Suzuki Atsushi

Inorganic phosphate (Pi) is required in many biological processes, including signaling cascades, skeletal development, tooth mineralization, and nucleic acid synthesis. Recently, we showed that Pi transport in osteoblasts, mediated by Slc20a1, a member of the type III sodium-dependent phosphate transporter family, is indispensable for osteoid mineralization in rapidly growing rat bone. In addition, we found that bone mineral density decreased slightly with dysfunction of Pi homeostasis in aged transgenic rats overexpressing mouse Slc20a1 (Slc20a1-Tg). Bone and tooth share certain common molecular features, and thus, we focused on tooth development in Slc20a1-Tg mandibular incisors in order to determine the role of Slc20a1 in tooth mineralization. Around the time of weaning, there were no significant differences in serologic parameters between wild-type and Slc20a1-Tg rats. However, histological analysis showed that Slc20a1-Tg ameloblasts formed clusters in the papillary layer during the maturation stage as early as 4 weeks of age. These pathologies became more severe with age and included the formation of cyst-like or multilayer ameloblast structures, accompanied by a chalky white appearance with abnormal attrition and fracture. Hyperphosphatemia was also observed in aging Slc20a1-Tg rats. Micro-computed tomography and electron probe microanalysis revealed impairments in enamel, such as delayed mineralization and hypomineralization. Our results suggest that enamel formation is sensitive to imbalances in Pit1-mediated cellular function as seen in bone, although these processes are under the control of systemic Pi homeostasis.

Sekiguchi S, Suzuki A, Asano S, Nishiwaki-Yasuda K, Shibata M, Nagao S, Yamamoto N, Matsuyama M, Sato Y, Yan K, Yaoita E, Itoh M. Phosphate overload induces podocyte injury via type III Na-dependent phosphate transporter. Am J Physiol Renal Physiol 300: F848-F856, 2011. First published February 9, 2011; doi:10.1152/ajprenal.00334.2010.-Uptake of Pi 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 Pi transporter Pit-1 to explore the role of extracellular Pi 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 anticonnexin 43 and anti-desmin antibodies suggested the progressive injury of podocytes in TG rats. TG rats showed higher Pi uptake in podocytes than wild-type rats, especially under low Pi 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.

・甲状腺クリーゼは「生命が危険となるような激しい症状を呈する甲状腺中毒症」であり、多臓器における非代償性状態を特徴とする。・甲状腺診療における救急の代表例である。・臨床症状に基づいて診断され、最近、日本における診断基準が樹立された。同診断基準に基づいて、わが国における全国疫学調査が実施された。・致死的疾患であるので、疑診の段階でも治療を開始することが肝要である。(著者抄録)

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.

The aim of this study was to determine whether a relatively low dose of pioglitazone or metformin was effective in diabetic patients with metabolic syndrome. Fifty diabetic patients with metabolic syndrome were randomly assigned to a low-dose pioglitazone (15mg/day) treatment group or a low-dose metformin (500mg/day) treatment group. Drugs were administered for 12 weeks. Systolic and diastolic blood pressure, heart rate, body mass index, triglyceride (TG), HDL and LDL-cholesterol, fasting plasma glucose (FPG), fasting plasma insulin (IRI), postprandial glucose, and HOMA-IR in the 75gOGTT, HbA1c, high-sensitivity CRP (hs-CRP) determined by cervical artery echography, and pulse wave velocity (PWV) were measured before/after 12-week drug administration. Significant decreases in HbA1c and HOMA-IR were noted in the pioglitazone group, along with significant decreases in TG, AST, ALT, blood pressure, hs-CRP and PWV. Significant decreases in HbA1c, HOMA-IR, BMI and waist circumference were noted in the metformin group. The pioglitazone group significantly improved the values for ALT, systolic blood pressure, hs-CRP and PWV compared to the metformin group. However, the metformin group demonstrated significant improvement in BMI compared with the pioglitazone group. Using a low dose regimen, pioglitazone significantly improved blood pressure and hepatic function and may be more effective than metformin to reduce risk factors in Japanese diabetic patients with metabolic syndrome at preventing atherosclerosis.

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 D-2 (PGD(2)) on Pi transport activity and its intracellular signaling mechanism in MC3T3-E1 osteoblast-like cells. PGD(2) 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 PGD(2) on Pi uptake. The selective inhibitors of mitogen-activated protein (MAP) kinase pathways such as ERK, p38 and Jun kinases suppressed PGD(2)-induced Pi uptake. The inhibitors of phosphaticlylinositol (PI) 3-kinase and S-6 kinase reduced this effect of PGD(2), while Akt kinase inhibitor did not. These results suggest that PGD(2) 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 S-6 kinase. (C) 2009 Elsevier Ltd. All rights reserved.

Telomerase (TA) activity is known to be present in malignant tumor cells, but not in most somatic differentiated cells. TA shows relatively high activity in thyroid cancer cells, but reports vary. This fact prompted us to elucidate whether cell component inhibitors of TA in the thyroid follicles can modulate its activity. The activity of TA extracted from Hela cells was inhibited by mixing with the supernatant fraction of human thyroid tissue extract. To examine the effect of iodine, thyroid hormones (-T3 and -T4) and human thyroglobulin (hTg) contained in the thyroid follicles, -T3, -T4 and hTg were added to the TRAP assay system in vitro, using TA from Hela cells. Iodine, -T3 and -T4 did not affect TA activity, but hTg inhibited the TA activity in a dose-dependent manner (IC50 of hTg: ca 0.45M: inhibiting concentration of hTg was from 0.15M to 3.0M). The hTg inhibition was not evident in the RT-PCR system, suggesting no effect of hTg on Taq DNA polymerase activity. The hTg inhibition of TA activity was attenuated by dNTP but not significantly by TS primer. These data suggest that hTg contained in thyroid follicular cells of various thyroid diseases may affect the TA activity measured in biopsied thyroid specimens, and that the reduction of the TA activity by hTg may induce slow progression and growth, and low grade malignancy of thyroid cancer, particularly differentiated carcinoma.