Intrinsic TGF-β2-triggered SDF-1-CXCR4 signaling axis is crucial for drug resistance and a slow-cycling state in bone marrow-disseminated tumor cells

Dormant or slow-cycling disseminated tumor cells (DTCs) in bone marrow (BM) are resistant to conventional therapy in various cancers including head and neck squamous cell carcinoma (HNSCC), although the molecular mechanisms remain largely unknown. This study aimed to identify the intrinsic molecular mechanisms underlying drug resistance in BM-DTCs. We used in vivo selection of the human HNSCC cell line HEp3, which mimics non-proliferative BM-DTCs in mice, to establish BM-DTC-derived (BM-HEp3) and lung metastases-derived (Lu-HEp3) sublines. Both sublines had higher migration activity and shortened survival in a murine xenograft model compared with parental (P-HEp3) cells. Slow-cycling BM-HEp3 cells had intrinsically enhanced cisplatin resistance compared with Lu-HEp3 cells, which also manifested this resistance but proliferated rapidly. The drug resistance and slow-cycling state of BM-HEp3 cells depended on enhanced positive feedback of the signaling axis of stromal cell-derived factor-1 (SDF-1)-C-X-C chemokine receptor-4 (CXCR4) via their overexpression. Interestingly, BM-DTCs highly expressed transforming growth factor-beta 2 (TGF-beta 2) to maintain SDF-1-CXCR4 overexpression. Inhibition of SDF-1-CXCR4 signaling by down-regulating TGF-beta 2 fully reversed the drug resistance of BM-HEp3 cells via reactivation of cell proliferation. These data suggest that the intrinsic TGF-beta 2-triggered SDF-1-CXCR4 signaling axis is crucial for drug resistance dependent on a slow-cycling state in BM-DTCs.