Parathyroid hormone stimulation of the renal 25-hydroxyvitamin D-1alpha-hydroxylase--effect of age and free radicals

Regulation of 1 and 24 hydroxylation of vitamin D metabolites in the proximal tubule

Calcium and phosphate are critical for numerous physiological processes. Consequently, the plasma concentration of these ions are tightly regulated. Calcitriol, the active form of vitamin D, is a positive modulator of mineralization as well as calcium and phosphate metabolism. The molecular and physiological effects of calcitriol are well documented. Calcitriol increases blood calcium and phosphate levels by increasing absorption from the intestine, and resorption of bone. Calcitriol synthesis is a multistep process. A precursor is first made via skin exposure to UV, it is then 25-hydroxylated in the liver to form 25-hydroxyitamin D. The next hydroxylation step occurs in the renal proximal tubule via the 1-αhydroxylase enzyme (encoded by CYP27B1) thereby generating 1,25-dihydroxyvitamin D, that is, calcitriol. At the same site, the 25-hydroxyvitamin D 24-hydroxlase enzyme encoded by CYP24A1 can hydroxylate 25-hydroxyvitamin D or calcitriol to deactivate the hormone. Plasma calcitriol levels are primarily determined by the regulated expression of CYP27B1 and CYP24A1. This occurs in response to parathyroid hormone (increases CYP27B1), calcitriol itself (decreases CYP27B1 and increases CYP24A1), calcitonin (increases or decreases CYP24A1 and increases CYP27B1), FGF23 (decreases CYP27B1 and increases CYP24A1) and potentially plasma calcium and phosphate levels themselves (mixed effects). Herein, we review the regulation of CYP27B1 and CYP24A1 transcription in response to the action of classic phophocalciotropic hormones and explore the possibility of direct regulation by plasma calcium.

Vitamin D status and its association with parathyroid hormone in 23,134 outpatients

In vitro studies indicate that 25-hydroxyvitamin D3 (25(OH)D3) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) inhibits the synthesis of parathyroid hormone (PTH). The degree of PTH inhibition in humans by circulating 25(OH)D and 1,25(OH)2D may be different. Moreover, age and sex as well as confounding factors like calcium and phosphate may likewise affect the relationship between vitamin D and PTH in humans. However, this was not done so far in adequately powered studies. We investigated the relationship between 25(OH)D as well as 1,25(OH)2D and intact parathyroid hormone (iPTH) in 23,134 outpatients (age mean: 59.81 years) from the Berlin-Brandenburg area of Germany with normal serum creatinine considering confounding factors like age, sex, calcium and phosphate. 25(OH)D and iPTH were inversely correlated (r = -0.17, p < 0.0001). The inverse linear correlation was observed over the entire spectrum of 25(OH)D concentrations - from low 25(OH)D concentrations to very high 25(OH)D concentrations. Multiple linear regression analysis revealed that this correlation was independent of age, sex, creatinine, calcium and phosphate (unstandardized coefficients B: -0.16, p < 0.0001). However, 1,25(OH)2D was only positively associated with iPTH in women (r = 0.05, p = 0.033) and in the subgroup of patients with lower 25(OH)D (25(OH)D< 40 ng/ml) (r = 0.09, p < 0.0001), which was also presented in multiple linear regression analysis (unstandardized coefficients B: 0.20, p = 0.001). Circulating 1,25(OH)2D does not contribute substantially to the regulation of PTH in middle aged and vitamin D sufficient outpatients from the Berlin-Brandenburg area of Germany with normal kidney function. Presumably, serum 25(OH)D that is converted to 1,25(OH)2D after uptake in the parathyroid chief cells plays the critical role.

Oleanolic Acid and Ursolic Acid Improve Bone Properties and Calcium Balance and Modulate Vitamin D Metabolism in Aged Female Rats

Oleanolic acid (OA) and ursolic acid (UA) are the major chemical constituents in Fructus Ligustri Lucidi (FLL), a kidney-tonifying Chinese herb that is previously shown to improve bone properties and enhance calcium balance in aged female rats. The present study was designed to study if OA and UA act as the active ingredients in FLL to exert the positive effects on bone and mineral metabolism in aged rats. Aged (13-month-old) Sprague-Dawley female rats were randomly assigned to four groups with oral administration of drug or vehicle treatment for 12 weeks: medium calcium diet (MCD, 0.6% calcium), high calcium diet (HCD, 1.2% calcium), MCD + FLL (700 mg/kg/day), MCD + OA (23.6 mg/kg/day) + UA (8.6 mg/kg/day). A group of mature (3-month-old) female rats fed with MCD was included as positive control. The results demonstrated that FLL and OA+UA increased bone mineral density and improved microarchitectural properties of aged female rats. The osteoprotective effects of FLL and OA+UA might be, at least in part, associated with their actions on enhancing calcium balance and suppressing age-induced secondary hyperparathyroidism in aged female rats. FLL and OA+UA also significantly induced renal CYP27B1 protein expression and OA+UA treatment decreased CYP24A1 mRNA and protein expressions in aged female rats. In addition, FLL and OA+UA significantly increased the promoter activity, mRNA and protein expressions of renal CYP27B1 in vitro in human proximal tubule HKC-8 cells. The present findings suggest that OA+UA can be regarded as the active ingredients of FLL and might be a potential drug candidate for prevention and treatment of osteoporosis.

Oleanolic Acid Exerts Osteoprotective Effects and Modulates Vitamin D Metabolism

Oleanolic acid (OA) is a triterpenoid with reported bone anti-resorption activities. The present study aimed to characterize its bone protective effects in vivo and to study its effects on vitamin D metabolism, both in vivo and in vitro. OA significantly increased bone mineral density, improved micro-architectural properties, reduced urinary Ca excretion, increased 1,25(OH)₂D₃ and renal CYP27B1 mRNA expression in mature C57BL/6 ovariectomised (OVX) mice. OA also improved bone properties, Ca balance, and exerted modulatory effects on renal CYP27B1 and CYP24A1 expressions in aged normal female Sprague–Dawley rats. In addition, OA significantly increased renal CYP27B1 mRNA and promoter activity, and suppressed CYP24A1 mRNA and protein expressions in human proximal tubule HKC-8 cells. OA exerted bone protective effects in mature OVX mice and aged female rats. This action on bone might be, at least in part, associated with its effects on Ca and vitamin D metabolism. The present findings suggest that OA is a potential drug candidate for the management of postmenopausal osteoporosis.

FGF-23 regulates CYP27B1 transcription in the kidney and in extra-renal tissues

The mitochondrial enzyme 25-hydroxyvitamin D 1α-hydroxylase, which is encoded by the CYP27B1 gene, converts 25OHD to the biological active form of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)₂D). Renal 1α-hydroxylase activity is the principal determinant of the circulating 1,25(OH)₂D concentration and enzyme activity is tightly regulated by several factors. Fibroblast growth factor-23 (FGF-23) decreases serum 1,25(OH)₂D concentrations by suppressing CYP27B1 mRNA abundance in mice. In extra-renal tissues, 1α-hydroxylase is responsible for local 1,25(OH)₂D synthesis, which has important paracrine actions, but whether FGF-23 regulates CYP27B1 gene expression in extra-renal tissues is unknown. We sought to determine whether FGF-23 regulates CYP27B1 transcription in the kidney and whether extra-renal tissues are target sites for FGF-23-induced suppression of CYP27B1. In HEK293 cells transfected with the human CYP27B1 promoter, FGF-23 suppressed promoter activity by 70%, and the suppressive effect was blocked by CI-1040, a specific inhibitor of extracellular signal regulated kinase 1/2. To examine CYP27B1 transcriptional activity in vivo, we crossed fgf-23 null mice with mice bearing the CYP27B1 promoter-driven luciferase transgene (1α-Luc). In the kidney of FGF-23 null/1α-Luc mice, CYP27B1 promoter activity was increased by 3-fold compared to that in wild-type/1α-Luc mice. Intraperitoneal injection of FGF-23 suppressed renal CYP27B1 promoter activity and protein expression by 26% and 60% respectively, and the suppressive effect was blocked by PD0325901, an ERK1/2 inhibitor. These findings provide evidence that FGF-23 suppresses CYP27B1 transcription in the kidney. Furthermore, we demonstrate that in FGF-23 null/1α-Luc mice, CYP27B1 promoter activity and mRNA abundance are increased in several extra-renal sites. In the heart of FGF-23 null/1α-Luc mice, CYP27B1 promoter activity and mRNA were 2- and 5-fold higher, respectively, than in control mice. We also observed a 3- to 10-fold increase in CYP27B1 mRNA abundance in the lung, spleen, aorta and testis of FGF-23 null/1α-Luc mice. Thus, we have identified novel extra-renal target sites for FGF-23-mediated regulation of CYP27B1.

Hachimijiogan (Ba-Wei-Di-Huang-Wan), a herbal medicine, improves unbalance of calcium metabolism in aged rats

AIM OF THE STUDY

Aged animals as well as elderly humans commonly exhibit calcium (Ca) shortage because of increased Ca excretion into urine and decreased intestinal Ca absorption, which induce elevation of serum PTH levels to maintain serum Ca levels between a normal physiological range. The most important organ that regulates this Ca homeostasis is the kidney. Hachimijiogan (HJG), a traditional herbal medicine in Japan and China, has been used for treating clinical diseases associated with kidney dysfunctions in elderly humans. However, the mechanisms of its pharmacological actions remain to be understood poorly. The present study was designed to examine whether HJG improves age-related unbalance of Ca metabolism at the systemic level using aged rats.

MATERIALS AND METHODS

HJG was administered to 21-month-old aged rats for 3 months, and several parameters associated with Ca metabolism in serum and urine were measured.

RESULTS

Although HJG as well as aging itself did not affect serum Ca levels compared to young (11-week-old) rats, HJG improved increase in urinary Ca excretion and elevation of serum parathyroid hormone (PTH) levels in aged rats. However, HJG did not improve marked reduction of intestinal Ca absorption in aged rats.

CONCLUSION

HJG showed regulating action for age-related unbalance of Ca metabolism at the systemic level. This finding would provide useful information for treating age-related several disorders associated with Ca unbalance.

Calcitonin, a regulator of the 25-hydroxyvitamin D3 1alpha-hydroxylase gene

Although parathyroid hormone (PTH) induces 25-hydroxyvitamin D(3) (25(OH)D(3)) 1alpha-hydroxylase (1alpha(OH)ase) under hypocalcemic conditions, previous studies showed that calcitonin, not PTH, has an important role in the maintenance of serum 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) under normocalcemic conditions. In this study we report that 1alpha(OH)ase transcription is strongly induced by calcitonin in kidney cells and indicate mechanisms that underlie this regulation. The transcription factor C/EBPbeta is up-regulated by calcitonin in kidney cells and results in a significant enhancement of calcitonin induction of 1alpha(OH)ase transcription and protein expression. Mutation constructs of the 1alpha(OH)ase promoter demonstrate the importance of the C/EBPbeta binding site at -79/-73 for activation of the 1alpha(OH)ase promoter by calcitonin. The SWI/SNF chromatin remodeling complex was found to cooperate with calcitonin in the regulation of 1alpha(OH)ase. Chromatin immunoprecipitation analysis showed that calcitonin recruits C/EBPbeta to the 1alpha(OH)ase promoter, and Re-chromatin immunoprecipitation analysis (sequential chromatin immunoprecipitations using different antibodies) showed that C/EBPbeta and BRG1, an ATPase that is a component of the SWI/SNF complex, bind simultaneously to the 1alpha(OH)ase promoter. These findings are the first to address the dynamics between calcitonin, C/EBPbeta, and SWI/SNF in the regulation of 1alpha(OH)ase and provide a mechanism, for the first time, for calcitonin induction of 1alpha(OH)ase. Because plasma calcitonin as well as 1,25(OH)(2)D(3) have been reported to be increased during pregnancy and lactation and in early development, these findings suggest a mechanism that may account, at least in part, for the increase in plasma 1,25(OH)(2)D(3) during these times of increased calcium requirement.

Seasonal variation of 1,25-dihydroxyvitamin D and its association with body mass index and age

Under most normal conditions the serum level of 1,25-dihydroxyvitamin D is constant throughout the year, due to tight biochemical regulation. In contrast to this, the level of 25-hydroxyvitamin D is variable through the year, being largest in late summer, due to photosynthesis in the skin. The vitamin D status is usually assessed by measuring the level of the latter vitamin D derivative, rather than that of the presumably most active derivative 1,25(OH)2 vitamin D.We here show that for persons with a high body mass index (BMI) there is a significant seasonal variation, not only of 25(OH) vitamin D, but also of 1,25(OH)2 vitamin D. The variation seems to be largest for those with the poorest vitamin D status. Furthermore, there seems to be a correlation between the levels of the two vitamin D metabolites, indicating that the regulation of 1,25(OH)2 vitamin D is not always tight, notably in persons with high BMI.