No abstract

Pi handling by osteogenic cells is important for bone mineralization. The role of Pi transport in BMP-2-induced matrix calcification was studied. BMP-2 enhances Pit-1 Pi transporters in osteogenic cells. Experimental analysis suggest that this response is required for bone matrix calcification.Introduction: Bone morphogenetic proteins (BMPs) are produced by osteogenic cells and play an important role in bone formation. Inorganic phosphate (Pi) is a fundamental constituent of hydroxyapatite, and its transport by osteogenic cells is an important function for primary calcification of the bone matrix. In this study, we investigated the role of Pi transport in BMP-2-induced matrix mineralization. Materials and Methods: Confluent MC3T3-E1 osteoblast-like cells were exposed to BMP-2 for various time periods. Pi and alanine transport was determined using radiolabeled substrate, Pit-1 and Pit-2 expression by Northern blot analysis, cell differentiation by alkaline phosphatase activity, matrix mineralization by alizarin red staining, and the characteristics of mineral deposited in the matrix by transmission electron microscopy, electron diffraction analysis, and Fourier transformed infrared resolution (FTIR). Results: BMP-2 time-and dose-dependently stimulated Na-dependent Pi transport in MC3T3-E1 cells by increasing the V max of the transport system. This effect was preceded by an increase in mRNA encoding Pit-1 but not Pit-2. BMP-2 also dose-dependently enhanced extracellular matrix mineralization, an effect blunted by either phosphonoformic acid or expression of antisense Pit-1. Enhanced Pi transport and matrix mineralization induced by BMP-2 were blunted by a specific inhibitor of the c-Jun-N-terminal kinase (JNK) pathway. Conclusions: Results presented in this study indicate that, in addition to its well-known effect on several markers of the differentiation of osteoblastic cells, BMP-2 also stimulates Pi transport activity through a selective increase in expression of type III Pi transporters Pit-1. In MC3T3-E1 cells, this effect is mediated by the JNK pathway and plays an essential role in bone matrix calcification induced by BMP-2.

The aim of this study was to investigate the in vivo and in vitro effects of exendin-4, a potent glucagon-like peptide 1 agonist, on the protection of the pancreatic b-cells against their cell death. In in vivo experiments, we used b-cell-specific calmodulinoverexpressing mice where massive apoptosis takes place in their b-cells, and we examined the effects of chronic treatment with exendin-4. Chronic and s.c. administration of exendin-4 reduced hyperglycemia. The treatment caused significant increases of the insulin contents of the pancreas and islets, and retained the insulin-positive area. Dispersed transgenic islet cells lived only shortly, and several endoplasmic reticulum (ER) stress-related molecules such as immunoglobulin-binding protein (Bip), inositol-requiring enzyme-1 a, X-box-binding protein-1 (XBP-1), RNA-activated protein kinase-like endoplasmic reticulum kinase, activating transcription factor-4, and C/EBP-homologous protein (CHOP) were more expressed in the transgenic islets. We also found that the spliced form of XBP-1, a marker of ER stress, was also increased in b-cellspecific calmodulin-overexpressing transgenic islets. In the quantitative real-time PCR analyses, the expression levels of Bip and CHOP were reduced in the islets from the transgenic mice treated with exendin-4. These findings suggest that excess of ER stress occurs in the transgenic b-cells, and the suppression of ER stress and resultant protection against cell death may be involved in the anti-diabetic effects of exendin-4.

No abstract

Context: Most patients with defective synthesis and/or secretion of thyroglobulin (Tg) present relatively high serum free T 3 (FT 3 ) concentrations with disproportionately low free T 4 (FT 4 ) resulting in a high FT 3 /FT 4 ratio. The mechanism of this change in FT 3 /FT 4 ratio remains unknown.Objective: We hypothesize that increased type 2 iodothyronine deiodinase (D2) activity in the thyroid gland may explain the higher FT 3 /FT 4 ratio that is frequently observed in patients with abnormal Tg synthesis.