Abstract
Krebs von den Lungen‑6 (KL‑6), a high‑molecular‑weight glycoprotein, is frequently elevated in patients with cancer; however, its precise role in the clinical progression of breast cancer (BC) remains unclear. Tumor hypoxia has been recognized as a critical driver of cancer progression. The present study aimed to investigate the effects of hypoxia on KL‑6 expression and its contribution to the invasive behavior of BC cells. Immunohistochemical analysis of KL‑6 and hypoxia‑inducible factor‑1α (HIF‑1α) was performed in 30 clinical BC tissue samples, and their expression levels were correlated with patient outcomes. BC cell lines (MCF‑7 and MDA‑MB‑231) were used for in vitro analyses, including immunofluorescence, western blotting, wound healing assays, and three‑dimensional spheroid cultures under normoxic and hypoxic conditions, as well as chemically induced hypoxia using cobalt chloride (CoCl(2)). The subcellular localization of KL‑6 was further examined through immunoelectron microscopy. In clinical specimens, high KL‑6 expression was significantly associated with increased recurrence or metastasis, as was elevated HIF‑1α expression. Although MCF‑7 cells exhibited higher basal KL‑6 expression, marked upregulation was observed in MDA‑MB‑231 cells under hypoxic spheroid conditions, where KL‑6 co‑localized with HIF‑1α, particularly within invasive cellular protrusions. Functional inhibition of KL‑6 suppressed the migration of MCF‑7 cells. Treatment with CoCl(2) significantly induced KL‑6 and HIF‑1α expression in MCF‑7 cells. Ultrastructural analysis confirmed the localization of KL‑6 on cell membranes and within protrusive structures. Collectively, these findings demonstrated that hypoxia enhances KL‑6 expression in BC cells, partly mediated by HIF‑1α, and that KL‑6 contributes to tumor cell invasion.