Vitamin D Binding Protein and the Biological Activity of Vitamin D

Rene F Chun; Albert Shieh; Carter Gottlieb; Vahe Yacoubian; Jeffrey Wang; Martin Hewison; John S Adams

doi:10.3389/fendo.2019.00718

Vitamin D Binding Protein and the Biological Activity of Vitamin D

Front Endocrinol (Lausanne). 2019 Oct 24:10:718. doi: 10.3389/fendo.2019.00718. eCollection 2019.

Authors

Rene F Chun¹, Albert Shieh², Carter Gottlieb¹, Vahe Yacoubian¹, Jeffrey Wang¹, Martin Hewison³, John S Adams¹

Affiliations

¹ Department of Orthopaedic Surgery, David Geffen School of Medicine at UCLA, Los Angeles, CA, United States.
² Department of Medicine, David Geffen School of Medicine at UCLA, Los Angeles, CA, United States.
³ Institute of Metabolism and Systems Research, University of Birmingham, Birmingham, United Kingdom.

Abstract

Vitamin D has a long-established role in bone health. In the last two decades, there has been a dramatic resurgence in research interest in vitamin D due to studies that have shown its possible benefits for non-skeletal health. Underpinning the renewed interest in vitamin D was the identification of the vital role of intracrine or localized, tissue-specific, conversion of inactive pro-hormone 25-hydroxyvitamin D [25(OH)D] to active 1,25-dihydroxyvitamin D [1,25(OH)₂D]. This intracrine mechanism is the likely driving force behind vitamin D action resulting in positive effects on human health. To fully capture the effect of this localized, tissue-specific conversion to 1,25(OH)₂D, adequate 25(OH)D would be required. As such, low serum concentrations of 25(OH)D would compromise intracrine generation of 1,25(OH)₂D within target tissues. Consistent with this is the observation that all adverse human health consequences of vitamin D deficiency are associated with a low serum 25(OH)D level and not with low 1,25(OH)₂D concentrations. Thus, clinical investigators have sought to define what concentration of serum 25(OH)D constitutes adequate vitamin D status. However, since 25(OH)D is transported in serum bound primarily to vitamin D binding protein (DBP) and secondarily to albumin, is the total 25(OH)D (bound plus free) or the unbound free 25(OH)D the crucial determinant of the non-classical actions of vitamin D? While DBP-bound-25(OH)D is important for renal handling of 25(OH)D and endocrine synthesis of 1,25(OH)₂D, how does DBP impact extra-renal synthesis of 1,25(OH)₂D and subsequent 1,25(OH)₂D actions? Are their pathophysiological contexts where total 25(OH)D and free 25(OH)D would diverge in value as a marker of vitamin D status? This review aims to introduce and discuss the concept of free 25(OH)D, the molecular biology and biochemistry of vitamin D and DBP that provides the context for free 25(OH)D, and surveys in vitro, animal, and human studies taking free 25(OH)D into consideration.

Keywords: CYP27B1; DBP; VDR; bone; free vitamin D; immunology; vitamin D.

Publication types

Review

Abstract

Publication types

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