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Vitamin D3 is metabolized in the liver to 25-hydroxyvitamin D3 (25-OH-D3) and then in the kidney to 1α, 25-dihydroxyvitamin D3 (1α,25-(OH)2D3). The action of vitamin D3 to mobilize calcium and phosphate from intestine and bone is thought to be mediated by the 1α,25-(OH)2D3 metabolite and therefore this sterol is considered to be the hormonal form of vitamin D (1). 1α,25-(OH)2D3 qualifies as a mineral regulating hormone based upon observations that its biosynthesis appears to be regulated by the calcium and phosphorus status of animals (2) and its biochemical action on target tissues such as intestine resembles the functioning of classic steroid hormones in their respective target organs (3). Thus 1α,25-(OH)2D3 stimulates intestinal calcium absorption through a mechanism involving transport of the hormone to the cell nucleus by a cytoplasmic receptor protein, enhancement of nuclear ENA synthesis and ultimate induction of functional proteins such as calcium binding protein (4, 5, 6).

1,25-Dihydroxyvitamin D3 receptors are cytosoluble proteins detectable in a variety of tissues responsive to 1,25(OH)2D3. They are DNA binding proteins analogous to other steroid receptors and it is this functional property which is likely involved in the activation of hormone-sensitive genes. Utilizing 1,25(OH)2D3 and DNA binding assays, as well as antireceptor monoclonal antibodies, we have probed the relationship between the 1,25(OH)2D3 receptor binding domains after selective cleavage with trypsin. These studies reveal that the hormone and DNA binding regions are separable, and are consistent with the finding that tissue resistance to 1,25(OH)2D3 is a result of structural defects in these domains. Recently, a primate model, the LLC-MK2 monkey kidney line, has been uncovered which may exemplify a hormone-binding defect. Here, 25hydroxyvitamin D3-24-hydroxylase induction, a 1,25(OH)2D3 bioresponse, requires 100-fold higher concentrations of the hormone for maximal response. Concomitantly, this cell contains a variant receptor form which displays a correspondingly lowered apparent affinity for the hormone despite its seemingly normal DNA binding characteristics. Taken together, these studies suggest that the 1,25(OH)2D3 receptor is a macromolecule with multiple domains each of which may produce modified cellular resistance to 1,25(OH)2D3 if structurally altered.

Unique cellular responses to steroid hormones occur through the latter’s capacity to alter the expression of certain genes (1). An analogous series of biochemical processes has been postulated to unfold during the action of the renal vitamin D3 hormone, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) (2,3). Accordingly, 1,25(OH)2D3 associates with an intracellular receptor protein, whereupon, via a largely unknown mechanism thought to involve genomic DNA, it modifies the transcriptional activity of hormone-sensitive genes (3). Receptor proteins for 1,25(OH)2D3 have been the recent focus of intense research, largely because it is clear that knowledge of their function is the key to understanding both the mechanism of vitamin D3 action as well as the molecular abnormalities which underlie tissue resistant disease states (4). This chapter describes our view of the physical and functional properties of 1,25(OH)2D3 receptors and documents certain cellular events which appear to be integral to the molecular action of this calcium regulating hormone.

We have previously reported (1,2) that a group of patients with idiopathic hypercalciuria (IH) exhibited certain metabolic characteristics. Compared with age- and sex-matched control subjects, the IH patients were normocalcemic, but their serum phosphate and immunoreactive parathyroid hormone (PTH) concentrations were lower, serum 1,25-dihydroxyvitamin D (1,25-diOHD) concentration, urinary calcium excretion, and fractional enteral calcium absorption were higher. Even though their serum phosphate concentration was lower than that of the controls, urinary phosphate excretion was normal in the IH patients due to a lower TTP/GFR (2).