Selective inhibition of the C5a chemotactic cofactor function of the Vitamin D binding protein by 1,25(OH)2 Vitamin D3

Abstract

The vitamin D binding protein (DBP) is a multifunctional plasma protein that can significantly enhance the chemotactic response to complement fragment C5a. The chemotactic cofactor function of DBP requires cell surface binding in order to mediate this process. The goal of this study was to investigate the effect of ligating DBP with its two primary physiological ligands, vitamin D and Gactin, on both binding to neutrophils and the ability to enhance chemotaxis to C5a. There was no difference in neutrophil binding between of the holo (bound) forms versus the apo (unbound) form of radioiodinated DBP, indicating that the cell binding region of DBP likely is distinct from the vitamin D sterol and G-actin binding sites. Likewise, G-actin, 25(OH)D 3 , and G-actin plus 25(OH) D 3 bound to DBP did not alter its capacity to enhance chemotaxis toward C5a. However, the active form of vitamin D (1,25(OH) 2 D 3 ) completely eliminated the chemotactic cofactor function of DBP. Dose response curves demonstrated that as little as 1 pM 1,25(OH) 2 D 3 significantly inhibited chemotaxis enhancement. Moreover, at physiological concentrations 1,25(OH) 2 D 3 needs to be bound to DBP to mediate the inhibitory effect. Neutrophil chemotaxis to optimal concentrations of C5a, formyl peptide, CXCL8 or leukotriene B 4 was not altered by 1,25(OH) 2 D 3 indicating that the active vitamin does not have a global inhibitory effect on neutrophil chemotaxis. Finally, inhibition of cell surface alkaline phosphatase with sodium orthovanadate completely reversed the inhibitory effect of 1,25(OH) 2 D 3 . These results indicate that the cell binding and co-chemotactic functions of DBP are not altered when the protein binds G-actin and/or vitamin D. Furthermore, the co-chemotactic signal from DBP can be eliminated or counteracted by 1,25(OH) 2 D 3 .

Vitamin-D deficiency is associated with chronic bacterial colonisation and disease severity in bronchiectasis

“…The relationship between VDBP in the airway and neutrophil mediated inflammation provides a potential mechanism for the link between vitamin-D and disease severity. Shah et al 30 demonstrated that treatment with vitamin-D prevented C5a/VDBP co-chemotactic activity, suggesting a mechanism whereby treatment with vitamin-D could reduce neutrophil recruitment. Our results extend the observations of Wood et al 17 who showed, in populations of patients with α-1-antitrypsin deficiency and chronic obstructive pulmonary disease (COPD), an association between high levels of VDBP and lung function impairment.…”

Behind the scenes of vitamin D binding protein: More than vitamin D binding

“…In contrast to DBP, DBP-actin complexes are not chemotactic for and do not activate human neutrophils [107]. The binding of 1,25(OH) 2 -vitamin D 3 to DBP abolishes the chemotactic cofactor function for human neutrophils [108] and oleic acid is one of the tonic inhibitors of chemotaxis in human plasma [109]. Due to its fatty acid binding capacity, DBP could scavenge the inhibitory oleic acid [100].…”

Identification of two distinct cell binding sequences in the vitamin D binding protein

“…Although SPR usually is employed to quantitate the interaction between two purified molecules, it also can be used to measure the relative binding of cells (suspended in media) to an immobilized ligand [28]. The human myeloid cell line U937 was utilized since these cells grow in suspension and previous data has shown that DBP binding to U937 cells essentially is identical to neutrophils obtained from peripheral blood [29, 30]. In addition, we have recently demonstrated that neutrophil binding to immobilized DBP using SPR [31] is almost identical to that of U937 cells reported herein.…”