Calcium-sensitive transcriptional activation of the proximal CCAAT regulatory element of the grp78/BiP promoter by the human nuclear factor CBF/NF-Y

Abstract

Transcription of the gene encoding GRP78/BiP, a calcium-binding molecular chaperone localized in the endoplasmic reticulum, is induced in mammalian cells through gradual depletion of the intracellular calcium stores. The multimeric CCAAT binding factor, CBF/NF-Y, binds to the most proximal CCAAT regulatory element (C1) of the grp78 promoter required for both basal level expression and stress response. Using an in vitro transcription system, we show through factor competition and immunodepletion that the grp78 C1-mediated enhancement of transcription requires primarily CBF. Correlating with the previous observation that CBF binding to the 78C1 site is enhanced by EGTA and EDTA, these divalent cation chelators specifically stimulate 78Cl-directed transcription. In contrast, increasing amounts of calcium ions are inhibitory. These results provide evidence that CBF is functionally important in transactivating the grp78 C1 transcriptional activity, and suggest a possible mechanism by which grp78 transcription is stimulated by calcium depletion. We further discovered that in addition to binding CBF, both the 78C1 element and the CBF binding site of the α2(I) collagen promoter interact weakly with the multifunctional transcription factor YY1. Our studies show that the binding sites for CBF and YY1 are distinct for the two promoter sites, suggesting that YY1 and other interacting factors could exert differential effects on individual promoters bearing the same CBF site.