The accurate diagnosis and effective treatment of LUAD are always challenging to clinical physicians and researchers because of its high incidence rate and mortality. Thus, it is imperative to explore more sensitive and specific biomarkers for early detection and personal treatment strategies for LUAD. In this study we performed comprehensive analysis of CCTs in LUAD based on TCGA and other public databases such as UALCAN, GEPIA2.0, TIMER2.0, The Human Protein Atlas, PrognoScan, cBioPortal, GEO and STRING. TCGA database began in 2006 which collected both gene expression and clinical data on 33 different tumor types ranging from solid to hematological types, from mildly to severely aggressive tumors(25). Many significant discoveries have been made using TCGA for identifying novel oncogenic biomarkers and establishing molecular subtypes(26, 27). Many other platforms such as UALCAN and GEPIA2.0 are developed based on the data from TCGA. Through these available databases, we identifying CCTs family genes may serve as novel and useful biomarkers for LUAD diagnosis and targets for LUAD treatment.
There are several studies about CCTs including CCT3, CCT5 and CCT6A in lung cancer especially in NSCLC. Upregulation of CCT3 levels was observed in NSCLC tumor tissues, which was linked to unfavorable patient prognosis(28). Another study revealed that CCT3 expression was increased in the cisplatin-resistant A549 cells compared to the cisplatin-sensitive A549 cells and CCT3 could promote cisplatin resistance via activating the JAK2/STAT3 pathway(29). In CCT3 deficient LUAD cells, glycolysis process was inhibited and ATP production reduced, resulting in decreased cell growth and metastasis(30). CCT6A were associated with the poor prognosis of LUAD patients and were coamplifying and coexpressing with EGFR(31). Interestingly, anti-CCT5 autoantibody was found in 51% of NSCLC patients, but only 2.5% in non-tumor controls, indicating CCT5 is a tumor associated antigen and can serve as diagnostic markers for NSCLC(32). In view of the gradually emerging role of CCTs in NSCLC, it is urgent to make a comprehensive understanding of CCTs to facilitate further research. Our study showed that CCTs except CCT6B had higher expression in NSCLC tumor tissues including LUAD and LUSC. However, high expression of all the CCTs except CCT6B predicted unfavorable outcome of LUAD patients rather than LUSC patients. Thus, CCTs subunits may be prognostic biomarkers specifically in LUAD, instead of LUSC.
Chaperones are initially defined by their ability to interact with other proteins and assist folding of substrate proteins, oligomeric assembly, protein trafficking and proteolytic degradation(33). The most extensively studied chaperones are the heat shock proteins (HSPs) which have multiple clinical implications as biomarkers for cancer diagnosis and anti-cancer treatment(34). CCTs is another novel family of molecular chaperones remains to be further studied. Cytoskeleton proteins, such as actin and tubulin, are known substrates for CCTs(35). One study indicated that loss of CCT8 impairs the formation of nuclear actin filaments of T cells, affecting T cell maturation and function(36). Furthermore, in esophageal squamous cell carcinoma, CCT8 influenced the migration and invasion of ESCC cells by regulating expression of α-actin and β-tubulin(14). Interactions between CCTs and other functional proteins are also reported. CCT3 bound to the β-strand rich, DNA-binding domain of Stat3, contributing to its biosynthesis and transcriptional activity(37). CCTs were confirmed to bind to Cdc20 and CCTs were essential for Cdc20-dependent cell cycle events such as sister chromatid separation and exit from mitosis(38). In metastatic cancer cells, CCT complex directly bound and stabilized XIAP and β-catenin expression, facilitating chemoresistance and metastasis(12). In LUAD, CCT3 interacted with eukaryotic translation initiation factor 3, affecting cytoplasmic translation(30). Moreover, CCTs interacted with PLK1(39), CyclinE(40), CyclinD(41) and p53(42) which were closely related with cancer progression. In the present study, we screened CCTs-related Hub genes and found these genes were upregulated in LUAD and predicted poor outcome of LUAD patients. Further studies were needed to verify whether these genes can serve as substrates of CCTs in LUAD.
The tumor microenvironment comprises a mass of heterogeneous cell types, including immune cells, endothelial cells, and fibroblasts. Avoiding immune destruction remains to be one important hallmark of cancer(43). The continuous interactions between tumor microenvironment and cancer cells promoted tumor progression, metastasis, and affect therapeutic response and clinical outcome(44). T cell-mediated immune response has attracted widespread interest and has been used as therapeutic targets in diverse pre-clinical and clinical models(45). Th1 and Th2 cells are the two firstly studied classes of CD4+ T cells and were characterized by the production of interferon gamma and interleukin-4, respectively(46). Cancer patients are in a state of dominant Th2 differentiation, and the cytokines produced by Th2 cells have an inhibitory effect on the proliferation and differentiation of Th1 cells and impair the function of cytotoxic T cells, resulting in weakening of the body's immunity against tumor(45). In this study, we found that expression of all the CCTs in LUAD had positive relationship with the infiltration of Th2 cells. This data was in agreement with one study which indicated that Th2 cell polarization and T cell metabolism are dependent on CCT8 expression. They used the Cct8 KO mouse model and found that CCT8 is essential for protective immunity against intestinal helminths(36). Thus, CCTs may be required for T cells to be Th2 polarized, weakening immune response against tumor cells.
As protein chaperones, HSPs have been widely studied and highly selective and specific inhibitors towards HSPs have been developed. Furthermore, some inhibitors have been subjected to clinical evaluation(47–49). Due to the significance of CCTs in cancer, researchers also tried to seek for inhibitors targeting CCTs. A compound was reported to impede the interaction between CCT2 and β-tubulin and inhibited migration and invasion of the LUAD cells(15). Furthermore, another compound anticarin-β showed a promising anti-osteosarcoma effect by specifically inhibiting CCT4 to impair proteostasis. Anticarin-β could strongly decreased CCT4-mediated STAT3 maturation and had outstanding antitumor efficacy in both orthotopic and patient-derived xenograft models of osteosarcoma(50). Collectively, it is promising to target CCTs for cancer treatment and more efforts are needed to develop CCTs inhibitors.