Mechanisms of altered vascular endothelial growth factor expression and angiogenesis due to decreased thioredoxin reductase activity

Deficiency in the micronutrient selenium (Se) contributes to the progression of various cancers. Although many possibilities may explain this association, one of the most intriguing is the observation that Se deficiency increases blood vessel development and the production of vascular endothelial growth factor (VEGF), a potent pro-angiogenic molecule regularly over-expressed in human tumors. Selenium mediates many of its biological activities through incorporation into selenoproteins, such as thioredoxin reductase (TrxR). Decreased activity of this enzyme during Se deficiency may alter endothelial cell (EC) metabolism and induce important changes that contribute to cancer progression. Therefore, the focus of this thesis was to determine the role of TrxR in regulating VEGF and angiogenesis. To accomplish this, we chemically inhibited TrxR in Se-sufficient bovine mammary EC (BMEC) to mimic the state of Se-deficient BMEC, and found that inhibiting TrxR activity in +Se BMEC increased VEGF and VEGF receptor expression, cell migration, proliferation, and angiogenesis to levels similar to those seen during Se deficiency.;Once we confirmed a role for TrxR in altering EC function, we evaluated the effects of TrxR on downstream factors, such as thioredoxin (Trx) and hypoxia-inducible factor-1 (HIF-1), which may be part of the pathway through which TrxR controls VEGF and angiogenesis. We showed that decreased TrxR activity decreases Trx activity and that chemical inhibition of Trx in +Se BMEC causes increased VEGF expression and angiogenesis. The importance of Trx was confirmed by the addition of recombinant Trx to -Se BMEC, which prevented the increases in VEGF and angiogenesis observed in these cells. Since HIF-1 is known to dramatically affect angiogenesis, we examined the regulation of this factor when TrxR or Trx activities were low. We found that reduced TrxR or Trx activity increase the protein levels of HIF-1α, as well as binding to the hypoxia response element, which is required for the activation of HIF-1-dependent genes, such as VEGF. Together, these results provide insight into one mechanism through which Se deficiency affects cancer progression by mediating changes in cells surrounding a tumor. This could significantly improve cancer treatment strategies by acknowledging the importance of the tumor's microenvironment and designing treatments accordingly.