ABSTRACT
Transmembrane 4 L6 family proteins have been known to promote cancer. In this study, we demonstrated that transmembrane 4 L6 family member 4 (TM4SF4), which is induced by γ-radiation in non-small cell lung cancer (NSCLC) cells, is involved in epithelial-to-mesenchymal transition (EMT) and cancer stem cell (CSC) properties of NSCLC through the regulation of osteopontin (OPN). Forced TM4SF4 overexpression in A549 cells increased the secretion of OPN, which activates CD44 or integrin signaling and thus maintains EMT-associated CSC-like properties. OPN, known as a downstream target of ß-catenin/T-cell factor 4 (TCF-4), was induced by up-regulated ß-catenin via TM4SF4-driven phosphorylation of glycogen synthase kinase 3b (GSK3ß). TCF4 complexed to promoter regions of OPN in TM4SF4-overexpressing A549 cells was also confirmed by chromatin immunoprecipitation. Knockout of either ß-catenin or TCF4-suppressed OPN expression, demonstrating that both factors are essential for OPN expression in NSCLC cells. OPN secreted by TM4SF4/GSK3ß/ß-catenin signaling activated the JAK2/STAT3 or FAK/STAT3 pathway, which also up-regulates OPN expression in an autocrine manner and consequently maintains the self-renewal and metastatic capacity of cancer cells. Neutralizing antibody to OPN blocked the autocrine activation of OPN expression, consequently weakened the metastatic and self-renewal capacity of cancer cells. Collectively, our findings indicate that TM4SF4-triggered OPN expression is involved in the persistent reinforcement of EMT or cancer stemness by creating a positive feedback autocrine loop with JAK2/STAT3 or FAK/STAT3 pathways.
ABSTRACT
Amyloid ß precursor protein binding family B member 1(APBB1) was first identified as a binding partner of amyloid precursor protein during brain development, but its function in the context of cancer remain unclear. Here we show for the first time that APBB1 is partly associated with intensifying cancer stem cell(CSC) and epithelial-to-mesenchymal transition (EMT) and enhancing radiation-resistant properties of lung cancer cells. We found that APBB1 was highly expressed in ALDH1high CSC-like cells sorted from A549 lung cancer cells. In APBB1-deficient H460 cells with forced overexpression of APBB1, the protein directly interacted with IGF1Rß, enhanced phosphorylation of IGF1Rß/PI3K/AKT pathway(activation) and subsequently induced the phosphorylation of GSK3ß(inactivation). This phosphorylation stabilized Snail1, a negative regulator of E-cadherin expression, and regulated ß-catenin-mediated ALDH1 expression, which are representative markers for EMT and CSCs, respectively. In contrast, suppression of APBB1 expression with siRNA yielded the opposite effects in APBB1-rich A549 cells. We concluded that APBB1 partly regulates the expression of ALDH1. We also found that APBB1 regulates activation of nuclear factor-κB, which is involved in reducing various stresses including oxidative stress, which suggests that APBB1 is associated with γ-radiation sensitivity. Our findings imply that APBB1 plays an important role in the maintenance of EMT-associated CSC-like properties and γ-radiation resistance via activation of IGF1Rß/AKT/GSK3ß pathway in lung cancer cells, highlighting APBB1 as a potential target for therapeutic cancer treatment.