Receptor mechanisms of rapid extranuclear signalling initiated by steroid hormones

In addition to their role as direct regulators of gene transcription mediated by classical nuclear hormone receptors, steroid hormones have also been described to exert rapid effects on intracellular signalling pathways independent of gene transcription. This chapter focuses on recent advances in our understanding of the receptors and mechanisms that mediate these rapid signalling actions of oestrogens and progesterone. Increasing evidence suggests that at least some of these rapid actions are mediated by a subpopulation of the classical nuclear oestrogen receptor (ER) and progesterone receptor (PR) that localize to the cytoplasm or associate with the plasma membrane. Human PR has been shown to mediate rapid progestin activation of the Src/Ras/Raf/mitogen-activated protein kinase signalling pathway in mammalian cells by a direct interaction with the Src homology 3 domain of Src tyrosine kinases through a Pro-Xaa-Xaa-Pro-Xaa-Arg motif located in the N-terminal domain of the receptor. Moreover, this is an extranuclear action of PR that is separable from its direct transcriptional activity. Additionally, a novel membrane protein unrelated to nuclear PR was recently identified that has properties of a G-protein-coupled receptor for progesterone and has been shown to be involved in mediating the extranuclear signalling actions of progesterone that promotes oocyte maturation in fish. The role of this membrane PR (mPR) in mammalian cells is less clear and the relationship of the membrane and classical nuclear PR in mediating rapid non-transcriptional signalling of progestins has not been explored. To date, a novel membrane ER unrelated to classical nuclear receptors has not been cloned and characterized, and many of the known rapid extranuclear signalling actions of oestrogen appear also to be mediated by a subpopulation of nuclear ER, or a closely related receptor. A novel protein termed modulator of non-genomic activity of ER (MNAR) has been identified that acts as an adaptor between ER and Src, and thus provides a mechanisms for coupling of oestrogen and ER with rapid oestrogen-induced activation of Src and the downstream mitogen-activated protein kinase signalling cascade. The physiological relevance of rapid extranuclear signalling by the classical ER has been provided by experiments showing that these actions contribute to the anti-apoptotic effect of oestrogen in bone in vivo and to the rapid effects of oestrogen on vasodilation and protection of endothelial cells against injury.