Renal cell carcinoma (RCC) is the most common kidney malignancy and has a poor prognosis owing to its resistance to chemotherapy. RCC cells overexpress the transcription factor, PAX2, normally expressed in fetal kidney but downregulated at birth. Since Pax2 suppresses apoptosis during renal development, we reasoned that PAX2 may confer resistance to cisplatin-induced apoptosis in RCC. Here, we show that PAX2 confers resistance to cisplatin-induced apoptosis in normal kidney cells and fetal kidney explants. Human embryonic kidney 293 cells transfected with a PAX2 expression vector and exposed to cisplatin (40 microM) exhibited 45 +/- 15% as much caspase-3 cleavage compared to control cells. Conversely, murine collecting duct cells stably transfected with PAX2 antisense cDNA had twofold increase in cisplatin-induced apoptosis. Murine fetal (embryonic day 15) kidney explants from PAX2(1Neu)+/- mice exposed to cisplatin (25 microM x 24 h) had 50% increased apoptosis (terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling staining). We then show that RCC cells (CAKI-1 (human, Caucasian, kidney, carcinoma) and ACHN (human, Caucasian, kidney, adenocarcinoma)) express PAX2 protein. PAX2-small interfering RNA (100 nM) reduces endogenous PAX2 protein (10% of baseline) and induces apoptosis (Annexin-V staining). Pax2 knockdown sensitized RCC cells to cisplatin-induced apoptosis, killing 50-60% of cisplatin-resistant ACHN and CAKI-1 cells. These findings suggest that PAX2 confers resistance to cisplatin-induced apoptosis in non-transformed kidney cells and fetal kidney explants. Similarly, Pax2 overexpression in RCC cells contributes to cisplatin resistance. Conceivably, a therapeutic strategy that inactivates Pax2 in vivo might enhance the efficacy of conventional cytotoxic drugs against RCC.