Mechanisms and treatment strategies to overcome resistance to non-cytotoxic therapy in cancer

Abstract

As anti-cancer agents, retinoid induce cell growth arrest and differentiation, while HDACIs cause cell differentiation, growth inhibition, death and inhibit angiogenesis in many cancer types. However, a proportion of patients respond poorly to these therapies.

My studies, presented here, aimed to improve the anti-cancer effects of these agents by identifying key factors which mediate cancer cell sensitivity or resistance to their action. In this study I have found that the clinically used retinoid, 13-cis RA, exerts its anti-cancer signal in a manner similar to atRA, by modulating the transcriptional response of retinoid-regulated genes.

HDACI-induced cytotoxicity is significantly enhanced when combined with IFNα in 8 out of 9 cancer cell lines from various organ origins. Sensitivity to the combination treatment correlated with an absence of basal p21 protein expression, and cell cycle arrest. Knocking-down p21 gene expression further sensitized cancer cells to the combination therapy. Moreover, IFNα and HDACI co-operatively inhibited pro-angiogenic gene expression in cancer cells, and the combination therapy decreased endothelial cell migration, invasion, and capillary tubule formation.

Further experiments on p21 as a resistance factor to anti-cancer treatment demonstrated that conditioned media from breast cancer MCF-7 cells transfected with p21 siRNA, induced significantly less endothelial cell migration, invasion and vascular sprouting, compared with media from cells transfected with scrambled siRNA. LC/MS analysis of the conditioned media revealed that Trx secretion was significantly reduced after p21 knockdown. The reduction in Trx secretion following p21 knockdown was due to a direct effect of p21 siRNA on the expression of intracellular TBP2 in neuroblastoma, prostate and lung cancer cells. Consistent with this result, media from MCF7 cells transfected with TBP2-specific siRNA alone, promoted endothelial cell invasion and vascular sprouting, Trx knockdown resulted in opposite effects, and the anti-angiogenic effect of p21 siRNA was offset by simultaneous TBP2 siRNA transfection. ChIP assay revealed that p21 directly bound to an E2F1-bindng site in the TBP2 gene promoter. These data indicate that p21 promoted tumour-driven angiogenesis through transcriptional repression of TBP2. Collectively, my experiments indicate several potential treatment targets directed toward enhancing the effectiveness of HDACIs and retinoids.