Angiogenesis, or new blood vessel formation, is a physiological response of tissues to hypoxia or ischemia. Vascular endothelial growth factor (VEGF) is a potent angiogenic factor that is up-regulated by hypoxia. The mechanisms responsible for hypoxic induction of VEGF are still not completely understood, though both transcriptional and post-transcriptional mechanisms are involved. In recent years, we have investigated cis-regulatory sequences and trans-acting factors which mediate the hypoxia-induced increase in VEGF mRNA stability. In particular, we have identified a 40 bp sequence motif in the 3'-untranslated region of VEGF mRNA, which is critical for the increase in VEGF mRNA stability with hypoxia and have shown that the RNA-binding protein, HuR, binds to this region. By means of indirect immunofluorescence experiments using monoclonal antibodies against HuR, we demonstrated that HuR localizes to the nucleus under hypoxia. As HuR binds to VEGF mRNA and appears to mediate stabilization of VEGF mRNA, it was of interest to show whether a fraction of VEGF protein localizes similarly to the nucleus. Double-labeling immunofluorescence showed that VEGF protein colocalizes with HuR in discrete nuclear compartments and nuclear VEGF protein was increased in hypoxia. These results indicate that VEGF may have a nuclear function, especially during hypoxia.