Microsomal receptor for XXXX hormones: functional implications for nuclear activity.

@entity172 @entity1420 @entity134 @entity969 @entity342 @entity35 @entity1329
Target tissues for @entity134 hormones are responsive by virtue of and to the extent of their content of functional intracellular receptors. Recent years have seen a shift in considerations of the cellular dynamics and distribution of these receptors, with current views favoring predominant intranuclear localization in the intact cell. This paper summarizes our analyses of the microsomal @entity172 and @entity342 binding capability of @entity35 uterine and ventral prostate tissue, respectively; these studies have revealed a set of high affinity sites that may act as a conduit for @entity172 traversing the cell en route to the nucleus. These sites have many properties in common with cytosolic receptors, with the salient difference of a failure to activate to a more avid DNA-binding form under conditions which permit such activation of cytosolic receptors. The microsomal @entity172 -binding proteins also have appreciable affinity for @entity1420 , another distinction from other known cellular @entity172 receptor species. Various experimental approaches were employed to demonstrate that the microsomal receptors were not simply cytosol contaminants; the most convincing evidence is the recent successful separation of the cytosolic and microsomal forms by differential @entity1329 precipitation. Discrete subfractionation of subcellular components on successive @entity969 gradients, with simultaneous assessments of binding capability and marker enzyme concentrations, indicates that the major portion of the binding is localized within the vesicles of the endoplasmic reticulum free of significant plasma membrane contamination. The microsomal receptors are readily solubilized by extraction with high- or low-salt-containing buffers or with @entity134 . The residual microsomes following such extraction have the characteristics of saturable acceptor sites for cytosolic @entity172 -receptor complexes. The extent to which these sites will accept the cytosolic complexes is equal to the concentration of microsomal binding sites extracted. These observations suggest three possible roles for the microsomal receptor-like proteins: (a) modulation of @entity172 access to nuclear binding sites; (b) formation of functional complexes which diffuse to other extranuclear sites to alter non-genomic cellular processes; (c) regulation of nuclear concentration of @entity172 -receptor complexes by virtue of producing microsomal acceptor sites for uptake of free or loosely associated nuclear complexes, previously thought to exist in the cytoplasm.
