Epidermal growth factor receptor inhibition sensitizes renal cell carcinoma cells to the cytotoxic effects of bortezomib

In renal cell carcinoma (RCC) models, maximal cytotoxicity of the proteasome inhibitor bortezomib is dependent on efficient blockade of constitutive nuclear factor kappaB (NF-kappaB) activity. Signaling through the epidermal growth factor receptor (EGFR) has been shown to result in NF-kappaB activation. Thus, we sought to investigate whether inhibition of the EGFR sensitizes RCC cells to the cytotoxic effects of bortezomib. We first established that constitutive NF-kappaB activity is dependent on signaling through the EGFR in RCC cells. Indeed, blockade of EGFR signaling with an EGFR tyrosine kinase inhibitor (TKI) resulted in inhibition of NF-kappaB activity. Using pharmacologic and genetic approaches, we also showed that EGFR-mediated NF-kappaB activation occurs through the phosphotidylinositol-3-OH kinase/AKT pathway. Combinations of the EGFR-TKI and bortezomib resulted in synergistic cytotoxic effects when RCC cells were pretreated with the EGFR-TKI, but an antagonistic interaction was observed with bortezomib pretreatment. Evaluation of the effects of drug sequencing on inhibition of NF-kappaB activity revealed that EGFR-TKI pretreatment markedly augmented the NF-kappaB inhibitory effect of bortezomib, whereas bortezomib preexposure resulted in suboptimal NF-kappaB blockade and thus provides a biochemical explanation for the drug interaction results. We conclude that the constitutive NF-kappaB activity observed in RCC cells is mediated, at least in part, through an EGFR/phosphotidylinositol-3-OH kinase/AKT signaling cascade. Pretreatment with an EGFR-TKI sensitizes to bortezomib-mediated cytotoxicity by inhibiting constitutive NF-kappaB activity. The combination of bortezomib and a currently approved EGFR inhibitor warrants clinical investigation.