Hypoxic Regulation of Angiopoietin-2 Expression in Endothelial Cells

Exposure of endothelial cells to hypoxia-induced angiopoietin-2 (Ang2) expression. The increase in Ang2 mRNA levels occurred by transcriptional regulation and by post-transcriptional increase in mRNA stability. Induction of Ang2 mRNA resulted in an increase of intracellular and secreted Ang2 protein levels. Since the transcriptional regulation of several genes involved in angiogenesis during hypoxia is mediated by hypoxia-inducible factor-1 (HIF-1), it was conceivable that Ang2 expression might be regulated by the same oxygen-dependent mechanism. However, our data showed that pharmacological HIF inducers, CoCl2 and DFO, did not affect Ang2 expression. Moreover, HIF-1-deficient hepatoma cell (Hepa1 c4) and its wild-type counterpart (Hepa1 c1c4) up-regulates Ang2 during hypoxia. These results indicated that hypoxia-driven Ang2 expression may be independent of the HIF pathway. Using neutralizing VEGF antibody or pharmacological inhibitors of VEGF receptors, we showed that hypoxia-induced VEGF participates but could not account completely for Ang2 expression during hypoxia. In addition, hypoxia elicited an increase of cyclooxygenase-2 (COX-2) expression and a parallel increase in prostanglandin E2 (PGE2) and prostacyclin (PGI2) production. COX-2 inhibitors decreased the hypoxic induction of Ang2 and the hypoxic induction of PGE2 and PGI2 in a dose-dependent manner. Similarly, COX-2 but not COX-1 antisense treatment decreased hypoxic induction of Ang2 expression, and this effect was reversed by exogenous PGE2. Finally, exogenous PGE2 and PGI2 were able to stimulate Ang2 under normoxic conditions. These findings suggest that COX-2-dependent prostanoids may play an important role in the regulation of hypoxia-induced Ang2 expression.