The hypoxic tumor microenvironment,particularly hypoxia-conditioned cancer-associated fibroblasts (CAFs),drives breast cancer (BC) progression and therapy resistance. However,the molecular mechanisms linking hypoxic CAFs to BC plasticity and chemoresistance remain elusive. Primary CAFs were isolated from high-grade BC tissues (Grade III) and characterized (α-SMA⁺/CD34⁻/pan-CK⁻),with normal fibroblasts (NFs) from reduction mammoplasty as controls. Hypoxic CAF-derived exosomal circSTAT3 stability was validated using RNase R resistance and actinomycin D assays. Exosomes were characterized via transmission electron microscopy (TEM),dynamic light scattering (DLS),and marker profiling (CD63⁺/TSG101⁺/Alix⁺,calnexin⁻). Functional effects of hypoxic CAF exosomes on TNBC cells (MDA-MB-231,SUM159) were assessed through proliferation/migration assays,stemness/epithelial-mesenchymal transition (EMT) marker analysis,and siRNA-mediated circSTAT3 knockdown. Mechanistic studies employed luciferase assays and RNA immunoprecipitation (RIP). Chemoresistance was evaluated by cisplatin half-maximal inhibitory concentration (IC₅₀). In vivo tumor growth and stemness enrichment were analyzed in xenografts. Clinical validation used BC tissues (n = 60) and plasma exosomes from BC patients (n = 40) versus healthy controls (n = 25). Hypoxic CAF-derived exosomes efficiently transferred circSTAT3 to TNBC cells,promoting proliferation,migration,EMT,and stemness marker expression. SiRNA-mediated circSTAT3 knockdown reversed these effects. Mechanistically,circSTAT3 acted as a competitive endogenous RNA (ceRNA),sponging miR-671-5p to derepress NOTCH1. Hypoxic CAF exosomes increased cisplatin IC₅₀ in TNBC cells,while circSTAT3 depletion restored chemosensitivity. In vivo,hypoxic CAF exosomes accelerated tumor growth,enriched CD44⁺/NOTCH1⁺ populations,and elevated circulating exosomal circSTAT3. Clinically,circSTAT3 was significantly upregulated in advanced BC tissues (p < 0.01) and patient plasma exosomes (p < 0.01),correlating with lymph node metastasis. This study identifies a hypoxia-driven feedforward loop wherein CAF-derived exosomal circSTAT3 promotes TNBC stemness and chemoresistance via miR-671-5p/NOTCH1 signaling. CircSTAT3 redefines stromal-tumor crosstalk as a circRNA-driven process and serves as both a circulating non-invasive biomarker and a promising therapeutic target to disrupt stromal-mediated resistance in aggressive TNBC. The online version contains supplementary material available at 10.1186/s12967-025-06794-8.
View Publication