Different intracellular signalling sensitivity and cell behaviour of porcine insulin with aging

Insulin has many important biological functions. Insulin interacts with the insulin receptor (IR) to play its physiological role and execute its functions. Here, we isolated porcine hepatocytes from young and aged pigs, which endogenously express the IR, as a model to study the intracellular signalling properties and cellular behaviour of insulin with aging. Firstly, we analysed the intracellular signal transduction that is triggered by insulin in porcine hepatocytes that were isolated from young and aged pigs and found that insulin can strongly activate insulin receptor subunit (IRS), protein kinase B (AKT), and GSK in a time- and dose-dependent manner in hepatocytes from young pigs. On the contrary, the signalling response to insulin in hepatocytes from aged pigs was significantly reduced compared to that of the young pig. Secondly, the different subcellular locations of insulin/insulin receptor (IR) may result in different biological activities, although nuclear-localized insulin/IR still could exhibit important functions and roles. We found that insulin can translocate into cell nuclei in the hepatocytes of the young pigs; however, insulin/insulin receptor fails to transports into the cell nucleus in hepatocytes from aged pigs, although insulin/insulin receptor could internalize into cell cytoplasm. In summary, in the current study, we explored and compared for the first time insulin's behaviour and signalling properties in the cells of young pig hepatocytes and aged pig hepatocytes. Furthermore, we found that the insulin signalling response in hepatocytes was significantly reduced with age; more importantly, we found that the cell behaviour of insulin was changed significantly in the hepatocytes from aged pigs compared to young pigs, and it is noteworthy that insulin/IR cannot translocate into the cell nuclei in the hepatocytes from the aged pig. This may be a potential new reason contributing to insulin resistance with aging, suggesting that we need to study the reason for insulin resistance from a new point of view.