Prostate cancer (PCa) is manageable in its initial stages and when localized, but as it progresses it metastasizes and also becomes resistant to conventional chemotherapies. Identifying mechanisms that are related to the rapid progression of the disease could result in development of appropriate intervention strategies. We analyzed protein and mRNA expression of Grb10 (also known as insulin receptor-binding protein) in various PCa cell lines and found a significant decrease when compared to normal prostate epithelial cells. We also analyzed the expression pattern of Grb10 in a human PCa tissue array and found significant loss of Grb10 mRNA in more than 90% of the samples. Based on Gleason Score, the loss of Grb10 mRNA correlated with the stage and grade of the disease. These initial observations led us to hypothesize that Grb10 may act as a tumor suppressor whose expression is lost during the progression of human PCa. Using immuno-histochemical analysis of human PCa tissue samples we found loss of Grb10 protein with increasing tumor grade. We next analyzed various components of insulin signaling, based on the evidence that Grb10 expression is negatively correlated with insulin signaling. We observed significant increase at both mRNA and protein levels of IR, IGF-1R, IRS-1 and IRS-2 in PCa cells and tissues that directly correlated with loss of Grb10. Recently, Grb10 expression was shown to be negatively correlated with PTEN levels in different cancer types including PCa. We determined the expression pattern of Grb10 mRNA in prostate tissue samples from 15-weeks old PTEN wild type (WT) and PTEN knockout (KO) mice and observed that Grb10 levels were significantly down-regulated in PTEN-KO mice suggesting that loss of PTEN results in loss of Grb10. Further analysis suggested increased insulin signaling in PTEN-KO mice compared to PTEN-WT mice confirming negative regulation of Grb10 with insulin signaling. To further understand the role of Grb10 in PCa, we overexpressed Grb10 in human PCa DU145 (PTEN-mutated) cells. Overexpression of Grb10 resulted in decrease in colony formation, migration and invasion that was associated with down-regulation of insulin signaling. In summary, our findings suggest that Grb10 is a negative regulator of insulin signaling whose expression is lost in PCa in humans and correlates well with disease progression and aggressiveness. We conclude that Grb10 is a tumor suppressor in human PCa whose loss promotes rapid progression of the disease. Identification of this novel role of Grb10 could open up new strategies for the management of human PCa.

Citation Format: Mohammad Imran Khan, Vaqar M. Adhami, Rahul Kumar Lall, Deeba Nadeem Syed, Imtiaz Ahmad Siddiqui, Bilal Bin Hafeez, Ajit Kumar Verma, Hasan Mukhtar. Grb10, a negative regulator of insulin signaling, is a tumor suppressor in prostate cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4312. doi:10.1158/1538-7445.AM2013-4312