Calmodulin regulates the transcriptional activity of estrogen receptors - Selective inhibition of calmodulin function in subcellular compartments

The steroid hormone estrogen elicits biological effects in cells by binding to and activating the estrogen receptor (ER). Estrogen binding induces a conformational change in the receptor, inducing nuclear translocation and transcriptional activation of ER. The ubiquitous Ca2+-binding protein calmodulin has been shown to interact directly with ER and enhance its stability. To further elucidate the functional sequelae of the association between calmodulin and ER, we examined the effect on ER transcriptional activation of specifically inhibiting calmodulin. The cell-permeable calmodulin antagonist CGS9343B prevented estrogen-induced transcriptional activation by ER, without altering basal transcription. The inhibition was dose-dependent and independent of the time of estrogen stimulation. To validate these findings, calmodulin function was also neutralized by targeted expression of a specific inhibitor peptide. By inserting localization signals, the inhibitor peptide was selectively targeted to different subcellular domains. Inactivation of calmodulin function in the nucleus virtually eliminated estrogen-stimulated ER transcriptional activation. By contrast, when membrane calmodulin was specifically neutralized, estrogen-stimulated transcriptional activation by ER was only slightly attenuated. Importantly, the inhibitor peptides did not significantly reduce the amount of ER in the cells. Together, these data demonstrate that calmodulin is a fundamental component of ER transcriptional activation.