Evading immune destruction is another critical hallmark of cancer [19]. An immunosuppressive milieu is one of great characteristics of NPC, which facilitates its progression[35]. Recently, emerging illustrations of immune evasion and immunotherapy have been reported in NPC. However, searching for proper immune-related target and evaluating NPC patients who might benefit from immunotherapy remain a great challenge.
In this study, we identified the downregulation of ACAA1 in NPC. In vitro and in vivo studies verified that overexpression of ACAA1 could inhibit the proliferation, migration, and invasion capabilities of NPC cells. Computational analysis showed that decreased ACAA1 was associated with poor survival of NPC patients. GO and KEGG analyses revealed an enrichment of immune-related pathways in the ACAA1 high-expressing NPC group. Furthermore, we found that NPC patients with higher expression of ACAA1 were tend to exhibit higher levels of infiltration of immune cells, and higher correlation with the expression of immune checkpoint-related genes, suggesting that ACAA1 might be a novel immune therapeutic target.
Under-expressed ACAA1 has been found in a panel of malignancies and negatively associated with Ki-67 expression [15, 17, 18]. It had been noted that lower expression of ACAA1 might contribute to the occurrence of hepatocellular carcinoma [36]. A decreased transcription of ACAA1 was associated with unfavorable overall survival of breast cancer patients [37]. The single nucleotide polymorphism (SNP) rs4988453, which maps to the promoter region shared by ACAA1 and toll-like receptors (TLR) downstream effector MYD88, is associated with decreased survival of colorectal cancer [38]. In addition, ACAA1 rs2239621 is a risk factor of gastric cancer [39]. In non-small cell lung cancer, it is noted that ACAA1 is downregulated by oncogenic KRAS through MAPK pathway[40]. In line with these, our data supported that the downregulation of ACAA1 was a prevalent phenomenon in NPC, and ROC analysis showed a favorable diagnosis efficiency of ACAA1 for NPC, both in mRNA and protein levels. This is worth further evaluation in a larger size of clinical samples. Besides, overexpression of ACAA1 could inhibit the malignant behaviours of NPC cells. One of the involving mechanisms might lie in the repression and redistribution of cellular actin filaments.
ACAA1 is related to several immune-related disease processes and is associated with the effect of anti-cancer treatment. For instance, a SNP (rs156265) in ACAA1 is found to modify the effect of endotoxin exposure on childhood asthma risk [41]. In lung tumor microenvironment, ACAA1 is positively associated with antigen presentation and correlated with infiltration of T cells, including CD4+, Th1, Th2, and Treg cells [18, 40]. In addition, the expression of ACAA1 could be applied as a biomarker for personal therapeutic assessment. Patients with higher expression of ACAA1 are proposed to be more sensitive to anti-cancer drugs such as EGFR inhibitor Erlotinib and VEGFR2/3 inhibitor ZD-6474[18]. Notably, Erlotinib might be an enhancer of radiotherapy in NPC by evoking G2/M phase cell cycle arrest, as well as an enhancer of chemoradiotherapy by impeding DNA damage repairment[42]. Consistently, we found that higher ACAA1 expression was positively associated with higher infiltration of immune cells, and lower expression of ACAA1 was associated with worse overall survival of NPC patients. Indicated by these data, we hypothesis that NPC cells stimulate their malignant behaviours and survive through regulating immune cell infiltration in the tumor microenvironment.
Tumor microenvironment and immune modulation are pivotal in cancer research. Numerous studies underscore the critical roles of TME, particularly infiltrating immune cells, in tumor promotion and progression [43, 44]. T lymphocytes, particularly CD4+ and CD8+ cells, are abundant in NPC tumors [45]. CD4+ T cells, which generating chemokine ligand 13 (CXCL13), critically contribute to the formation of tertiary lymphoid structures by interacting with B cells, correlating with better survival of NPC patients[46]. CD8+ T cells, known for their ability to eliminate target cells through anti-tumor cytokines and cytotoxic molecules, exhibit exhaustion and dysfunction, and subsequently promote immune evasion of NPC cells [45, 47, 48]. Furthermore, recurrent NPC is associated with increased immunosuppression of T cells and exacerbated dysfunctional cytotoxicity in CD8+ T cells [49]. In addition to T cells, macrophages also contribute to the regulation of tumor infiltration. Macrophage phenotypes in NPC exhibit significant differences, with M1 macrophages mainly residing in tumor nests and M2 macrophages predominantly presenting in tumor stroma [50]. Tumor-derived fibroblast growth factor 2 (FGF-2) has been identified as a recruiter of macrophages and a promoter of M2 macrophage polarization through the upregulation of CXCL14 in NPC [51]. Notably, plasmacytoid dendritic cells (pDCs) aggregate the tumor stroma of NPC, and this phenomenon is significantly associated with improved survival outcomes [52]. Our study demonstrated a negative association between ACAA1 and immunosuppressive cells, alongside a positive association with effector cells, suggesting ACAA1’s potential role in regulating immune cells and counteracting tumor suppression in NPC.
EBV maintains a persistent latent infection in the human population by establishing a balance with the host’s immune system. Interestingly, reports have documented that EBV-encoded genes, such as EBNA1, EBNA2, EBV-encoded miRNAs BART11 and BART17-3p, facilitate tumor immune evasion through a complex network of pathways [53–55]. For instance, EBNA1 activates the JAK2/STAT1/IRF-1 signalling pathway and suppresses the promoter activity of PD-L1[53]. EBNA2 inhibits miR-34a through the downregulation of the transcription factor EBF1, consequently enhancing the expression of PD-1[54]. EBV-miRNAs BART11 and EBV-miR-BART17-3p stimulate the expression of PD-L1 by repressing FOXP1 and PBRM1, respectively[55]. RPMS1 and A73 are also important members of EBV BRATs implicated in NPC tumorigenesis [56, 57]. In the current study, we have described a negative association between the expression of ACAA1 and EBV-encoded genes (ie., EBNA1, RPMS1, and A73), indicating that the decrease of ACAA1 might heighten the infection of EBV. In addition, findings from the POLARIS-02 and CAPTAIN-1st trials have shown that an early decrease in plasma EBV titer or DNA is associated with a more favorable response to immunotherapy, implying that dynamic alterations in plasma EBV may serve as a promising biomarker [58, 59]. In this regard, we propose that ACAA1 might play a role in facilitating the immune response of NPC by modulating EBV infection. However, the precise relationship between ACAA1 and EBV infection remains uncertain and warrants further investigation.
The primary strategies for immunotherapy consist of immune checkpoint blockade, adoptive cell therapy, and vaccination. Among these, immune checkpoint inhibitors (ICIs) are employed to obstruct the activity of immune checkpoint proteins, boosting the immune response and alleviating immune suppression. Targeting T cell exhaustion-associated co-stimulatory signals (e.g., PD-1/PD-L1, CTLA-4, and LAG-3) and inhibiting tumor-infiltrating lymphocytes (TILs) are key objectives in immunotherapy [60]. This offers a novel and promising approach for the treatment of NPC. Clinical trials involving PD-1 inhibitors in recurrent and metastatic NPC patients have shown promising anti-tumor efficacy and favorable safety profiles [61–64]. Furthermore, patients receiving combined treatment with PD-1 inhibitors and chemotherapy have achieved extended overall survival (OS) and progression free survival (PFS) [59, 65]. Besides PD-1 inhibitors, dual immune checkpoint inhibitors, such as CTLA-4/PD-L1, LAG-3/PD-L1, and TIM-3/PD-L1, have undergone developed and evaluation in clinical trials [66]. However, these novel treatment approaches have not been approved by the FDA or NMPA, and the efficacy of PD-1/PD-L1 therapies remains constrained. In our study, we have observed a positive correlation between ACAA1 expression and several immune checkpoint-related genes, including PDCD1 (encoding PD-1 protein), CTLA4, CD80, CD86, and LAG3. Therefore, modulation of ACAA1 expression represents another important target for anti-cancer therapy.
In conclusion, we have identified ACAA1 as a novel tumor suppressor in NPC. ACAA1 is notably downregulated in NPC. Restoring ACAA1 effectively inhibits the proliferation, migration, and invasion of NPC cells, by suppressing Ki-67 expression and altering action filaments. Moreover, decrease of ACAA1 expression indicates poor survival of NPC patients, potentially due to immune evasion. These findings are anticipated to shed light on novel perspectives regarding NPC diagnosis markers, personalized immunotherapeutic strategies, and prognosis.