Membrane receptors for steroid hormones: a case for specific cell surface binding sites for vitamin D metabolites and estrogens.

Steroid hormones, including vitamin D metabolites and estrogens, activate target cells through specific receptors that discriminate among ligands based upon recognition of distinct structural features. For both classes of ligands, cell surface and nuclear receptors co-exist in many target cells. Upon ligand binding, these receptors generate both rapid and long lasting responses. While the structure of the nuclear receptors and their function as transcriptional activators of specific target genes is generally understood, the identity of the membrane receptors remains elusive. Using pharmacological, functional and biochemical approaches, new insights are being gained into nature of the cell surface receptors for both vitamin D metabolites and estrogens.