Regulation of the extrarenal CYP27B1-hydroxylase

Abstract

Provided here is a collective review of research on the extrarenal CYP27B1-hydroxylase that shapes our current and expanding vision of the role this enzyme plays in the intracrinology and paracrinology, as opposed to the traditional endocrinology, of vitamin D to regulate the innate and adaptive immune responses, particularly in human granuloma-forming diseases like tuberculosis. Special emphasis is placed on soluble factors (i.e., cytokines) in the local microenvironment of these human diseases that coordinate amplification and feedback inhibition of the macrophage CYP27B1-hydroxylase. Principal among these factors are Type I and Type II interferons (IFNs); the Type II IFN, IFN-γ, stimulates the production of 1,25-dihydroxyvitamin D (1,25(OH)2D) from 25-hydroxyvitamin D (25OHD) by the granuloma-forming disease-activated macrophage, while the Type I IFNs, IFN-α and IFN-β, block the hydroxylation reaction. The Type I IFN response is associated with more aggressive disease, while the Type II IFN response, the one that promotes 1,25(OH)2D production by the macrophage, is associated with more confined disease. Tilting the balance in the human immune response toward a confined disease phenotype is enabled by the presence of sufficient extracellular 25OHD to modulate IFN-γ-promoted and substrate 25OH-driven intracellular synthesis of 1,25(OH)2D. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Keywords: Hydroxylase; Immunology; Innate immunity; Intracrine; Macrophage; Vitamin D.

Figure 1

Intracrine synthesis and action of 1,25-dihydroxyvitamin D (1,25-D). The upper panel of the cartoon demonstrates dependence on the serum vitamin D binding protein (DBP) to deliver substrate 25-hydroxyvitamin D (25-D) to its target cell and the movement of 25-D off the DBP and of free 25-D into the cell. Chaperone-mediated transport of 25-D (not depicted) to the inner mitochondrial membrane delivers substrate 25-D to the CYP27B1-hydroxylase for synthesis of 1,25-D. This event is followed by the binding of 1,25-D to VDR in an intracrine mode and dimerization with the retinoid X receptor to drive transactivation of genes bearing cis-acting vitamin D responsive elements. The lower panel shows an alternative mode of delivery of 25-D to the target cell via endocytic internalization of megalin-bound DBP and subsequent diffusion of 25-D across the endosomal membrane to the cell interior; the latter sequence of megalin-mediated events occurs in renal and mammary epithelial cells (see Table 1).

Figure 2

Human macrophages express the CYP27B1-hydroxylase. Macrophages harvested from the alveolar or pleural space of patients with active granuloma-forming disease of the lung (left panel; e.g., sarcoidosis and tuberculosis) and plated in monolayer at high density (right panel) synthesize and secrete 1,25-dihydroxyvitamin D3 (1,25D3) when incubated with physiologically relevant concentrations of substrate 25-hydroxyvitamin D3 (25D3). The specific activity of the 1-hydoxylation reaction is significantly increased if monolayer cultures of macrophages are preconditioned in medium containing rhIFN-γ or patient-derived pulmonary alveolar fluid enriched in IFN-γ.

Figure 3

Regulation of the vitamin D-dependent antimycobacterial response in human monocytederived macrophages. Panel A shows that Mycobacterium tuberculosis (mTB) stimulation of the Toll-like receptor (TLR) 2/1 leads to induction of co-expression of the CYP27B1-hydroxylase and vitamin D receptor (VDR), intracrine induction of the cathelicidin gene product LL-37 expression and mycobacterial killing inside phagolysosomes. The key element for the occurrence of these serial events is the provision of adequate free, non-serum vitamin D binding protein ((DBP)bound) 25-hydroxyvitamin D (25D) substrate to the mitochondrial CYP27B1. Panel B shows that TLR activation leads to MYD88-coupled, NFκB-directed IL-15 and IL-1ß expression. IL-15 drives expression of the CYP27B1, VDR and 1,25DVDR-directed cathelicidin gene. The circuit is amplified by IL-15 and IL-1ß-induced TH1 cell proliferation with subsequent IFN-γ production to drive STAT-1-mediated increases interferon regulatory factor (IRF) and NF-kB signaling. The net result is i) more IL-15-powered CYP27B1 and VDR expression, ii) increased cathelicidin and ß defensin-4 expression and iii) recruitment of the cell’s autophagy and inflammasome capacity to support microbial killing; inflammasome activation increases the production and release of mature IL-1ß. Feedback inhibition of this amplification loop occurs through i) IL-1ß stimulated TH2 cell proliferation, ii) IFN-ß-driven IL-4 and IL-10 synthesis, iii) blockade endogenous 1,25D synthesis and iv) release of 1,25D into the extracellular environment in quantities sufficient to preferentially inhibit the proliferation of TH1 over TH2 cells, promoting suppression of the adaptive immune response.

Figure 3

Regulation of the vitamin D-dependent antimycobacterial response in human monocytederived macrophages. Panel A shows that Mycobacterium tuberculosis (mTB) stimulation of the Toll-like receptor (TLR) 2/1 leads to induction of co-expression of the CYP27B1-hydroxylase and vitamin D receptor (VDR), intracrine induction of the cathelicidin gene product LL-37 expression and mycobacterial killing inside phagolysosomes. The key element for the occurrence of these serial events is the provision of adequate free, non-serum vitamin D binding protein ((DBP)bound) 25-hydroxyvitamin D (25D) substrate to the mitochondrial CYP27B1. Panel B shows that TLR activation leads to MYD88-coupled, NFκB-directed IL-15 and IL-1ß expression. IL-15 drives expression of the CYP27B1, VDR and 1,25DVDR-directed cathelicidin gene. The circuit is amplified by IL-15 and IL-1ß-induced TH1 cell proliferation with subsequent IFN-γ production to drive STAT-1-mediated increases interferon regulatory factor (IRF) and NF-kB signaling. The net result is i) more IL-15-powered CYP27B1 and VDR expression, ii) increased cathelicidin and ß defensin-4 expression and iii) recruitment of the cell’s autophagy and inflammasome capacity to support microbial killing; inflammasome activation increases the production and release of mature IL-1ß. Feedback inhibition of this amplification loop occurs through i) IL-1ß stimulated TH2 cell proliferation, ii) IFN-ß-driven IL-4 and IL-10 synthesis, iii) blockade endogenous 1,25D synthesis and iv) release of 1,25D into the extracellular environment in quantities sufficient to preferentially inhibit the proliferation of TH1 over TH2 cells, promoting suppression of the adaptive immune response.