Claudin-1 mediates progression by regulating EMT through AMPK/TGF-β signaling in head and neck squamous cell carcinoma

Claudin-1 (CLDN1), a major component of tight junction complexes in the epithelium, maintains cellular polarity, and plays a critical role in cell-to-cell communication as well as epithelial cell homeostasis. Although the role of CLDN1 has been widely studied in cancer, its role in the progression and the exact regulatory mechanisms, remain controversial. Using next-generation sequencing, we first analyzed the expression profiles of tumor/non-tumor paired tissue in patients with head and neck squamous cell carcinoma (HNSC) from public and local cohorts and found out that CLDN1 is upregulated in tumors compared to normal tissues. Next, its correlation with lymph node metastasis and poor prognosis was validated in the retrospective cohort, which collectively suggests CLDN1 as an oncogene in HNSC. As expected, the knockdown of CLDN1 inhibited invasive phenotypes by downregulating epithelial-to-mesenchymal transition (EMT) in vitro. To ascertain the regulatory mechanism of CLDN1 in HNSC analysis of GO term enrichment, KEGG pathways, and curated gene sets were used. As a result, CLDN1 was negatively associated with AMP-activated protein kinase (AMPK) and positively associated with transforming growth factor-β (TGF-β) signaling. In vitro mechanistic assay showed that CLDN1 inhibited AMPK phosphorylation by regulating AMPK upstream phosphatases, which led to inhibition of Smad2 activity. Intriguingly, the invasive phenotype of cancer cells increased by CLDN1 overexpression was rescued by AMPK activation, indicating a role of the CLDN1/AMPK/TGF-β/EMT cascade in HNSC. Consistently in vivo, CLDN1 suppression significantly inhibited the tumor growth, with elevated AMPK expression, suggesting the novel observation of oncogenic CLDN1-AMPK signaling in HNSC.