRNA increased the cytotoxic effects of TSA and IFNalpha. In vitro assays of endothelial cell function showed that TSA and IFNalpha decreased endothelial cell migration, invasion, and capillary tubule formation, without affecting endothelial cell viability. TSA and IFNalpha co-operatively inhibited gene expression of some pro-angiogenic factors: vascular endothelial growth factor, hypoxia-inducible factor 1alpha and matrix metalloproteinase 9, in neuroblastoma cells under hypoxic conditions. Combination TSA and IFNalpha therapy markedly reduced tumour angiogenesis in neuroblastoma-bearing transgenic mice.
Conclusion: Our results indicate that combination TSA and IFNalpha therapy has potent co-operative cytotoxic and anti-angiogenic activity. High basal p21WAF1 expression appears to be acting as a resistance factor to the combination therapy.