Enhancing the anticancer effect of the histone deacetylase inhibitor by activating transglutaminase

Histone deacetylase (HDAC) inhibitors have shown promising anticancer effects in clinical trials. However, a proportion of patients do not respond to HDAC inhibitor therapy. We have previously demonstrated that tissue transglutaminase (TG2) is one of the genes commonly up-regulated by HDAC inhibitors in vitro and in vivo, and that two structurally distinct TG2 protein isoforms, the full-length (TG2-L) and the short form (TG2-S), exert opposing effects on cell differentiation due to difference in transamidation activity. Here we show that the HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) transcriptionally activates the expression of both TG2-L and TG2-S, and that up-regulation of TG2-L renders neuroblastoma cells less sensitive to SAHA-induced cytotoxicity. Combination therapy with SAHA and the transamidation activator Naringenin, a natural product found in citrus fruits, synergistically enhanced transamidation activity and SAHA-induced cytotoxicity in neuroblastoma cells, but not in normal non-malignant cells. In tumour-bearing N-Myc transgenic mice, SAHA and Naringenin synergistically suppressed tumour progression. Taken together, our data demonstrate that SAHA-induced TG2-L over-expression renders cancer cells less sensitive to SAHA therapy, and suggest the addition of Naringenin to SAHA and probably also other HDAC inhibitors in future clinical trials in cancer patients.