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Selective biological response by target organs (intestine, kidney, and bone) to 1,25-dihydroxyvitamin D3 and two
A W Norman1, I N Sergeev, J E Bishop
1Division of Biomedical Sciences, University of California, Riverside 92521.
Abstract:
The hormonally active form of vitamin D, 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] stimulates biological responses related to calcium homeostasis, cell differentiation, and immunomodulation in many target cells, including leukemic cells. Most of these responses are dependent upon 1 alpha,25(OH)2D3 interaction with a nuclear receptor protein. Structural analogues of 1 alpha,25(OH)2D3 might allow for separation of biological function, avoiding adverse calcemic effects. This report quantitates intestinal calcium absorption, bone calcium resorption, induction of intestinal and renal calcium-binding protein (CaBP), and occupancy of the intestinal and renal nuclear 1 alpha,25(OH)2D3 receptor in vitamin D-deficient chicks after a single dose of 1 alpha,25(OH)2D3, 1 alpha,25-dihydroxyvitamin-16-ene-23-yne-D3 (analogue V), or 22-[m-(dimethylhydroxymethyl)phenyl]-23,24,25,26,27- pentanor-1 alpha-hydroxy-vitamin D3 (analogue EV). The interaction of these compounds with chick intestinal nuclear 1 alpha,25(OH)2D3 receptor and chick plasma vitamin D-binding protein was determined in vitro; analogues V and EV bound 68% and 62% [1 alpha,25(OH)2D3 receptor] and 8% and 13% (vitamin D-binding protein), respectively, as well as 1 alpha,25(OH)2D3 (100%). 1 alpha,25(OH)2D3 doses (0.075-1.2 nmol) generated responses in intestinal calcium absorption, bone calcium resorption, intestinal CaBP, and renal CaBP. When analogue V (1.2-300 nmol) was administered, increases in bone calcium resorption and renal CaBP were noted. However, a significant response in intestinal calcium absorption and intestinal CaBP appeared only after a 300-nmol dose. Unoccupied nuclear 1 alpha,25(OH)2D3 receptor in the intestine and kidney was determined in vivo after doses of 1 alpha,25(OH)2D3, analogue V, or analogue EV. Doses (0.25-6.0 nmol) of 1 alpha,25(OH)2D3 and analogue EV reduced unoccupied receptor to 24% and 59% (intestine) and to 13% and 41% (kidney), respectively. Analogue V (6.0-600 nmol) decreased unoccupied receptor in the kidney. In the intestine analogue V (300-600 nmol) reduced unoccupied receptor only to 75%. These results confirm that some vitamin D analogues can generate selective biological responses and different levels of target organ receptor occupancy.
Insights
New vitamin D analogues show potential for selective biological responses, separating therapeutic effects from adverse calcemic impacts. This research explores their efficacy and receptor binding in chicks.
Area of Science:
- Endocrinology
- Molecular Biology
- Calcium Metabolism
Background:
- The active form of vitamin D, 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3], influences calcium homeostasis, cell differentiation, and immunomodulation.
- These effects are mediated through interaction with a nuclear vitamin D receptor.
- Structural analogues of 1 alpha,25(OH)2D3 are being investigated to achieve therapeutic benefits while minimizing adverse calcemic effects.
Purpose of the Study:
- To quantify the biological responses and receptor occupancy of novel vitamin D analogues in vitamin D-deficient chicks.
- To compare the efficacy of 1 alpha,25(OH)2D3, analogue V, and analogue EV in stimulating calcium absorption, bone resorption, and calcium-binding protein (CaBP) induction.
- To assess the in vitro binding affinities of analogues V and EV to the vitamin D receptor and vitamin D-binding protein.
Main Methods:
- Single-dose administration of 1 alpha,25(OH)2D3, analogue V, or analogue EV to vitamin D-deficient chicks.
- Quantification of intestinal calcium absorption, bone calcium resorption, and induction of intestinal and renal CaBP.
- In vitro assessment of analogue binding to chick intestinal nuclear 1 alpha,25(OH)2D3 receptor and plasma vitamin D-binding protein.
- In vivo determination of unoccupied nuclear 1 alpha,25(OH)2D3 receptor in the intestine and kidney.
Main Results:
- Analogues V and EV exhibited significant binding to the 1 alpha,25(OH)2D3 receptor (68% and 62%, respectively) and lower binding to vitamin D-binding protein (8% and 13%).
- 1 alpha,25(OH)2D3 effectively stimulated calcium absorption, bone resorption, and CaBP induction.
- Analogue V induced bone resorption and renal CaBP at lower doses, but required a higher dose (300 nmol) for intestinal effects.
- Analogue V showed limited reduction of unoccupied intestinal receptor even at high doses, indicating selective action.
Conclusions:
- Novel vitamin D analogues can elicit selective biological responses.
- These analogues demonstrate differential target organ receptor occupancy compared to the parent compound.
- The findings support the development of vitamin D analogues with separated biological functions for therapeutic applications.