Calreticulin modulates the in vitro DNA binding but not the in vivo transcriptional activation by peroxisome proliferator-activated receptor/retinoid X receptor heterodimers

Calreticulin is a ubiquitous calcium binding/storage protein found primarily in the endoplasmic reticulum. Calreticulin has been shown to inhibit DNA binding and transcriptional activation by glucocorticoid and androgen hormone receptors by binding to the conserved sequence KXFF(K/R)R, present in the DNA-binding domains of all known members of the steroid/nuclear hormone receptor superfamily. To determine whether calreticulin might be a general regulator of hormone-responsive pathways, we examined its effect on DNA binding in vitro and transcriptional activation in vivo by heterodimers of the peroxisome proliferator-activated receptor (PPAR) and the 9-cis retinoic acid receptor (RXR alpha). We show here that purified calreticulin inhibits the binding of PPAR/RXR alpha heterodimers and of other nuclear hormone receptors, to peroxisome proliferator-responsive DNA elements in vitro. However, overexpression of calreticulin in transiently transfected cultured cells had little or no effect on transactivation mediated by PPAR/RXR alpha. Therefore, while calreticulin inhibits the binding of both nuclear and steroid hormone receptors to cognate response elements in vitro, our findings suggest that calreticulin does not necessarily play an important role in the regulation of all classes of hormone receptors in vivo.