Regulation of vitamin D metabolism: factors influencing the rate of formation of 1,25-dihydroxycholecalciferol by kidney homogenates.
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ABSTRACT:
SUBMITTER: Colston KW
PROVIDER: S-EPMC1177880 | biostudies-other | 1973 Jul
REPOSITORIES: biostudies-other
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Project description:Many factors influence the production of 1,25(OH)2D3 (1,25-dihydroxycholecalciferol) by the kidney. One important factor seems to be feedback regulation by 1,25(OH)2D3 itself. Administration of 1,25(OH)2D3 to vitamin D-deficient chicks abolishes renal 25(OH)D3(25-hydroxycholecalciferol)1-hydroxylase activity and induces the appearance of 25(OH)D3 24-hydroxylase activity. It is likely that these effects are mediated via a nuclear effect, as they are prevented by pretreatment with actinomycin D and alpha-amanitin. Further, 1,25(OH)2D3 has a marked effect on gene transcription in the kidney cell, as assessed by measurement of RNA polymerase activities. RNA polymerase I and II activities are 80-90% inhibited by 12.5nmol of 1,25(OH)2D3 within 30min of subcutaneous administration, indicating an immediate and massive decrease in total gene transcription. By 4h RNA polymerase II activity has returned to control values, but RNA polymerase I activity is markedly enhanced. These results are consistent with the view that regulation of cholecalciferol metabolism in the kidney is associated with an effect of the active metabolite on the kidney nucleus.
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Project description:Primary cultures of chick kidney cells convert 25-hydroxycholecalciferol into more-polar metabolites. Cells from vitamin D-deficient chicks have high 25-hydroxycholecalciferol 1 alpha-hydroxylase (1 alpha-hydroxylase) activity, but no 25-hydroxycholecalciferol 24-hydroxylase (24-hydroxylase) activity. Physiological concentrations of 1,25-dihydroxycholeclaciferol suppress 1 alpha-hydroxylase and induce 24-hydroxylase activity. The inhibition of 1 alpha-hydroxylase preceded the induction of 24-hydroxylase. In contrast, oestradiol-17 beta had no effect on the activity of either hydroxylase under a variety of experimental conditions. These results clearly demonstrate that 1,25-dihydroxycholecalciferol, but not oestrogen, acts directly on the kidney cells to regulate the metabolism of 25-hydroxycholecalciferol.
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Project description:Stimulation of intestinal calcium transport by the hormone 1,25-dihydroxycholecalciferol appears to involve RNA transcriptions and the synthesis of new proteins. Although one of these proteins has been identified as calcium-binding protein, no RNA molecules specifically induced by the hormone in the nucleus have been identified. Nuclear RNA from intestine of vitamin D-deficient chicks before and at various time intervals after treatment with the hormone or cholecalciferol was tested for its ability to code for calcium-binding protein in a cell-free system. Calcium-binding-protein mRNA could only just be detected in the intestinal nuclei 2h after dosing with these steroids which is the same time that it was first observed in the polyribosomes. Thus 1,25-dihydroxycholecalciferol induces the production of new calcium-binding protein by stimulating the formation and rapid release from the nucleus of new mRNA molecules for this protein. Polyribosomal translation of the mRNA continued only as long as it was being synthesized, and the maximum rate of synthesis following a pulse dose of 125ng of the hormone was the same as that observed after prolonged stimulation with cholecalciferol. The possibility that other 1,25-dihydroxycholecalciferol-dependent events may be occurring in the nucleus in the lag period between accumulation of the hormone in the intestine and the appearance of active calcium-binding-protein mRNA, and that these may ultimately control the synthesis of that mRNA, is discussed.
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Project description:(23S)-23,25-Dihydroxycholecalciferol was converted into a polar metabolite in a calciferol-deficient chick kidney homogenate. The metabolite was identified as (23S)-1,23,25-trihydroxycholecalciferol by absorbance spectroscopy and mass spectrometry, and by formation of derivatives. (23S)-1,23,25-Trihydroxycholecalciferol was also observed as a 1,25-dihydroxycholecalciferol metabolite in intestinal cells isolated from 1,25-dihydroxycholecalciferol-treated rat. The trihydroxy metabolite was 50-fold less potent than 1,25-dihydroxycholecalciferol in the chick intestinal 1,25-dihydroxycholecalciferol receptor assay.
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Project description:The location of intestinal cells taken from the base of the crypt to the tip of the villus responsive to calcitriol (1,25-dihydroxycholecalciferol) and the distribution of [3H]calcitriol within the intestinal epithelium has been determined in vitamin D-deficient rats. The calcitriol responses examined were CaBP (Ca2+-binding protein) levels as measured by immunodiffusion and alkaline phosphatase levels as determined cytochemically. Calcitriol had no effect on villus structure or on enterocyte kinetics. This made it possible to compare levels of CaBP and alkaline phosphatase activity in enterocytes at different ages in rats at known times after hormone injection. Cells from both the crypt and villus synthesized CaBP in response to calcitriol. Alkaline phosphatase activity was not detectable in crypt cells, although a pool of precursor was produced in these cells in response to calcitriol. Enzyme activity was increased in all villus cells in response to calcitriol, but the quantitative description of this effect was very different from that found for calcitriol effects on CaBP synthesis. Calcitriol injected into vitamin D-deficient rats was detected, within 2h, in all cells of the crypt and villus. Most of the binding was to sites having a high affinity for the injected hormone.
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Project description:The synthesis of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] and 24,25-dihydroxycholecalciferol [24,25(OH)2D3] from 25-hydroxycholecalciferol [25(OH)D3] has previously been shown to occur in cells isolated from bone. The main findings of the present study are that the enzyme systems which catalyse these syntheses are: (1) active at 'in vitro' substrate concentrations over the range of 2-50 nM; (2) regulatable in a complex way by 1,25(OH)2D3, 24,25(OH)2D3, 25,26-dihydroxycholecalciferol and 25(OH)D3, but not by cholecalciferol ('vitamin D3'); and (3) have relatively short half-lives (approx. 5 h).
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Project description:A specific 1,25-dihydroxycholecalciferol-binding protein has been detected in high-salt cytosols prepared from human medullary thyroid carcinomas. The binding protein had the same equilibrium dissociation constant (Kd = 0.17 +/- 0.05 nM; n = 4) and sedimentation coefficient on sucrose gradients (3.7S) as than seen in established vitamin D target tissues. This protein was not detected in normal thyroid cytosols, which may reflect the low proportion of C-cells within the gland.
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Project description:BACKGROUND Our kidney transplant waitlist includes 20% re-transplantations (TX2). Knowing what to expect is a clinical obligation. MATERIAL AND METHODS We compared graft and patient survival of all 162 TX2 patients, transplanted 2000 to 2009, with 162 patients after first transplantation (TX1) matched for age, sex, living/non-living donation, and transplantation date. Patient follow-up was 10 years. RESULTS TX2 graft and patient survivals were inferior to TX1 (p<0.001 and p=0.047). TX2 patients had a longer cumulative dialysis vintage, more human leucocyte antigen (HLA) mismatches, more panel-reactive HLA antibodies, more often received induction therapy with rabbit-antithymocyte globulin (rATG), and had a lower body mass index (all p<0.05). Death from infection and graft failure by rejection was more frequent after TX2 (both p<0.05) but not after TX1. Multivariable Cox regression analysis revealed that both cohorts had graft failure and death risk associated with infection and cardiovascular disease, and graft failure by humoral rejection. However, only TX2 patients had an additional risk of graft failure with early inferior graft function and of patient death with ≥2 comorbidities. Moreover, Kaplan-Meier analysis showed that TX2 and not TX1 patients had a lower graft and patient survival associated with infection and with ≥2 comorbidities (all p<0.05). CONCLUSIONS Re-transplantation is associated with worse graft outcomes mainly because of immunologic and graft-quality reasons, although the high number of comorbidities and infection severities aside from cardiovascular disease drive mortality. The more frequent rATG induction of TX2 patients could promote infection by enhancing immunosuppression. By addressing comorbidities, outcomes could possibly be improved.
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