SEC61G Facilitates Brain Metastases via Antagonizing PGAM1 Ubiquitination and Immune Microenvironment Remodeling in Non-Small Cell Lung Cancer

<b>Background:</b> Brain metastases are a leading cause of mortality in non-small cell lung cancer (NSCLC), yet their molecular mechanisms remain unclear. SEC61G, a subunit of the SEC61 translocon, has been implicated in tumor progression but its role in brain metastases is unknown. This study explores how SEC61G contributes to brain metastases by driving metabolic reprogramming and immune microenvironment remodeling. <b>Methods:</b> Brain-metastatic NSCLC cell lines were established through <i>in vivo</i> selection in a mouse model. SEC61G expression was analyzed via transcriptomics, immunohistochemistry, multiplex immunofluorescence, and patient datasets. Functional assays were used to assess SEC61G's role in glycolysis, TLS formation, and immune interactions, with a focus on the SEC61G-PGAM1 axis. Pharmacological inhibitors and co-culture systems were employed to validate findings. <b>Results:</b> SEC61G was identified as a key upregulated gene in brain metastases based on transcriptomic data from patient-derived samples and mouse models. Higher SEC61G expression in brain metastases correlated with advanced tumor stages and poor survival in NSCLC patients. Mechanistically, SEC61G promoted brain metastasis by stabilizing the key glycolytic enzyme PGAM1. This occurred through a novel mechanism of competitive inhibition of PGAM1 ubiquitination: SEC61G directly antagonized the E3 ubiquitin ligase UBE3C, preventing PGAM1 degradation via the proteasome pathway. Stabilized PGAM1 enhanced glycolysis and regulated oxidative phosphorylation, driving metabolic reprogramming that supported brain metastatic colonization. Moreover, SEC61G reshaped the tumor immune microenvironment by promoting microglial M2 polarization and suppressing M1 polarization, accompanied by increased secretion of IL-6 and IL-10. These immune effects were dependent on PGAM1, as its pharmacological inhibition reversed SEC61G-induced M2 polarization and restored CD8⁺ T cell infiltration. <i>In vivo</i> and clinical studies confirmed that high SEC61G expression in brain metastases correlated with excessive M2 microglia, reduced immune surveillance, and poor patient outcomes. Immunoprofiling revealed a striking gradient of SEC61G expression across tertiary lymphoid structures (TLS) maturation stages: SEC61G levels were highest in TLS-absent samples and CD206⁺ microglia infiltration, intermediate in samples with immature TLS, and lowest in those with mature TLS. <b>Conclusion:</b> In conclusion, this study identifies a novel mechanism in which SEC61G drives NSCLC brain metastases by competitively inhibiting UBE3C-mediated ubiquitination of PGAM1, stabilizing PGAM1 and enhancing glycolysis. In addition to metabolic reprogramming, SEC61G impairs TLS maturation, suppresses adaptive immune responses, and facilitates immune evasion, contributing to brain metastatic colonization. These findings highlight SEC61G as a key regulator of brain metastasis and a promising therapeutic target for NSCLC patients with brain metastases.