Tissue specificity and mechanism of vitamin D receptor up-regulation during dietary phosphorus restriction in the rat.
Dietary phosphorus restriction up-regulates intestinal vitamin D receptor (VDR), but the tissue specificity of the up-regulation and the mechanism of receptor accumulation remain unknown. Therefore, the effects of low phosphorus diet (LPD) on VDR content in intestine, kidney, and splenic monocytes/macrophages were examined. Male Sprague-Dawley rats weighing 50-100 g were fed a normal diet (NPD; 0.6% Ca, 0.65% P) as controls followed by an LPD (0.6% Ca, 0.1% P) for 1-10 days (D1-D10). LPD rapidly decreased serum P levels by D1 from 11.11 +/- 0.19 mg/dl (mean +/- SE) to 4.98 +/- 0.37 mg/dl (n = 9). LPD increased total serum Ca from 10.54 +/- 0.09 mg/dl to 11.63 +/- 0.15, 12.17 +/- 0.15, and 12.39 +/- 0.18 mg/dl by D1, D2, and D3, respectively, and then remained stable. Serum 1,25-(OH)2D3 rapidly increased from 123 +/- 5.4 pg/ml to 304 +/- 35 pg/ml by D1, reached a plateau through D5, and then gradually increased to 464.9 +/- 27.7 pg/ml by D10. Intestinal VDR quantitated by ligand binding assay increased 3.5-fold from 169.6 +/- 13.7 fmol/mg of cytosol protein in rats fed NPD (n = 12) to a peak of 588.3 +/- 141.88 fmol/mg of protein by D3 (n = 6; p < 0.001) and then decreased to a plateau level of 2.5-fold greater than NPD (p < 0.05) during D5 to D10. In contrast, LPD did not up-regulate kidney or splenic monocyte/macrophage VDR. Northern blot analysis showed that intestinal VDR mRNA increased 2-fold by D2 (n = 3) of LPD and then gradually decreased to control levels after D5. In contrast, kidney VDR mRNA levels did not change during the first 5 days of P restriction and then subsequently decreased to 50% of NPD controls. The results of these studies indicate that VDR up-regulation during dietary phosphorus restriction is tissue-specific and that the mechanism of the up-regulation is time-dependent. Acutely (D1-D5), phosphorus restriction up-regulates intestinal VDR through increased VDR gene expression, whereas chronic (D5-D10) phosphorus restriction appears to alter VDR metabolism through nongenomic mechanisms that are consistent with prolongation of the half-life of the receptor. The nature of the tissue-specific regulation of VDR during phosphorus restriction remains to be determined.
Sriussadaporn
S
S
Department of Medicine, University of Chicago Pritzker School of Medicine, Illinois, USA.
Wong
M S
MS
Pike
J W
JW
Favus
M J
MJ
D000818:Animals; D015152:Blotting, Northern; D015153:Blotting, Western; D002118:Calcium / Q000097:blood; D002851:Chromatography, High Pressure Liquid; D007413:Intestinal Mucosa / Q000378:metabolism; D007422:Intestines / Q000187:drug effects; D007668:Kidney / Q000187:drug effects / Q000378:metabolism; D008264:Macrophages / Q000187:drug effects; D008297:Male; D009000:Monocytes / Q000187:drug effects; D010758:Phosphorus / Q000172:deficiency*; D016226:Phosphorus, Dietary / Q000008:administration & dosage; D012333:RNA, Messenger / Q000032:analysis; D011869:Radioligand Assay; D011897:Random Allocation; D051381:Rats; D017207:Rats, Sprague-Dawley; D018167:Receptors, Calcitriol / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D013154:Spleen / Q000166:cytology / Q000187:drug effects; D014018:Tissue Distribution; D015854:Up-Regulation / Q000502:physiology
D016226:Phosphorus, Dietary; D012333:RNA, Messenger; D018167:Receptors, Calcitriol; D010758:Phosphorus; D002118:Calcium