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.