J W Pike

1,25-dihydroxyvitamin D3 inhibits the clonogenic growth of transformed cells via its receptor

Anchorage-independent growth in soft agar is a unique property of transformed cells which is known to be correlated with tumorigenicity. We report here that 1,25-dihydroxyvitamin D3 suppresses colony formation by a number of cultured cancer cell lines in soft agar in a dose dependent manner with an ID50 of 5-7 X 10(-10) M. This effect is also achieved with analogues of 1,25-dihydroxyvitamin D3 in accordance with their binding affinity for the hormones receptor. Only cells with 1,25-dihydroxyvitamin D3 receptor protein are inhibited in their colony formation by vitamin D analogs indicating that the hormone receptor complex may be integrally involved in the in vitro suppression of the anchorage-independent phenotype.

1,25-Dihydroxyvitamin D3 receptors: altered functional domains are associated with cellular resistance to vitamin D3.

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 anti-receptor 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, 25-hydroxyvitamin 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.

Receptor-Mediated Action of the Vitamin D Hormone

1,25-Dihydroxyvitamin D3(1,25(OH)2D3) is now considered to be the active hormonal sterol derived from the sunlight vitamin, vitamin D3. As depicted in Fig. 1, 1,25(OH)2D3 is formed in the kidney according to the calcium and phosphorus needs of the organism (Haussler and McCain, 1977). Its main functions are the stimulation of intestinal calcium and phosphate absorption as well as bone remodeling. In addition to its mineral conservation effects in the kidney, 1,25(OH)2D3 induces a 24-OHase enzyme that appears to initiate a catabolic cascade for side chain cleavage and metabolic elimination of both the hormone (Fig. 1) and its 25(OH)D3 precursor (Chandler et al., 1984). Thus the 1,25(OH)2D3 hormone is dynamic in the sense that its production is controlled by calcium/parathyroid hormone (PTH) and phosphate status, and its biodegradation is self-initiated.