Abstract 3957: SRC inhibitors dephosphorylate and reactivate the DLC1 tumor suppressor: A new biomarker of clinical response
Authors: Tripathi Brajendra K. , Anderman Meghan , Qian Xiaolan , Zhou Ming , Wang Dunrui , Papageorge Alex G. , Lowy Douglas R. 
Published: August 2018

Abstract
Although activation of oncogenes and inactivation of tumor suppressor genes frequently play cooperative roles in pathogenesis of cancer, most targeted cancer treatments are focused on oncogene inhibitors, with relatively little consideration given to their concomitant effects on tumor suppressors. In this study, we have identified a direct relationship between the SRC kinase and the tumor suppressor DLC1, determined that SRC phosphorylates and attenuates the tumor suppressor activity of DLC1, and explored whether reactivation of DLC1 may be used as a biomarker for the clinical response to SRC inhibition. Several human cancers have high SRC activity and constitutive up-regulation of RhoA-GTP, which is a pro-oncogenic driver that is implicated in several processes of tumor formation and its progression to metastasis. We determined that SRC directly phosphorylates two Tyrosines in DLC1, Y451 in the linker region and Y701 in the Rho-GAP domain, and inhibits the Rho-GAP activity and tumor suppressor functions of DLC1. SRC phosphorylation of DLC1-Y701 attenuates the hydrolysis of active RhoA-GTP to inactive RhoA-GDP, resulting in an increase in RhoA-GTP levels and its downstream signaling, while SRC phosphorylation of DLC1-Y451 abolishes the binding of tensin to DLC1, which further reduces DLC1 tumor suppressor activity independently of RhoA-GTP. The ability of SRC to phosphorylate its target Tyrosines in DLC1 is facilitated by the ERK-dependent phosphorylation of DLC1-S129, which enhances the binding of the SRC SH3 domain to DLC1 and increases the SRC-dependent phosphorylation of Y451 and Y701 in DLC1. The potentially reversible nature of the SRC phosphorylations of DLC1 may be useful as a biomarker of clinical response. In two models that have high SRC activity and express DLC1 (the mouse MMTV-PyMT breast cancer model and xenografts from a human lung cancer cell line), a 5-day treatment with the SRC inhibitors Saracatanib and Bosutinib have potent antitumor activity (~75% reduction in tumor weight) in conjunction with reducing phosphorylation of SRC target Tyrosines in DLC1 and reactivating DLC1 Rho-GAP activity, which reduces cellular RhoA-GTP levels and its downstream signaling. However, Saracatanib and Bosutinib have much less antitumor activity (~20% reduction in tumor weight) in isogenic versions of the lung cancer xenografts if the SRC inhibitors do not reactivate DLC1, as occurs when DLC1 is not expressed or the SRC phosphorylatable Tyrosines in DLC1 are mutated. Our results highlight the potential importance of DLC1 tumor suppressor reactivation as a biomarker for predicting and monitoring the response to SRC inhibitors.
Citation Format: Brajendra K. Tripathi, Meghan Anderman, Xiaolan Qian, Ming Zhou, Dunrui Wang, Alex G. Papageorge, Douglas R. Lowy. SRC inhibitors dephosphorylate and reactivate the DLC1 tumor suppressor: A new biomarker of clinical response [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3957.