Ad.mda-7 induces MDR reversal and MDR1 gene over-expression confers increased sensitivity to mda-7/IL-24 in human colorectal cancer cells Free

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Inappropriate expression of the multidrug resistance (MDR1) gene, encoding the P-glycoprotein (P-gp), is frequently implicated in resistance of cancer cells to diverse chemotherapeutic drugs. However, high toxicity of P-gp inhibitors mandates alternative approaches for modulating MDR1 and promoting death in drug-resistant cancer cells. In this study, the effect of melanoma differentiation-associated gene-7/interleukin-24 (mda-7/IL-24), which exhibits cancer-specific apoptosis-inducing properties, was studied in drug-sensitive (SW620/WT) and drug-resistant (SW620/DOX) colorectal carcinoma cell lines. mda-7/IL-24, when administered by means of a replication-incompetent adenovirus, Ad.mda-7, at a multiplicity of infection of 25-50 pfu/cell, significantly increased the sensitivity of SW620/DOX cells to doxorubicin. However, the cytotoxic effect of doxorubicin in the SW620/WT sensitive cell line was not significantly altered by Ad.mda-7 infection. Annexin V binding assays revealed that Ad.mda-7 infection effectively reversed resistance to doxorubicin-induced apoptosis in SW620/DOX cells. In addition, infection of SW620/DOX cells with Ad.mda-7 increased intracellular accumulation of doxorubicin and significantly decreased P-gp expression. Surprisingly, we observed that P-gp overexpressing cells (SW620/DOX) displayed increased apoptosis following Ad.mda-7-infection compared to parental SW620/WT cells. Western blot analysis indicated more MDA-7/IL-24 protein in SW620/DOX than SW620/WT cells following infection with Ad.mda-7, which might explain the increased sensitivity of P-gp overexpressing cells to Ad.mda-7 infection. Furthermore, overexpression of MDR1 in SW620/WT cells increased apoptosis and MDA-7/IL-24 protein following Ad.mda-7 infection, while down modulation of MDR1 in SW620/DOX cells by siRNA led to decreased apoptosis and MDA-7/IL-24 protein following Ad.mda-7 infection. These findings reveal that mda-7/IL-24 is a potent MDR reversal agent, preferentially causing apoptosis in P-gp overexpressing MDR cells, suggesting significant clinical implications for the use of mda-7/IL-24 in treating neoplasms that have failed chemotherapy mediated by the P-gp multidrug resistance mechanism.