Substrate and Enzyme Trafficking as a Means of Regulating 1,25-Dihydroxyvitamin D Synthesis and Action: The Human Innate Immune Response

Abstract

Tissue availability of the active vitamin D metabolite, 1,25‐dihydroxyvitamin D [1,25(OH)₂D] is dependent on expression of the activating enzyme 1α‐hydroxylase (CYP27b1) and its catabolic counterpart 24‐hydroxylase (CYP24). The activity of these two enzymes is in turn controlled by factors including affinity of the serum vitamin D–binding protein (DBP) for 25‐hydroxyvitamin D [25(OH)D]; the availability of enzyme cofactors; and the relative amount of hydroxylase gene product expressed. In recent years, it has become clear that directed trafficking of substrate and enzyme is also a pivotal component of the regulated process of hormone synthesis by both renal and extrarenal tissues expressing the CYP27b1 and CYP24 genes. Extracellular regulatory trafficking events are defined by the quantity of substrate 25(OH)D entering the circulatory pool. Entry into some target cells in vivo, such as the macrophage and proximal renal tubular epithelial cells, requires 25(OH)D binding to serum DBP, followed by recognition, internalization, and intracellular release. The “released” intracellular substrate is moved to specific intracellular destinations (i.e., the mitochondrial CYP enzymes and the vitamin D receptor [VDR]) by the hsc70 family of chaperone proteins. Synthesis of 1,25(OH)₂D is also regulated by CYP24 and its metabolically inactive splice variant CYP24‐SV. Finally, initiation of transcription of 1,25(OH)₂D‐regulated genes, such as the CYP24, requires movement of the CYP27b1 product, 1,25(OH)₂D, to the VDR in the same cell for intracrine action or export to another cell for paracrine action. In either case, the 1,25(OH)₂D ligand is required for the VDR to heterodimerize with the retinoid x receptor and compete away the dominant‐negative acting, heterogeneous nuclear ribonucleoprotein (hnRNP)‐related, vitamin D response element–binding proteins that inhibit hormone‐directed transactivation of genes. In this review, we use vitamin D–directed events in the human innate immune response to Mycobacterium tuberculosis as a physiologically relevant model system in which to highlight the importance of these intracellular traffic patterns.