Diverse actions of calcitonin gene-related peptide on intracellular free Ca2+ concentrations in UMR 106 osteoblastic cells

Distinguish fatty acids impact survival, differentiation and cellular function of periodontal ligament fibroblasts

Alveolar bone (AB) remodeling is necessary for the adaption to mechanical stimuli occurring during mastication and orthodontic tooth movement (OTM). Thereby, bone degradation and assembly are strongly regulated processes that can be altered in obese patients. Further, increased fatty acids (FA) serum levels affect bone remodeling cells and we, therefore, investigated whether they also influence the function of periodontal ligament fibroblast (PdLF). PdLF are a major cell type regulating the differentiation and function of osteoblasts and osteoclasts localized in the AB. We stimulated human PdLF (HPdLF) in vitro with palmitic (PA) or oleic acid (OA) and analyzed their metabolic activity, growth, survival and expression of osteogenic markers and calcium deposits. Our results emphasize that PA increased cell death of HPdLF, whereas OA induced their osteoblastic differentiation. Moreover, quantitative expression analysis of OPG and RANKL revealed altered levels in mechanically stimulated PA-treated HPdLF. Furthermore, osteoclasts stimulated with culture medium of mechanical stressed FA-treated HPdLF revealed significant changes in cell differentiation upon FA-treatment. For the first time, our results highlight a potential role of specific FA in the function of HPdLF-modulated AB remodeling and help to elucidate the complex interplay of bone metabolism, mechanical stimulation and obesity-induced alterations.

Calcitonin and Bone Physiology: In Vitro, In Vivo, and Clinical Investigations

Calcitonin was discovered as a peptide hormone that was known to reduce the calcium levels in the systemic circulation. This hypocalcemic effect is produced due to multiple reasons such as inhibition of bone resorption or suppression of calcium release from the bone. Thus, calcitonin was said as a primary regulator of the bone resorption process. This is the reason why calcitonin has been used widely in clinics for the treatment of bone disorders such as osteoporosis, hypercalcemia, and Paget's disease. However, presently calcitonin usage is declined due to the development of efficacious formulations of new drugs. Calcitonin gene-related peptides and several other peptides such as intermedin, amylin, and adrenomedullin (ADM) are categorized in calcitonin family. These peptides are known for the structural similarity with calcitonin. Aside from having a similar structure, these peptides have few overlapping biological activities and signal transduction action through related receptors. However, several other activities are also present that are peptide specific. In vitro and in vivo studies documented the posttreatment effects of calcitonin peptides, i.e., positive effect on bone osteoblasts and their formation and negative effect on osteoclasts and their resorption. The recent research studies carried out on genetically modified mice showed the inhibition of osteoclast activity by amylin, while astonishingly calcitonin plays its role by suppressing osteoblast and bone turnover. This article describes the review of the bone, the activity of the calcitonin family of peptides, and the link between them.

Immature osteoblastic MG63 cells possess two calcitonin gene-related peptide receptor subtypes that respond differently to [Cys(Acm)(2,7)] calcitonin gene-related peptide and CGRP(8-37)

Calcitonin gene-related peptide (CGRP) is clearly an anabolic factor in skeletal tissue, but the distribution of CGRP receptor (CGRPR) subtypes in osteoblastic cells is poorly understood. We previously demonstrated that the CGRPR expressed in osteoblastic MG63 cells does not match exactly the known characteristics of the classic subtype 1 receptor (CGRPR1). The aim of the present study was to further characterize the MG63 CGRPR using a selective agonist of the putative CGRPR2, [Cys(Acm)(2,7)]CGRP, and a relatively specific antagonist of CGRPR1, CGRP(8-37). [Cys(Acm)(2,7)]CGRP acted as a significant agonist only upon ERK dephosphorylation, whereas this analog effectively antagonized CGRP-induced cAMP production and phosphorylation of cAMP response element-binding protein (CREB) and p38 MAPK. Although it had no agonistic action when used alone, CGRP(8-37) potently blocked CGRP actions on cAMP, CREB, and p38 MAPK but had less of an effect on ERK. Schild plot analysis of the latter data revealed that the apparent pA2 value for ERK is clearly distinguishable from those of the other three plots as judged using the 95% confidence intervals. Additional assays using 3-isobutyl-1-methylxanthine or the PKA inhibitor N-(2-[p-bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide hydrochloride (H-89) indicated that the cAMP-dependent pathway was predominantly responsible for CREB phosphorylation, partially involved in ERK dephosphorylation, and not involved in p38 MAPK phosphorylation. Considering previous data from Scatchard analysis of [125I]CGRP binding in connection with these results, these findings suggest that MG63 cells possess two functionally distinct CGRPR subtypes that show almost identical affinity for CGRP but different sensitivity to CGRP analogs: one is best characterized as a variation of CGRPR1, and the second may be a novel variant of CGRPR2.

Immature human osteoblastic MG63 cells predominantly express a subtype 1-like CGRP receptor that inactivates extracellular signal response kinase by a cAMP-dependent mechanism

Although accumulated data suggest that calcitonin gene-related peptide (CGRP) produces anabolic effects in skeletal tissue by directly acting on osteogenic cells, neither the distribution of CGRP receptor subtypes nor the associated cellular signaling pathways are well understood. In this study, we have pharmacologically and biochemically characterized CGRP-binding sites in immature human osteoblastic MG63 cells. In a [125I]CGRP whole-cell-binding assay, nonlinear regression curve-fitting analysis demonstrated a single binding site (K(D)=405+/-29 pM; 13,100+/-223 sites per cell). Immunocytochemical and Western blot analyses demonstrated that 48-, 52-, and 120-kDa forms of the calcitonin receptor-like receptor (CRLR) and a 15-kDa form of the receptor-activity-modifying protein-1 (RAMP-1) was expressed on the plasma membrane. CGRP strongly stimulated cellular cAMP production and this effect was antagonized not only by an antagonist of the subtype-1 CGRP (CGRP(1)) receptor, CGRP-(8-37), but by an agonist of the putative subtype-2 CGRP (CGRP(2)) receptor, [Cys(Acm)(2,7)]-CGRP, that also itself acted as a weak agonist. In contrast to published data, CGRP dose- and time-dependently dephosphorylated and inactivated extracellular signal response kinase (ERK). This action was blocked by CGRP-(8-37), by an inhibitor of cAMP-dependent protein kinase (H-89), or by an inhibitor of protein phosphatases (vanadate). Prolonged CGRP treatments significantly suppressed DNA synthesis at 27 h, but up-regulated type I collagen. Both these actions were blocked by CGRP-(8-37) and mimicked by a specific inhibitor of ERK (PD98059). In summary, our data suggest that the CGRP receptors in MG63 cells meet many, but not all, of the classical criteria used to define CGRP(1) receptors. These receptors that functioned in a pharmacologically distinct manner could inhibit cell proliferation, and were substantially more sensitive to a CGRP(2) receptor agonist than are typical CGRP(1) receptors. These receptor proteins were not exactly matched with the known components of a CGRP(1) receptor that have been reported. Therefore, it is possible that the CGRP receptors expressed in immature osteoblastic human MG63 cells represent a variation of the known CGRP(1) receptor.

Activation of phospholipase C-beta1 via Galphaq/11 during calcium mobilization by calcitonin gene-related peptide

Interaction of calcitonin gene-related peptide (CGRP) with its receptors leads to stimulation of adenylyl cyclase and/or phospholipase C (PLC). While regulation of adenylyl cyclase is thought to involve the G-protein Gs, it is not known whether activation of PLC results from coupling the receptor to Gq family proteins or whether beta gamma subunits released from receptor-activated Gs activate PLC. We used human bone cells OHS-4 bearing CGRP receptors in which CGRP activates only the PLC signaling pathway to determine how CGRP acts. CGRP increased the concentration of intracellular calcium ([Ca2+]i) within 5 s via a Ca2+ influx through voltage-gated calcium channels and by mobilizing calcium from the endoplasmic reticulum. The activation of effectors, like PLC coupled to G-proteins, is the early event in the pathway leading to inositol 1,4,5-trisphosphate formation, which is responsible for Ca2+ mobilization. Western blotting demonstrated a range of PLC-beta isoforms (beta1, beta3, beta4, but not beta2) and G-proteins (Galphaq/11 and Galphas). Only phospholipase C-beta1 is involved in the mobilization of Ca2+ from the endoplasmic reticulum of Fura-2-loaded confluent OHS-4 cells and the formation of inositol 1,4,5-trisphosphate by CGRP; PLC-gamma have no effect. Activation of PLC-beta1 by CGRP involves the Galphaq/11 subunit, which is insensitive to pertussis toxin, but not Gbeta gamma subunits. We therefore believe that CGRP causes the activation of two separate G-proteins.

Effects of amylin on human osteoblast-like cells

Amylin has been reported to have bone-conserving effects. In the present study we evaluated the possible activity of the peptide on human osteoblast-like (hOB) cells in primary culture. Amylin between 10(-9) and 10(-6) M, dose-dependently stimulated cell proliferation with a maximal effect (200%) at 10(-6) M. In addition, amylin increased osteocalcin production when hOB cells were exposed to 1,25(OH)2D3 (10(-8)M) but there was a nonsignificant upward trend on alkaline phosphatase activity. The present results suggest that amylin could be included among the group of peptides endowed with osteogenic activity.