17β-Estradiol-induced interaction of estrogen receptor a and human atrial essential myosin light chain modulates cardiac contractile function

Chronic increased workload of the human heart causes ventricular hypertrophy, re-expression of the atrial essential myosin light chain (hALC-1), and improved contractile function. Although hALC-1 is an important positive inotropic regulator of the human heart, little is known about its regulation. Therefore, we investigated the role of the sex hormone 17 beta-estradiol (E2) on hALC-1 gene expression, the underlying molecular mechanisms, and the impact of this regulatory process on cardiac contractile function. We showed that E2 attenuated hALC-1 expression in human atrial tissues of both sexes and in human ventricular AC16 cells. E2 induced the nuclear translocation of estrogen receptor alpha (ER alpha) and hALC-1 in AC16 cells, where they cooperatively regulate the transcriptional activity of hALC-1 gene promoter. E2-activated ER alpha required the estrogen response element (ERE) motif within the hALC-1 gene promoter to reduce its transcriptional activity (vehicle: 15.55 +/- 4.80 vs. E2: 6.51 +/- 3.69; similar to 2 fold). This inhibitory effect was potentiated in the presence of hALC-1 (vehicle: 11.13 +/- 3.66 vs. E2: 2.18 +/- 1.10; similar to 5 fold), and thus, hALC-1 acts as a co-repressor of ER alpha-mediated transcription. Yeast two-hybrid screening of a human heart cDNA library revealed that ER alpha interacts physically with hALC-1 in the presence of E2. This interaction was confirmed by CoImmunoprecipitation and immunofluorescence in human atrium. As a further novel effect, we showed that chronic E2-treatment of adult mouse cardiomyocytes overexpressing hALC-1 resulted in reduced cell-shortening amplitude and twitching kinetics of these cells independent of Ca2+ activation levels. Together, our data showed that the expression of hALC-1 gene is, at least partly, regulated by E2/ER alpha, while hALC-1 acts as a co-repressor. The inotropic effect of hALC-1 overexpression in cardiomyocytes can be significantly repressed by E2.