Estrogen acts via receptors

Abstract

Since estrogen belongs to the family of steroids and estrogen receptors are part of the steroid receptor superfamily the following summary applies to steroid receptors, in general. As introduced, in contrast to protein hormones, which act via membranous hormone receptors, steroid molecules readily cross the cellular membrane because of their high lipophilicity. Although diffusion is believed to be the main mechanisms of the penetration, for certain steroids and in certain tissues a facilitated diffusion has been proposed. Early investigations have addressed the question whether estrogen’s entry into cells of the uterus is not a simple diffusion but is rather a protein-mediated facilitated process (Milgrom et al., 1973). Indeed, such a facilitated or enhanced uptake process would introduce an additional level of regulation of estrogen activity since it would be the “transport-protein” that controls the entry of the steroids into the cells.