In vitro regulation of fibroblast growth factor 23 by 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D synthesized by osteocyte-like MC3T3-E1 cells.

Fibroblast growth factor 23 (FGF23) is produced and secreted by osteocytes and is essential for maintaining phosphate homeostasis. One of the main regulators of FGF23, 1,25-dihydroxyvitamin D (1,25(OH)2D3), is primarily synthesized in the kidney from 25-hydroxyvitamin D (25(OH)D) by 1α-hydroxylase (encoded by CYP27B1). Hitherto, it is unclear whether osteocytes can convert 25(OH)D and thereby allow for 1,25(OH)2D3 to induce FGF23 production and secretion locally. Here, we differentiated MC3T3-E1 cells toward osteocyte-like cells expressing and secreting FGF23. Treatment with 10-6 M 25(OH)D resulted in conversion of 25(OH)D to 150 pmol/L 1,25(OH)2D3 and increased FGF23 expression and secretion, but the converted amount of 1,25(OH)2D3 was insufficient to trigger an FGF23 response, so the effect on FGF23 was most likely directly caused by 25(OH)D. Interestingly, combining phosphate with 25(OH)D resulted in a synergistic increase in FGF23 expression and secretion, likely due to activation of additional signaling pathways by phosphate. Blockage of the vitamin D receptor (VDR) only partially abolished the effects of 25(OH)D or 25(OH)D combined with phosphate on Fgf23, while completely inhibiting the upregulation of cytochrome P450 family 24 subfamily A member 1 (Cyp24a1), encoding for 24-hydroxylase. RNA sequencing and in silico analyses showed that this could potentially be mediated by the nuclear receptors Retinoic Acid Receptor ß (RARB) and Estrogen Receptor 2 (ESR2). Taken together, we demonstrate that osteocytes are able to convert 25(OH)D to 1,25(OH)2D3, but this is insufficient for FGF23 activation, implicating a direct effect of 25(OH)D in the regulation of FGF23, which occurs at least partially independent from its cognate VDR. Moreover, phosphate and 25(OH)D synergistically increase expression and secretion of FGF23, which warrants investigating consequences in patients receiving a combination of vitamin D analogues and phosphate supplements. These observations help us to further understand the complex relations between phosphate, vitamin D, and FGF23.

A human vitamin D receptor mutation causes rickets and impaired Th1/Th17 responses.

We present a brother and sister with severe rickets, alopecia and highly elevated serum levels of 1,25-dihydroxyvitamin D (1,25-(OH)2D3). Genomic sequencing showed a homozygous point mutation (A133G) in the vitamin D receptor gene, leading to an amino acid change in the DNA binding domain (K45E), which was described previously. Hereditary vitamin D resistant rickets (HVDRR) was diagnosed. Functional studies in skin biopsy fibroblasts confirmed this. 1,25-(OH)2D3 reduced T helper (Th) cell population-specific cytokine expression of interferon γ (Th1), interleukins IL-17A (Th17) and IL-22 (Th17/Th22) in peripheral blood mononuclear cells (PBMCs) from the patient's parents, whereas IL-4 (Th2) levels were higher, reflecting an immunosuppressive condition. None of these factors were regulated by 1,25-(OH)2D3 in PBMCs from the boy. At present, both patients (boy is 23 years of age, girl is 7) have not experienced any major immune-related disorders. Although both children developed alopecia, the girl did so earlier than the boy. The boy showed complete recovery from the rickets at the age of 17 and does not require any vitamin D supplementations to date. In conclusion, we characterized two siblings with HVDRR, due to a mutation in the DNA binding domain of VDR. Despite a defective T cell response to vitamin D, no signs of any inflammatory-related abnormalities were seen, thus questioning an essential role of vitamin D in the immune system. Despite the fact that currently medicine is not required, close monitoring in the future of these patients is warranted for potential recurrence of vitamin D dependence and diagnosis of (chronic) inflammatory-related diseases.

Opposing actions of rosiglitazone and resveratrol on mineralization in human vascular smooth muscle cells.

Arteriosclerotic vascular disease is a major cardiac health problem in westernized countries and the primary cause of mortality in diabetic patients. Recent data have raised serious safety concerns with the antidiabetic rosiglitazone, a thiazolidinedione with peroxisome proliferator-activated receptor γ (PPAR-γ) agonistic activity, in regard to cardiovascular risks. A common feature of atherosclerosis is vascular mineralization. The latter is formed by vascular smooth muscle cells (VSMC) through complex processes that are similar to mineralization in bone. The aim of the current study was to investigate the effect of rosiglitazone on mineralization in cultured human VSMCs. We found that rosiglitazone stimulated mineralization by, at least in part, induction of caspase-dependent apoptosis. Furthermore, rosiglitazone-induced oxidative stress was correlated with stimulated osteoblast-like differentiation of VSMCs. Treatment of rosiglitazone-supplemented VSMC cultures with the caloric restriction mimetic and antioxidant resveratrol diminished rosiglitazone-induced oxidative stress, osteoblast-like differentiation and mineralization. In conclusion, this study reveals novel insights into the relationship of rosiglitazone and cardiovascular events by providing a model that links rosiglitazone-induced osteoblast-like differentiation, oxidative stress and apoptosis with mineralization in VSMCs. In addition, we position resveratrol in this model acting to reduce rosiglitazone-induced oxidative stress, osteoblast-like VSMC differentiation and mineralization.