Receptor-mediated genomic interactions

Receptor-mediated genomic interactions 3.2.1. l,25(OH)2D3 receptor characteristics 

25(OH)2D3 receptor of the chick intestine has been extensively characterized by our laboratory and several others, and its biochemical properties reveal many similarities to classical steroid hormone receptors [5,6], The l,25(OH)2D3 receptor is a protein with a molecular mass of approximately 67000 Da when occupied, unoccupied or in the absence of protease inhibitors. The receptor has a high affinity for l,25(OH)2D3 with a Kd in the range of 1-50 x 10 n M. The specificity of the receptor for binding of l,25(OH)2D3 metabolites and analogues, as determined by competitive binding studies, parallels the biological activity of these compounds. 

Unlike the estrogen, progesterone and glucocorticoid receptors, it appears that the l,25(OH)2D3 steroid-receptor complex does not require a temperature-de-pendent transformation step, i.e., the binding of l,25(OH)2D3 does not appear to result in a dramatic change in the shape and size of the receptor. It has recently been demonstrated, however, that the binding of l,25(OH)2D3 to its receptor exhibits positive cooperativity the binding of one molecule of l,25(OH)2D3 to the re- 

Early experiments demonstrated that actinomycin D and alpha-amanitin could block induction of calbindin-D28K by l,25(OH)2D3 [9]. It was later demonstrated that l,25(OH)2D3 was able to stimulate general RNA synthesis in the chick intestine [11] as well as specifically inducing calbindin-D28K mRNA [12]. In addition, 

25(OH)2D3-stimulated transport of calcium across the intestinal epithelium has been extensively studied in our laboratory. l,25(OH)2D3 can stimulate rapid transport of calcium in vascularly perfused chick duodenal loops [19] before the ap- 

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The mechanism of steroid receptor-mediated changes in transcription has occupied center stage over the last 40 years. Serious attention is only just beginning to be paid to the role of steroid receptors in other, often more rapid, processes that occur within minutes (15, 16), such as the estradiol-mediated activation of eNOS that proceeds in about 5 min (15). These types of responses are frequently called nongenomic to indicate that the steroid receptor does not interact with genomic... 

PTKs can be subdivided into two large families, receptor tyrosine kinases (RTKs) and non-RTKs. The human genome encodes for a total of 90 tyrosine kinases of which 32 are nonreceptor PTKs that can be placed in 10 subfamilies (Fig. 1). All nonreceptor PTKs share a common kinase domain and usually contain several additional domains that mediate interactions with protein-binding partners, membrane lipids, or DNA (Table 1). These interactions may affect cellular localization and the activation status of the kinase or attract substrate proteins for phosphorylation reactions. 

Some recent studies [19-21] suggest that not all of the actions of l,25(OH)2D3 are explained by l,25(OH)2D3 receptor interactions with the genome. Rapid effects of l,25(OH)2D3 on stimulating intestinal calcium transport have been demonstrated which occur too quickly (within 4-6 minutes) to involve genome activation and have led to the hypothesis that some of the actions of l,25(OH)2D3 may be mediated at the membrane or by extranuclear subcellular components. 

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