Fibroblast growth factor 23 inhibits extrarenal synthesis of 1,25-dihydroxyvitamin D in human monocytes

Abstract

Vitamin D is a potent stimulator of monocyte innate immunity, with this effect being mediated via intracrine conversion of 25-hydroxyvitamin D (25OHD) to 1,25-dihydroxyvitamin D (1,25(OH)2D). In the kidney synthesis of 1,25(OH)2D is suppressed by fibroblast growth factor 23 (FGF23), via transcriptional suppression of the vitamin D-activating enzyme 1α-hydroxylase (CYP27B1). We hypothesized that FGF23 also suppresses CYP27B1 in monocytes, with concomitant effects on intracrine responses to 1,25(OH)2D. Monocytes from healthy donor peripheral blood mononuclear cells (PBMCm) and from peritoneal dialysate effluent from kidney disease patients (PDm) were assessed at baseline to confirm the presence of mRNA for FGF23 receptors (FGFRs), with Klotho and FGFR1 being more strongly expressed than FGFR2/3/4 in both cell types. Immunohistochemistry showed co-expression of Klotho and FGFR1 in PBMCm and PDm, with this effect being enhanced following treatment with FGF23 in PBMCm but not PDm. Treatment with FGF23 activated MAP kinase (MAPK) and Akt pathways in PBMCm, demonstrating functional FGFR signaling in these cells. FGF23 treatment of PBMCm and PDm decreased expression of mRNA for CYP27B1. In PBMCm this was associated with downregulation of 25OHD to 1,25(OH)2D metabolism, and concomitant suppression of intracrine induced 24-hydroxylase (CYP24A1) and antibacterial cathelicidin (LL37). FGF23 suppression of CYP27B1 was particularly pronounced in PBMCm treated with interleukin-15 to stimulate synthesis of 1,25(OH)2D. These data indicate that FGF23 can inhibit extra-renal expression of CYP27B1 and subsequent intracrine responses to 1,25(OH)2D in two different human monocyte models. Elevated expression of FGF23 may therefore play a crucial role in defining immune responses to vitamin D and this, in turn, may be a key determinant of infection in patients with CKD.

Low Vitamin D and High Fibroblast Growth Factor 23 Serum Levels Associate with Infectious and Cardiac Deaths in the HEMO Study

“…15,16 In addition, it is possible that FGF23 also regulates the innate immunity. 21,22 A plausible hypothesis is that FGF23 inhibits CYP27B1 in monocytic cells, with subsequent effects on intracellular synthesis of 1,25(OH) 2 D. 21,22,28 In fact, treatment of peripheral blood mononuclear cells isolated from peritoneal dialysate effluents of uremic patients decreased the mRNA expression of CYP27B1 and conversion of 25(OH)D to 1,25(OH) 2 D. 21,22 These biologic effects of FGF23 on monocytic cells support the observation in the present study, which is the first clinical evidence of a relationship between higher serum FGF23 levels and infectious events. Previous observational studies have found similar relationship of serum FGF23 concentration with all-cause mortality but less is known about the longitudinal relationship between serum FGF23 levels with cardiac events in hemodialysis patients.…”

“…21 Recent studies reported that FGF23 treatment of mononuclear cells isolated from healthy human peripheral blood and peritoneal dialysis effluents from uremic patients decreased the mRNA expression of CYP27B1. 21,22 Accordingly, it is reasonable to hypothesize that circulating FGF23 excess decreases the intracrine production of 1,25(OH) 2 D, which consequently decreases the transcription and the production of cathelicidins 14 leading to an increase in infectious outcomes.…”

FGF23 signaling impairs neutrophil recruitment and host defense during CKD

“…Although various effects of increased FGF23 levels with respect to phosphate homeostasis regulation and myocardial hypertrophy have been described, the available evidence for a direct effect of FGF23 on the immune system is very scarce (17). It has been described that FGF23 may inhibit the extrarenal expression of vitamin D-activating enzyme 1α-hydroxylase in monocytes, thus regulating 1,25-dihydroxyvitamin D synthesis (32,33). Moreover, a recently published study among chronic hemodialysis patients demonstrated that increased FGF23 levels were independently associated with a higher infection rate and higher all-cause mortality (18).…”

Suppression of Iron-Regulatory Hepcidin by Vitamin D

“…Expression of ferroportin protein was assessed by Western blot analyses using previously reported protocols, 46 involving overnight incubation with primary ferroportin antibody (1/1000; Amgen, Thousand Oaks, CA). A b-actin antibody (Sigma-Aldrich, St. Louis, MO) was used as a loading control.…”

“…Immunofluorescence analysis of ferroportin and ferritin was carried out using adaptations of previously described methods. 46 Briefly, monocytes or hepatocytes were seeded on 4-well glass slides, and incubated for 1 hour with antibodies to ferroportin (1/200; Amgen) or ferritin (1/200; Abcam). Secondary antibodies labeled with Alexa 594 were applied for 1 hour, and finalized with 49,6-diamidino-2-phenylindole (1/10,000, 5 minutes).…”