The Combination of PF-543 and TRAIL Effectively Induces Apoptotic Cell Death and Inhibits Stem Cell-Like Properties Through the SPHK1/S1PR1/STAT3 Pathway in TRAIL-Resistant Colorectal Cancer Cells

Purpose: Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptosis in various tumor cell types. Although TRAIL can directly trigger cell death, cancer cells may acquire resistance to its apoptotic effects during treatment. Sphingosine kinase 1 (SPHK1) is a key regulator of cancer progression and resistance to therapy. In this study, we determined whether combining TRAIL with PF-543, a specific SPHK1 inhibitor, could circumvent TRAIL resistance in previously established TRAIL-resistant colorectal cancer cells (HCT116-TR cells). Methods: HCT116-TR cells were treated with TRAIL and/or PF-543. Apoptotic cell death and signaling pathways were evaluated using MTT assay, colony formation assay, and flow cytometry analysis. Cell aggressiveness and cancer stemness were assessed through wound healing assay, Matrigel-coated Transwell assay, and tumorsphere formation assay. The underlying molecular mechanisms were examined by Western blotting. Results: Combined treatment with PF-543 significantly enhanced TRAIL-induced apoptosis in HCT116-TR cells and exhibited a synergistic effect. Mechanistically, PF-543 sensitized HCT116-TR cells to TRAIL by activating the mitochondrial apoptosis pathway. Moreover, PF-543 increased TRAIL sensitivity by regulating DcR1 and DR5 through the SPHK1/S1PR1/STAT3 pathway. In addition, combination treatment reduced the aggressiveness and cancer stemness of HCT116-TR cells by modulating the epithelial-mesenchymal transition (EMT) pathway as well as cancer stemness markers. Conclusion: We identified the molecular mechanisms underlying acquired TRAIL resistance in CRC cells and suggest that targeting SPHK1 represents a potential strategy to overcome TRAIL resistance and inhibit CRC metastasis.