Vitamin D-binding protein prevents vitamin D deficiency and presents vitamin D for its renal activation.

Two mouse models in which the vitamin D-binding protein (DBP) or megalin, a DBP receptor, were knocked out have recently confirmed that DBP is essential for normal vitamin D homeostasis, and have revealed a major pathway for vitamin D activation (1, 2). DBP is also known as the group-specific component of serum (Gc-globulin). It is a highly polymorphic 58 kDa glycoprotein, which has been used as a serum protein marker in studies of population genetics. Several biological functions have been associated with DBP, which is a member of a gene family including albumin and a-fetoprotein. DBP can prevent polymerization of G-actin into F-actin after traumas by binding and sequestering G-actin, and can activate macrophages and enhance chemotaxis. Most importantly, it binds almost all calcidiol (25-hydroxyvitamin D3) molecules in the circulation and leaves only 0.003% of the metabolite in the free form, whereas calcitriol (1,25dihydroxyvitamin D3) and vitamin D3 are bound with an affinity which is approximately ten times lower (4 × 10 M) (3). According to the ‘free hormone’ hypothesis, the sterols bound to DBP are a reservoir that can be made available to vitamin D receptors or enzymes that modify the sterols by dissociation from the binding protein (4). Although sera from many thousands of individuals have been studied, not a single person has been identified without DBP. Consequently, DBP has been regarded as essential to human survival. However, when Safadi et al. (1) used targeted mutagenesis to generate mice without DBP, normal viability, fertility and fecundity were observed. The mice had normal growth curves and showed no histological or other abnormalities. Analyses of serum revealed that the homozygous knock-out mice (DBP) had no detectable DBP, whereas the heterozygous mice (DBP) had approximately two-thirds the DBP levels of the wild type animals (DBP). On a standard diet, serum levels of calcidiol and calcitriol in DBP mice were approximately 10% and 15% of the DBP mice respectively, whereas the heterozygotes had intermediate levels. Calcium and parathyroid hormone (PTH) levels were not significantly different among the genotypes. In this situation, equilibrium was established between the low total levels of extracellular vitamin D and the activation of the intracellular vitamin D receptor. Increasing or decreasing the supply of vitamin D in the diet changed the situation dramatically. On a vitamin D-deficient diet, serum levels of calcidiol and calcitriol decreased to the limit of detection in both DBP and DBP animals, serum calcium remained constant, but PTH levels doubled in DBP mice compared with DBP mice. Skeletal abnormalities were observed in the DBP mice, with increased osteoblastic activity and undermineralization of the newly synthesized bone matrix as in hypovitaminosis D osteopathy. The hyperparathyroidism induced increased osteolysis. Studies of the serum half-life of calcidiol confirmed that DBP prevented a rapid metabolization and excretion of vitamin D in urine. In the absence of DBP the uptake of cholecalciferol in the liver increased 15-fold and the sterol was preferentially metabolized to watersoluble products that were excreted in the urine. On the other hand, these metabolic pathways protected the DBP animals from the toxic effects of high-level administration of vitamin D. The capacity of the organism to save vitamin D by DBP may be fatal in a situation of vitamin D excess. DBP binding to cell surfaces has been reported. This may be a mechanism that facilitates the entry of vitamin D into target cells. DBP may also slow the release of calcitriol and regulate the level of free sterol available. Safadi et al. (1) showed that in the absence of DBP the kinetics of the mRNA expression of a vitamin D-regulated gene, calbindin-D9k, were more rapid, whereas the level of expression was the same as in DBP animals. The primary role of DBP in the study by Safadi et al. (1) was related to the sequestration of vitamin D sterols in the circulation. Evolution has clearly been in favour of the conservation of vitamin D rather than protecting the organism from vitamin D toxicity. Nykjaer et al. (2) recently showed that DBP is also involved in other aspects of vitamin D metabolism. In the kidneys calcidiol is converted to the more potent calcitriol by 1a-hydroxylation, which is regulated mainly by PTH (5). It has been suggested that calcidiol is absorbed at the basolateral side of the tubules from the small non-protein-bound fraction (3). In the glomerulus, DBP is filtered into the preurine and is reabsorbed by endocytosis in the proximal tubules. It has been shown that megalin, a multifunctional receptor and a member of the low-density European Journal of Endocrinology (1999) 141 321–322 ISSN 0804-4643 H IG H L IG H T