Chemotaxis
Chemotaxis is an important process of neutrophil recruitment during inflammation. DBP has no intrinsic chemotactic effect; however, when bound to complement component 5a (C5a), it enhances C5a-mediated neutrophil and macrophage chemotaxis [17]. The action requires binding to cell surface receptor, which is mediated by the residues 130–149 in the protein [12]. Interestingly, binding to vitamin D metabolites (25[OH]D or 1,25[OH]2D) reduces DBP chemotactic action due to binding site competition [2]. The 3 major polymorphisms (GC1F, GC1S, and GC2) have comparable chemotactic activity.
DBP Serum Concentrations
The relative abundance of DBP in serum permits the use of simple immunochemical techniques for measurements. Radioimmunoassay, rocket immune electrophoresis, single radial immunodiffusion, turbidimetry, nephelometry, and ELISA have been used [2]. Normal DBP levels can vary according to the used biochemical methodology but usually range from 200 to 600 mg/L in healthy subjects [21, 22]. There is no season-, weight-, or age-related variance of DBP levels [12, 23] but a circadian rhythm of DBP serum concentrations has been observed, with lower levels in the morning, followed by a rapid increase until reaching a plateau during the day [18]. The correction of vitamin D deficiency by supplementation does not change DBP levels [24]. The serum concentrations are lower in men than women, possibly due to the estrogen effect on its synthesis [23–25]. Furthermore, a hormonal contraceptive is associated with 13–25% higher concentrations of DBP, but, apparently, there are no differences in free hormone levels [26].
Decreased DBP levels are found in advanced liver disease (decreased production) [27], nephrotic syndrome, and diabetes mellitus with nephropathy (probably due to urinary loss). DBP is filtered through the glomerulus and in normal conditions, is reabsorbed by megalin-mediated endocytosis and catabolized by proximal tubule epithelial cells, reducing the urinary excretion to trace amounts. Acute tubular necrosis and glomerulonephropathies can disturb this process and DBP can be found in urine, as an early biomarker for kidney injury [28]. Lower levels of DBP were also found in inflammatory states, like acute injury or sepsis [29]. Compared to controls, individuals with musculoskeletal inflammatory disorders also showed lower levels of DBP, measured by two-dimensional gel electrophoresis. The reasons for the connection between inflammatory states and decreased DBP levels are still unknown, but it may be speculated that DBP, as a precursor for MAF, could be consumed by the increased formation of MAF and ensuing activation of macrophages and neutrophils at inflammation sites [30].
DPB and Vitamin D Deficiency
Vitamin D deficiency has been described worldwide, defined by 25(OH)D levels, and there is a large discussion about the adequate threshold [31, 32]. Furthermore, the circulating levels of 25(OH)D are strongly related to the amount of the carrier protein, which might be influenced by multiple conditions, as discussed above [33]. In addition, the affinity for the ligand varies between the different polymorphisms, and the relevance of all these aspects in the vitamin D physiology is still being debated. Among the DBP phenotypes, the carriers of the Gc1F allele in homozygosis are more prone to have lower levels of 25(OH)D.
The free hormone hypothesis, valid for diffusible hormones such as steroids and L-thyroxine, suggests that only the unbound or weakly bound (to albumin, for instance) molecules would be able to get into the cells and induce their biological effects. If this is true for vitamin D, the DBP polymorphism, with different affinities for 25(OH)D and 1,25(OH)2D, could modify free hormone availability and, consequently, the biological effects on target tissues [33]. Nevertheless, this hypothesis cannot be easily transferred to vitamin D metabolism. In the apical membrane of renal tubular cells the importance of the endocytic megalin/cubilin pathway for the internalization of the entire complex DBP-25(OH)D from the glomerular filtrate has been demonstrated. When inside the cell, 25(OH)D undergoes the 1α-hydroxylation in the mitochondria, resulting in the production of 1,25(OH)2D, which is subsequently released into circulation, while DBP is degraded [6, 34]. Megalin and cubilin knockout mice lose DBP and 25(OH)D in the urine and develop vitamin D deficiency and bone disease [33]. Megalin was found in other tissues including placenta, mammary gland, and parathyroid glands, sites where the 1α-hydroxylase is also present, but the importance of this pathway for the metabolism of vitamin D outside the kidney remains inconclusive.
In addition, in the experimental absence of DBP in knockout mice, the 1,25(OH)2D concentrations in tissues were equivalent to those found in wild-type counterparts, despite its reduced circulating levels, suggesting that the DBP is not necessary for the internalization of vitamin D metabolites in target tissues [35].
Although surrounded by controversies regarding DBP measurements, Powe et al. [36 ]have reported that as much as 9.9% of variation in 25(OH)D levels is explained by genetic polymorphisms. It could be associated with particularities in the production and/or metabolism of each isoform. Besides its influence in DBP absolute levels, the multiple polymorphisms have also been associated with differences in 25(OH)D absolute and bioavailable levels [36]. The greater affinity of GcF1F isoform to vitamin D metabolites makes it a more efficient transporter, but, on the other hand, less likely to leave them free and bioavailable. It is possible that these features could provide a similar concentration of bioavailable vitamin D metabolites with lower 25(OH)D and DBP serum levels in those with this variant [36, 37]. Multiple genome-wide association studies have shown other SNPs that are related to 25(OH)D concentrations, and they are involved in the vitamin D metabolic pathway [18].
Recent data suggest caution in the interpretation of DPB measurements by immunoassays, because some monoclonal antibodies might not recognize all polymorphic forms of the molecule in the same way. This alert was raised after the publication of Powe et al. [36], who found a remarkable difference in the DBP concentrations between black and white Americans. They found much lower