Abstract 1923: Cytoplasmic localization of KRAS in Her2-amplified colorectal cancer induces an altered dependency of the oncogenic signaling pathway from RAS/MAPK to PI3K/AKT

Abstract Nowadays, KRAS-G12C specific inhibitor has been used for the treatment of patients with KRAS mutated non-small cell lung cancer, but not for colorectal cancer (CRC) patients. Recent clinical trials have shown that the efficacy of a single KRAS inhibitor treatment was limited for CRC due to multiple primary/adaptive resistance mechanisms. In the CRC, clinical trial with dual inhibition of KRAS-G12C and EGFR has demonstrated that combination therapy had improved partial response rate, suggesting that primary resistance to KRAS inhibitor is mainly caused by EGFR-mediated adaptation. Meanwhile, more than half of the primary resistance mechanisms are still unknown. To understand the intrinsic resistance mechanisms to KRAS inhibitors in CRC, KRAS G12C- or G12D-positive CRC patient-derived cells (PDCs) were established from surgically resected tumor specimens. Analysis of cell proliferation under KRAS inhibitor treatment or KRAS knockdown revealed these PDCs were not solely dependent on KRAS mediated growth signaling. To clarify the compensatory activated pathways in KRAS inhibitor, inhibitor library screening and sequencing analysis were performed among 8 KRAS-mutated PDCs. As a result, PDCs were categorized into EGFR inhibitor with KRAS inhibitor sensitive group and PI3K pathway activating group harboring PIK3CA mutation or/and ERBB2 amplification with KRAS mutation. To explore the novel intrinsic resistance mechanisms, we deeply analyzed one of the PDC named JC261, harboring PIK3CA, ERBB2, and KRAS alterations. We observed that PI3K pathway was highly activated in JC261 cells, and cell proliferation was mainly dependent on PI3K/AKT/mTOR pathway regardless the existence of KRAS-G12C mutation. To examine whether Her2 amplification contributes to the PI3K pathway dependency, we established Her2 knockout JC261 cells. Notably, Her2 knockout suppressed Her2-Her3-PI3K signaling and restored KRAS-MAPK dependency. To analyze the mechanism of how KRAS-MAPK pathway was activated in Her2 knockout cells, we focused on KRAS localization, since the MAPK activation is known to initiate from plasma membrane localization of KRAS after appropriate modifications. Remarkably, immunofluorescence staining of KRAS revealed that KRAS was aberrantly localized in the cytoplasm in JC261, while KRAS localization was changed to the plasm membrane in Her2 knockout JC261 cells. In addition, the cytoplasmic KRAS localization was also observed in Her2-amplified, KRAS-WT PDC (JC69). These results suggest that Her2 amplification may induce aberrant KRAS localization, resulting in low dependence on the KRAS-MAPK pathway. Thus, KRAS localization may regulate the balance between MAPK and PI3K signaling intensity. Citation Format: Kohei Maruyama, Satoshi Nagayama, Ryohei Katayama. Cytoplasmic localization of KRAS in Her2-amplified colorectal cancer induces an altered dependency of the oncogenic signaling pathway from RAS/MAPK to PI3K/AKT abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 1923.