The emerging role of SPHK1 at the immune-metabolic interface: a pan-cancer integrative analysis.

Cancer remains a major global health challenge, with incidence and mortality rates continuing to rise. Metabolic reprogramming, a hallmark of cancer, not only sustains rapid proliferation but also shapes an immunosuppressive tumor microenvironment. Among metabolic enzymes, sphingosine kinase 1 (SPHK1) plays a key role in sphingolipid signaling by regulating the balance between sphingosine-1-phosphate and sphingosine. This regulation influences both cell fate and immune responses. However, the role of SPHK1 as a potential "metabolic immune checkpoint" across various cancers, as well as its implications for prognosis and immunotherapy, remains insufficiently explored. In this pan-cancer study, we analyzed SPHK1 expression using RNA-seq data from The Cancer Genome Atlas, which includes 33 cancer types. We also examined its clinical association. We then validated SPHK1 expression at the mRNA and protein levels in clinical samples of head and neck squamous cell carcinoma (HNSC), stomach adenocarcinoma (STAD), and liver hepatocellular carcinoma (LIHC) using RT‑qPCR and immunohistochemistry, and assessed its effect on cancer cell viability using the CCK‑8 assay. Furthermore, we conducted integrated analyses to evaluate the relationship between SPHK1 expression and key immunological features, including immune cell infiltration, tumor mutation burden (TMB), microsatellite instability (MSI), and immune checkpoint gene expression. These analyses aimed to delineate SPHK1's role in immune modulation. We observed significant upregulation of SPHK1 in multiple cancers, especially in HNSC, STAD, and LIHC. We also confirmed its ability to enhance cancer cell viability. High SPHK1 expression is consistently associated with poor patient survival, supporting its prognostic value. Importantly, comprehensive immunological analyses revealed that SPHK1 expression is closely linked to immunosuppressive features across cancers, including altered immune cell infiltration and elevated expression of established immune checkpoint molecules, positioning SPHK1 as a key regulator linking metabolic dysregulation to immune evasion. Our findings suggest that SPHK1 acts as an oncogene and prognostic biomarker. Additionally, it functions as a novel "metabolic immune checkpoint" across multiple cancer types. SPHK1 may bridge sphingolipid metabolism with tumor immune suppression and represents a potential promising integrated target for metabolically informed immunotherapy strategies.