Integrin α7 is a functional cancer stem cell surface marker in esophageal squamous cell carcinoma

Abstract

Esophageal squamous cell carcinoma (ESCC), the major histologic subtype of esophageal cancer, is one of the most common malignancies and ranked as the sixth leading cause of cancer-related death worldwide. ESCC is characterized by its remarkable geographic distribution with particularly high-risks in Northern China. Despite advances in diagnosis and treatment, the 5-year survival rate of ESCC is still less than 18%, mainly due to tumor metastasis and chemoresistance. Current chemotherapies against ESCC are usually developed against the bulk of the tumor mass, where although they are able to initially shrink the size of the primary tumor, they ultimately fail to eradicate the lesion in full, thus resulting in tumor relapse. Recently, increasing evidences suggest that cancer stem cells (CSCs) represent an important subset of cancer cells that are biologically distinct from the others and have the capability to maintain tumor formation and growth, and resist traditional treatment. The stem cell-like characteristics of these cells and their limited number within the tumor mass are believed to account for their capability to escape therapies. Non-CG methylation has been associated with stemness regulation in embryonic stem cells and it has been defined as a characteristic of stem cell state. As CSCs share many common properties with stem cells, we hypothesize that non-CG methylation also predominantly exists in CSCs but not in other differentiated cancer cells. By comparing differentially expressed genes affected by non-CG methylation between tumor and corresponding non-tumor tissues, Integrin α7 (ITGA7) was found to be sporadically expressed in ESCC and its roles in esophageal CSCs were further investigated. The expression level ITGA7 was detected in clinical specimens of ESCC and adjacent non-tumor tissues by immunohistochemistry. The data showed that ITGA7 was maintained to be a rare number in ESCC (<1% in 203/262 cases), and a high-frequency (>0.6%) of 〖ITGA7〗^+ cells in ESCC tissues was significantly associated with poor differentiation, lymph node metastasis and worst prognosis. Functional studies demonstrated that both sorted 〖ITGA7〗^+ cells and ITGA7 overexpressing cells displayed enhanced stemness features, including elevated expression of stemness-associated genes and epithelial-mesenchymal transition (EMT) features, as well as increased abilities to self-renew, differentiate, resist chemotherapy and form tumor in vivo. As few as 5,000 〖ITGA7〗^+ cells were sufficient for tumor development in NOD/SCID mice while at least 20× as many 〖ITGA7〗^- cells were necessary to generate a tumor in the same model. While silencing ITGA7 expression with shRNA had the opposite effects. Mechanism study found that ITGA7 regulated CSC properties through the activation of the FAK/MAPK/ERK signaling pathway; these effects were reversed with a FAK inhibitor. Both animal model and clinical data suggest that 〖ITGA7〗^+ cells were more resistant to chemotherapeutic agents cisplatin and 5-fluorouracil by the inhibition of apoptosis via activating FAK/Akt signaling pathway. In summary, we report here that ITGA7 is a novel CSC marker in ESCC with function in stemness regulation and CSCs maintenance. Further characterization of ITGA7 may provide a novel therapeutic approach by targeting esophageal CSCs to achieve better clinical outcome.