LUAD is a type of malignant lung tumor that originates from the bronchial mucosal glandular epithelium. Early diagnosis is difficult in most patients. LUAD is characterized by inconspicuous early symptoms, and LUAD is a lung tumor with a significant rate of malignant recurrence, metastasis, and unsatisfactory prognosis. Currently, targeted therapy, chemotherapy, immunotherapy, radiation therapy, and surgery remain the mainstays of therapy for LUAD, but the limitations of these therapies necessitate the development of effective approaches. Despite the remarkable importance of LUAD, a disease that affects millions of people worldwide, its exact pathogenesis is not fully characterized, and many aspects of the disease remain controversial.
ADRB2 is ubiquitously expressed in multiple tissues, including the smooth muscle of the human bronchi, cardiovascular system, central nervous system, and gastrointestinal tract. It has been proposed that gene expression, receptor function, and ligand response could be potentially affected by genetic polymorphisms. In recent years, polymorphisms in ADRB2 gene at different loci have been studied in various diseases, and increasing evidence shows that ADRB2 has a vital place in the occurrence and development of diverse range of cancers. Zhang et al. 11 found that the mRNA expressions of ADRB2 were higher in gastric cancers compared with normal tissues. Moreover, patients with gastric cancer with positive ADRB2 expression exhibited larger tumor size, late clinical stage, lower differentiation, and distant metastasis. In addition, high ADRB2 expression can promote the angiogenic switch in prostate cancer and prevent or delay the dominant role of pro-angiogenic factors, leading to tumor progression 12. β2-AR is encoded by ADRB2 and can bind specifically to endogenous catecholamines (such as adrenaline and noradrenaline), and promotes the production and release of cyclic adenosine phosphate (cAMP). cAMP can further activate and phosphorylate protein kinase A and C to activate downstream signal transduction pathways and promote the proliferation, migration, and metastasis of lung cancer cells 13. The positive β2-AR expression can occur in several cancers, including hepatocellular carcinoma, colorectal cancer, melanoma, and gastric cancer, and is often indicative of poor prognosis 5,11,14−16. Nevertheless, in oral squamous cell carcinoma, patients with higher β2-AR had a significant longer DSS and OS 17,18. Yazawa et al.19 retrospectively analyzed 328 surgically-resected patients with NSCLC and found that positive β2-AR expression was found in 29% of LUAD tissues, which markedly increased compared with in non-adenocarcinoma tissues. A high level of β2-AR expression was associated with vascular invasion, tumor cell proliferation, and poor prognosis in patients with LUAD. Nevertheless, Wang et al. 10 searched the gene expression synthesis (GEO) to obtain data showing that ADRB2 is down-regulated in LUAD, and ADRB2 mRNA levels declined with stage progression. ADRB2 mRNA expression levels and its gene product, β2-AR, differ. Gene Polymorphism plays an indispensable function in tumorigenesis and prognosis of malignancies. Due to polymorphism, there is significant genetic variation in β2-AR structure in human populations. Dennis W. McGraw et al.20 found that the C variant at rs1042711 is association with the down-regulation of β2-AR expression. Polymorphisms of ADRB2 in the 5′UTR and promoter region would causes a down-modulation of ADRB2 expression which in turn promotes onset of malignancies 21. The ADRB2 gene polymorphism affects gene expression, receptor function, and response to ligands8. Further studies are needed to analyze ADRB2 mRNA and β2-AR expression in LUAD tissues and the biological mechanisms regulating the transcription and translation of the ADRB2 gene in lung cancer cells.
The present study attempts to ascertain the prognostic significance of ADRB2 expression in LUAD and further explore its correlation with immune cell infiltration. ADRB2 was down-regulated in LUAD patients according to the TCGA database. Moreover, ADRB2 levels were significantly lower in male patients and smokers. Premised on this, further studies revealed that ADRB2 in LUAD was strikingly associated with sex, smoking status, T classification, lymph node metastasis, and pathologic stage.
The survival analysis revealed a favorable survival in high ADRB2 expression group compared to those with low expression. Univariate analysis revealed that pathological stage, T stage, N stage, M stage, and ADRB2 expression influenced OS. To further determine the diagnostic capacity of ADRB2 in LUAD, ROC curves were used to confirm that ADRB2 is sensitive to the diagnosis and prognosis of LUAD. Altogether, these findings illustrate that ADRB2 is a potential prognostic marker for LUAD.
The tumor immune microenvironment (TME) is very important in cancer pathogenesis 22. Immune cells are vital elements of the TME 23. The correlation between ADRB2 expression and the infiltration of 24 immunocytes was further explored to elucidate the mechanisms responsible for ADRB2 to predict clinical prognosis. Further correlation analysis indicated that the infiltration of 19 immune cells was significantly associated with ADRB2 expression. ADRB2 expression was positively correlated with aDCs, B cells, cytotoxic cells, DCs, eosinophils, iDCs, macrophages, mast cells, neutrophils, NK cells, pDCs, T cells, T helper cells, Tcm, Tem, TFH, Th1 cells, and Th17 cells. B cells are dominant in the progression of lung cancers 24,25 and can be observed at the individual stages of carcinogenesis 26. B cells can prolong the survival of cancer patients by inhibiting tumor progression and preventing metastasis. In addition, antibodies produced by B cells are essential mediators of tumor cell death 27. Cytotoxic T lymphocytes (TILs) are major players in antitumor immunity and can lead to apoptosis of cancer cells through a series of steps; therefore, high infiltration of TILs is a favorable prognostic marker for many cancers. DCs are the most effective antigen-presenting cells to induce primary tumor immune response (20), and in NSCLC patients, an increased DC count was significantly associated with an increase in DSS (21). The role of macrophages in cancer progression is still controversial. Tumor-associated macrophages promote tumor progression by facilitating tumor stroma formation and angiogenesis 28. In patients with NSCLC with prolonged survival, macrophage-infiltrating tumors are mainly of the M1 type 29. Mast cells, which have cytotoxic effects on cancer cells, can enhance the immunity of patients with LUAD against cancer cells and improve their postoperative prognosis 30. NK cells are cytotoxic and it is essential in the immune monitoring of cancers 31. Carrega et al.32 found that in resected LUAD tissues, increasing numbers of infiltrating NK cells were associated with favorable patient survival outcomes. T cells are the most abundant monocytes infiltrating the NSCLCs 33. T cells can secrete cytokines to inhibit tumor stroma formation and use cytotoxic molecules to kill epithelial nuclear stromal cells. Al-Shibli et al.34 reported that T cell infiltration is associated with better DSS, and T cells are an independent indicator of survival. T helper cells play an important role in cancer immunity by secreting cytokines 35. Both Th1 and Th17 cells produce proinflammatory factors, and their extensive infiltration can significantly improve clinical outcomes in a variety of cancers 36–38. Large infiltration of cytotoxic T cells in tumor tissues is associated with longer survival 36. Studies have shown that TFH has an antitumor response, and IL-21 secreted by TFH induces the activation, proliferation, and differentiation of B cells 39,40. ADRB2 expression may up-regulate the levels of infiltrating immune cells to limit the development of LUAD. In contrast, ADRB2 expression was negatively correlated with Th2 cells and Tgd. Th2 cells have many pro-neoplastic activities and take part in cancer progression by cytokine release. Th2 cells are dominant in lymphocytes from malignant pleural effusion in patients with lung cancer 41,42. Current studies have revealed that Tgd has a pro-tumor effect, which can inhibit innate and adaptive immunity by inducing immunosenescence 43–45.
To summarize, low ADRB2 expression is associated with poor prognosis of LUAD. These results show that ADRB2 expression level affects the immunity activity in the TME, and ADRB2 might be a valuable biomarker for the immune status in LUAD patients.
To further explore the mechanism of ADRB2 in LUAD, the signaling pathways involved in ADRB2 was screened. In the ADRB2 high-expression group, ADRB2 associated genes were significantly enriched in immune signaling pathways (such as B cell receptor signaling pathway, T cell receptor signaling pathway, and NSCLC, NK-cell-mediated cytotoxicity, chemokine signaling pathway, and Jak STAT signaling pathway), in KEGG analysis. Those are significant in the tumorigenesis, development, and invasion of malignancies 46. On the other hand, in the ADRB2 low-expression group, ADRB2 correlated genes were enriched in metabolism-related pathways, including RNA polymerase, citrate cycle, pentose phosphate pathway, oxidative phosphorylation, cysteine and methionine metabolism, and amino sugar and nucleotide sugar metabolism, implying that ADRB2 up-regulated the signaling pathways associated with immune response and induced antitumor efficiency. Therefore, ADRB2 expression was down-regulated as LUAD progressed, and the TME switched from an immune-active state to a metabolic state. The ADRB2 expression can be considered a biomarker to predict immune response.
At present, most studies on the relationship between ADRB2 and the occurrence and progression of LUAD are based on its gene expression product, β2-AR, and its signaling pathway. This study revealed ADRB2 as a key gene in the immune microenvironment of LUAD by performing a bioinformatics analysis, to provide evidence for ADRB2 as a potential prognostic marker for LUAD. However, some limitations arise in the research. Firstly, the present research was limited by the small number of cases, and a large cohort is needed to validate the results of this research. Secondly, this research primarily focused on the expression of ADRB2 mRNA from TCGA, and without involving β2-AR levels in LUAD tissues. Thus, this study still needs large-sample, multi-center, multi-ethnic clinical trials and basic experimental studies to prove the prognostic value of ADRB2 in LUAD.