Induction of calreticulin expression in HeLa cells by depletion of the endoplasmic reticulum Ca²⁺ store and inhibition of N-linked glycosylation

Calreticulin is now considered to be a multifunctional Ca²⁺-binding protein. Its primary role is as a Ca²⁺ storage protein within the lumen of the endoplasmic reticulum (ER), where it also seems to assist in the correct folding and assembly of proteins. We have investigated whether agents that affect these processes can alter calreticulin expression in HeLa cells. Perturbation of intracellular Ca²⁺ levels by prolonged exposure to either thapsigargin or ionomycin induced calreticulin mRNA, both in the presence and absence of extracellular Ca²⁺, consistent with the proposal that sustained depletion of the ER Ca²⁺ store can trigger these increases. The mechanism underlying the induction seems to be transcriptional up-regulation as both agents increased calreticulin promoter-driven firefly luciferase expression in transfected cells to the same degree as the observed increases in calreticulin mRNA. Experiments with a truncated promoter construct showed that the sequences that confer this inducibility reside within the 225 bp immediately upstream of the putative major transcriptional start site. We also examined the effect of tunicamycin, which inhibits N-linked glycosylation in the ER thereby interfering with protein processing. This caused increases in calreticulin mRNA greater than those with either thapsigargin or ionomycin, but failed to transactivate the calreticulin promoter. Thus either additional cis sequences that reside outside our promoter region are necessary for transcriptional activation by tunicamycin, or the increases in calreticulin mRNA occur post-transcriptionally. This suggests that there are probably different mechanisms by which calreticulin expression can be induced in response to agents that affect normal ER functioning.