Low concentrations of doxorubicin sensitizes human solid cancer cells to tumor necrosis factor‐related apoptosis‐inducing ligand (TRAIL)‐receptor (R) 2‐mediated apoptosis by inducing TRAIL‐R2 expression

There is accumulating evidence suggesting that tumor necrosis factor‐related apoptosis‐inducing ligand (TRAIL)‐receptor (R) 2 is a promising molecular target for cancer therapy. Therefore, we investigated the effect of chemotherapeutic agents on TRAIL‐R2‐mediated apoptosis and cytotoxicity in various human solid cancer cells. Treatment of the ACHN human renal cell carcinoma (RCC) cell line with agonistic TRAIL‐R2 antibody (lexatumumab) in combination with 5‐fluorouracil, vinblastine, paclitaxel, or docetaxel did not overcome resistance to these agents. However, treatment with lexatumumab in combination with doxorubicin had a synergistic cytotoxicity. Synergy was also achieved in two other human RCC cell lines, Caki‐1 and Caki‐2, and in eight primary RCC cell cultures. Sequential treatment with doxorubicin followed by lexatumumab induced significantly more cytotoxicity than reverse treatment or simultaneous treatment. Low concentrations of doxorubicin (0.1 and 1 µg/mL) significantly increased TRAIL‐R2 expression at both the mRNA and protein levels. Furthermore, the combination of doxorubicin and lexatumumab significantly enhanced caspase 8 activity, Bid cleavage, Bcl‐xL decrease, release of cytochrome c, and caspase 9 and caspase 3 activity, and induced synergistic apoptosis. The activation of caspases and apoptosis induced with lexatumumab and doxorubicin was blocked by the human recombinant DR5:Fc chimeric protein. In addition, synergistic cytotoxicity was also observed in human prostate, bladder, and lung cancer cells, but was inhibited by the DR5:Fc chimeric protein. These findings suggest that doxorubicin sensitizes solid cancer cells to TRAIL‐R2‐mediated apoptosis by inducing TRAIL‐R2 expression, and that the combination treatment with lexatumumab and doxorubicin might be a promising targeted therapy for cancers, including RCC, prostate, bladder, and lung cancers. (Cancer Sci 2007; 98: 1969–1976)