Endocrine Abstracts

Background: IGF1R targeting emerged in recent years as a promising therapeutic approach in prostate cancer (PCa). The insulin receptor (IR) shares high structure homology with the IGF1R and activates similar signaling cascades. In recent studies we have shown that insulin’s mitogenic activities in PCa cells are mediated via the IR. Given the central role of IR in regulation of metabolism, most anti-IGF1R therapeutic strategies were designed to specifically inhibit IGF1R, but not the IR. Therefore, the aim of this study was to investigate IR ability to compensate and mediate IGF1 mitogenic signals after IGF1R inhibition.

Methods: We employed P69 and C4-2 prostate cancer-derived cell lines, which express both IGF1R and IR. To specifically inhibit IGF1R we used monoclonal antibody IMAC-A12 (ImClone) or Thyrphostin (AG1024). Activation of receptors was assessed by immunoprecipitation assays. Expression levels of receptors and activation of signaling cascades were measured by western immunoblotting. Cells viability was measured by MTT assays.

Results: IGFI mainly activates its own receptor but also led to significant cross-activation of the IR. Results of IP assays showed that specific inhibition of IGF1R in C4-2 cells did not affect cross-activation of IR by IGF1. In P69 cells, on the other hand, there was a minor inhibition of cross-activation. In neither cell line, IGF1R inhibition affected the activity of signaling cascade proteins Akt and ERK. Finally, MTT assays revealed that, despite a significant reduction in IGF1R abundance on cell membranes, IMAC-A12 had no effect on cells viability.

Conclusion: In our model, specific inhibition of IGF1R had no effect on the activation of IR by IGFI, providing evidence for a compensation mechanism by IR following IGF1R inhibition. The evidence for a compensatory mechanism between the two receptors may be of future use for the local targeting of both IGF1R and IR in PCa.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.