Molecular Dynamics Reveals New Insights into Activation of the Insulin-Family Proteins and their Binding Specificity to the Insulin Receptor

Abstract

The insulin-family proteins, which include insulin, as well as insulin-like growth factors I (IGF-I) and II (IGF-II), share high homology in their structure and sequence. Insulin and IGFs are implicated in chronic diseases such as diabetes and cancer, respectively. Interestingly, while the insulin-family proteins bind to their own receptors, IGF-II can also bind to the A-isoform of the insulin receptor (IR-A), activating unique and alternative signalling pathways, from those of insulin, to stimulate cancer cell growth. This provides a mechanism by which cancer cells can appear to be resistant to treatments targeting the IGF-1 receptor. Consequently, the way in which IGFs interact with and bind to the IR-A is of particular interest. Although extensive studies of insulin have revealed that its activation is associated with the opening of the B-chain-C-terminal (BC-CT), the activation mechanisms of the IGFs still remain unknown, yet are essential for the prediction and design of therapeutic antibodies. To this end, we performed Molecular Dynamics (MD) simulations to elucidate the activation dynamics and energetics of IGFs, to identify the differences and similarities in their activation behaviour, and to contrast them against those of insulin. Our MD simulations revealed, in particular, that insulin and IGFs exhibit stochastic dynamics during their activation, which involves two opening locations in the IGFs compared to the only one in insulin. Moreover, the occurrence of these two openings is simultaneous in IGF-I, but not in IGF-II, which exhibits two independently occurring openings. This suggests that the IGFs undergo different activation mechanisms for receptor binding. The probabilities of the active/inactive states of the proteins are further suggestive of IGF-II having a hyperactive nature compared to IGF-I and insulin. Summarising, our MD simulations have elucidated the crucial differences and similarities in the activation mechanisms of the insulin-family proteins, providing new insights into their binding specificity to the insulin receptor that could not readily be gleaned experimentally.