CD44 serves a dual role in supporting tumor survival and promoting invasion. Claudin‑low breast cancer, characterized by a CD44+/CD24‑ phenotype and epithelial‑mesenchymal transition (EMT), displays aggressive behavior. The present study investigated the interaction between CD44 and TGF‑β signaling, and assessed the cellular effects of their combined inhibition. CD44 was knocked down in claudin‑low breast cancer cell lines (SUM159 and MDA‑MB‑231), and the TGF‑β receptor (TGFBR) inhibitor LY2109761 (LY‑61) was applied for treatment. Cell viability (MTT assay), apoptosis (annexin V assay), invasion (Transwell assay), colony formation and Smad2 phosphorylation (western blotting) were evaluated. CD44 knockdown reduced viability and increased apoptosis but did not markedly suppress invasion. Although TGF‑β stimulation enhanced Smad2 phosphorylation, CD44 knockdown alone did not increase Smad2 activation, indicating that it does not directly regulate Smad2. However, LY‑61 inhibited TGF‑β‑induced Smad2 phosphorylation, effectively counteracting pro‑invasive signaling. Notably, while CD44 knockdown alone had a negligible impact on invasion, its combination with LY‑61 markedly reduced the invasive capacity and colony formation of cells compared with the control (control cells transduced with non‑targeting short hairpin RNA without LY‑61 treatment). LY‑61 induced S phase accumulation, which was more pronounced in SUM159 cells than in MDA‑MB‑231 cells, indicating cell line‑specific effects on cell‑cycle regulation. Clinical data indicated that low CD44 expression was associated with improved survival in patients with claudin‑low breast cancer, despite its potential to enhance EMT signaling. These findings suggested that CD44 knockdown enhanced the response to TGFBR inhibition. Although CD44 depletion may increase EMT‑related signaling, invasion was primarily suppressed by TGF‑β blockade, and the combination with CD44 knockdown further enhanced the inhibition of proliferative phenotypes compared with either treatment alone. This dual‑targeting approach warrants further investigation in claudin‑low breast cancer.
Keywords: CD44; TGF‑β; claudin‑low breast cancer.