Tumor cells can adapt to a hostile environment with reduced oxygen supply. Chronic or repetitive acute hypoxia, in turn, may foster neoplastic transformation or malignant progression of tumor cells. The present study aimed to identify molecules that confer resistance to hypoxia by the use of immortalized proximal convoluted tubule (PT) cells microdissected from newborn mouse kidney. Partially hypoxia-resistant PT cells were selected by exposing PT cultures to repetitive cycles of hypoxia and reoxygenation. Thereafter, hypoxic stress-induced changes in mRNA (quantitative RT-PCR) and protein expression (immunoblotting), inner mitochondrial membrane potential (DY_m), reactive oxygen species (ROS) formation, and apoptotic cell death (flow cytometry with TMRE, CM-H₂DCFDA, and propidium iodide as DY_m-, redox- and DNA-specific fluorescence dyes, respectively) were compared between hypoxia-selected and control PT cultures. As a result, hypoxia-selected PT cells constitutively up-regulated mRNAs associated with oxidative defense, DNA-repair and apoptosis. In addition, hypoxia-selected PT cells (but not control cultures) exhibited highly hypoxic-stress inducible UCP-3 mRNA and protein besides chronically upregulated UCP-3 mRNA. UCP-3 reportedly short-circuits DY_m. Accordingly, hypoxia-selected PT cells exhibited a lower hypoxic stress-induced rise in DY_m. Moreover, hypoxia-selected PT cells produced less ROS upon hypoxic stress than control cells. Consequently, hypoxic stress triggered break-down of DY_m and DNA degradation in a lower percentage of hypoxia-selected than control PT cells. Finally, to test for UCP-3 protein expression in renal cell carcinoma, protein abundance of human tumor specimens and benign renal tissue was compared by immunoblotting. As a result, renal cell carcinoma expressed UCP-3 protein. Highest expression was seen close to necrotic tumor areas suggesting hypoxia-induced UCP-3 up-regulation. Taken together, our data demonstrate that repetitive hypoxia/reoxygenation cycles select cells with higher oxidative defense and lower hypoxic-stress stimulated ROS formation. The latter is achieved by upregulating UCP-3 uncoupling protein which confines hypoxia/reoxygenation-stimulated hyperpolarization of DY_m. Most importantly, UCP-3 expression in hypoxic areas of human renal cell carcinoma suggests that UCP-3 may play a similar role in the adaptation of tumor cells to hypoxia.