Background

Aims/hypothesis Insulin analogues were developed to improve the pharmacological properties of injected insulin and to better mimic endogenous insulin output. However, certain insulin analogues have been suggested to display IGF-I-like biological activities. Furthermore, several recent epidemiological studies have suggested a potential increase in cancer risk for treatment of diabetes patients with longacting analogue insulin glargine (A21Gly,B31Arg,B32Arg human insulin). Additional studies, however, reported no increased cancer risk. The purpose of the present study was to identify the receptor(s) and signal transduction pathways responsible for the biological actions of insulin glargine and insulin detemir (B29Lys[ε-tetradecanoyl],desB30 human insulin). Methods The colon cancer-derived cell line HCT116 was treated with increasing doses of insulin glargine, insulin detemir, regular insulin or IGF-I, and receptor activation was evaluated by immunoprecipitation assays. IGF-I receptor (IGF-IR) internalisation following insulin glargine treatment was assessed by confocal microscopy. Activation of the Akt and extracellular signal-regulated kinase pathways was evaluated by western blots. The anti-apoptotic effect of the analogues was measured by poly-(ADP ribose) polymerase antibody and annexin assays. Results We found evidence for dual activation of the insulin receptor and IGF-IR by the analogues. Dosedependency experiments showed that insulin glargine was able to phosphorylate the IGF-IR at fivefold lower doses than those required to activate the insulin receptor. We also showed that insulin glargine can lead to prolonged activation of the receptors and therefore promote abnormal signalling. Confocal imaging experiments showed that insulin glargine, but not regular insulin induced IGF-IR internalisation similarly to IGF-I. Finally, both analogues displayed IGF-I-like anti-apoptotic activities and stimulated cell cycle progression. Conclusions/interpretation Our data indicate that insulin glargine and insulin detemir display atypical signalling activities that differ from those elicited by regular insulin and involve activation of the anti-apoptotic IGF-IR.

The introduction of insulin analogues has had a major impact in diabetes care. However, the benefit of some of these new insulins for the patient has yet to be demonstrated. Furthermore, research is needed to clarify whether insulin glargine is more strongly associated with cancer risk compared with other insulins.

We thank the team of Eli Lilly and Company for their critical appraisal of our study and for giving us the opportunity to clarify a number of points [1]. We agree with Kazda et al. that randomized clinical trials will best address any direct relationship between insulin analogues and cancer in man. We do not question the fact that tight control of blood glucose has dramatically advanced the lives of patients with diabetes. This article is protected by copyright. All rights reserved.