The role of plant extracts on islet function in vivo and in vitro
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 Gymnema sylvestre (GS) and Costus pictus (CP) are reported to have antidiabetic activity in vivo. The exact molecular mode of action(s) of GS and CP is unclear but the antihyperglycemic effect seen in animal studies was associated with dramatic increases in insulin secretion so in my thesis I have examined the effect of alcoholic aqueous GS extract, named OSA®, on blood glucose and plasma insulin levels in vivo from animals and humans with T2DM and I have measured the effect of GS aqueous alcoholic and CP methanolic extracts on insulin secretion in vitro from the MIN6 β-cell line and isolated mouse and human islets. The in vivo data showed that OSA® increased insulin levels and reduced blood glucose in patients with T2DM and ameliorated glucose intolerance in animal model of diabetes. The in vitro data demonstrated that OSA® and CP have a direct stimulatory effect on insulin secretion which was not associated with compromised membrane integrity or decreased β-cell viability. OSA®-, but not CP-, induced insulin secretion was coupled with elevations in insulin gene expression suggesting that GS may reserve β-cell insulin store following chronic stimulation of insulin. Single cell calcium microfluorimetry measurements showed that OSA® and CP elevated intracellular Ca2+ concentrations ([Ca2+]i) in Fura-2-loaded β-cell, 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 OSA® and CP is through stimulating insulin secretion which may be mediated, in part, by the ability of OSA® and CP to increase [Ca2+]i levels through VGCC. In addition, OSA®-induced insulin secretion was partially associated with protein kinase activation and was independent of classical PKC, CaMKII or cAMP activation. OSA® was also found to protect β-cells from cytokine-induced apoptosis. Our measurement of caspase-3 and -7 production showed partial reduction in OSA®-treated MIN6 cells and mouse islets following cytokines exposure. These data were further supported by our microarray analysis of mouse islets when challenged with cytokines in the presence of OSA®. Enrichment analysis indicated that OSA® reduced caspase-3 gene expression most likely through activation of PI3K/AKT pathway and subsequent downstream effectors including MnSOD.
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