Effects and interactions of 24R,25(OH)2D3 and 1,25(OH)2D3 on bone

1,25(OH)2D3 ameliorates doxorubicin‑induced cardiomyopathy by inhibiting the NLRP3 inflammasome and oxidative stress

Doxorubicin (DOX), as a chemotherapy agent with marked therapeutic effect, can be used to treat certain types of cancer such as leukemia, lymphoma and breast cancer. However, the toxic effects of DOX on cardiomyocytes limit its clinical application. Oxidative stress has been documented to serve a pivotal role in DOX-induced cardiomyopathy. Previous studies have reported that 1,25(OH)2D3 has antioxidant and anti-inflammatory effects and can inhibit the renin-angiotensin system. However, the effects of 1,25(OH)2D3 on the pathophysiological processes of DOX-induced cardiomyopathy and its mechanisms remain poorly understood. To investigate these potential effects, C57BL/6J mice were used to construct a DOX-induced cardiomyopathy model and treated with 1,25(OH)2D3. At 4 weeks after the first injection of DOX, cardiac function and myocardial injury were evaluated by echocardiograph and ELISA. Masson's trichrome staining and RT-qPCR were used to assess myocardial fibrosis, and immunohistochemistry and western blotting were performed to analyze expression levels of inflammation and oxidative stress, and the NLRP3 inflammasome pathway. ChIP assay was used to assess the effects of 1,25(OH)2D3 on histone modification in the NLRP3 and Nrf2 promoters. The results showed that 1,25(OH)2D3 treatment increased LVEF and LVFS, reduced serum levels of BNP and cTnT, inhibited the collagen deposition and profibrotic molecular expression, and downregulated the levels of inflammatory cytokines in DOX-induced cardiomyopathy. ROS and antioxidant indices were also ameliorated after 1,25(OH)2D3 treatment. In addition, 1,25(OH)2D3 was found to inhibit the NLRP3 inflammasome and KEAP-Nrf2 pathways through regulation of the levels of H3K4me3, H3K27me3 and H2AK119Ub in the NLRP3 and Nrf2 promoters. In conclusion, the present study demonstrated that 1,25(OH)2D3 regulated histone modification in the NLRP3 and Nrf2 promoters, which in turn inhibits the activation of NLRP3 inflammasome and oxidative stress in cardiomyocytes, alleviating DOX-induced cardiomyopathy. Therefore, 1,25(OH)2D3 may be a potential drug candidate for the treatment of DOX-induced cardiomyopathy.

Nephrogenic diabetes insipidus, cystinosis, and vitamin D

We describe a patient with early diagnosed cystinosis who presented with nephrogenic diabetes insipidus in addition to proximal tubular dysfunction. Another feature in this patient was abnormally low serum concentration of 24,25 dihydroxy vitamin D3 (24,25(OH)2D3) with normal 25 hydroxy vitamin D3 (25(OH)D3) and relatively low 1,25 dihydroxy vitamin D3 (1,25(OH)2D3).

Effect of 24,25-dihydroxyvitamin D3 on 1,25-dihydroxyvitamin D3 metabolism in calcium-deficient rats

The effect of 24,25-dihydroxyvitamin D3 [24,25(OH)2D3] on 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] metabolism was examined in rats fed on a low-calcium diet. These rats exhibit hypocalcaemia, high urinary cyclic AMP excretion, a markedly elevated serum 1,25(OH)2D concentration and low serum concentrations of both 24,25(OH)2D and 25(OH)D. When the rats are treated orally with 1, 5 or 10 micrograms of 24,25(OH)2D3/100 g every day, there is a dramatic decrease in serum 1,25(OH)2D concentration in a dose-dependent manner concomitant with an increase in serum 24,25(OH)2D concentration. Serum calcium concentration and urinary cyclic AMP excretion are not significantly affected by the 24,25(OH)2D3 treatment, which suggests that parathyroid function is not affected by the 24,25(OH)2D3 treatment. The 25(OH)D3 1 alpha-hydroxylase activity measured in kidney homogenates is markedly elevated in rats on a low-calcium diet but is not affected by any doses of 24,25(OH)2D3. In contrast, recovery of intravenously injected [3H]1,25(OH)2D3 in the serum is decreased in 24,25(OH)2D3-treated rats. Furthermore, when [3H]1,25(OH)2D3 is incubated in vitro with kidney or intestinal homogenates of 24,25(OH)2D3-treated rats there is a decrease in the recovery of radioactivity in the total lipid extract as well as in the 1,25(OH)2D3 fraction along with an increase in the recovery of radioactivity in the water-soluble phase. These results are consistent with the possibility that 24,25(OH)2D3 has an effect on 1,25(OH)2D3 metabolism, namely that of enhancing the degradation of 1,25(OH)2D3. However, because a considerable proportion of the injected 24,25(OH)2D3 is expected to be converted into 1,24,25(OH)3D3 by renal 1 alpha-hydroxylase in 24,25(OH)2D3-treated rats, at least a part of the decrease in serum 1,25(OH)2D concentration may be due to a competitive inhibition by 24,25(OH)2D3 of the synthesis of 1,25(OH)2D3 from 25(OH)D3. Thus the physiological importance of the role of 24,25(OH)2D3 in regulating the serum 1,25(OH)2D concentration as well as the mechanism and metabolic pathway of degradation of 1,25(OH)2D3 remain to be clarified.

Effects of 1,25-dihydroxycholecalciferol on fracture healing. Calcium, phosphate, and zinc in callus and serum

The incorporation of calcium, phosphate, and zinc into the callus of closed tibial fractures was studied in adult rats fed a standard diet. Low doses (60 ng/kg per day) of 1,25(OH)2D3 5 days a week greatly increased early callus mineralization. This was not related to an increased serum calcium-phosphate molar product but rather to a decreased ratio. The incorporation of zinc into callus seemed to be correlated to the mineralization process but not to the 1,25(OH)2D3 administration as such. The question of a direct, indirect, or a complex role of 1,25(OH)2D3 in bone formation and mineralization is discussed.

Stimulation of creatine kinase BB activity by 1 alpha,25-dihydroxycholecalciferol and 24R,25-dihydroxycholecalciferol in rat tissues

Vitamin D metabolites stimulate creatine kinase BB activity in organs of vitamin D-deficient rats. In epiphyses of long bones, creatine kinase BB activity increases 2.6-fold 24 h after injection of 24R,25-dihydroxycholecalciferol but not of 1 alpha,25-dihydroxycholecalciferol. Contrariwise, 1 alpha,25-dihydroxycholecalciferol, but not 24R,25-dihydroxycholecalciferol, increases creatine kinase BB activity in diaphyses and in kidney. Neither metabolite affects creatine kinase activity in duodenal mucosa.

Calcitriol but no other metabolite of vitamin D is essential for normal bone growth and development in the rat

To determine the relative importance of different metabolites of vitamin D in bone growth and development, weanling male rat pups suckled by vitamin D-deficient mothers were given either calcitriol (1,25-dihydroxycholecalciferol) by continuous subcutaneous infusion, oral calcidiol (25-hydroxycholecalciferol), or oral 24,24-difluoro-25-hydroxycholecalciferol, a synthetic compound that can undergo 1-hydroxylation but not 24-hydroxylation, as their sole source of vitamin D for 40 d. Pups raised in the same manner, but given no vitamin D, served as controls. The three metabolites compared were given in doses that restored normal plasma calcium levels and normal increments in body weight. After in vivo double tetracycline labeling, bone histomorphometry by standard methods was performed on one femur and one tail vertebra. There were no significant differences between the three metabolite-treated groups in length, periosteal or endosteal diameter, cortical cross-sectional area, cortical porosity, osteoid thickness and volume, appositional rate and bone formation rate in the femur, or in qualitative and quantitative indices of endochondral ossification in the tail vertebra. All three groups differed markedly from the untreated controls with respect to all measurements. Collectively, the data indicate that neither calcidiol nor any 24-hydroxylated metabolite of calcidiol is needed in the rat (other than as a precursor) for longitudinal or transverse bone growth, for normal endochondral ossification, or for normal periosteal and endosteal formation, mineralization, and resorption of bone. Calcitriol was fully active with respect to each of the indices listed when given in a manner resembling its continuous endogenous production by the kidney, suggesting that previous reports of incomplete skeletal response to calcitriol result from its rapid clearance and infrequent oral administration. We demonstrated that calcitriol is the only metabolite that is both necessary and sufficient for normal bone growth and development in the rat, but our data do not indicate the extent to which its beneficial skeletal effects were mediated by direct action on bone, either of calcitriol itself or of some metabolite thereof, or by restoration of normal plasma levels of calcium and phosphate.

A new semiautomatic method for quantitative static and dynamic bone histology

A new semiautomatic technique combining advantages of the manual and fully automatic methods is described for obtaining quantitative static and dynamic histologic data of bone. The hardware consists of a photomicroscope, digitizing platen, digitizer, plotter/printer, floppy disc drive, and computer. The microscope is equipped with a drawing tube through which the image of the digitizing platen is projected over the optical field. The investigator selects and traces all histologic structures to be measured by moving a cursor on the digitizing platen which is visible by its projection over the histologic field. The results on accuracy and static and dynamic precision of this method show that static and dynamic parameters of bone are obtained with a degree of error (less than 20%) well within the acceptable range for biologic measurements. Comparison of this method with the grid technique according to Merz and Schenck showed that for almost all micromorphometric parameters comparable absolute data are obtained. Due to the higher precision of our method, however, the number of optical fields evaluated in obtaining these comparable data could be reduced to 25% of the number of fields evaluated by the Merz and Schenck technique. The time requirements for quantitative evaluation of a histologic slide of bone by our technique are 40-50 min; 20-25 min is needed for quantitative evaluation of osteocytes.

Bone response to phosphate and vitamin D metabolites in the hypophosphatemic male mouse

The hypophosphatemic male mouse (Hyp/y), the proposed model for human vitamin D-resistant rickets (VDRR), is characterized by chronic hypophosphatemia, dwarfism, and rachitic and osteomalacic bone lesions. We have reported that treatment of Hyp/y mice with phosphate salts (Pi) heals rickets but does not correct the defective endosteal bone mineralization. In an attempt to cure osteomalacia, mutant male animals were treated with Pi combined with 25-hydroxyvitamin D3 (25OHD3, 1 microgram/kg/day), 24,25-dihydroxyvitamin D3 [24,25(OH)2D3, 0.5 microgram/kg/day], or 1,25-dihydroxyvitamin D3 [1,25(OH)2D3, 0.05--0.25 microgram/kg/day] infused constantly for 3 weeks. The biochemical and skeletal effects of treatment were assessed by analytical methods and bone histomorphometry. The results show that only 1,25(OH)2D3 produced a dose-dependent elevation of serum calcium and phosphorus, and greatly improved bone mineralization at doses high enough to increase serum calcium and phosphorus concentrations within or above the normal range. Better improvement of bone mineralization was obtained when Pi was combined to 1,25(OH)2D3. In conjunction with the correction of hypocalcemia, Pi + 1,25(OH)2D3 suppressed the stimulation of bone turnover induced by Pi supplementation. The results show that, as in VDRR children, 1,25(OH)2D3 produces beneficial effects on bone lesions in Hyp/y mice, mainly through enhancement of mineral availability. However, the persistence of osteomalacia despite correction of serum mineral concentrations suggests that there is a specific bone cell resistance to mineral and/or hormonal influences in Hyp/y mice.

Suppressive effects of 24,25-dihydroxycholecalciferol on bone resorption induced by acute bilateral nephrectomy in rats

The possible suppressive effects of 24,25-dihydroxycholecalciferol on secondary hyperparathyroidism and increased bone resorption were investigated in adult rats raised on a diet normal in calcium, phosphorus, and vitamin D, and subjected to acute bilateral nephrectomy. The animals had received subcutaneous radiocalcium 4 wk before the experiment. 5 h after nephrectomy an increase in serum total calcium, (45)Ca-specific activity, citrate, phosphorus, and magnesium concentrations were observed. Serum immunoreactive parathyroid hormone increased, while serum calcitonin decreased. The osteoclast count in the tibial metaphyses was augmented. The biochemical and histological changes observed were partly parathyroid hormone and calcitonin independent, as they also occurred in parathyroidectomized hypocalcemic rats. Pretreatment with 650 pmol of 24,25-dihydroxycholecalciferol 16 h before nephrectomy prevented bone calcium mobilization and diminished the rise in serum total calcium and citrate both in parathyroid-intact and in parathyroidectomized animals. In parathyroid-intact rats, serum immunoreactive parathyroid hormone and calcitonin remained normal in spite of the fall in serum-ionized calcium, and the number of osteoclasts did not increase. In parathyroidectomized rats, 24,25-dihydroxycholecalciferol did not prevent the postnephrectomy rise in the osteoclast count. This latter observation suggests that this metabolite exerts its effect on bone either by acting on cells other than osteoclasts, i.e., the osteocytes, or by inhibiting cell activity. At equimolar dosage 1,25-dihydroxycholecalciferol had a potent stimulatory effect on bone resorption. This effect of 1,25-dihydroxycholecalciferol was partly blocked by the simultaneous administration of 24,25-dihydroxycholecalciferol. The potential clinical significance of these observations remains to be determined.