| Summary: | Cellular messenger RNAs (mRNAs) exist almost exclusively in the context of ribonucleoprotein complexes (RNPs), which are largely responsible for the coordinated regulation of mRNA fate, and in particular, the post-transcriptional regulation of mRNA stability and translation. RNA- binding proteins, antisense RNAs, and microRNAs represent three major classes of post- transcriptional regulatory factors that interact with target mRNAs. Significantly, these interactions are dynamically regulated under both basal and stress conditions, such as hypoxia. Given the prominent contributions of post-transcriptional regulation to overall gene expression, a more comprehensive understanding of the underlying mechanisms is required.
In this thesis, we present exciting new evidence for the functional importance of post- transcriptional gene regulation, especially in the vascular endothelium. Firstly, we show that the formation of hnRNP E1-containing RNPs contributes significantly to the remarkable basal stability of endothelial nitric oxide synthase (eNOS) mRNAs in endothelial cells by protecting them from inhibitory post-transcriptional forces. However, hypoxia impairs such RNP formation through hnRNP E1 serine phosphorylation and nuclear localization. Together, these mechanisms contribute significantly to decreased eNOS expression and activity in chronic hypoxia.
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Secondly, we reveal an important functional relationship between the microRNA pathway and the HIF-mediated cellular hypoxic response. Specifically, the down-regulation of Dicer and an important number of Dicer-dependent microRNAs in chronic hypoxia represents an important adaptive mechanism that serves to maintain the cellular hypoxic response through HIF-α- and microRNA-dependent mechanisms, with significant implications for the development of RNAi- based therapies. Finally, we provide evidence that the up-regulation of specific microRNAs in acute hypoxia is a potentially important mechanism that serves to suppress global translation initiation in order to conserve energy and ensure cellular survival.
Collectively, the findings presented in this thesis provide important new mechanistic insight into the post-transcriptional regulation of eNOS, as well as the functional integration of the microRNA and the cellular hypoxic response pathways.
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