While experimental evidence demonstrates that estrogen protects vascular cells, clinical trials on hormone replacement therapies (HRT) fail to report cardiovascular benefits. This discrepancy may indicate that estrogen signaling during HRT may not be fully effective in vascular cells, possibly due to the way of delivering estrogens to vascular tissues. We therefore, tested whether a different kinetics of exposure of endothelial cells to estrogens may alter the balance between transcriptional and non-transcriptional signaling.
Methods and results: Cultured human umbilical vein endothelial cells (HUVEC) were exposed to equal amounts of 17beta-estradiol administered transiently (7 nM for 1 h) or continuously (0.29 nM for 24 h), using endothelial nitric oxide synthase (eNOS) expression and activity as functional endpoints. Transient and continuous administrations equally increased eNOS expression, implying that a short contact between hormone and cells is sufficient to trigger genomic pathways. In agreement, in ovariectomized rats pulse-like changes in plasma estradiol or stable levels equally increased aortic eNOS. More importantly, we extensively show that transient estradiol administration results in preferential recruitment of non-genomic pathways, leading to more vivid activation of MAPK and phosphatidylinositol-3 kinase (PI3K) cascades.
Conclusions: Our data suggest that the kinetics of contact of estrogen with the cells could represent a sensor for estrogen receptor (ER) signaling towards non-genomic or genomic pathways. These findings deepen our understanding of estrogen signaling, and have important implications for the design of safer and more effective HRT.