Membrane Estrogen Receptors in Human Spermatozoa: An Example of a Non-Classic Steroid Receptor Located in the Membrane

Abstract

Estrogens have been described to act through binding to their specific receptors (ERα and ERβ) localized at the cytosolic and nuclear level. Ligand binding results in estrogen receptor (ER) release from the bound heat shock proteins and induces ER dimerization. ER dimers exist in three different isoforms: the homdimers α/α and β/β and the heterodimers α/β. Upon dimerization, the estradiol (E₂)-ER complex translocates to the cellular nucleus were it interacts through DNA binding domain with specific DNA sequences regulating transcription of target genes (Fig.l). Recently, in contrast to these well known genomic effects at the transcriptional level mediated by the classical nuclear receptors ERα and ERβ. rapid effects of estrogens arising between seconds to few minutes from stimulation, have been described in different cellular models [for review see 1–3]. Indeed, both 17βE₂ and its cell-impermeant conjugate E₂-BSA induce activation of intracellular second messengers such as calcium, nitric oxide formation, and activation of kinases, in neuronal, vascular and bone systems [1–3] (Fig.l). Such rapid nongenomic effects are initiated at the plasma membrane, but the nature and characteristics of the receptor(s) involved is still a matter of debate [4].