miRNA networks modulate human endothelial cell adaptation to cyclic hypoxia.

Solid tumor microenvironments are often subjected to various levels of hypoxia. Although regulation of gene expression has been examined extensively, most studies have focused on prolonged hypoxia. The tumor microenvironment, however, experiences waves of hypoxia and reoxygenation that stimulate the expression of pro-angiogenic factors that promote blood vessel formation. In this study, we examined human umbilical vascular endothelial cells (HUVECs) under waves of intermittent (cyclic) hypoxia to determine how this process compares to prolonged hypoxia, and more importantly, how this influences the microRNA profiles that potentially affect the posttranscriptional regulation of angiogenic genes. The rationale for these studies is that cancer cells subjected to cyclic hypoxia appear to have increased metastatic potential and endothelial cells exhibit a higher radiation resistance and greater migration potential. This indicates that gene regulatory networks in cyclic hypoxia may be different from prolonged hypoxia. Here we examined the consequences of cyclic hypoxia on miRNA gene expression and how these changes in miRNA expression influence angiogenesis. Using Next Generation Sequencing, our results demonstrate that cyclic hypoxia has very different effects on the miRNA networks compared to prolonged hypoxia, and that the in silico predicted effects on the certain mRNA target genes are more similar than might be expected. More importantly, these studies indicate that identifying potential miRNAs (including hsa-miR-19a-5p) as therapeutic targets for inhibiting angiogenesis and tumor progression will require this type of physiologically relevant analysis.