Role of adenosine in the hypoxic induction of vascular endothelial growth factor in porcine brain derived microvascular endothelial cells.

Hypoxia induced the mRNA expression of vascular endothelial growth factor (VEGF) in porcine brain derived microvascular endothelial cells (BMEC) in a time-dependent manner. Corresponding to the mRNA induction the protein level of VEGF was elevated during hypoxia. The adenosine A1 receptor antagonist 8-phenyltheophylline (8-PT) reduced the hypoxia-induced VEGF mRNA and protein expression significantly. The treatment of BMEC with cobalt chloride-known to activate an oxygen sensing mechanism similar to the one used by the erythropoietin gene-also induced the VEGF mRNA expression, but 8-PT did not reduce this VEGF induction. Although, earlier studies revealed that agents like phorbolester induced the VEGF mRNA expression, the specific inhibitor of the proteinkinase C (PKC) bisindolylmaleimide (BIM) did not reduce but enhanced the hypoxia-induced VEGF mRNA expression. These results indicate that the VEGF induction in BMEC can proceed through PKC-dependent and -independent pathways (like those acting via the putative oxygen sensor). Hypoxia in BMEC probably activates the PKC-dependent pathway mainly via adenosine which might be formed during hypoxia and thereby inhibits activation of PKC-independent, oxygen sensing, pathways. This suggestion was supported by the fact that hypoxia as well as adenosine increased the VEGF mRNA expression post-transcriptionally by enhancing the stability of the VEGF mRNA [corrected].