SLC16A9-Mediated Carnitine Uptake Enhances Radiotherapy Resistance in Colorectal Cancer via Lipid Metabolic Reprogramming

Background: Radiation resistance severely impairs the therapeutic efficacy of radiotherapy in colorectal Cancer (CRC). Lipid metabolic reprogramming, particularly the activation of fatty acid oxidation (FAO), has been well recognized to be closely implicated in radioresistance, whereas its underlying regulatory mechanism remains poorly elucidated. This study aimed to explore the functional role of the transporter SLC16A9 in CRC radioresistance and its correlation with carnitine metabolism as well as the FAO signaling pathway.

Methods: Differentially expressed genes associated with radioresistance were screened by analyzing public transcriptomic databases and further validated using clinical specimens. CCK-8 assay, colony formation assay, comet assay, and cell Apoptosis analysis were performed to evaluate cellular radiosensitivity. RT-qPCR, Western blot, and immunofluorescence staining were adopted to detect SLC16A9 expression levels and DNA damage status. Metabolic flux analysis, ATP measurement, and radioactive carnitine uptake assay were utilized to assess carnitine uptake capacity and FAO activity. A nude mouse xenograft tumor model was constructed to determine the in vivo influence of SLC16A9 on tumor response to radiotherapy.

Results: Elevated expression of SLC16A9 was observed in radioresistant CRC cell lines and clinical samples. SLC16A9 facilitated carnitine uptake and augmented FAO pathway activity, which was accompanied by reduced radiation-induced DNA damage, decreased Reactive Oxygen Species (ROS) levels, and enhanced cellular survival capability. Inhibition of FAO partially reversed the above biological effects. In the in vivo model, upregulated SLC16A9 was correlated with attenuated tumor suppression efficacy of radiotherapy, while SLC16A9 inhibition markedly elevated radiosensitivity.

Conclusion: SLC16A9 contributed to the development of CRC radioresistance by promoting carnitine uptake and enhancing FAO activity. This study provided novel evidence supporting the association between metabolic reprogramming and radiosensitivity and suggested that targeting SLC16A9 or the FAO pathway held promising potential for clinical therapeutic application.