BMK1/ERK5 Is a Novel Regulator of Angiogenesis by Destabilizing Hypoxia Inducible Factor 1α

Abstract

Abstract-Big MAP kinase 1 (BMK1 or ERK5) is a key mediator of endothelial cell (EC) function as shown by impaired embryonic angiogenesis and vascular collapse in BMK1 knockout mice. Hypoxia inducible factor 1␣ (HIF1␣), a potent mediator of angiogenesis, is positively regulated by the MAP kinases, ERK1/2. Because BMK1 deficiency is associated with impaired angiogenesis we hypothesized that BMK1 might regulate HIF1␣. To test this hypothesis, bovine lung microvascular ECs (BLMECs) were transfected with HIF1␣ and BMK1 cDNAs, and stimulated by hypoxia. HIF1␣ activity was measured by a reporter gene assay in which luciferase expression was driven by HIF1␣ activation. Understanding the specific mechanisms that contribute to each step of the angiogenic response may offer the best approach to developing therapies that will augment angiogenesis in diseases such as stroke and myocardial infarction or inhibit angiogenesis in cancer. An important master regulator of angiogenesis (especially induced by hypoxia) is hypoxia inducible factor-1␣ (HIF1␣), which controls critical proangiogenic genes such as vascular endothelial growth factor (VEGF). [3][4][5][6] Targeted inactivation of HIF1␣ in mice results in abnormal vascular development and embryonic lethality, demonstrating its key role in the angiogenic processes. 7 Multiple mechanisms regulate HIF1␣ including both transcriptional and posttranslational processes. In EC, posttranslational mechanisms include both protein degradation (mediated by proline hydroxylases and ubiquitination) and protein phosphorylation. 8 -11 Previous studies suggest that HIF1␣ function is regulated by mitogen activated protein (MAP) kinases. 6,12,13 Specifically it was shown that ERK1/2 was activated by hypoxia, phosphorylated HIF1␣ directly, and enhanced HIF1␣ transcriptional activity as measured by VEGF expression. 6,12 The mechanisms by which ERK1/2 stimulates HIF1␣ transcriptional activity remain unknown because HIF1␣ phosphorylation by ERK1/2 did not change HIF1␣ protein stability. However, the ERK1 knockout mouse did not exhibit any defects in angiogenesis. 14 In contrast, several recent articles 15-17 suggest a critical role for the closely related MAP kinase, BMK1, in angiogenesis. Sohn et al showed in the BMK1 knockout mouse that initial vascularization occurred normally but subsequent remodeling and maintenance of the vasculature was adversely affected. 15 A similar result was reported by Hayashi et al who showed in a conditional BMK1 knockout that vascular integrity was compromised and EC apoptosis increased. 16 We hypothesized that the abnormalities in BMK1 knockout vasculature may reflect BMK1-mediated regulation of HIF-1␣. This hypothesis derived from an article showing that VEGF expression was upregulated in BMK1 knockout embryonic fibroblasts and overexpression of BMK1 cDNA inhibited hypoxia induced VEGF promoter activity. 15 The authors suggested that the phenotype of BMK1 null mice was possibly because of disregulation of HIF activity, which disrupted normal vasculogenesis and an...

ERK5 regulation in naïve T‐cell activation and survival

“…Inducible deletion of ERK5 in adult mice also results in death, which correlates with disintegration of the blood vessels. In the same study, endothelial-specific deletion of ERK5 demonstrated an essential role for ERK5 in the survival of these cells in the embryo [23], a finding in line with previous cell-based studies [24,25]. Roles for ERK5 in neuronal and cardiac development have also been proposed [26,27], however, both neuronal and cardiac development are superficially normal in ERK5 conditional knockouts [23] (Wang and Tournier, unpublished data).…”

Global Gene Expression Analysis of ERK5 and ERK1/2 Signaling Reveals a Role for HIF-1 in ERK5-mediated Responses

“…Two previous studies have shown that ERK5 is phosphorylated in response to low oxygen tension and also functions to promote angiogenesis (79,80). Interestingly, both reports showed that ERK5 activation inhibited hypoxia-induced vascular endothelial growth factor expression, most likely by suppression of HIF-1␣ (79,80). These studies, which were carried out in bovine lung microvascular endothelial cells and mouse embryonic fibroblast cells, showed behavior opposite that of the RPE system used in our study, where ERK5 instead promotes HIF-1/HRE signaling.…”

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