Introduction
Lung cancer is a highly prevalent and deadly form of malignant tumor (Bray et al.
2018). Non-small cell lung cancer (NSCLC), which comprises approximately 80–85% of all lung cancer cases, can be further categorized into two main subtypes: lung adenocarcinoma (LUAD) and lung squamous cell carcinoma. LUAD accounts for approximately 40% of all lung cancers and is the most prevalent form of lung cancer. Despite the development of treatments to enhance patient outcomes, prognosis remains unfavorable for patients with LUAD. The 5-year survival rate of patients with advanced cancer is 10–15% (Kim et al.
2020; Siegel et al.
2012). Currently, surgery, radiotherapy, chemotherapy, and targeted therapy are the primary therapeutic approaches for LUAD. However, these methods insufficiently improve the symptoms and outcomes of patients with LUAD and survival rates. Hence, continuous exploration of novel and promising molecular indicators is imperative to identify targets to enhance the survival and prognosis of patients with LUAD.
FSTL3 belongs to the follistatin-like protein family. The
FSTL3 gene contains five exons and is located on chromosome 19p13. FSTL3 is expressed in various tissues throughout the human body, including the heart, placenta, gonads, and pancreas. The regulation of FSTL3 mRNA transcription is controlled by transforming growth factors and is indirectly enhanced by activin-A via SMAD family proteins (Huret et al.
2001). FSTL3 can affect myocyte development by directly inhibiting myogenin and activin-A through SMAD-dependent mechanisms (Nam et al.
2015; Smith and Lin
2013).
Typically, FSTL3 inhibits the activities of activin-A and myostatin (Brandt et al.
2014). Nevertheless, there have been limited reports on FSTL3 expression and its role in tumors, aside from its regulation of biological functions. The expression of FSTL3 in hepatocellular carcinoma tissues is lower than that in normal liver tissues (Grusch et al.
2006). Although breast cancer tissue and serum have shown high levels of FSTL3 (Bloise et al.
2009; Li et al.
2017), it is not significantly associated with patient prognosis (Couto et al.
2017). A study on lung cancer revealed that FSTL3 expression is elevated in both NSCLC tissues and cell lines. These increased levels are associated with poor prognoses in patients with NSCLC (Gao et al.
2020). Nevertheless, the expression of FSTL3 in LUAD and its influence on prognosis remains unclear.
Using The Cancer Genome Atlas (TCGA) database and clinical samples from our hospital, we discovered that FSTL3 holds significant prognostic significance and is linked to the infiltration of immune cells in individuals with LUAD. We also investigated the predictive significance of molecular controls and genes associated with FSTL3 in individuals diagnosed with LUAD, offering novel insights and avenues for prognostic prediction in patients with this disease.
Discussion
FSTL3, encoded by the
FSTL3 gene that is targeted by novel chromosomal rearrangements, belongs to the FSTL protein family (Kralisch et al.
2017). In 1998, FSTL was identified in the B cells of individuals with chronic lymphocytic leukemia (Hayette et al.
1998). Recently, FSTL3 has been shown to regulate numerous BP such as cellular differentiation, aging, obesity development, atherosclerosis progression, and tumor progression (Hayette et al.
1998; Sidis et al.
2002,
2005). FSTL3 also plays a role in the development of different types of malignancies, including hepatocellular carcinoma (Grusch et al.
2006), breast cancer (Bloise et al.
2009), and NSCLC (Nishimura et al.
1982). Tumor development is heavily regulated by FSTL3. In this study, we investigated FSTL3 expression and its prognostic significance in LUAD by utilizing TCGA database, our center's validation dataset, the TIMER database, and R software. In both TCGA and our validation set datasets, FSTL3 expression was notably reduced in tumor tissues compared to that in normal tissues. Furthermore, increased FSTL3 expression was linked to unfavorable prognosis in patients with LUAD. Patients with LUAD and exhibiting elevated levels of FSTL3 expression demonstrated a higher propensity for lymph node metastasis and presented with inferior clinicopathological staging compared to LUAD patients with lower FSTL3 expression. GSEA, GO, and KEGG pathway enrichment analyses were used to investigate the possible biological roles of FSTL3. This analysis revealed that FSTL3 expression was associated with nuclear division and spindle and microtubule binding. We then further examined the association between FSTL3 expression and prognosis as well as immune components in LUAD. Furthermore, immunohistochemistry was used to investigate the protein expression levels of FSTL3 in an external validation cohort at our institution. Overall, the findings of this research indicate that FSTL3 shows potential as a predictive marker for LUAD.
Tumor biology studies have increasingly focused on the relationship between tumors and the tumor microenvironment (TME) in recent years, which is crucial for understanding tumor pathogenesis and the effectiveness of immunotherapy (Chakravarthy et al.
2018; Henke et al.
2019). The effectiveness of immunotherapy in various forms of cancer is dependent on the TME. Nevertheless, LUAD presents a significant obstacle to personalized therapy due to its varied characteristics and unfavorable prognosis. The cells in the TME demonstrate flexibility by altering their antigenicity, evading immune detection, and persistent proliferation. Consequently, these cells can stimulate the activity of signal effectors, ultimately promoting the progression of tumors. Intratumor heterogeneity continues to pose a challenge in immunotherapy (Spella and Stathopoulos
2021). Cancerous and host cells in LUAD tumors form a diverse environment containing inflammatory signals, including cytokines, chemokines, and their corresponding receptors (Allavena et al.
2011). Activation of this inflammation signaling results in the recruitment of different types of immune cells, such as macrophages associated with tumors, lymphocytes that are reactive to tumors, suppressor cells derived from myeloid cells, neutrophils associated with tumors, and mast cells. These immune cells interact with tumor cells, leading to the development of a highly immunosuppressive TME. This environment reduces the effectiveness of immunotherapy to kill tumor cells and promotes tumor growth (Bronte et al.
2006; Zaynagetdinov et al.
2011; Ostrand-Rosenberg et al.
2012). Conversely, within the TME, the arrangement and role of infiltrating immune cells in tumors can exhibit minor differences based on the stage of the tumor and the immune condition of the host (Xiong et al.
2018).
In this study, we examined the relationship between FSTL3 and immune infiltration in LUAD and found a significant correlation between the expression of FSTL3 and B cells, CD4 + T cells, macrophages, neutrophils, and CD8 + T cells, indicating that the expression of FSTL3 is associated with the presence of immune cells in LUAD tissues. Various types of immune cells, such as Th1 cells, Tregs, CD8 + T cells, DC, macrophages, neutrophils, and NK cells, were present in the tumor immune microenvironment of LUAD tissues. The expression of FSTL3 was significantly correlated with the expression of marker genes of various immune cells. Correlation analysis revealed a significant association between FSTL3 expression and B cell infiltration, as well as markers of different types of T cells, demonstrating a strong correlation between FSTL3 expression and the infiltration of immune cells in LUAD.
After analyzing genes that showed positive and negative correlations with FSTL3 expression, we found that alterations in the expression of six genes, KRT6A, VEGFC, KRT14, SNORD17, SNORA12, and KRT81, had a notable impact on the prognosis of patients with LUAD. These findings may guide future studies on the molecular mechanisms associated with the impact of FSTL3 on the prognosis of patients with LUAD.
Our investigation revealed a notable diminution in FSTL3 expression among lung adenocarcinoma patients, which exhibited a significant correlation with N stage, pathological stage, and overall survival metrics. Moreover, we observed associations between FSTL3 levels and the presence of B cells, T cells, NK cells, and neutrophils. These findings imply that FSTL3 potentially exerts its influence by modulating the tumor microenvironment, especially in terms of immune cell infiltration. Gene Set Enrichment Analysis further elucidated a strong linkage of FSTL3 with critical signaling pathways, notably those involved in DNA replication and cell cycle regulation. This underscores the pivotal role of FSTL3 in cellular proliferation and survival, thereby impacting tumor initiation and progression.
Looking forward, we aim to implement animal model experiments to substantiate the function and mechanisms of FSTL3 in lung adenocarcinoma more robustly. We plan to investigate the impact of FSTL3 downregulation or overexpression on tumor growth, metastasis, and immune evasion in lung adenocarcinoma. Furthermore, the prospect of FSTL3 as a viable target for immunotherapy is a primary focus of our future research. Considering the association of FSTL3 with diverse immune cell infiltrates, we intend to assess the therapeutic efficacy of immunotherapy strategies that target FSTL3 in treating lung adenocarcinoma. Concurrently, we will delve into the interactions between FSTL3 and other pertinent genes, such as KRT6A, VEGFC, KRT14, KRT17, SNORA12, and KRT81, to enhance our understanding of its role in lung adenocarcinoma progression and to pinpoint novel therapeutic targets. However, our study has certain limitations. We examined pertinent genes and molecular mechanisms using sequencing data from public databases, which require validation through future studies involving in vivo and ex vivo experiments.
In summary, FSTL3 expression is crucial for the prediction of LUAD prognosis and plays a crucial role in the progression of LUAD by regulating the infiltration of immune cells such as T lymphocytes and B lymphocytes in the TME. The associated genes identified here could potentially serve as novel targets for future studies on the diagnosis and treatment of LUAD.
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