Reduced expression of the murine HLA-G homolog Qa-2 is associated with malignancy, epithelial-mesenchymal transition and stemness in breast cancer cells

Abstract

Qa-2 is believed to mediate a protective immune response against cancer; however, little is known about the role of Qa-2 in tumorigenesis. Here, we used 4T1 breast cancer cells to study the involvement of Qa-2 in tumor progression in a syngeneic host. Qa-2 expression was reduced during in vivo tumor growth and in cell lines derived from 4T1-induced tumors. Tumor-derived cells elicited an epithelial-mesenchymal transition associated with upregulation of Zeb1 and Twist1/2 and enhanced tumor initiating and invasive capacities. Furthermore, these cells showed increased stem characteristics, as demonstrated by upregulation of Hes1, Sox2 and Oct3/4, and enrichment of CD44high/CD24median/low cells. Remarkably, Qa-2 cell-surface expression was excluded from the CD44high/CD24median/low subpopulation. Tumor-derived cells showed increased Src activity, and treatment of these cells with the Src kinase inhibitor PP2 enhanced Qa-2 but reduced Sox2 and CD44high/CD24median/low expression levels, suggesting that Src signaling, while positively associated with stemness, negatively regulates Qa-2 expression in breast cancer. Finally, overexpression of the Qa-2 family member Q7 on the cell surface slowed down in vivo tumor growth and reduced the metastatic potential of 4T1 cells. These results suggest an anti-malignant role for Qa-2 in breast cancer development, which appears to be absent from cancer stem cells.

Tumor-associated macrophage-derived cytokines enhance cancer stem-like characteristics through epithelial–mesenchymal transition

“… 63 – 65 Tumor cells after EMT express high levels of stem surface markers, indicating that these cells have become stem-like cells. 66 – 68 One interesting study revealed that breast CSCs originate from the fusion of M2-TAMs and breast cancer cells; these hybrid cells overexpress mesenchymal-associated genes and stemness markers. 48 Therefore, it can be said that tumor cells after EMT are likely becoming CSCs to some extent.…”

New Insights Into the Role of Qa-2 and HLA-G Non-classical MHC-I Complexes in Malignancy

“…4T1 tumor cells are representative of the highly aggressive triple-negative subtype of human breast cancer (62). In the 4T1 cell line, only a small percentage of cells (about 4%) express Qa-2 on the cell surface, and Qa-2 protein levels were further reduced during in vivo tumor growth and in tumor-derived cultured cells (63), suggesting that Qa-2 expression is downregulated during breast cancer development. Cell lines derived from tumors induced by 4T1 in the back skin, or in the mammary fat pad, of syngeneic Balb/c mice elicited a partial epithelial-mesenchymal transition (EMT) and exhibited increased stem cell characteristics and enhanced tumor-initiating and invasive capacities that correlated with reduced Qa-2 expression.…”

“…Cell lines derived from tumors induced by 4T1 in the back skin, or in the mammary fat pad, of syngeneic Balb/c mice elicited a partial epithelial-mesenchymal transition (EMT) and exhibited increased stem cell characteristics and enhanced tumor-initiating and invasive capacities that correlated with reduced Qa-2 expression. In fact, Qa-2 expression was completely lost in a CD44 high /CD24 med/low cancer stem cell subpopulation (64) isolated from these cell lines (63). This striking result suggests the possibility that Qa-2 is excluded from cancer stem cells, thus contributing to their evasion from immunosurveillance.…”

“…Downregulation of Qa-2 in cancer stem cells appears to be mediated, at least partially, by the Src signaling pathway. Indeed, pharmacological inhibition of Src kinase activity enhanced Qa-2 expression and concomitantly reduced stemness of 4T1 tumor-derived cell lines (63). Increased Src activity is a general characteristic of malignancy associated with EMT, stemness, invasion and metastasis (65).…”

“…Q7 is a key member of the Qa-2 family in Balb/c mice (42). 4T1 cells overexpressing Q7 produced tumors that grew slower and were less metastatic than control or parental cells (63).…”