TRAF6 is upregulated in colon cancer and promotes proliferation of colon cancer cells

Abstract

TNF receptor associated factor 6 (TRAF6) plays a critical role in the regulation of innate immune responses and adaptive immune responses. Though recent studies indicate that TRAF6 is involved in cancer, its precise role in cancer including colon cancer has not been extensively investigated and remains largely unknown. The purpose of this work is to determine the expression of TRAF6 in colon cancer as well as the possible role of it in proliferation and apoptosis of colon cancer cells. Fifty colon cancer tissues paired with their adjacent non-cancerous tissues were analyzed. TRAF6 expression is upregulated in cancers (P = 0.000), which is correlated with tumor grades (P = 0.012). The tumor tissue-array analysis also indicates that expression of TRAF6 is upregulated in colon cancer (P = 0.000), but the TRAF6 upregulation has no association with patients’ survival rate (P = 0.055). We found that knockdown of TRAF6 blocks proliferation of colon cancer cells through cyclin D1. Different from other reports, in our experiments knockdown of TRAF6 alone has little effect on survival of colon cancer cells examined. Knockdown of TRAF6 sensitizes the cells to treatment of the conventional anti-cancer drugs 5-fluorouracil and etoposide. Thus, inhibition of TRAF6 may improve the therapeutic treatment of these drugs. Together, our data suggest that TRAF6 promotes proliferation of colon cancer cells and it may serve as a potential target for therapy of colon cancer.

Introduction

TNF receptor associated factor 6 (TRAF6) is a key activator of NF-κB. In response to pro-inflammatory cytokines, it activates IκB kinase (IKK)/NF-κB signal pathway. TRAF6 is an E3 ubiquitin ligase and catalyzes K63 polyubiquitination of TAK1 that is required for IKK activation (Chen, 2012, Skaug et al., 2009). TRAF6 mediates not only the signaling from the members of the TNF receptor superfamily, such as CD40 and RANK (Hostager, 2007, Darnay et al., 2007), but also from Toll-like receptor/IL-1 receptor family (Choi, 2005, Bradley and Pober, 2001). It also interacts with various protein kinases including IRAK1, SRC and PKCzeta, which provides a link between distinct signaling pathways. In addition to regulating the activity of IKK/NF-κB signaling, TRAF6 also modifies other proteins or signaling. TRAF6 was found to mediate activation of AKT (Yang et al., 2009), JNK and p38 (Yamashita et al., 2008). Linares et al. (2013) demonstrated that TRAF6 was involved in mTOR activation through K63 polyubiquitination. Wei et al. (2012) showed that TRAF6 negatively regulated the JAK-STAT signaling. We reported recently that TRAF6 stabilized hypoxia-inducible factor 1α (HIF-1α) (Sun et al., 2013).

A few studies indicate that TRAF6 may play a role in cancer. A recent study identifies TRAF6 as a commonly amplified oncogene bridging RAS and NF-κB in lung cancer (Starczynowski et al., 2011). Overexpression of TRAF6 in primary mouse marrow cells resulted in a myelodysplastic syndrome that develops into a fatal acute myeloid leukemia (Starczynowski et al., 2010). A positive relationship of TRAF6 in tumor margin cells with the histological grade and the mode of tumor invasion was found in laryngeal carcinoma (Starska et al., 2009). TRAF6 mediates oncogenesis of marginal zone B cell lymphoma of mucosa-associated lymphoid tissue (Hirata et al., 2006). It promotes proliferation and regulates apoptosis of esophageal cancer, osteosarcoma, glioma and lung adenocarcinoma cells (Ma et al., 2011a, Ma et al., 2011b, Yao et al., 2013, Meng et al., 2012, Peng et al., 2013, Zhong et al., 2013). We recently reported that TRAF6 stabilized HIF-1α and promoted tumor angiogenesis (Sun et al., 2013). In addition, TRAF6 exhibited the higher and more consistent expression in human cancer cell lines (Zapata et al., 2000, Rajandram et al., 2012). And TRAF6 protein levels were found higher in myelodysplastic syndrome patients (Starczynowski et al., 2010). It was reported that amplification of the TRAF6 locus was a somatic and frequent event in several human cancer types (Beroukhim et al., 2010).

Despite these findings, the precise role of TRAF6 protein in cancers including colon cancer has not been extensively investigated. In this manuscript, we demonstrate that expression of TRAF6 is upregulated in colon cancer, which is associated with tumor grades. We show that TRAF6 promotes proliferation of colon cancer cells via cyclin D1. TRAF6 functions to enhance resistance of colon cancer cells to treatment of drug including 5-fluorouracil (5-Fu) or etoposide. Our results suggest that TRAF6 plays an important role in development of colon cancer. And it may serve as a potential target for colon cancer chemotherapy.