MAPK ERK5 is a novel regulator of MHC-I in cancer cells.

Downregulation of major histocompatibility complex class I (MHC-I) molecules is a frequent mechanism of tumor immune evasion, impairing recognition and elimination of cancer cells by cytotoxic CD8+ T lymphocytes. This phenotype, associated with poor prognosis and resistance to immune checkpoint blockade, is often driven by oncogenic pathways that reversibly suppress MHC-I. The MAPK ERK5 and its only upstream activating kinase MEK5 configure a unique intracellular signaling that regulates cell proliferation, differentiation and survival, and it has emerged as an oncogenic driver of different tumors. In this work, we investigated whether the ERK5 pathway contributes to cellular immunity. We used a panel of human cancer cell lines representing three well-established oncogenic contexts linked to reversible MHC-I downregulation (MYCN amplification, PTEN/PIK3CA mutations, and androgen receptor signaling), together with control cellular models exhibiting constitutively high MHC-I expression. We found that MEK5 or ERK5 pharmacologic inhibition or ERK5 targeted degradation increased MHC-I surface and total expression in low-MHC-I cells, without affecting PD-L1 levels. Conversely, ERK5 overexpression impaired MHC-I levels. Moreover, systemic administration of an ERK5 inhibitor also enhanced MHC-I expression in tumor xenografts. Mechanistically, RT-qPCR analysis showed that ERK5 or MEK5 inhibition did not significantly modify transcription of classical HLA-I genes or antigen-processing machinery components, and RNA-seq analysis did not render enrichment of MHC-I transcriptional programs in response to ERK5 inhibition. In contrast, trafficking experiments implicated the ERK5 pathway in the regulation of MHC-I lysosomal degradation, suggesting that ERK5 controls surface MHC-I through post-translational mechanisms. Notably, functional assays were carried out in co-cultures of cancer cells with tumor-specific human CD8+ T cells, where ERK5 inhibition sensitized MYCN-amplified or PTEN/PI3KCA-mutated cancer cells to CD8 + T cell-mediated apoptosis. These results identify ERK5 as a novel regulator of MHC-I expression in cancer cells, by regulating antigen presentation across diverse oncogenic contexts, and support a rationale for the use of ERK5 inhibitors as a strategy to improve the efficacy of immune checkpoint blockade-based immunotherapy.