Abstract 1908: Kinome rewiring upon acquisition of resistance to PI3K inhibitors in ER+ breast cancer

Abstract

Abstract Hyperactivation of the phosphatidylinositol-3 kinase (PI3K) pathway is a critical step in oncogenesis and plays a role in the development of treatment resistance in estrogen receptor-positive (ER+) breast cancer. Targeted inhibition of PI3K (PI3Ki) was recently approved in combination with endocrine therapy for advanced ER+ breast cancer. As most tumor-targeted therapies are encumbered by the development of secondary resistance, there remains a need to identify novel therapeutic strategies to prevent and overcome PI3Ki resistance. We treated mice bearing anti-estrogen-resistant ER+ breast cancer xenografts with the anti-estrogen fulvestrant +/- the pan-PI3Ki GDC-0941. The drug combination induced partial regression of tumors, followed by regrowth despite continued drug treatment. We harvested tumors before, during, and after acquisition of resistance to the drug combination and performed quantitative phosphoproteomic profiling. Using LC-MS/MS, we identified 203 phosphopeptides and 227 unique phosphorylation sites in the drug-resistant tumors, with clusters of 87 upregulated and 116 downregulated phosphopeptides. The NetPhorest analysis platform predicted hyperactivation of 6 kinases and hypoactivation of 16 kinases in drug-resistant tumors. Notably, two hyperactive kinases, MAPK1 and HIPK2, were predicted to target substrates MARKS (at Ser27 in the calcium binding domain) and NS1BP (at Ser322 in the homeodomain kelch repeat). MARCKS is a filamentous actin cross-linking protein, while the kelch repeats of NS1BP are known to associate with F-actin to stabilize actin filaments. In addition, the most significant predicted kinase among the cluster of 116 downregulated phosphopetides in the drug-resistant tumors was “PKCiota”, predicted to target PARD3 at Ser852. Previous work showed that PARD3 is central for the formation of tight junctions. These data collectively suggest a dynamic role of kinase-substrate interactions involved in resistance to combined anti-estrogen/PI3Ki treatment, including filamentation actin formation and stabilization, as well as the deregulation of tight junction formation. Citation Format: Jacqueline Reedy Griffin, Nicole Traphagen, Arminja Kettenbach, Todd Miller. Kinome rewiring upon acquisition of resistance to PI3K inhibitors in ER+ breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1908.