Proproliferative function of adaptor protein GRB10 in prostate carcinoma

Growth factor receptor-binding protein 10 (GRB10) is a well-known adaptor protein and a recently identified substrate of the mammalian target of rapamycin (mTOR). Depletion of GRB10 increases insulin sensitivity and overexpression suppresses PI3K/Akt signaling. Because the major reason for the limited efficacy of PI3K/Akt-targeted therapies in prostate cancer (PCa) is loss of mTOR-regulated feedback suppression, it is therefore important to assess the functional importance and regulation of GRB10 under these conditions. On the basis of these background observations, we explored the status and functional impact of GRB10 in PCa and found maximum expression in phosphatase and tensin homolog (PTEN)-deficient PCa. In human PCa samples, GRB10 inversely correlated with PTEN and positively correlated with pAKT levels. Knockdown of GRB10 in nontumorigenic PTEN null mouse embryonic fibroblasts and tumorigenic PCa cell lines reduced Akt phosphorylation and selectively activated a panel of receptor tyrosine kinases. Similarly, overexpression of GRB10 in PTEN wild-type PCa cell lines accelerated tumorigenesis and induced Akt phosphorylation. In PTEN wild-type PCa, GRB10 overexpression promoted mediated PTEN interaction and degradation. PI3K (but not mTOR) inhibitors reduced GRB10 expression, suggesting primarily PI3K-driven regulation of GRB10. In summary, our results suggest that GRB10 acts as a major downstream effector of PI3K and has tumor-promoting effects in prostate cancer.-Khan, M. I., Al Johani, A., Hamid, A., Ateeq, B., Manzar, N., Adhami, V. M., Lall, R. K., Rath, S., Sechi, M., Siddiqui, I. A., Choudhry, H., Zamzami, M. A., Havighurst, T. C., Huang, W., Ntambi, J. M., Mukhtar, H. Proproliferatve function of adaptor protein GRB10 in prostate carcinoma.