CASTOR1 : A Novel Tumor Suppressor Linking mTORC1 and KRAS Pathways in Tumorigenesis and Resistance to KRAS-Targeted Therapies in Non-Small Cell Lung Cancer

Cytosolic arginine sensor for mTORC1 Subunit 1 (CASTOR1) functions as a key regulator of mechanistic target of rapamycin complex 1 (mTORC1) signaling. Despite its frequent dysregulation in cancers via mechanisms such as KSHV microRNA-mediated inhibition or AKT-driven phosphorylation and degradation, the impact of CASTOR1 loss on tumor initiation and progression remains poorly understood. Here, we identify CASTOR1 as a critical tumor suppressor in non-small cell lung cancer (NSCLC) by demonstrating that its genetic ablation amplifies tumorigenesis in a KRAS -driven genetically engineered mouse model (GEMM;LSL- KRAS ^G12D ). CASTOR1 deficiency markedly enhances lung tumor incidence, accelerates tumor progression, and increases proliferative indices in KRAS ^G12D -driven tumors ( KRAS ^G12D ; C1 ^KO ) compared to CASTOR1 wild type (WT) tumors ( KRAS ^G12D ; C1 ^WT ). Advanced-stage tumors exhibit elevated phosphorylated CASTOR1 (pCASTOR1) and reduced total CASTOR1 levels, suggesting active degradation during tumorigenesis. Mechanistically, CASTOR1 loss amplifies mTORC1 signaling, as evidenced by heightened phosphorylation of downstream effectors 4EBP1 and S6, while also augmenting AKT and ERK activation, uncovering a crosstalk between the PI3K/AKT/mTORC1 and KRAS/ERK pathways. Furthermore, CASTOR1 ablation induces genome instability, which may contribute to enhanced tumor incidence and progression. Importantly, CASTOR1 deficiency confers resistance to KRAS ^G12D -specific inhibitors, while over half of KRAS ^G12D ; C1 ^WT tumors also display resistance. Organoids derived from KRAS ^G12D ; C1 ^KO and KRAS ^G12D ; C1 ^WT tumors reveal a correlation between KRAS inhibitor resistance and hyperactivation of mTORC1, with mTORC1 and PI3K inhibitors sensitizing resistant tumors to KRAS ^G12D -targeted therapies. These findings position CASTOR1 as a novel tumor suppressor that modulates mTORC1 and KRAS signaling to constrain NSCLC progression. Our study further highlights the therapeutic potential of combining mTORC1 or ERK inhibitors with KRAS-targeted therapies for NSCLC characterized by hyperactive KRAS signaling and impaired CASTOR1 activity.