MSN/STAT3 drives cancer stemness and chemoresistance via IL-6/LPAR1 ligand receptor complex in triple-negative breast cancer

Background: Resistance to chemotherapy remains a major clinical challenge in triple-negative breast cancer (TNBC), an intrinsic subtype with limited available therapeutic options. The expression of moesin (MSN) is upregulated in TNBC patients, but little is known about the role of MSN in breast carcinogenesis. Methods: We investigated the MSN-dependent autocrine loop between extracellular interleukin 6 (IL-6) and NF-κB, along with a signaling cascade involving GTPase-mediated STAT3 phosphorylation. Various in vitro and in vivo assays were used to evaluate tumor initiation, growth, and stemness properties in TNBC models. Results: High MSN expression was correlated with shorter overall and disease-free survival in TNBC patients. In vivo, MSN promotes tumor initiation and growth. Mechanistically, MSN-mediated IL-6/NF-κB autoregulatory feedback enhances IL-6 transcription. IL-6 binding to LPAR1 activated MSN phosphorylation, which then sequentially phosphorylated the CDC42-PAK4 complex, triggering nuclear translocation of the pSTAT3-MSN complex. This led to pSTAT3-mediated activation of cancer stemness genes (IGFN1, EML1, and SRGN), contributing to Adriamycin resistance. Notably, combination treatment with the FDA-approved STAT3 inhibitor Atovaquone and Adriamycin restored drug sensitivity. Conclusions: Our findings uncover the critical role of MSN in regulating STAT3-mediated cancer stemness via the IL-6/NF-κB signaling axis. These results provide a strong rationale for repositioning STAT3 inhibitors such as Atovaquone as a therapeutic strategy in Adriamycin-resistant TNBC patients exhibiting pSTAT3-MSN complex upregulation.