Membrane-Initiated Steroid Actions and the Proteins that Mediate Them

Membrane-initiated or nongenomic activities of steroids contribute to the effects of steroids on a wide variety of target organs, including those with low receptor numbers, recently discovered due to the increased sensitivity of new measurement techniques. Membrane-initiated responses are the cell's rapid and first response to steroids. The responses that emanate from the membrane can have direct functional consequences, such as secretion of other peptide hormones or rapid behavioral changes. Other rapid responses are prerequisites for subsequent genomic responses. The wide variety of signal transduction schemes employed by various tissues and hormones are summarized and discussed in terms of the identity of proteins that mediate these responses. Probable mixed binding systems for steroids in plasma membranes are compared to similar multiple hormone-binding protein systems in extracellular fluids and inside cells. These issues are related to steroid-dependent tumor growth, developmental and therapeutic apoptosis, and the actions of endocrine disrupters. The integration of membrane-initiated effects with genomic mechanisms results in the complete cellular response to steroids.