RhoA Silencing Reverts the Resistance to Doxorubicin in Human Colon Cancer Cells

The efficacy of doxorubicin in the treatment of cancer is limited by its side effects and by the onset of drug resistance. Reverting such resistance could allow the decrease of the dose necessary to eradicate the tumor, thus diminishing the toxicity of the drug. We transfected doxorubicin-sensitive (HT29) and doxorubicin-resistant (HT29-dx) human colon cancer cells with RhoA small interfering RNA. The subsequent decrease of RhoA protein was associated with the increased sensitivity to doxorubicin in HT29 cells and the complete reversion of doxorubicin resistance in HT29-dx cells. RhoA silencing increased the activation of the nuclear factor-kappa B pathway, inducing the transcription and the activity of nitric oxide synthase. This led to the tyrosine nitration of the multidrug resistance protein 3 transporter (MRP3) and contributed to a reduced doxorubicin efflux. Moreover, RhoA silencing decreased the ATPase activity of P-glycoprotein (Pgp) in HT29 and HT29-dx cells as a consequence of the reduced expression of Pgp. RhoA silencing, by acting as an upstream controller of both MRP3 nitration and Pgp expression, was effective to revert the toxicity and accumulation of doxorubicin in both HT29 and HT29-dx cells. Therefore, we suggest that inactivating RhoA has potential clinical applications and might in the future become part of a gene therapy protocol. (Mol Cancer Res 2008;6(10):1607-20)