Dysregulation of renal vitamin D metabolism in the uremic rat
2010; Elsevier BV; Volume: 78; Issue: 5 Linguagem: Inglês
10.1038/ki.2010.168
ISSN1523-1755
AutoresChristian Helvig, Dominic Cuerrier, Christopher M. Hosfield, Breanna Ireland, Aza Z. Kharebov, Jae‐Weon Kim, Navindra J. Ramjit, Kara Ryder, Samir P. Tabash, Andrew M. Herzenberg, Tina Epps, Martin Petkovich,
Tópico(s)Pharmacological Effects and Toxicity Studies
ResumoThe progressive decline in kidney function and concomitant loss of renal 1α-hydroxylase (CYP27B1) in chronic kidney disease (CKD) are associated with a gradual loss of circulating 25-hydroxyvitamin D3 (25(OH)D3) and 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3). However, only the decrease in 1α,25(OH)2D3 can be explained by the decline of CYP27B1, suggesting that insufficiency of both metabolites may reflect their accelerated degradation by the key catabolic enzyme 24-hydroxylase (CYP24). To determine whether CYP24 is involved in causing vitamin D insufficiency and/or resistance to vitamin D therapy in CKD, we determined the regulation of CYP24 and CYP27B1 in normal rats and rats treated with adenine to induce CKD. As expected, CYP24 decreased whereas CYP27B1 increased when normal animals were rendered vitamin D deficient. Unexpectedly, renal CYP24 mRNA and protein expression were markedly elevated, irrespective of the vitamin D status of the rats. A significant decrease in serum 1α,25(OH)2D3 levels was found in uremic rats; however, we did not find a coincident decline in CYP27B1. Analysis in human kidney biopsies confirmed the association of elevated CYP24 with kidney disease. Thus, our findings suggest that dysregulation of CYP24 may be a significant mechanism contributing to vitamin D insufficiency and resistance to vitamin D therapy in CKD. The progressive decline in kidney function and concomitant loss of renal 1α-hydroxylase (CYP27B1) in chronic kidney disease (CKD) are associated with a gradual loss of circulating 25-hydroxyvitamin D3 (25(OH)D3) and 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3). However, only the decrease in 1α,25(OH)2D3 can be explained by the decline of CYP27B1, suggesting that insufficiency of both metabolites may reflect their accelerated degradation by the key catabolic enzyme 24-hydroxylase (CYP24). To determine whether CYP24 is involved in causing vitamin D insufficiency and/or resistance to vitamin D therapy in CKD, we determined the regulation of CYP24 and CYP27B1 in normal rats and rats treated with adenine to induce CKD. As expected, CYP24 decreased whereas CYP27B1 increased when normal animals were rendered vitamin D deficient. Unexpectedly, renal CYP24 mRNA and protein expression were markedly elevated, irrespective of the vitamin D status of the rats. A significant decrease in serum 1α,25(OH)2D3 levels was found in uremic rats; however, we did not find a coincident decline in CYP27B1. Analysis in human kidney biopsies confirmed the association of elevated CYP24 with kidney disease. Thus, our findings suggest that dysregulation of CYP24 may be a significant mechanism contributing to vitamin D insufficiency and resistance to vitamin D therapy in CKD. Vitamin D insufficiency is commonly observed in patients with chronic kidney disease (CKD) and is causally related to secondary hyperparathyroidism, a disorder characterized by elevated serum-intact parathyroid hormone (iPTH) levels, parathyroid gland hyperplasia and imbalances in bone and mineral metabolism.1.DeLuca H.F. Overview of general physiologic features and functions of vitamin D.Am J Clin Nutr. 2004; 80: 1689S-1696SPubMed Google Scholar, 2.Horl W.H. The clinical consequences of secondary hyperparathyroidism: focus on clinical outcomes.Nephrol Dial Transplant. 2004; 19: V2-V8Crossref PubMed Scopus (84) Google Scholar, 3.Holick M.F. Vitamin D for health and in chronic kidney disease.Semin Dial. 2005; 8: 266-275Crossref Scopus (137) Google Scholar Low vitamin D levels have also been linked to the pathogenesis of other diseases related to CKD, including diabetes,4.Mathieu C. Gysemans C. Giulietti A. et al.Vitamin D and diabetes.Diabetologia. 2005; 48: 1247-1257Crossref PubMed Scopus (470) Google Scholar hypertension,5.Barri Y.M. Hypertension and kidney disease: a deadly connection.Curr Hypertens Rep. 2008; 10: 39-45Crossref PubMed Scopus (43) Google Scholar and obesity.6.Sarafidis P.A. Obesity, insulin resistance and kidney disease risk: insights into the relationship.Curr Opin Nephrol Hypertens. 2008; 17: 450-456Crossref PubMed Scopus (44) Google Scholar, 7.Ting S.M. Nair H. Ching I. et al.Overweight, obesity and chronic kidney disease.Nephron Clin Pract. 2009; 112: c121-c127Crossref PubMed Scopus (55) Google Scholar External factors, such as lack of sunlight and inadequate vitamin D intake, are recognized as important factors contributing to vitamin D insufficiency in CKD patients;8.Taskapan H. Wei M. Oreopoulos D.G. 25(OH) vitamin D3 in patients with chronic kidney disease and those on dialysis: rediscovering its importance.Int Urol Nephrol. 2006; 38: 323-329Crossref PubMed Scopus (39) Google Scholar however, disturbances in the regulation of key cytochrome P450 enzymes involved in the synthesis (1α-hydroxylase; CYP27B1) and catabolism (24-hydroxylase; CYP24) of vitamin D metabolites may also be implicated. Vitamin D3 is synthesized in human skin from 7-dehydrocholesterol after ultraviolet light exposure and is metabolized in the liver to form the prohormone, 25-hydroxyvitamin D3 (25(OH)D3). Circulating 25(OH)D3 provides substrate for conversion to the biologically active hormone 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3) by 1α-hydroxylase CYP27B1 primarily expressed in renal proximal and distal convoluted tubules.9.Kawashima H. Torikai S. Kurokawa K. Localization of 25-hydroxyvitamin D3 1 alpha-hydroxylase and 24-hydroxylase along the rat nephron.Proc Natl Acad Sci USA. 1981; 78: 1199-1203Crossref PubMed Scopus (138) Google Scholar, 10.Zehnder D. Bland R. Walker E.A. et al.Expression of 25-hydroxyvitamin D3-1alpha-hydroxylase in the human kidney.J Am Soc Nephrol. 1999; 10: 2465-2473PubMed Google Scholar Although kidneys produce the bulk of circulating hormones, extra-renal expression of CYP27B1 is thought to be important for localized production of 1α,25(OH)2D3.11.Zehnder D. Bland R. Williams M.C. et al.Extrarenal expression of 25-hydroxyvitamin d3-1 alpha-hydroxylase.J Clin Endocrinol Metab. 2001; 86: 888-894Crossref PubMed Scopus (791) Google Scholar, 12.Bikle D. Extra renal synthesis of 1,25-dihydroxyvitamin D and its health implications.Clinic Rev Bone Miner Metab. 2009; 7: 114-125Crossref Scopus (45) Google Scholar The effects of 1α,25(OH)2D3 are mediated by the vitamin D receptor expressed in target organs, including those involved in the maintenance of calcium/phosphate homeostasis and normal bone mineralization, immunomodulation, as well as the regulation of cell growth and differentiation, insulin secretion, cardiovascular function, and blood pressure regulation.13.Bikle D. Nonclassic actions of vitamin D.J Clin Endocrinol Metab. 2009; 94: 26-34Crossref PubMed Scopus (616) Google Scholar Vitamin D insufficiency observed in CKD is associated with morbidity, which extends well beyond compromised bone and mineral metabolism,14.Chocano-Bedoya P. Ronnenberg A.G. Vitamin D and tuberculosis.Nutr Rev. 2009; 67: 289-293Crossref PubMed Scopus (73) Google Scholar, 15.Deeb K.K. Trump D.L. Johnson C.S. 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Beaulieu M. et al.Variability and risk factors for kidney disease progression and death following attainment of stage 4 CKD in a referred cohort.Am J Kidney Dis. 2008; 52: 661-671Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar, 20.Negri A.L. Association of oral calcitriol with improved survival in non-dialysed and dialysed patients with CKD.Nephrol Dial Transplant. 2009; 24: 341-344Crossref PubMed Scopus (6) Google Scholar Declining renal mass and concomitant loss of renal CYP27B1 capacity in CKD are commonly associated with reductions in circulating levels of both 1α,25(OH)2D3 and 25(OH)D3.21.Mawer E.B. Taylor C.M. Backhouse J. et al.Failure of formation of 1,25-dihydroxycholecalciferol in chronic renal insufficiency.Lancet. 1973; 1: 626-628Abstract PubMed Scopus (151) Google Scholar, 22.Satomura K. Seino Y. Yamaoka K. et al.Renal 25-hydroxyvitamin D3-1-hydroxylase in patients with renal disease.Kidney Int. 1988; 34: 712-716Abstract Full Text PDF PubMed Scopus (21) Google Scholar However, observations of low serum 1α,25(OH)2D3 have not been consistently linked with decreases in renal CYP27B1 expression, as levels of CYP27B1 mRNA may in some cases remain unchanged in CKD patients deficient in 1α,25(OH)2D3.23.Zehnder D. Quinkler M. Eardley K.S. et al.Reduction of the vitamin D hormonal system in kidney disease is associated with increased renal inflammation.Kidney Int. 2008; 74: 1343-1353Abstract Full Text Full Text PDF PubMed Scopus (120) Google Scholar Moreover, diminishing CYP27B1 expression levels cannot directly account for the progressive loss of 25(OH)D3. These findings suggest that additional intrinsic mechanisms may underlie declining vitamin D metabolites, namely, 25(OH)D3 and 1α,25D(OH)2D3, in renal disease. Apart from disturbances in 1α,25(OH)2D3 synthesis, accelerated catabolism may also have a role in lowering circulating 1α,25(OH)2D3 and 25(OH)D3 levels in CKD patients. The mitochondrial cytochrome P450 enzyme CYP24 has a unique role in the catabolism of both 1α,25(OH)2D3 and 25(OH)D3.24.Akiyoshi-Shibata M. Sakaki T. Ohyama Y. et al.Further oxidation of hydroxycalcidiol by calcidiol 24-hydroxylase. A study with the mature enzyme expressed in Escherichia coli.Eur J Biochem. 1994; 224: 335-343Crossref PubMed Scopus (136) Google Scholar, 25.Beckman M.J. Tadikonda P. Werner E. et al.Human 25-hydroxyvitamin D3-24-hydroxylase, a multicatalytic enzyme.Biochemistry. 1996; 35: 8465-8472Crossref PubMed Scopus (174) Google Scholar Deletion of the CYP24 gene significantly increases the half-lives of circulating 1α,25(OH)2D3 and 25(OH)D3 and renders CYP24-null animals hypersensitive to vitamin D, thus confirming the importance of CYP24 in vitamin D homeostasis.26.St-Arnaud R. Arabian A. Travers R. et al.Deficient mineralization of intramembranous bone in vitamin D-24-hydroxylase-ablated mice is due to elevated 1,25-dihydroxyvitamin D and not to the absence of 24,25-dihydroxyvitamin D.Endocrinology. 2000; 141: 2658-2666Crossref PubMed Scopus (201) Google Scholar, 27.Masuda S. Byford V. Arabian A. et al.Altered pharmacokinetics of 1alpha,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 in the blood and tissues of the 25-hydroxyvitamin D-24-hydroxylase (Cyp24a1) null mouse.Endocrinology. 2005; 146: 825-834Crossref PubMed Scopus (135) Google Scholar Normally, CYP24 protein seems to be most abundant in the proximal tubule of the kidney, with lower expression observed in distal segments.28.Iwata K. Yamamoto A. Satoh S. et al.Quantitative immunoelectron microscopic analysis of the localization and induction of 25-hydroxyvitamin D3 24-hydroxylase in rat kidney.J Histochem Cytochem. 1995; 43: 255-262Crossref PubMed Scopus (17) Google Scholar, 29.Yang W. Friedman P.A. 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Given the important functional role of CYP24 in tightly regulating the biological activity of 1α,25(OH)2D3 and 25(OH)D3, overexpression of this enzyme in kidney can also have a significant impact on vitamin D status. To determine whether CYP24 and CYP27B1 expression is altered in uremia, we investigated the regulation of these enzymes in normal and adenine-induced uremic rats, as well as in renal biopsy tissue from patients with kidney disease. Our findings suggest that dysregulation of CYP24 may be a significant mechanism contributing to vitamin D insufficiency and resistance to vitamin D therapy in CKD. The effects of uremia on the expression of renal CYP24 and CYP27B1 mRNA and protein were examined using the adenine rat model of CKD. Previous studies using adenine-treated rats have shown that this model exhibits all key features of CKD pathology, including elevated creatinine, iPTH and fibroblast growth factor 23 (FGF23), hypocalcemia, hyperphosphatemia, and reduced serum 1α,25(OH)2D3.40.Katsumata K. Kusano K. Hirata M. et al.Sevelamer hydrochloride prevents ectopic calcification and renal osteodystrophy in chronic renal failure rats.Kidney Int. 2003; 64: 441-450Abstract Full Text Full Text PDF PubMed Scopus (128) Google Scholar, 41.Terai K. Mizukami K. Okada M. Comparison of chronic renal failure rats and modification of the preparation protocol as a hyperphosphataemia model.Nephrology (Carlton). 2008; 13: 139-146Crossref PubMed Scopus (21) Google Scholar, 42.Terai K. Nara H. Takakura K. et al.Vascular calcification and secondary hyperparathyroidism of severe chronic kidney disease and its relation to serum phosphate and calcium levels.Br J Pharmacol. 2009; 156: 1267-1278Crossref PubMed Scopus (38) Google Scholar Uremia in adenine-treated rats was evident from elevated mean serum creatinine levels of 1.86±0.20 mg/dl compared with 0.39±0.17 mg/dl in normal rats (P<0.001). Plasma iPTH and serum FGF23 levels were elevated in uremic rats, serum calcium was decreased, serum phosphorus was increased (Table 1), and serum 1α,25(OH)2D3 declined (59.20±9.80 pg/ml nonuremic vs 15.20±3.16 pg/ml uremic; P<0.01; Figure 1d). Although serum 25(OH)D3 levels remained unchanged (23.90±2.09 ng/ml nonuremic vs 25.90±1.90 ng/ml uremic; Figure 1e) 1 week after adenine treatment, a decline of about 20% in 25(OH)D3 levels was observed at 6 and 8 weeks after treatment compared with normal control animals (Figure 1f). These findings are consistent with the accelerated elimination of 25(OH)D3, raising the possibility that elevated CYP24 may have a role in declining vitamin D status in CKD patients.Table 1Biochemical parameters measured in normal vs uremic ratsNonuremic vehicleUremic vehicleUremic +1α,25(OH)2D3Creatinine (mg/dl)0.39±0.17 (8)1.86±0.20 (10)***(P<0.001); significant from uremic vehicle1.38±0.06 (5)**(P<0.01)iPTH (pg/ml)199±91.3 (10)521±158.4 (10)29.6±17.3 (7)†(P<0.05)FGF23 (ng/ml)0.41±0.02 (10)21.80±10.3 (8)138±39.2 (7)***(P<0.001); significant from uremic vehicle(P<0.01)Calcium (mg/dl)11.04±0.24 (10)9.72±0.10 (10)**(P<0.01)13.96±0.41 (7)***(P<0.001); significant from uremic vehicle†(P<0.001).Phosphorus (mg/dl)10.41±0.16 (10)14.24±0.98 (10)*(P<0.05)19.04±2.01 (7)***(P<0.001); significant from uremic vehicle†(P<0.05)Abbreviations: FGF23, fibroblast growth factor 23; iPTH, intact parathyroid hormone; 1α,25(OH)2D3, 1α,25-dihydroxyvitamin D3.Significant from nonuremic vehicleData are presented as mean±s.e.m.; (n) denotes sample size.* (P<0.05)** (P<0.01)*** (P<0.001); significant from uremic vehicle† (P<0.05)†† (P<0.01)††† (P<0.001). Open table in a new tab Abbreviations: FGF23, fibroblast growth factor 23; iPTH, intact parathyroid hormone; 1α,25(OH)2D3, 1α,25-dihydroxyvitamin D3. Significant from nonuremic vehicle Data are presented as mean±s.e.m.; (n) denotes sample size. Examination of renal mRNA revealed a greater than fivefold increase in CYP24 expression after adenine treatment (P<0.001; Figure 1a). Consistent with this finding, an increased CYP24 protein expression was also observed in uremic kidneys (Figure 1b). CYP27B1 mRNA expression increased nearly twofold in uremic kidney (Figure 1a; P<0.01). Concordant with mRNA, CYP27B1 protein expression was clearly elevated in uremic kidney (Figure 1b), indicating that translation of CYP27B1 mRNA was not impaired in this model.43.Yuan B. Xing Y. Horst R.L. et al.Evidence for abnormal translational regulation of renal 25-hydroxyvitamin D-1alpha-hydroxylase activity in the hyp-mouse.Endocrinology. 2004; 145: 3804-3812Crossref PubMed Scopus (23) Google Scholar We next investigated the regulatory effect of 1α,25(OH)2D3 on these enzymes in uremia. It is well established that 1α,25(OH)2D3 treatment induces CYP24 and attenuates CYP27B1 expression in vitamin D target tissues, including kidney.9.Kawashima H. Torikai S. Kurokawa K. Localization of 25-hydroxyvitamin D3 1 alpha-hydroxylase and 24-hydroxylase along the rat nephron.Proc Natl Acad Sci USA. 1981; 78: 1199-1203Crossref PubMed Scopus (138) Google Scholar, 28.Iwata K. Yamamoto A. Satoh S. et al.Quantitative immunoelectron microscopic analysis of the localization and induction of 25-hydroxyvitamin D3 24-hydroxylase in rat kidney.J Histochem Cytochem. 1995; 43: 255-262Crossref PubMed Scopus (17) Google Scholar, 44.Henry H.L. Regulation of the hydroxylation of 25-hydroxyvitamin D3 in vivo and in primary cultures of chick kidney cells.J Biol Chem. 1979; 254: 2722-2729Abstract Full Text PDF PubMed Google Scholar In the uremic kidney, CYP24 mRNA levels were approximately threefold greater than levels of CYP27B1 (Figure 1c). Administration of 1α,25(OH)2D3 markedly increased the expression of CYP24 by approximately 12-fold relative to CYP27B1 expression, which increased only slightly (Figure 1c). Administration of 1α,25(OH)2D3 (0.50 μg/kg) to uremic rats increased mean serum 1α,25(OH)2D3 (15.20±3.16 pg/ml uremic to 78.0±12.09 pg/ml uremic+1α,25(OH)2D3; P<0.001; Figure 1d), reduced serum 25(OH)D3 (25.90±1.90 ng/ml uremic to 18.10±2.53 ng/ml uremic+1α,25(OH)2D3; P<0.05; Figure 1e), improved creatinine and iPTH, and elevated calcium, FGF23 and phosphorus (Table 1). To examine the effect of vitamin D status and uremia on CYP24 and CYP27B1 expression, rats were fed either a normal or vitamin D-deficient diet and treated with adenine or vehicle through oral gavage. Adenine-gavaged animals exhibited serum chemistries comparable to those observed in adenine-diet treated animals (data not shown).41.Terai K. Mizukami K. Okada M. Comparison of chronic renal failure rats and modification of the preparation protocol as a hyperphosphataemia model.Nephrology (Carlton). 2008; 13: 139-146Crossref PubMed Scopus (21) Google Scholar Serum 1α,25(OH)2D3 and 25(OH)D3 levels fell below the limit of detection in rats fed a vitamin D-deficient diet independent of renal status (Table 2). As expected, uremic rats fed a normal diet exhibited elevated levels of renal CYP24 mRNA (Figure 2a; P<0.05). Renal CYP24 mRNA levels in nonuremic vitamin D-deficient rats dropped to approximately 25% of those observed in nonuremic rats fed a normal diet, whereas CYP27B1 mRNA levels more than doubled (Figure 2a and b). Unexpectedly, CYP24 mRNA remained significantly elevated in vitamin D-deficient uremic rats (Figure 2b; P<0.01). Moreover, CYP24 protein expression was augmented in vitamin D-deficient renal tissue from uremic animals (Figure 2c). Changes in vitamin D status did not induce any appreciable changes in CYP27B1 mRNA levels in uremic rats (Figure 2b).Table 2Serum levels of vitamin D metabolites in nonuremic or uremic rats fed a normal or vitamin D-deficient dietNonuremic vehicleUremic vehicleNormalVDDNormalVDD1α,25(OH)2D3 (pg/ml)250±49.5 (8)BLD (7)140±65.0 (4)BLD (9)25(OH)D3 (ng/ml)14.4±2.92 (10)BLD (10)19.2±2.99 (4)BLD (9)Abbreviations: BLD, below the limit of detection; VDD, vitamin D-deficient diet; 1α,25(OH)2D3, 1α,25-dihydroxyvitamin D3; 25(OH)D3, 25-hydroxyvitamin D3.Data are presented as mean±s.e.m.; (n) denotes sample size. Open table in a new tab Abbreviations: BLD, below the limit of detection; VDD, vitamin D-deficient diet; 1α,25(OH)2D3, 1α,25-dihydroxyvitamin D3; 25(OH)D3, 25-hydroxyvitamin D3. Data are presented as mean±s.e.m.; (n) denotes sample size. Histological examination of kidney tissue showed no pathological abnormalities in periodic acid-Schiff- (Figure 3a), hematoxylin and eosin- (Figure 3b), and trichrome-stained sections analyzed using light microscopy in nonuremic rats, irrespective of vitamin D status (Figure 3A and B). However, uremic (Figure 3aC and bC) and uremic vitamin D-deficient (Figure 3aD and bD) rats showed marked intraluminal tubular deposition of brown adenine crystals. This deposition was accompanied by marked interstitial inflammation, interstitial fibrosis, and acute tubular injury on periodic acid-Schiff stain (Figure 3aC and aD). Glomeruli seemed diffusely ischemic, with shrunken glomerular tufts and thickened corrugated glomerular capillary walls. Hematoxylin and eosin staining showed that the interstitial inflammatory infiltrate consists of mixed infiltrating leukocytes, including mononuclear cells and occasional neutrophils (Figure 3bC and bD). On trichrome staining, there was marked interstitial fibrosis in uremic and uremic vitamin D-deficient rats (data not shown). This was seen in parallel to severe tubular dilatation, microcystic change, and foci of tubular atrophy. Normal control and nonuremic vitamin D-deficient rats showed proximal tubular staining of CYP24 (2+) that was predominantly located along the cell membrane at the apical (luminal) aspect of epithelial cells (Figure 4a and b). There was no significant glomerular or vascular staining for CYP24. In contrast, kidneys from uremic and uremic vitamin D-deficient rats showed not only apical staining of proximal tubular epithelial cells (2+) but also cytoplasmic staining of the same cells (2+ to 3+) (Figure 4c and d). In addition, thick ascending distal tubular epithelial cells showed immunoperoxidase staining (2+) in these groups of animals. Kidney biopsy samples from patients with CKD and normal age-matched controls were examined for CYP24 renal expression by immunohistochemistry. CYP24 staining was highly localized to the apical membrane (1+ to 2+) in proximal tubules in control tissue (Figure 5aA), whereas CKD tissue showed marked (2+ to 3+) and diffuse cytoplasmic staining in the proximal tubular (Figure 5aB), as well as cortical (Figure 5bB) and medullary (Figure 5cB) distal tubules in six of eight biopsy samples. In most biopsy samples, mural staining in the interlobular arteries was evident and seemed to be intracytoplasmic in arterial wall smooth muscle cells. Interstitial staining in three of eight biopsy samples was focally present (data not shown). The intensity of immunostaining of CYP24 was not associated with the clinical parameters outlined in Table 3.Table 3Biochemical parameters measured in patients with CKDData are presented as mean±s.e.m.; (n) denotes sample size.Clinical characteristicsControlCKDPhosphorus (mg/dl)3.37±0.34 (8)3.78±0.28 (8)Calcium (mg/dl)9.64±0.32 (8)9.30±0.48 (8)Creatinine (mg/dl)1.20±0.2 (8)2.40±0.40 (8)*(P<0.05).Abbreviations: CKD, chronic kidney disease. Measured in serum; significant from control* (P<0.05). Open table in a new tab Abbreviations: CKD, chronic kidney disease. Measured in serum; significant from control Vitamin D insufficiency is highly prevalent in CKD patients and may contribute significantly to the morbidity14.Chocano-Bedoya P. Ronnenberg A.G. Vitamin D and tuberculosis.Nutr Rev. 2009; 67: 289-293Crossref PubMed Scopus (73) Google Scholar, 15.Deeb K.K. Trump D.L. Johnson C.S. Vitamin D signalling pathways in cancer: potential for anticancer therapeutics.Nat Rev Cancer. 2007; 7: 684-700Crossref PubMed Scopus (1039) Google Scholar, 16.Raghuwanshi A. Joshi S.S. Christakos S. Vitamin D and multiple sclerosis.J Cell Biochem. 2008; 105: 338-343Crossref PubMed Scopus (56) Google Scholar and mortality associated with this disease.17.Inaguma D. Nagaya H. Hara K. et al.Relationship between serum 1,25-dihydroxyvitamin D and mortality in patients with pre-dialysis chronic kidney disease.Clin Exp Nephrol. 2008; 12: 126-131Crossref PubMed Scopus (43) Google Scholar, 18.Kovesdy C.P. Ahmadzadeh S. Anderson J.E. et al.Association of activated vitamin D treatment and mortality in chronic kidney disease.Arch Intern Med. 2008; 168: 397-403Crossref PubMed Scopus (251) Google Scholar, 19.Levin A. Djurdjev O. Beaulieu M. et al.Variability and risk factors for kidney disease progression and death following attainment of stage 4 CKD in a referred cohort.Am J Kidney Dis. 2008; 52: 661-671Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar, 20.Negri A.L. Association of oral calcitriol with improved survival in non-dialysed and dialysed patients with CKD.Nephrol Dial Transplant. 2009; 24: 341-344Crossref PubMed Scopus (6) Google Scholar Although reduced exposure to sunlight and vitamin D intake are important factors contributing to insufficiency, disruptions in the synthesis and catabolism of 25(OH)D3 and 1α,25(OH)2D3 may also have considerable etiological roles. Altered expression or activity of CYP27B1 has previously been implicated in reduced blood levels of 1α,25(OH)2D3 in patients21.Mawer E.B. Taylor C.M. Backhouse J. et al.Failure of formation of 1,25-dihydroxycholecalciferol in chronic renal insufficiency.Lancet. 1973; 1: 626-628Abstract PubMed Scopus (151) Google Scholar, 22.Satomura K. Seino Y. Yamaoka K. et al.Renal 25-hydroxyvitamin D3-1-hydroxylase in patients with renal disease.Kidney Int. 1988; 34: 712-716Abstract Full Text PDF PubMed Scopus (21) Google Scholar and animal models of CKD;45.Murayama A. Takeyama K. Kitanaka S. et al.Positive and negative regulations of the renal 25-hydroxyvitamin D3 1alpha-hydroxylase gene by parathyroid hormone, calcitonin, and 1alpha,25(OH)2D3 in intact animals.Endocrinology. 1999; 140: 2224-2231Crossref PubMed Google Scholar however, recent evidence suggests that aberrant CYP27B1 expression cannot entirely account for low 1α,25(OH)2D3 levels or explain 25(OH)D3 depletion in kidney disease.23.Zehnder D. Quinkler M. 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