Our previous study showed that iron deprivation decreases the proliferation of NPC cells. TfR is a cellular iron gate that plays a critical role in maintaining cellular iron homeostasis. This study showed that TFRC was overexpressed in NPC tissues. Moreover, we found that TFRC knockdown led to iron deprivation and inhibited proliferation, migration, invasion, and EMT via the PI3K/Akt/mTOR signaling pathway. To our knowledge, this is the first study to assess TfR expression and the effects of TFRC knockdown on NPC progression.
Tumors require sufficient iron input to drive their proliferation and progression. The iron input receptor TfR is overexpressed in a variety of solid tumor types and is associated with a worse prognosis. However, carcinoid, prostate, and testicular cancers show low expression (26). Cui et al. found that TFRC downregulation promotes cancer progression (27). Thus, the role of TFRC in cancer progression remains unclear. In our study, we found a similar expression in most tumors, and TFRC overexpression was also observed in NPC. In addition, TFRC overexpression can be associated with poor prognosis and is considered an effective prognostic marker in different types of cancer, such as esophageal squamous cell carcinoma (21), breast cancer (28, 29), renal cell carcinoma (30), and hepatocellular carcinoma (31). Interestingly, patients with non-Hodgkin’s lymphoma (NHL) who are positive for human immunodeficiency virus (HIV) present with a higher level of TFRC than do patients with NHL who are HIV-negative. Another study found that targeting anti-TfR antibodies is a promising approach to prevent Epstein–Barr Virus (EBV) carcinogenesis (32). TfR appears to be associated with viral infections. It is well known that elevated plasma EBV is related to NPC (33). Further research is required to clarify the relationship between TfR and EBV infection in NPC.
Elevated TfR levels and its core role in cancer pathology provide new insights into cancer therapies. Currently, there are two methods of TfR-targeted cancer therapy. Indirect methods use TfR antibodies conjugated with anticancer drugs such as chemotherapeutics, toxins, and nucleic acids. Direct methods include direct silencing of TFRC or activation of antibody-mediated effector functions such as antibody-dependent cell-mediated cytotoxicity (ADCC), antibody-dependent cell-mediated phagocytosis (ADCP), and complement-dependent cytotoxicity (CDC) (34–37). Direct methods limit iron uptake, impair TfR function, and lead to iron deprivation. Direct anticancer activity of TfR has been confirmed in many types of cancer (38–40). Our research also found TFRC knockdown by siRNA could significantly inhibit the growth, migration, and invasion of NPC cells, which is consistent with previous studies on most cancers (18, 41, 42). These findings indicate that TFRC knockdown may be a promising therapeutic target for NPC.
The mechanism by which TfR affects cancer progression varies among cancer types. Huang et al. found that TfR promotes proliferation and metastasis by upregulating AXIN2 expression in epithelial ovarian cancer cell (18). Jeong et al. reported that TfR promotes the growth of human pancreatic ductal adenocarcinoma by increasing ROS and mitochondrial respiration (43). O’Donnell et al. confirmed that TfR is a downstream target of the c-Myc oncogene in B-cell lymphoma (44). To clarify the possible mechanism by which TfR affects NPC progression, we analyzed the gene ontology (GO) and KEGG pathways of TfR using the DEGs after RNA-sequencing. GO analysis showed that proliferation, apoptosis, and cell adhesion may be related to TFRC knockdown. These findings were consistent with our experimental results. By sequencing, we identified a series of cancer-related pathways, including the PI3K/Akt signaling pathway, cytokine–cytokine receptor interaction, human papillomavirus infection, and the MAPK signaling pathway. KEGG pathway analysis showed that the PI3K/Akt signaling pathway was the most affected pathway after TFRC knockdown. Activation of the PI3K/AKT signaling pathway promotes mTOR activation. Therefore, mTOR expression was investigated using western blotting. We found that TFRC knockdown inhibited cancer progression via the PI3K/Akt/mTOR signaling pathway. The PI3K/AKT/mTOR pathway is one of the most frequently altered pathways in cancer. Previous studies have shown that the PI3K/AKT/mTOR pathway modulates cell cycle, apoptosis, autophagy, angiogenesis, EMT, and chemoresistance in cancers (45, 46). Additionally, drugs targeting the PI3K/AKT/mTOR pathway in combination with chemotherapeutic drugs are considered promising treatment approaches (47). Several genes have been reported to influence cell cycle, apoptosis, migration, and invasion via the PI3K/AKT/mTOR signaling pathway in cancer (48–50). We hypothesized that TFRC knockdown inhibits NPC progression by inhibiting the PI3K/AKT/mTOR pathway. We confirmed that p-Akt and mTOR, which are important markers of the PI3K/AKT/mTOR pathway, were decreased in TFRC-knockdown cells compared with the levels in control cells. These results indicated that TFRC knockdown may inhibit cancer progression via the PI3K/Akt/mTOR signaling pathway.