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.

miR-200b downregulates Kruppel Like Factor 2 (KLF2) during acute hypoxia in human endothelial cells.

The role of microRNAs in controlling angiogenesis is recognized as a promising therapeutic target in both cancer and cardiovascular disorders. However, understanding a miRNA's pleiotropic effects on angiogenesis is a limiting factor for these types of therapeutic approaches. Using genome-wide next-generation sequencing, we examined the role of an antiangiogenic miRNA, miR-200b, in primary human endothelial cells. The results indicate that miR-200b has complex effects on hypoxia-induced angiogenesis in human endothelia and importantly, that many of the reported miR-200b effects using miRNA overexpression may not be representative of the physiological role of this miRNA. We also identified the antiangiogenic KLF2 gene as a novel target of miR-200b. Our studies indicate that the physiological changes in miR-200b levels during acute hypoxia may actually have a proangiogenic effect through Klf2 downregulation and subsequent stabilization of HIF-1 signaling. Moreover, we provide a viable approach for differentiating direct from indirect miRNA effects in order to untangle the complexity of individual miRNA networks.