Extranuclear Steroid Receptors: Nature and Actions

Rapid effects of steroid hormones result from the actions of specific receptors localized most often to the plasma membrane. Fast-acting membrane-initiated steroid signaling (MISS) leads to the modification of existing proteins and cell behaviors. Rapid steroid-triggered signaling through calcium, amine release, and kinase activation also impacts the regulation of gene expression by steroids, sometimes requiring integration with nuclear steroid receptor function. In this and other ways, the integration of all steroid actions in the cell coordinates outcomes such as cell fate, proliferation, differentiation, and migration. The nature of the receptors is of intense interest, and significant data suggest that extranuclear and nuclear steroid receptor pools are the same proteins. Insights regarding the structural determinants for membrane localization and function, as well as the nature of interactions with G proteins and other signaling molecules in confined areas of the membrane, have led to a fuller understanding of how steroid receptors effect rapid actions. Increasingly, the relevance of rapid signaling for the in vivo functions of steroid hormones has been established. Examples include steroid effects on reproductive organ development and function, cardiovascular responsiveness, and cancer biology. However, although great strides have been made, much remains to be understood concerning the integration of extranuclear and nuclear receptor functions to organ biology. In this review, we highlight the significant progress that has been made in these areas.