Receptor activator of NF-κB ligand induces cell adhesion and integrin α2 expression via NF-κB in head and neck cancers

Head and neck squamous cell carcinoma (HNSCC) constitutes the sixth most common malignancy worldwide1. Despite therapeutic and diagnostic advances, the 5-year survival rate for this disease remains at approximately 50%, due to a high degree of local invasiveness and metastasis to cervical lymph nodes2. It has been widely accepted that such malignant characteristics are a consequence of communication between cancer cells and their cognate environment. Moreover, given that the head and neck region is an organ that is challenged by a wide variety of environmental irritants, it is particularly important to consider the effect of the microenvironment on the development and promotion of HNSCC for a better understanding of the pathogenesis and for the establishment of efficient therapeutics. Indeed, due to a lack of such therapeutic strategies, HNSCC treatment largely depends on radical surgery, which results in significant functional and cosmetic defects as well as a significant reduction in patient quality of life. Therefore, to improve patient quality of life, it is essential to develop sensitive and reliable biomarkers that predict aggressive HNSCC and eventually provide the most suitable (i.e., necessary and sufficient) treatment for each individual patient3. Unfortunately, there are currently few available biomarkers that satisfy these criteria4. The extracellular matrix (ECM) is one of the first environmental factors encountered by epithelial cancer cells migrating from the tumor mass. These cancer cells interact with the surrounding ECM through integrins, a family of primary ECM receptors. Integrins are required for a wide variety of cellular processes, including cell growth and differentiation, tissue repair, intracellular signaling, and tumorigenesis5,6,7. To date, EGFR activation has been demonstrated to be critical for integrin α2-mediated adhesion and motility in colon cancer8. Indeed, several studies have demonstrated the significance of adhesion between cancer cells and the ECM9,10. The expression and distribution patterns of integrin family members have also been investigated in oral, pancreatic, breast, lung, and colon cancers7,11,12,13,14, and integrins have been shown to play significant roles in malignant tumor invasion, migration, and metastasis15,16,17. We recently reported that receptor activator of NF-κB ligand (RANKL) expression induces epithelial mesenchymal transition (EMT) and angiogenesis in HNSCC in vivo and correlates with histological differentiation in human HNSCC specimens18. Despite such aggressive phenotypes in vivo, RANKL could not enhance either cell proliferation or cell migration/invasion, indicating that RANKL is a specific marker of tumor progression in vivo. In this study, we investigated the detailed molecular mechanisms by which RANKL evokes malignant phenotypes and unveiled a previously unknown function of RANKL in the regulation of cell adhesion. RANKL elicited RANK-NF-κB signaling to upregulate integrin α2 expression and subsequent cell adhesion to type I collagen by facilitating active integrin trafficking. The enhanced cell adhesion resulted in increased survival in collagen-rich environments, including a collagen gel and tumor tissues with a poorly differentiated phenotype in vivo. Furthermore, RANKL and integrin α2 expression levels were significantly correlated in human oral cancer tissues. These findings highlight that RANKL and its downstream signaling may be functional biomarkers and, presumably, attractive therapeutic targets in HNSCC.