High prevalence of hypovitaminosis D in pregnant Japanese women with threatened premature delivery.

Serum 25-hydroxyvitamin D (25-OHD) concentrations are thought to accurately reflect vitamin D stores, and vitamin D deficiency causes secondary hyperparathyroidism, irreversible bone loss, and increased risk of fracture. Recent studies suggest that decrease of serum 25-OHD level in mothers could increase the risk of preeclampsia, cesarean section, and craniotabes. Furthermore, this deficiency may affect bone mass and the incidence of neuromuscular diseases of their children in the future. In the present study, the serum concentration of 25-OHD in 93 pregnant women after the 30th week of their gestation was determined by direct radioimmunoassay. Mean 25-OHD levels in spring, summer, fall, and winter were 14.3 ± 5.1, 15.7 ± 6.4, 13.7 ± 5.1, and 13.9 ± 4.2 ng/ml, respectively. Severe vitamin D deficiency (25-OHD < 10 ng/ml) was found in 10 of these 93 women. Overall, hypovitaminosis D, which was defined as serum 25-OHD concentration equal to or less than 20 ng/ml, was revealed in 85 mothers (89.5%). Serum 25-OHD levels were not associated with either intact parathyroid hormone or corrected calcium concentrations, but were negatively associated with serum type I collagen N-terminal telopeptide and bone-specific alkaline phosphatase in these subjects. Mothers with threatened premature delivery had significantly lower 25-OHD levels (11.2 ± 3.2 ng/ml) than those in mothers with normal delivery (15.6 ± 5.1 ng/ml). In conclusion, the present data suggest a high prevalence of hypovitaminosis D in perinatal pregnant Japanese women throughout the year, which seems to affect bone metabolism and to be associated with threatened premature delivery.

Phosphate overload induces podocyte injury via type III Na-dependent phosphate transporter.

Uptake of P(i) at the cellular membrane is essential for the maintenance of cell viability. However, phosphate overload is also stressful for cells and can result in cellular damage. In the present study, we investigated the effects of the transgenic overexpression of type III P(i) transporter Pit-1 to explore the role of extracellular P(i) in glomerular sclerosis during chronic renal disease. Pit-1 transgenic (TG) rats showed progressive proteinuria associated with hypoalbuminemia and dyslipidemia. Ultrastructural analysis of TG rat kidney by transmission electron microscopy showed a diffuse effacement of the foot processes of podocytes and a thickening of the glomerular basement membrane, which were progressively exhibited since 8 wk after birth. TG rats died at 32 wk of age due to cachexia. At this time, more thickening of the glomerular basement membrane and segmental sclerosis were observed in glomeruli of the TG rats. Immunohistochemical examination using anti-connexin 43 and anti-desmin antibodies suggested the progressive injury of podocytes in TG rats. TG rats showed higher P(i) uptake in podocytes than wild-type rats, especially under low P(i) concentration. When 8-wk-old wild-type and TG rats were fed a 0.6% normal phosphate (NP) or 1.2% phosphate (HP) diet for 12 wk, HP diet-treated TG rats showed more progressive proteinuria and higher serum creatinine levels than NP diet-treated TG rats. In conclusion, our findings suggest that overexpression of Pit-1 in rats induces phosphate-dependent podocyte injury and damage to the glomerular barrier, which result in the progression of glomerular sclerosis in the kidney.

Effects of transgenic Pit-1 overexpression on calcium phosphate and bone metabolism.

The type III inorganic phosphate (Pi) transporter Pit-1 was previously found to be preferentially expressed in developing long bones. Several studies also described a regulation of its expression in cultured bone cells by osteotropic factors, suggesting a role of this transporter in bone metabolism. In the present study, we investigated the effects of the transgenic overexpression of Pit-1 in Wistar male rats on calcium phosphate and bone metabolism. A threefold increase and doubling of Pi transport activity were recorded in primary cultured osteoblastic cells derived from calvaria of two transgenic (Tg) lines compared with wild-type littermates (WT), respectively. Skeletal development was not affected by the transgene, and bone mass, analyzed by DXA, was slightly decreased in Tg compared with WT. Enhanced Pi uptake in calvaria-derived osteoblasts from Pit-1 Tg was associated with a significantly decreased expression of alkaline phosphatase activity and a normal deposition and calcification of the collagenous matrix. In 4-month-old adult Tg rats, serum Pi and renal Pi transport were increased compared with WT. The decrease of serum Ca concentration was associated with increased serum parathyroid hormone levels. Variations in serum Pi in Pit-1 Tg rats were negatively correlated with serum fibroblast growth factor-23, whereas 1,25-dihydroxyvitamin D(3) was not affected by Pit-1 overexpression. In conclusion, transgenic Pit-1 overexpression in rats affected bone and calcium phosphate metabolism. It also decreased alkaline phosphatase activity in osteoblasts without influencing bone matrix mineralization as well as skeletal development.

Effects of prostaglandin D2 on Na-dependent phosphate transport activity and its intracellular signaling mechanism in osteoblast-like cells.

Inorganic phosphate (Pi) transport probably represents an important function of bone-forming cells in relation to extracellular matrix mineralization. In the present study, we investigated the effect of prostaglandin D2 (PGD2) on Pi transport activity and its intracellular signaling mechanism in MC3T3-E1 osteoblast-like cells. PGD2 stimulated Na-dependent Pi uptake time- and dose-dependently in MC3T3-E1 cells during their proliferative phase. A protein kinase C (PKC) inhibitor calphostin C partially suppressed the stimulatory effect of PGD2 on Pi uptake. The selective inhibitors of mitogen-activated protein (MAP) kinase pathways such as ERK, p38 and Jun kinases suppressed PGD2-induced Pi uptake. The inhibitors of phosphatidylinositol (PI) 3-kinase and S6 kinase reduced this effect of PGD2, while Akt kinase inhibitor did not. These results suggest that PGD2 stimulates Na-dependent Pi transport activity in the phase of proliferation of osteoblasts. The mechanisms responsible for this effect are activation of PKC, MAP kinases, PI 3-kinase and S6 kinase.

Pharmacological topics of bone metabolism: recent advances in pharmacological management of osteoporosis.

The prevention of osteoporotic fracture is an essential socioeconomical priority, especially in the developed countries including Japan. Estrogen, selective estrogen-receptor modulators (SERMs), and bisphosphonate are potent inhibitors of bone resorption; and they have clinical relevance to reduce osteoporotic fractures in postmenopausal women. However, we can prevent at most 50% of vertebral fractures with these agents. For the better compliance of aminobisphosphonate, the use of a daily bisphosphonate regimen is moving to a weekly or monthly bisphosphonate regimen. Both cathepsin K inhibitors and modulators of the RANK-RANKL system, which can reduce bone resorption, are the candidates for the future treatment of osteoporosis. As well as bone resorption, we need to increase bone formation to prevent osteoporotic fractures, particularly in elderly patients with low bone turnover. In the U.S., Europe, and Australia, they have already started intermittent parathyroid hormone injection and/or oral strontium ranelate to stimulate bone formation. We still need to discover new agents to reduce osteoporotic fractures for the better quality of life without fractures.

Subclinical hypothyroidism is related to lower heel QUS in postmenopausal women.

Recent findings suggest that thyroid stimulating hormone (TSH) is a negative regulator of skeletal remodeling by reducing both differentiation of osteoblasts and formation of osteoclasts. In addition, increased fracture risk in untreated hypothyroid patients has been reported to begin up to 8 years before diagnosis. The aim of the present study was to evaluate the effect of subclinical hypothyroidism on bone structure by using the heel QUS. Subjects were outpatients without any past or present history of thyroid disease. Among 210 postmenopausal women, 22 of 33 patients (Hypo), who had elevated serum TSH concentration (TSH>or=4 microU/ml) with normal serum free thyroxine (FT4) concentration, agreed to join to this study. We also randomly selected 24 control subjects (Cont) from 176 postmenopausal women with normal thyroid status. Calcaneus osteo sono assessment indices (OSI) of right feet were measured using the ultrasound bone densitometry AOS-100. Serum TSH concentrations in Hypo patients (5.31 +/- 1.3 microU/ml) were higher than those in Cont patients (2.05 +/- 1.1 microU/ml), and there was significant difference of FT(4) concentrations (Cont 1.33 +/- 0.15 ng/dl; Hypo 1.19 +/- 0.17 ng/dl). OSI and its Z-score in Hypo subjects (OSI, 2.138 +/- 0.152; Z-Score -0.322 +/- 0.504 SD, Mean SD) were significantly lower than those in Cont subjects (OSI, 2.347 +/- 0.243; Z-Score 0.322 +/- 0.91 SD, Mean +/- SD). Simple regression statistical analysis showed that OSI decreased according to the increase of serum TSH concentration (n = 47, P<0.037). In addition, multiple regression analysis showed that the elevation of serum TSH concentration was associated with the decrease of OSI. These results suggest that the elevation of serum TSH concentration in subclinical hypothyroidism affects not bone turnover but bone structure as assessed by QUS.

Vasopressin stimulates Na-dependent phosphate transport and calcification in rat aortic smooth muscle cells.

We investigated the effect of arginine vasopressin (AVP) on inorganic phosphate (Pi) transport in A-10 rat aortic vascular smooth muscle cells (VSMCs). AVP time- and dose-dependently stimulated Na-dependent Pi transport in A-10 cells. This stimulatory effect of AVP on Pi transport was markedly suppressed by V1 receptor antagonist. A protein kinase C (PKC) inhibitor calphostin C partially suppressed the stimulatory effect of AVP. The selective inhibitors of c-Jun-NH2-terminal mitogen-activated protein (MAP) kinase (Jun kinase) attenuated AVP-induced Pi transport, but Erk kinase or p38 MAP kinase inhibitors did not. Wortmannin, a phosphatidylinositol (PI) 3-kinase inhibitor, suppressed AVP-induced Pi transport. Rapamycin, a selective inhibitor of S6 kinase, reduced this effect of AVP, while Akt kinase inhibitor did not. The combination of inhibitors for PKC, Jun kinase and PI 3-kinase completely suppressed the AVP-enhanced Pi transport. Furthermore, AVP rescued the VSMC from high phosphate-induced cell death and enhanced mineralization of these cells. In summary, these results suggest that AVP stimulates both Na-dependent Pi transport and mineralization in VSMCs. The mechanism is mediated by the activation of multiple signaling pathways including PKC, PI 3-kinase, S6 kinase and Jun kinase.