Osteopathy and resistance to vitamin D toxicity in mice

Vitamin D binding protein (DBP), also known as the group-specific component of serum (Gc-globulin), is a member of the albumin, α-fetoprotein, and α-albumin multigene family (1, 2). DBP is a highly polymorphic serum protein predominantly synthesized in the liver as a single-chain glycoprotein of ∼58 kDa (3). The serum concentration of human DBP is 4–8 μΜ and its serum halflife is 2.5–3 days. DBP has a high-affinity binding site for 25(OH)D (5 × 108 M−1), the major circulating form of vitamin D that is generated by 25-hydroxylation of vitamin D in the liver. This site also binds 1,25(OH)2D, the active form of the vitamin, as well as the parental vitamin D itself, both with somewhat lower affinity (4 × 107 M−1) (4). Vitamin D sterols are necessary to maintain normal serum calcium homeostasis and bony development. Deficiency of vitamin D results in the bone diseases of osteomalacia and rickets, diseases characterized by formation of poorly calcified and structurally impaired bones. DBP has several biological activities in addition to its ability to bind vitamin D. DBP binds avidly to G-actin (2 × 109 M−1) via a binding domain in its carboxy terminus (5, 6); this binding can sequester circulating monomeric G-actin, preventing polymerization into F-actin after cellular trauma (7). DBP can activate macrophages in vitro (8) and enhances C5a-mediated chemotaxis (9). A definitive approach to testing the multiple function(s) of DBP in vivo, and most particularly its role in vitamin D metabolism and action, has not been available. The role of DBP in vitamin D action is not defined. However, it is likely that it functions in accordance with the “free-hormone” hypothesis. This hypothesis suggests that plasma sterols are trapped in the vascular compartment via their association with a corresponding set of serum binding proteins. These complexes provide an available reservoir of sterol hormones; the sterols can be made available to cells by their dissociation from the binding protein (10). Additional roles of the binding proteins might include facilitation of sterol/steroid entry into the cell, as suggested by the identification of cell surface receptors for certain serum steroid binding proteins (11, 12); facilitation of its intracellular bioactivity and gene activation; or protection from excess free hormone (13). The highly polymorphic structure of DBP has led to its use as a serum protein marker for population-genetics studies. Sera from many thousands of individuals from all five continents have been studied for variations in the electrophoretic mobility of DBP (14). Despite this intensive analysis, no individuals have been identified who are null for DBP. This observation has led to the suggestion that one or more functions of DBP may be essential to human viability (4). In this report, a mouse line lacking DBP was successfully established by targeted disruption of the endogenous murine DBP gene.