The extracellular calcium-sensing receptor (CaR) is usually associated with systemic Ca 2C homeostasis, but the CaR is also expressed in many other tissues, including pancreatic islets of Langerhans. In the present study, we have used human islets and an insulin-secreting cell line (MIN6) to investigate the effects of CaR activation using the calcimimetic R-568, a CaR agonist that activates the CaR at physiological concentrations of extracellular Ca 2C . CaR activation initiated a marked but transient insulin secretory response from both human islets and MIN6 cells at a sub-stimulatory concentration of glucose, and further enhanced glucose-induced insulin secretion. CaR-induced insulin secretion was reduced by inhibitors of phospholipase C or calcium-calmodulindependent kinases, but not by a protein kinase C inhibitor. CaR activation was also associated with an activation of p42/44 mitogen-activated protein kinases (MAPK), and CaR-induced insulin secretion was reduced by an inhibitor of p42/44 MAPK activation. We suggest that the b-cell CaR is activated by divalent cations co-released with insulin, and that this may be an important mechanism of intra-islet communication between b-cells.

Leaves of the Gymnema sylvestre (GS) plant have been used to treat diabetes mellitus for millennia, but the previously documented insulin secretagogue effects of GS extracts in vitro may be non-physiological through damage to the β-cells. We have now examined the effects of a novel GS extract (termed OSA) on insulin secretion from the MIN6 β-cell line and isolated human islets of Langerhans. Insulin secretion from MIN6 cells was stimulated by OSA in a concentration-dependent manner, with low concentrations (0.06-0.25mg/ml) having no deleterious effects on MIN6 cell viability, while higher concentrations (≥0.5mg/ml) caused increased Trypan blue uptake. OSA increased β-cell Ca²⁺ levels, an effect that was mediated by Ca²⁺ influx through voltage-operated calcium channels. OSA also reversibly stimulated insulin secretion from isolated human islets and its insulin secretagogue effects in MIN6 cells and human islets were partially dependent on the presence of extracellular Ca²⁺. These data indicate that low concentrations of the GS isolate OSA stimulate insulin secretion in vitro, at least in part as a consequence of Ca²⁺ influx, without compromising β-cell viability. Identification of the component of the OSA extract that stimulates regulated insulin exocytosis, and further investigation of its mode(s) of action, may provide promising lead targets for Type 2 diabetes therapy.

Type 1 diabetes mellitus (T1DM) is an autoimmune disease caused by immune-mediated destruction of insulin-secreting β cells of the pancreas. Near complete dependence on exogenous insulin makes T1DM very difficult to control, with the result that patients are exposed to high blood glucose and risk of diabetic complications and/or intermittent low blood glucose that can cause unconsciousness, fits and even death. Allograft transplantation of pancreatic islets restores normoglycemia with a low risk of surgical complications. However, although successful immediately after transplantation, islets are progressively lost, with most of the patients requiring exogenous insulin within 2 years post-transplant. Therefore, there is an urgent requirement for the development of new strategies to prevent islet rejection. In this study, we explored the importance of human regulatory T cells in the control of islets allograft rejection. We developed a pre-clinical model of human islet transplantation by reconstituting NOD-scid IL2rγnull mice with cord blood-derived human CD34+ stem cells and demonstrated that although the engrafted human immune system mediated the rejection of human islets, their survival was significantly prolonged following adoptive transfer of ex vivo expanded human Tregs. Mechanistically, Tregs inhibited the infiltration of innate immune cells and CD4+ T cells into the graft by down-regulating the islet graft-derived monocyte chemoattractant protein-1. Our findings might contribute to the development of clinical strategies for Treg therapy to control human islet rejection. We also show for the first time that CD34+ cells-reconstituted NOD-scid IL2rγnull mouse model could be beneficial for investigating human innate immunity in vivo.

Plant-derived extracts have been used as folk remedies for Type 2 diabetes mellitus (T2DM) for many centuries, and offer the potential of cheap and readily available alternatives to conventional pharmaceuticals in developing countries. Extracts of Costus pictus (CP), a plant belonging to the Costaceae family, are reported to have antidiabetic activity in vivo. The exact molecular mode of action(s) of CP is unclear but the antihyperglycemic effect seen in animal studies was associated with dramatic increases in insulin secretion so in our study we have measured the effect of aqueous CP extract on insulin secretion in vitro from the MIN6 Β-cell line and isolated mouse and human islets. Our data demonstrate that CP has a direct stimulatory effect on insulin secretion at basal but not stimulatory glucose concentrations which was not associated with compromised membrane integrity or decrease Β-cell viability. Single cell calcium microfluorimetry measurements showed that CP caused elevations in Β-cell intracellular Ca²⁺ concentrations ([Ca²⁺]_i), an effect which was completely abolished by the removal of extracellular Ca²⁺ or blockade of voltage-gated Ca²⁺ channels (VGCC). These in vitro observations suggest that one mode of action of CP is through stimulating insulin secretion which may be mediated, in part, by the ability of CP to increase [Ca²⁺]_i levels through VGCC. CP extracts may provide an affordable and inexpensive alternative for treating patients with T2DM.

The mechanism by which ErbB2 exerts its oncogenic effect is poorly defined. In this article we show that ErbB2 co-expression slows ligand-dependent growth factor receptor downregulation in NIH 3T3 transfectants. Ligand dependence of cell growth and MAP kinase signaling are retained in epidermal growth factor receptor (EGFR) transfectants but are abolished in ErbB2-expressing cells, which grow and signal constitutively. In association with this phenomenon, we have noticed that ErbB2-expressing cells contain increased amounts of EGFR, which is hyperphosphorylated. EGFR overexpressors do not contain increased levels of ErbB2, however, tending to exclude a transfection artifact caused by saturation of receptor processing. EGF treatment of EGFR transfectants results in more rapid EGFR downregulation than occurs in ErbB2 transfectants, but Northern blot analysis demonstrates reduced basal EGFR gene expression in ErbB2 transfectants. We conclude that ErbB2 expression impairs EGFR downregulation via a posttranscriptional mechanism and propose that ErbB2 overexpression may sensitize tumor cells to the mitogenic effects of heterologous growth factors by retarding degradation of liganded heterodimers.