Introduction
Endometrial cancer (EC) is a common malignant tumor worldwide, and its incidence and mortality rates are increasing [
1,
2]. Over the preceding two decades, a conspicuous increase in the incidence of endometrial cancer has been observed, with a notable predilection for impacting younger people. This form of cancer accounts for approximately 20–30% of gynecological malignancies, highlighting a worrisome trend [
3]. The 5-year overall survival (OS) of patients with EC is contingent upon the diagnostic stage. Early detection often leads to favorable prognoses, with a five-year OS rate exceeding 90% [
4]. Regrettably, distant metastasis is frequently encountered and significantly impacts survival rates. Once regional spread or metastasis occurs, the 5-year survival drops to 60% for stage III patients and 20% for stage IV patients [
5]. Accurate and efficient evaluation of novel clinically applicable biomarkers is crucial for improving EC treatment.
Treatment strategies for endometrial cancer encompass various options, including surgery, radiotherapy, chemotherapy, and immunotherapy. Currently, the standard first-line chemotherapy for treating EC mainly involves combination therapy comprising carboplatin and paclitaxel, but half of the patients still experience chemotherapy resistance [
6]. Therefore, new approaches need to be developed to enhance the sensitivity of patients to chemotherapeutic drugs in the face of drug resistance. With the integration of immuno-oncology with other immunotherapies, chemotherapies, or targeted therapies, there has been a significant transformation in treatment approaches for late-stage patients. Recently, immunotherapeutic advancements have been seen in the realm of endometrial cancer treatment, demonstrating potential therapeutic effects in clinical trials with PD-1 and PD-L1 inhibitors. Such comprehensive treatment strategies provide individual patients with more precise and effective therapeutic options [
7].
Threonine and tyrosine kinase (TTK), alternatively referred to as Mps1, is a dual-specificity kinase that can catalyze the phosphorylation of both serine/threonine and tyrosine residues [
8]. TTK plays a key role in safeguarding chromosome segregation fidelity, participating in the assembly of mitotic checkpoint complexes, regulating cytokinesis, responding to DNA damage, and promoting accurate chromosome alignment [
9]. The spindle assembly checkpoint (SAC) represents the primary mechanism for preserving chromosomal stability during cytokinesis [
10]. The SAC pathway, involving TTK, Polo, Aurora, Bub, BubR and Mad, detects misoriented chromosomes, establishes proper bipolar connections with the spindle, and diminishes chromosomal mismatch errors prior to late-stage initiation [
8]. The mitotic spindle checkpoint is controlled by TTK, which monitors the tension and bipolar connections of all chromosomes attached to spindle microtubules [
11]. This ensures faithful segregation of sister chromatids at the kinetochore. The cell cycle pauses during metaphase when improperly attached chromosomes are detected until all chromosomes achieve proper attachment before proceeding to anaphase [
12]. In addition to regulating the mitotic SAC, TTK is involved in centrosome replication, the DNA damage response and organ development [
13]. Notably, TTK is rarely detected in normal tissues except for the testis and placenta. However, TTK is overexpressed in many cancers, promoting tumor growth in glioblastoma, breast cancer and other cancers [
14‐
16]. In triple-negative breast cancer, increased TTK expression correlates with tumor tropism and metastatic behavior [
17]. Similarly, TTK knockdown in hepatocellular carcinoma inhibited tumor migration, proliferation and cellular activity. Recent evidence also indicates that TTK downregulation impedes cancer cell migration [
18]. In summary, TTK has emerged as a prospective biomarker and therapeutic target for cancer.
The exact prognostic importance of TTK in endometrial cancer, as well as its connection to immune infiltration, remains incompletely understood. Given the well-documented overexpression of TTK in cancer and its ability to inhibit tumor cell migration upon TTK depletion, we developed a hypothesis suggesting a plausible correlation between TTK expression and survival outcomes in patients with endometrial cancer. To investigate and evaluate our hypothesis, we conducted a comprehensive assessment of the prognostic importance of TTK in endometrial cancer using The Cancer Genome Atlas (TCGA) dataset. In our study, we detected upregulated TTK expression in endometrial cancer. Notably, there were associations between TTK upregulation and unfavorable clinical characteristics and risk factors. Additionally, we explored the diagnostic and prognostic value of TTK in endometrial cancer as well as its correlation with immune infiltration. Our results demonstrated an association between high TTK and worse survival in patients with endometrial cancer.
Materials and methods
TCGA datasets
The transcriptional expression data of TTK along with pertinent clinical information were obtained from the official website of TCGA [
19]. After the initial provision of RNA-Seq gene expression data in the FPKM workflow format, the data were further converted into the TPM format and subjected to log2 transformation for subsequent analysis. Given that all the data utilized in this study were obtained from the TCGA, ethics committee approval was not needed.
RNA-sequencing data of TTK in endometrial carcinoma
The RNA-seq expression data for TTK in endometrial carcinoma were also obtained from the TCGA. A total of 545 endometrial carcinoma samples and 23 adjacent normal tissue samples were retained for analysis. The selected samples included TTK gene expression data and relevant clinical information, including age, sex, smoking status, TNM stage, and tumor location. The mRNA expression data were characterized using the mean ± standard deviation (SD) format.
Clinical proteomic tumor analysis consortium (CPTAC), GEPIA2 and UALCAN
The CPTAC employs mass spectrometry for the analysis of tumor specimens, enabling the quantification and identification of proteins to characterize the proteome [
20]. GEPIA2 uses a standardized pipeline to analyse RNA-seq data from 9,736 tumours and 8,587 normal samples in TCGA and Genotype-Ex (GTEx) cohorts [
21]. UALCAN is an online tool for analysing public cancer data. We used UALCAN to evaluate TTK protein expression using CPTAC data.
Quantitative real-time PCR (qRT‒PCR)
Total RNA was extracted from tissues, cultured cells were purified using TRIzol reagent (Invitrogen, USA), and the RNA concentration was quantified. Equal amounts of RNA (2 µg) were reverse transcribed into complementary DNA (cDNA) using Superscript Reverse Transcriptase (Applied Biosystems, USA). Quantitative real-time PCR (qRT–PCR) was performed on an ABI 7500 real-time PCR system (Applied Biosystems, USA) using PowerUP SYBR Green Master Mix (Applied Biosystems, USA). The primer sequences used for TTK and ACTB were as follows: TTK forward, 5′-TGCCCTGCGGAATTTAAACC-3′ and reverse, 5′-CTGTAAATGCCCAAGTGAACCG-3′; ACTB forward, 5′-GGCACCCAGCACAATGAAG-3′ and reverse, 5′-CCGATCCACACGGAGTACTTG-3′.
Western blot analysis
The protein concentration was measured using a BCA kit (Pierce, USA) after lysing tissues or cells with cell lysis buffer (Cell Signaling Technology, USA) containing protease inhibitors (Roche, Switzerland). Equal amounts of protein (20 µg) were separated via 10% SDS‒PAGE and transferred to nitrocellulose membranes (Invitrogen, USA). The membranes were blocked using 5% nonfat milk at room temperature for 2 h, followed by an overnight incubation with primary antibodies at 4 °C. Following TBST washes, the membranes were incubated with HRP-conjugated secondary antibodies for 2 h at room temperature. GAPDH was used as an internal loading control. For signal detection, an enhanced chemiluminescence (ECL) kit was used, while band intensity was quantified using ImageJ software (NIH, Bethesda, USA). The primary antibodies used in this study were as follows: TTK (Santa Cruz [SANTA CRUZ], sc-56,968) and GAPDH (Proteintech, 60004-1-Ig).
Protein–protein interaction (PPI) networks and functional enrichment analysis
STRING is a web-based database (version 11.05) that facilitates the retrieval of interacting genes and networks [
22]. In our study, we utilized STRING to search for coexpressed genes and construct PPI networks, with interactions > 0.4. Gene Ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses provided functional insights into the coexpressed genes. The “ClusterProfiler” package was used for these analyses, and the results were visualized using the “ggplot2” package.
Tumor immune estimation resource (TIMER) database
TIMER, an invaluable online resource, allows for the systematic analysis of immune infiltrates across various types of cancer [
23]. In our study, we used TIMER to investigate the relationship between TTK expression and infiltration of immune cells, including B cells, CD4 + T cells, CD8 + T cells, neutrophils, macrophages, and dendritic cells, in uterine corpus endometrial carcinoma.
Tumor-immune system interaction database (TISIDB)
TISIDB serves as an online web repository portal dedicated to the exploration of tumor-immune system interactions [
24]. In our study, we utilized TISIDB to assess the expression of TTK and tumor-infiltrating lymphocytes (TILs) across various cancers. By leveraging the gene expression profiles, we employed gene set variation analysis to estimate the relative abundance of TILs. Spearman’s test was subsequently used to evaluate the correlation between TTK expression and the presence of TILs.
Statistical analysis
All the statistical analyses were conducted using R (version 3.6.3). The ggplot2 package was used to visualize differences in expression. To determine the disparities between uterine corpus endometrial carcinoma tissues and adjacent normal tissues, paired t tests and Mann‒Whitney U tests were utilized. The pROC package was used to perform receiver operating characteristic (ROC) curve analysis, which identified the cutoff value for TTK [
25]. K-M analysis and log-rank tests were carried out using the survminer package to evaluate the impact of TTK on survival outcomes.
Discussion
The prognosis of advanced EC is less optimistic than that of early endometrial cancer, and there is an urgent need to introduce new biomarkers for early detection [
6]. Concurrently, the deployment of innovative methodologies is designed to enhance the sensitivity of chemotherapeutic agents, thereby substantially augmenting the prospects of treatment success for EC. This comprehensive strategy is expected to not only improve the cure rate but also have a positive impact on patient survival and quality of life.
In this study, we observed an upregulation of TTK mRNA expression across various tumors. The heightened expression of TTK in endometrial cancer was validated through multiple databases and cell lines. Clinicopathological characteristics and ROC curve analysis of TTK expression revealed positive correlations between TTK mRNA expression and tumor metastasis and between TTK mRNA expression and high TNM stage. This finding underscores the potential of TTK mRNA expression as a diagnostic biomarker for distinguishing endometrial cancer tissue from normal tissue. Additionally, Kaplan‒Meier analysis revealed that increased TTK mRNA expression was associated with a poorer prognosis, positioning TTK as a prospective biomarker for poor prognosis in patients with endometrial cancer. Our findings also suggest a unique role for TTK in immune infiltration in endometrial cancer.
TTK is a key spindle assembly checkpoint enzyme that regulates tumor cell proliferation across organisms. Aberrant TTK overexpression has been noted across cancer types, implicating its oncogenic role [
26]. Numerous studies have demonstrated the carcinogenic effects of TTK in various human cancers, including triple-negative breast cancer, pancreatic cancer, and hepatocellular carcinoma [
27‐
29]. Furthermore, TTK upregulation has been reported in different cancers and is associated with unfavorable prognosis [
30]. However, the expression and prognostic value of TTK in endometrial cancer have not been extensively explored. In our study, using a pancancer analysis, we observed aberrant TTK mRNA expression in patients with various cancers, consistent with previous reports.
Recent studies and our results strongly indicate that TTK is a biomarker of poor prognosis across cancers [
31]. In pancreatic ductal adenocarcinoma (PDAC), TTK knockdown substantially reduced proliferation and increased apoptosis in cell lines, highlighting its role in pancreatic cancer growth [
28]. In non-small cell lung cancer (NSCLC), elevated TTK expression has been linked to an unfavorable prognosis [
32]. Similarly, knockdown of TTK inhibited cell proliferation, migration, and tumorigenesis in NSCLC. Likewise, in other types of lung cancer, heightened TTK expression has been reported to promote cell migration and epithelial-mesenchymal transition (EMT), thereby enhancing metastatic potential and facilitating tumor metastasis [
33,
34]. Additionally, TTK expression was elevated in hepatocellular carcinoma tissue compared to normal tissue, and functional studies confirmed TTK’s oncogenic activity. Notably, this intervention is associated with increased senescence and autophagy in tumor cells [
35].
However, the prognostic value of TTK in endometrial cancer has not been explored. The positive correlation between TTK upregulation or metastasis and advanced TNM stage suggested that TTK is involved in endometrial cancer progression. Our Kaplan‒Meier and log-rank analyses revealed reduced survival in patients with high versus low TTK expression. Taken together, we conclude that TTK can indeed serve as a biomarker for predicting poor prognosis in patients with endometrial cancer.
Tumor development is closely linked to alterations in the tumor microenvironment (TME) [
36]. Cancer cells can reprogram their microenvironment by releasing various factors that modify neighboring immune cells [
37]. This interplay between cancer and immune cells creates a conducive environment that fosters tumor growth, promoting tumor survival and progression [
38]. However, the relationship between TTKs and immune infiltration has not been explored. In this study, we observed a positive correlation between high TTK expression in EC and increased infiltration of helper T2 and helper T cells as well as a negative correlation with CD56
bright NK cells. Additionally, higher TTK expression was inversely correlated with the enrichment of various infiltrating immune cell types. Moreover, the influence of immune cell infiltration on TTK expression has a significant impact on EC patient survival. These findings suggest that TTK might serve as a promising target for future combination immunotherapy trials. Nonetheless, further research is warranted to elucidate the precise role of TTKs in the tumor immune microenvironment.
The role of TTK inhibitors has been extensively documented in numerous articles, and these inhibitors are currently in clinical use [
12,
27,
29,
39‐
41]. The dual TTK/CLK2 inhibitor CC-671 provides a theoretical basis for overcoming ABCG2-mediated MDR in lung cancer patients [
40]. In multiple myeloma, the TTK inhibitor OSU-13 can inhibit tumor growth in vitro and in vivo, improving patient prognosis [
12]. In liver cancer, the TTK inhibitor AZ3146 can enhance DNA damage, increasing the sensitivity of cancer cells to radiation [
41]. Additionally, the orally active small molecule inhibitor of TTK (CFI-402,257) effectively inhibits tumor growth and promotes the action of immune cells [
29]. Some studies have also explored the impact of combining TTK inhibition with other treatment strategies, such as chemotherapy or targeted therapy. Theoretically, these combination approaches can enhance the efficacy of cancer treatment, potentially overcoming resistance mechanisms. In endometrial cancer, whether using immune checkpoint inhibitors alone or in combination with different drugs, significant breakthroughs can be achieved in cancer treatment, thereby extending the survival period of tumor patients [
42,
43].
This study has several limitations that should be acknowledged. First, the investigation of TTK expression and its prognostic importance relied on an online public database; thus, it is crucial to validate these findings through further studies involving clinical samples. Second, in vivo or in vitro experiments are needed to fully elucidate the mechanistic role of TTK in immune infiltration. These experiments could enable further investigations of the specific underlying mechanisms involved.
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