Involvement of c-Fos in signaling grp78 induction following ER calcium release

Release of calcium from the endoplasmic reticulum (ER) signals an increase in transcription of both the early response gene, c-fos, and the late respo nse gene, grp78. We hale used thapsigargin (TG), an ER calcium-ATPase pump inhibitor that induces calcium release from the ER, to investigate the poss ible involvement of c-Fos, a component of the AP-1 transcription factor, in grp78 induction. Tno cell lines with markedly different responses to TG tr eatment were employed: the WEHI7,2 mouse lymphoma line in which TG fails to induce grp78, and the MDA-MB-468 mammary epithelial line in which TG induc es grp78. In WEHI7,2 cells, TG-induced calcium release triggers a rapid inc rease in c-fos mRNA, but the level of c-Fos protein decreases due to degrad ation by the multicatalytic proteasome, C-Fos DeltaC, a proteasome resistan t c-Fos mutant with AP-1 activity similar to that of wild type c-Pos, resto res grp78 induction in WEHI7.2 cells, detected by both Northern hybridizati on and a grp78 promoter-luciferase reporter assay. In MDA-MB-468 cells, TG- mediated calcium release induces a sustained elevation of c-Fos protein tha t precedes grp78 induction. A region of the grp78 promoter containing both ERSE and CORE regions, but missing TRE and CRE regions, is sufficient to me diate induction of reporter luciferase activity, Induction of this reporter was blocked by A-Fos, a dominant negative inhibitor of c-Fos, Also, the in duction of grp78-luciferase reporter activity was inhibited by c-fos antise nse mRNA. In summary, the findings indicate that c-Fos is involved in signa ling grp78 induction following TG treatment, and that grp78 induction is in hibited by proteasome-mediated c-Fos degradation.