Selective and nonselective toxicity of TRAIL/Apo2L combined with chemotherapy in human bone tumour cells vs. normal human cells

Although TRAIL/Apo2L preferably induces apoptosis in tumour cells without toxicity in normal cells, many tumour cell types display TRAIL/Apo2L resistance. Whether TRAIL/Apo2L in combination with chemotherapy may overcome TRAIL/Apo2L resistance while maintaining tumour selectivity remains to be determined. Here, we report that while ActD, DOX and CDDP sensitised both OS and Ewing's tumour cell lines and normal cells (hOBs, synovial cells, fibroblasts) to TRAIL/Apo2L-induced apoptosis, the combination of etoposide (VP16) and TRAIL/Apo2L was selectively active on tumour cells without affecting normal cells. Sensitisation of OS cells and hOBs to TRAIL/Apo2L did not correlate with a compatible change in the gene expression profile of the receptors for TRAIL/Apo2L determined by quantitative real-time RT-PCR. Also, sensitisation of the TRAIL/Apo2L death pathway did not rely entirely on the chemotherapy-induced, caspase-dependent cytotoxicity. Further, chemotherapy did not cause a compatible change in expression levels of proteins such as Bcl-2, Bcl-x(L), Bax, cIAP2, XIAP and survivin. However, ActD, DOX and CDDP downregulated expression of cFLIP in OS cells as well as expression of p21 in normal hOBs. Interestingly, while VP16 also extinguished cFLIP in OS cells, which were sensitised for TRAIL/Apo2L by VP16, VP16 induced cFLIP and enhanced p21 levels in normal hOBs, which remained refractory to VP16 plus TRAIL/Apo2L. Together, our data reveal that TRAIL/Apo2L combined with certain chemotherapeutic drugs is toxic to bone tumour and normal human cells and suggest that cotreatment with TRAIL/Apo2L and VP16 provides an attractive approach for selective killing of tumour cells while leaving unaffected normal cells.