Disruption of angiogenesis by methyl tert-butyl ether (MTBE) is mediated by a dysregulation of the vascular endothelial growth factor (VEGF) pathway

Vascular endothelial growth factor (VEGF) is an essential mitogenic factor required for normal angiogenesis. In development, the primary inducer of this protein is hypoxia via hypoxia inducible factors (HIFs). Methyl tert-butyl ether (MTBE) was hypothesized to disrupt angiogenesis by altering the HIF-VEGF pathway. Exposure of zebrafish embryos to MTBE caused vascular lesions, specifically pooled blood in the common cardinal vein (CCV), cranial hemorrhages, and abnormal intersegmental vessels (ISV). These lesions occurred throughout development and were preceded by a critical period between 6-somites and Prim-5 stages during which there was a significant decrease in mRNA transcript levels of vegf-a, vegf-c and vegf receptor 2 (vegfr2). Lesions other than those associated with the vasculature were not observed. Embryonic exposure to the two primary metabolites, tert-butyl alcohol and formaldehyde, did not induce vascular lesions, indicating the parent chemical was responsible for the anti-angiogenesis. When embryos were exposed to two structurally related chemicals, ethyl tert-butyl ether iii (ETBE) or tert-amyl methyl ether (TAME), some vascular lesions were observed, but zebrafish also exhibited lesions in the heart, whole body edema, and craniofacial abnormalities. Unlike MTBE, ETBE, and TAME exposure did not significantly alter vegf expression. Of the 3 structurally similar ethers, MTBE appears unique in its ability to target developing endothelial cells. An analysis of the global gene expression changes in zebrafish exposed to MTBE during the critical period identified the cardiovascular system was among the most altered pathways affected by MTBE toxicity. Finally, manipulation of the HIF-VEGF pathway to rescue the specific MTBE-induced vascular lesions, via an over-expression of vegf-a and inhibition of HIF degradation, convincingly demonstrated that MTBE toxicity is mediated by the down regulation of VEGF at a critical time during cardiovascular development. Understanding the underlying mechanisms of angiogenesis is important to developing new therapies used to quell solid tumor growth, enhance wound repair, and reduce diabetes induced vascular damage, among others. Chemicals with anti-angiogenic properties, such as MTBE, can be used to advance the science of angiogenesis in both a disease state and during development.Ph. D.Includes bibliographical referencesby Josephine Ann Bonventr