Sustained signalling from the insulin receptor after stimulation with insulin analogues exhibiting increased mitogenic potency.
The metabolic and mitogenic potencies of six different insulin analogues were determined by measuring glucose transport in primary adipocytes and DNA synthesis in CHO cells respectively. Three analogues showed a disproportionately high mitogenic potency compared with their metabolic potency, and were up to 7 times more mitogenically than metabolically potent when compared with human insulin. The mitogenic/metabolic potency ratio of the analogues was found to be inversely correlated with the insulin receptor dissociation rate constant (Kd) in an exponential fashion (r=0.99), with a disproportionately greater increase in mitogenic potential compared with metabolic potential for analogues with Kd values of less than 40% of that of human insulin. To investigate the molecular mechanisms behind the correlation between the increased half-life of the receptor-ligand complex (low Kd) and mitogenicity, 3 h time-course experiments were performed. Slow ligand dissociation from the insulin receptor induced a parallel sustained activation of the insulin receptor tyrosine kinase. A similar pattern was observed for insulin receptor autophosphorylation and Shc phosphorylation, whereas the duration of insulin receptor substrate-1 phosphorylation with low-Kd analogues and with insulin was similar Thus the increased half-life of the ligand-receptor complex induces sustained activation of the insulin receptor tyrosine kinase and sustained phosphorylation of Shc, which may be the cause of the disproportionately high mitogenic potency seen for some insulin analogues.