Investigation of potential intervention targets to improve insulin action: a therapeutic approach
Impaired insulin action (insulin resistance) is a key factor in the pathogenesis of diabetes mellitus. To investigate therapeutic targets against insulin resistance, this thesis explores the mechanism of action of pharmacological agents and exogenous peptides known or suspected to modify insulin action. These included leptin, a hormone primarily involved in the regulation of body weight; sibutramine, an antiobesity agent; plant-derived compounds (pinitol and chamaemeloside) and agents known to affect insulin sensitivity, e.g. metformin, tolbutamide, thiazolidinediones, vanadyl sulphate and thioctic acid. Models used for investigation included the L6 skeletal muscle cell line and isolated skeletal muscles. In vivo studies were undertaken to investigate glycaemia, insulinaemia, satiety and body weight in streptozotocin-induced diabetic mice and obese (ob/ob) mice. Leptin acutely altered insulin action in skeletal muscle cells via the short form of the leptin receptor. This direct action of leptin was mediated via a pathway involving PI 3-kinase but not Jak2. The active metabolites of sibutramine had antidiabetic properties in vivo and directly improved insulin sensitivity in vitro. This effect appeared to be conducted via a non-PI 3-kinase-mediated increase in protein synthesis with facilitated glucose transport, and was independent of the serotonin and noradrenaline reuptake inhibition produced by sibutramine. Pinitol (a methyl inositol) had an insulin mimetic effect and was an effective glucose-lowering agent in insulinopenic states, acting directly on skeletal muscle. Conversely chamaemeloside appeared to improve glucose tolerance without directly altering glucose transport. Metformin directly increased basal glucose uptake independently of PI 3-kinase, possibly via an increase in the intrinsic activity of glucose transporters. Neither tolbutamide nor thiazolidinediones directly altered insulin sensitivity in L6 skeletal muscle cells: however vanadyl sulphate and thioctic acid increased glucose transport but appeared to exert toxic effects at therapeutic concentrations. Examination of glucose transport in skeletal muscle in this thesis has identified various components of post-receptor insulin signalling pathways which may be targeted to ameliorate insulin resistance.