Vitamin D homeostasis is compromised due to increased urinary excretion of the 25-hydroxycholecalciferol-vitamin D-binding protein complex in the Zucker diabetic fatty rat

Abstract

[1,25-dihydroxycholecalciferol (1,25D 3)] have been linked to non-insulin-dependent diabetes mellitus (NIDDM). However, a mechanistic basis for this occurrence has not been fully elucidated. Normally, renal reabsorption of vitamin D-binding protein-bound 25D 3 absolutely requires receptor-mediated endocytosis via a receptor complex containing megalin, cubilin, and disabled-2 (Dab2), whereas an absence of megalin or its endocytic partners can lead to a marked urinary loss of 25D and severe vitamin D deficiency. Therefore, we hypothesized that reduced serum vitamin D status in NIDDM may be due to reduced expression of megalin and/or its endocytic partners and increased urinary excretion of protein-complexed 25D 3. In the present study, we utilized Zucker diabetic fatty Rats (ZDF) to demonstrate that renal reuptake of the 25D 3-DBP complex was compromised in ZDF animals, which was reflected by a reduction in expression of megalin and Dab2. Moreover, serum levels of both 25D 3 and 1,25D3 were reduced, and urinary 25D 3, 1,25D3, and DBP excretion were elevated in the ZDF animals compared with their lean controls regardless of vitamin D levels in the diet. Taken together, these are the first reports to our knowledge that associate compromised renal reabsorption of the 25D3-DBP complex with expression of megalin and its endocytic partners in NIDDM, which in turn can lead to compromised vitamin D status. diabetes; kidney OPTIMAL VITAMIN D STATUS HAS BEEN ASSOCIATED with improved long-term health outcomes in cardiovascular disease and cancer, complications that occur at higher incidences in individuals with non-insulin-dependent diabetes mellitus (NIDDM). Poor renal function, also a consequence of poorly controlled type 2 diabetes, results in hypertension, increased epithelial cell damage, and increased risk for cardiovascular disease (3,11). In addition to data from case control studies that indicates that maintenance of optimal serum 25-hydroxycholecalciferol (25D 3 ) concentrations (Ͼ90 nmol/l) is preventative against many types of cancer, research has suggested that optimal vitamin D status may be protective against hypertension and nephron damage through the suppression of renin production in the kidney (13, 36). Furthermore, clinical and epidemiological studies have suggested that type 2 diabetics and individuals with chronic kidney disease are more likely to be in what is considered the suboptimal range (Ͻ80 nmol/l) with respect to serum vitamin D status (7, 15), although the mechanistic basis for this occurrence has not been elucidated. Therefore, fully understanding all factors influencing vitamin D homeostasis in this population may reveal opportunities to improve outcomes and comorbidities associated with type 2 diabetes, especially those with renal complications.Cells of the renal proximal tubule are responsible for reabsorption of the major circulating form of vitamin D (25D 3 ) from the glomerular filtrate and are the primary site of activation of 25D 3 to the active hormone 1,25-dihydroxycholecalciferol ...

Vitamin D insufficiency in neonatal hypoxic–ischemic encephalopathy

“…Renal reabsorption of 25(OH)D-bound DBP occurs in proximal tubular cells upon binding to megalin, a transmembrane receptor found on many cell types, including brain capillary endothelial cells, neurons and astrocytes (27). Ischemia reperfusion down-regulates renal megalin expression (28), which may result in excessive urinary losses of vitamin D and DBP (29). As DBP binds 25(OH)D with higher affinity than 1,25(OH) 2 D, renal injury after HI birth may not have the same effect on serum 1,25(OH) 2 D (12).…”

High glucose upregulates CYP24A1 expression which attenuates the ability of 1,25(OH)2D3 to increase NGF secretion in a rat Schwann cell line RSC96

“…A study has shown that exaggerated renal excretion of vitamin D metabolites and vitamin D binding proteins may induce vitamin D insufficiency in type 2 diabetic rats (Anderson et al, 2010). Another possibility for this vitamin D insufficiency may be due to the increased metabolism by elevated CYP24A1 expression.…”

The Role of Vitamin D in the Metabolic Homeostasis of Diabetic Bone

“…100 In the Zucker rat, a model of T2D, serum concentrations of 25OHD and 1,25(OH) 2 D are also reduced, while urinary 25OHD, 1,25(OH) 2 D and DBP excretion are increased compared to lean controls. 101 Thus, exaggerated renal losses of vitamin D metabolites and the carrier protein, DBP, provide a possible mechanistic explanation in animal models of T1D and T2D by which vitamin D clearance is amplified in diabetes. We can speculate that a similar mechanism may be operative in humans.…”