79 © Springer International Publishing Switzerland 2017 R. Meidan (ed.), The Life Cycle of the Corpus Luteum, DOI 10.1007/978-3-319-43238-0_5 Chapter 5 Steroid Hormone Receptors in the Corpus Luteum Robert Rekawiecki, Magdalena K. Kowalik, and Jan Kotwica R. Rekawiecki • M.K. Kowalik • J. Kotwica (*) Department of Physiology and Toxicology of Reproduction, Institute of Animal Reproduction and Food Research, The Polish Academy of Sciences, Olsztyn 10-748, Poland E-mail: r.rekawiecki@pan.olsztyn.pl; m.kowalik@pan.olsztyn.pl; j.kotwica@pan.olsztyn.pl Abstract The function of the corpus luteum (CL) is to produce progesterone (P4), which is the main regulator of estrous cycle duration and creates suitable conditions for embryo implantation and development. The CL also synthesizes moderate amounts of estradiol (E2). The action of these steroid hormones on target cells are evoked by specific nuclear receptors that belong to the family of receptor-dependent transcrip- tion factors. The physiological effect of P4 upon target cells is mediated through inter- action of this hormone with nuclear progesterone receptor (PGR) isoforms A (PGRA) and B (PGRB) and that of E2 through the alpha (ERα) and beta (ERβ) receptors. Steroids may also affect cells through a nongenomic mechanism, which involves the membrane steroid-binding proteins such as the progesterone receptor membrane com- ponent (PGRMC) 1 and 2 and the membrane progestin receptors (mPR) alpha (mPRα), beta (mPRβ), and gamma (mPRγ), and the G protein-coupled estrogen receptor (GPR30). These proteins rapidly activate the appropriate intracellular signal transduc- tion pathways, and subsequently they can initiate specific cell responses or modulate genomic cell responses. The diversity of nuclear and membrane steroid hormone receptors enhances their regulatory influence on the CL function. Keywords Corpus luteum • Progesterone receptor • Estradiol receptor • Steroid receptor isoforms