Global gene expression analysis of ERK5 and ERK1/2 signaling reveals a role for HIF-1 in ERK5-mediated responses.

ERK5 is a recently characterized MAPK, which is most similar to the well studied ERK1/2 subfamily but uses distinct mechanisms to elicit responses. To understand the specificity of signaling through ERK5 versus ERK1/2, we examined global gene expression changes in response to each pathway. Microarray measurements in retinal pigment epithelial cells revealed 36 genes regulated by ERK5, all which were novel targets for this pathway. 39 genes were regulated by ERK1/2, which included 11 known genes. Of these genes, 19 were regulated by both pathways. Inspection of the 17 genes uniquely regulated by ERK5 revealed that 14 genes (82%) were previously associated with hypoxia via regulation by HIF-1. In contrast, 16 genes (84%) regulated by either ERK5 or ERK1/2 were implicated in hypoxia, most through mechanisms independent of HIF-1. Of the 20 genes regulated by ERK1/2, only 9 were implicated in hypoxia and were not well characterized hypoxia targets. Thus, unlike ERK5, a mechanistic link between ERK1/2 and HIF-1/HRE could not be established on the basis of gene regulation. Activation of both pathways enhanced transcription from a hypoxia-response element and increased HIF-1alpha protein expression. In contrast, ERK5 but not ERK1/2 elevated transcription through GAL4-HIF-1. Most interestingly, ERK5 is not significantly activated by hypoxia in retinal pigment epithelial cells, indicating that ERK5 regulation of these genes is relevant in normoxia rather than hypoxia. Thus, ERK5 and ERK1/2 differ in their mechanisms of gene regulation, and indicate that ERK5 may control hypoxia-responsive genes by a mechanism independent of HIF-1alpha expression control.