SummaryWe have recently shown that leptin mimicks insulin effects on glucose transport and glycogen synthesis through a phosphatidylinositol-3 (PI) kinase dependent pathway in C2C12 myotubes. The aim of the present study was to identify the signalling path from the leptin receptor to the PI-3 kinase. We stimulated C2C12 myotubes with insulin (100 nmol/l, 5 min) or leptin (0.62 nmol/l, 10 min) and determined PI-3 kinase activity in immunoprecipitates with specific non-crossreacting antibodies against insulinreceptor substrate (IRS 1/IRS 2) and against janus kinase (JAK 1 and JAK 2). While insulin-stimulated PI-3 kinase activity is detected in IRS-1 and IRS-2 immunoprecipitates, leptin-stimulated PI-3 kinase activity is found only in IRS-2 immunoprecipitates, suggesting that the leptin signal to PI-3 kinase occurs via IRS-2 and not IRS-1. Leptin-, but not insulin-stimulated PI-3 kinase activity is also detected in immunoprecipitates with antibodies against JAK-2, but not JAK-1. The data suggest that JAK-2 and IRS-2 couple the leptin signalling pathway to the insulin signalling chain. Since we have also detected leptin-stimulated tyrosine phosphorylation of JAK-2 and IRS-2 in C2C12 myotubes it can be assumed that leptin activates JAK-2 which induces tyrosine phosphorylatjon of IRS-2 leading to activation of PI-3 kinase. As we could not detect the long leptin receptor isoform in C2C12 myotubes we conclude that this signalling pathway is activated by a short leptin receptor isoform. [Diabetologia (1997[Diabetologia ( ) 40: 1358[Diabetologia ( -1362 Keywords leptin, leptin receptor, insulin receptor, phosphatidylinositol kinase, janus kinaseThe ob -gene product leptin has been defined as a regulator of food intake and energy expenditure [1]. Identification and cloning of specific leptin receptors, which exist in different isoforms, has recently provided new insights into the mechanism and physiological function of leptin signalling in different tissues [2,3]. It was shown that effects of leptin on food intake

Aims/hypothesis: Present guidelines for the treatment of type 2 diabetes recommend HbA 1 c values of less than 7%. As beta cell function worsens during progress of the disease, insulin therapy is often necessary to achieve this ambitious goal. However, due to peripheral insulin resistance, many patients need rather high insulin dosages. In the light of the extremely high cardiovascular risk of diabetic patients, it is important to determine whether high concentrations of insulin or its frequently used analogues are harmful to the cardiovascular system. We therefore investigated the modulatory effects of regular human insulin and its analogue glargine on proliferation and apoptosis of human coronary artery endothelial cells (HCAECs) and human coronary artery smooth muscle cells (HCASMCs). Methods: Cells were treated with regular human insulin or insulin glargine. Proliferation was determined by [ 3 H]thymidine incorporation and by flow cytometric analysis of Ki-67 expression. Apoptosis was assessed by flow cytometry (cell cycle analysis and annexin V staining) and determination of caspase-3 activity. Results: HCAECs and HCASMCs treated with regular human insulin or insulin glargine did not show significant increases in DNA synthesis or Ki-67 expression. Administration of regular human insulin or insulin glargine did not modulate the extent of apoptotic events. No influence of insulin on lipoapoptotic vascular cell death could be detected.Conclusions/interpretation: Taken together, neither regular human insulin nor insulin glargine influences growth and apoptosis of human coronary artery cells in vitro. Our data do not suggest that regular human insulin or insulin glargine promote atherosclerosis through mechanisms affecting the cellularity of human coronary arteries.

In recent years, recombinant DNA technology has been used to design insulin molecules that overcome the limitations of regular insulin in mealtime supplementation. However, safety issues have been raised with these alternatives, as the alteration of the three-dimensional structure may alter the interaction with the insulin and/or IGF-I receptors and therefore lead to the activation of alternate metabolic as well as mitogenic signaling pathways. It is therefore essential to carefully study acute and long-term effects in a preclinical state, as insulin therapy is meant to be a lifelong treatment. In this study, we determined in vivo the insulin receptor signaling characteristics activated by insulin glulisine (Lys B3 , Glu B29 ) at the level of insulin receptor phosphorylation, insulin receptor substrate phosphorylation, and downstream signaling elements such as phosphatidylinositol (PI) 3-kinase, AKT, and mitogenactivated protein kinase. C57BL/6 mice were injected with insulin glulisine or regular insulin and Western blot analysis was performed for liver and muscle tissue. The extent and time course of insulin receptor phosphorylation and activation of downstream signaling elements after insulin glulisine treatment was similar to that of human regular insulin in vivo. Moreover, insulin signaling in hypothalamic tissue determined by PI 3-kinase activity was comparable. Therefore, insulin glulisine may be a useful tool for diabetes treatment.

Aims/hypothesis: New insulin analogues have been created by amino-acid exchange to provide an improved pharmacokinetic profile. However, safety issues have been raised regarding their use, as amino-acid exchange of insulin may induce altered metabolic and mitogenic effects. For example, the insulin analogue Asp(B10) causes breast cancer in rodents. B10). Materials and methods:We analysed insulin receptor binding characteristics and dissociation kinetics, as well as insulin-induced receptor auto-and dephosphorylation kinetics, in rat-1 fibroblasts overexpressing the human insulin receptor isoform B. Mitogenic activity was tested in the non-malignant cell line MCF10. Results: Regular insulin, HMR1964 and HMR1423 showed no significant differences in receptor association, dissociation and receptor binding affinity, while Asp(B10) displayed markedly increased insulin receptor affinity. All of the analogues induced rapid insulin receptor autophosphorylation, reaching a maximum 10 min after stimulation (10 −9 mmol/l insulin). In contrast, Asp(B10) induced a prolonged phosphorylation and dephosphorylation state of the 95 kDa insulin receptor β-subunit. With respect to [ 3 H]thymidine incorporation, the new analogues had similar (HMR1423) or even lower (HMR1964) effects than regular insulin in the mammary epithelial cell line MCF10, while Asp(B10) showed increased [ 3 H]thymidine incorporation. Conclusions/interpretation: HMR1964 and HMR1423 displayed the same association, dissociation and insulin receptor affinity kinetics as regular insulin, and might therefore be useful for the treatment of diabetes.

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