Normal fibroblasts induce the C/EBP beta and ATF-4 bZIP transcription factors in response to anoxia

Fibroblasts exhibit a complex response to temporary anoxia; this response, which is associated with no loss of viability, includes enhanced secretion of proteases and endonucleases along with an increase in glycolytic metabolism. Physiologically, this response is activated during the early stages of wound healing. We have been investigating the mechanisms by which gene expression is regulated in fibroblasts during conditions of oxygen deprivation. We show that normal fibroblasts in response to anoxia induce several members of the C/EBP and ATF subfamilies of the bZIP (basic/leucine zipper domain) transcription factor class. Although anoxia substantially elevated the levels of the corresponding proteins, we detected only slight changes in the activity of reporters containing consensus bZIP binding sites. In the case of C/EBP beta this was apparently related to the failure to detect an increase in the DNA binding of the induced factor to its consensus binding site. In contrast, we show that anoxia strongly increases ATF-4 or an immunologically related protein's DNA binding activity on a variant site, although this variant sequence by itself is insufficient for anoxia-inducible transactivation. This suggests that anoxic exposure of fibroblasts may promote the formation of variant bZIP proteins in nonpermissive conformations or they require the interaction with additional transcription factors at adjacent promoter elements for functional activity.