Altered serum concentrations of the major circulating form of vitamin D [25-hydroxycholecalciferol (25D3)] and its active hormone derivative [1,25-dihydroxycholecalciferol (1,25D3)] 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 25D3 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 25D3. In the present study, we utilized Zucker diabetic fatty Rats (ZDF) to demonstrate that renal reuptake of the 25D3-DBP complex was compromised in ZDF animals, which was reflected by a reduction in expression of megalin and Dab2. Moreover, serum levels of both 25D3 and 1,25D3 were reduced, and urinary 25D3, 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.
Investigating the Role of Functional Polymorphism of Maternal and Neonatal Vitamin D Binding Protein in the Context of 25-Hydroxyvitamin D Cutoffs as Determinants of Maternal-Neonatal Vitamin D Status Profiles in a Sunny Mediterranean Region
