Cervical cancer remains the fourth most common cancer among women and accounts for 527,624 new diagnosed cases and 265,672 deaths in 2018 (Bray et al. (2018)). Cervical cancer continues to be the first or second leading cause of cancer-related death among women for many low- and middle-income countries (LMICs) (Wang et al. (2018)). Persistent HPV infection, especially types 16 and 18, is a high-risk factor but not the only one for cervical cancer (Revathidevi et al. (2020)). Host genetic factors may also be involved in tumor development. The major treatments for cervical cancer patients include surgery, chemotherapy, and radiotherapy. For patients with early-stage cervical cancer, 5-year survival is up to 91.5%, while the treatment of advanced cervical cancer is not ideal (Luan and Wang (2018)). The median survival time of metastatic cervical cancer patients is about 8–13 months, and the 5-year overall survival rate is only around 16.5% (Ferlay et al. (2013); van Meir et al. (2014)). Therefore, it is urgent to find more accurate biomarkers for early detection of cervical cancer and monitoring the disease progression.

Secreted phosphoprotein 1 (SPP1) is a secreted multifunctional phosphoprotein located in 4q13 with seven exons and six introns. SPP1, also known as osteopontin-like protein or early T-lymphocyte activation 1 protein, is a member of the small integrin-binding ligand N-linked glycoprotein (SIBLING) family which can specifically bind and activate matrix metalloproteinases (MMPs) in cancer (Su et al. (2020)). Its main biological functions are involved in immune response, biomineralization, and tissue remodeling. SPP1 is also related to the growth, proliferation, migration, apoptosis, and chemotaxis of cells. Previous studies have proven that SPP1 is overexpressed in a variety of cancers and can be used to predict the adverse consequences, including ovarian cancer (Zeng et al. (2018)), glioblastoma (Kijewska et al. (2017)), hepatocellular carcinoma (Wang et al. (2019)), and gastric cancer (Song et al. (2019)). Recently, the relationship between the expression of SPP1 and chemotherapy resistance, such as prostate cancer and hepatocellular carcinoma, has also attracted the attention of researchers (Liu et al. (2016); Pang et al. (2019)), while no study has explored the correlation between SPP1 and cervical cancer. Therefore, our study aimed to explore the expression of SPP1 in cervical cancer tissues and its potential clinical values.

In our research, we utilized the cervical cancer RNA-seq data from The Cancer Genome Atlas (TCGA), Gene Expression Omnibus (GEO), and Genotype-Tissue Expression databases to compare the differential expression of SPP1 between normal cervical tissues and cervical cancer samples. Next, we investigated the relationship between SPP1 expression levels and clinical pathological features of cervical cancer. Furthermore, we explored the prognostic value of SPP1 in cervical cancer. Besides, we performed gene enrichment analysis to reveal its potential functions. Finally, we analyzed the relationship between SPP1 expression and immune infiltration and comprehensively explored its mechanism in inducing and promoting cervical cancer.

Invasive cervical cancer remains the leading cause of cancer death among women worldwide (Shen et al. (2020)). Thus, it is necessary to find more accurate biomarkers to detect at an early stage and monitor disease progression. According to the previous studies, SPP1 is overexpressed in various cancer types (Xu et al. (2017); Choe et al. (2018); Zhang et al. (2020)) and identified as a prognostic factor (Li et al. (2018); Chen J et al. (2019); Guo et al. (2020)), while to our knowledge, no study has explored the relationship of SPP1 expression and cervical cancer. In our study, we attempted to explore the potential mechanism of SPP1 in promoting cervical cancer and its feasibility as a molecular biomarker.

In pan-cancer analysis, we found that SPP1 was upregulated in most cancer types. Further exploration revealed that higher SPP1 expression was associated with reduced overall survival (OS) in cervical cancer patients. We performed logistic regression to evaluate the relationship between the SPP1 expression level and the clinicopathologic characteristics of cervical cancer. The result showed that SPP1 was significantly correlated with clinical stages. In addition, univariate and multivariate Cox analyses indicated that SPP1 was an independent factor to predict prognosis of patients. All these aforementioned results and ROC analysis suggest that SPP1 may be a promising prognostic biomarker for cervical cancer patients.

The tumor microenvironment (TME), composed of various types of immune cells, played an important role in tumor progression, metastasis, and treatment resistance (Usui et al. (2016)). The composition of tumor-infiltrating immune cells strongly influenced the tumor microenvironment and the behavior of the tumor. Our gene enrichment analysis revealed that the main biological function of SPP1 was mainly involved in immune response. We next confirmed that SPP1 expression correlated with immune cell infiltration. Hence, we hypothesized that SPP1 may affect the tumor microenvironment by changing proportions of specific immune cell types, thereby promoting tumor progression and metastasis. It was, indeed, the case that SPP1 had recently been shown to be an important component in maintaining the tumor microenvironment in AML (Ruvolo et al. (2019)). Our research demonstrated the significant positive correlation between macrophages and the expression of SPP1. Macrophages are important components of the tumor microenvironment, and tumor-associated macrophages play complex roles in cancer pathophysiology (Gibson et al. (2019)). A previous study found that SPP1 was involved in the function, migration, and differentiation of macrophages (Zhang et al. (2017); Wei et al. (2019); Jaitin et al. (2019); Srirussamee et al. (2019)). A recent study also showed that SPP1 was essential for M2-like macrophage, the tumor-associated macrophage, and promoted tumor growth (Chen P et al. (2019)). Furthermore, we found that the increased level of macrophages and DC infiltration were correlated with poor prognosis. Our results were supported by the findings of similar studies about this topic (Long et al. (2016); Ndiaye et al. (2019)). Certainly, the tumor microenvironment had a high level of complexity in its regulation; other immune cell types in the tumor microenvironment may also influence tumor cell survival, including iDC, neutrophils, NK CD56 bright cells, Th1 cells, DC, and pDC. Future studies were needed to further explore the relationship between SPP1 expression and these cells.

In conclusion, we demonstrated that SPP1 expression was upregulated in cervical cancer and significantly related to poor survival outcome. In addition to this, SPP1 might participate in the occurrence and development of cervical cancer by influencing the infiltration level of immune cells. Therefore, our study revealed the role of SPP1 in cervical cancer and identified a promising prognostic biomarker.

Although our study is the first work to explore the relationship between SPP1 expression and cervical cancer, it also has some limitations. First, all of the data analyzed by bioinformatics methods in this study were downloaded directly from public databases, so it requires further validation by experimental investigations; second, the number of normal samples used as controls was considerably different from that of patients with tumor in the TCGA database; therefore, further studies based on an equal balance of sample size are necessary. Third, further validation studies with a long-term follow-up and larger cohorts of patients are needed to definitely validate SPP1 as an OS predictor. Last but not least, our study laid the foundation for detailed studies of the correlation between SPP1 and the tumor-associated immune microenvironment. However, more studies are required to explore the hypothesis in depth.

The cervical cancer cell lines (Siha and Hela) present in this study were obtained from the Scientific Research Center of Zhongnan Hospital of Wuhan University. And normal cervical epithelial cell (END1) was donated by Wuhan University Basic Medical College.