SCLC is a rather malignant lung cancer that exhibits a low degree of differentiation, grows rapidly, has high vascularity, and undergoes early extensive dissemination, with extremely poor associated survival [17]. Those with extensive disease have a survival of only eight to thirteen months, with a two-year survival rate of approximately 5% [18]. The first-line therapy of SCLC has not changed for decades in the clinic, and an effective therapeutic option for such recurrent-prone disease is still lacking [18, 19]. Although numerous studies have indicated that several molecules regulate the progression of SCLC, the underlying carcinogenesis mechanism remains unclear.
Herein, through the integrated analysis of SCLC, we detected 236 common DEGs and 104 common DEMs and conducted functional enrichment analysis of the former, revealing these DEGs to be significantly enriched in multiple signaling pathways, such as 'cell cycle', 'DNA replication', 'human T-cell leukemia virus 1 infection', ‘oocyte meiosis’, and ‘p53 signaling pathway’. The PPI network filtered five molecules and twenty hub genes, which were considered to be the key genes for the development of SCLC. Subsequent integrated analysis of the DEG regulatory pairs revealed a total of 54 overlapping genes between the DEM-target genes and the DEGs. Particularly, TTK and CDCA8 were identified as the hub genes among the 54 overlapping genes and were targeted by miR-455-3p, miR-140-5p, and miR-133b.
Thr/Tyr kinase (TTK) phosphorylates serine, tyrosine, and threonine residues [20, 21]. TTK is important for mitosis as it influences the precise segregation of chromosomes and the duplication of centrosomes [22, 23]. The expression level of TTK changes dynamically in the cell cycle, and it increases during the G1/S cell cycle phase and peaks in the G2/M phase [24]. Thus, the expression of TTK is closely related to the cell cycle. It is worth mentioning that the cell cycle was also demonstrated as the most significant signaling pathway of SCLC in this integrated investigation. TTK has been well demonstrated as an oncogene, and the dysregulation of TTK is linked to several cancers, including neuroendocrine lung cancer [25-27]. Du et al. believed that TTK could play a tumorigenic role in neuroendocrine lung cancer in combination with LMO1 [28]. Furthermore, TTK inhibitors have been used in antitumor therapy [29-31]. Zheng et al. showed that TTK inhibitors induced pronounced anticancer effects by augmenting polyploidy and apoptosis in NSCLC [32]. Herein, TTK was identified as a hub gene among the overlapping genes between the DEGs and DEM target genes. Importantly, the investigation not only demonstrated the overexpression of TTK in SCLC but also suggested a marked reduction in the expression of miRNAs (miR-445-3p and miR-140-5p) that regulate TTK.
MiR-455-3p is an important regulator of multiple tumors [33, 34]. MiR-455-3p has been suggested to serve as an anti-oncogene or oncogene in different cancers. For example, a previous study revealed that upregulated miR-455-3p enhanced the progression of breast cancer by regulating EI24, indicating that miR-455-3p plays a role as a tumor promoter in breast cancer [35]. In contrast, miR-455-3p has been suggested to play an important role as a cancer inhibitor in NSCLC by directly targeting HOXB5, with decreased expression of miR-455-3p identified as associated with poorer overall survival in patients with NSCLC [36]. In addition, Ning et al. also found a significant correlation between the overall survival of patients with lung squamous cell carcinoma and miR-455-3p expression level [37]. In general, miR-455-3p is dysregulated in several human tumors, which is in accordance with our findings in SCLC.
MiR-140-5p, as a tumor-suppressor, has been extensively studied recently because it is involved in the tumorigenesis of multiple kinds of tumors, including gastric cancer, breast cancer, colorectal cancer, etc. [38-40]. Furthermore, numerous studies have been performed regarding miR-140-5p in lung cancer; in one study, miR-140-5p was shown to repress the proliferation of NSCLC cells through the MMD/Erk signaling pathway [41]. Yang et al. revealed that miR-140-5p regulates the invasion and migration of NSCLC by targeting VEGFA [40]. The miR-140-5p target genes can serve as biomarkers of NSCLC, contributing to diagnosis and prognostic prediction [42, 43]. Nevertheless, to the best of our knowledge, neither miR-455-3p nor miR-140-5p has yet been identified as associated with SCLC, and we speculate that the aberrant regulation of TTK by miR-455-3p and miR-140-5p may be an underlying regulator of the pathogenesis of SCLC.
Human cell division cycle associated 8 (CDCA8) was another hub gene among the overlapping DEGs. As a chromosomal passenger complex component [44], CDCA8 upregulation is related to the carcinogenesis of a variety of tumors [45, 46]. CDCA8 has been proven to be a tumor promoter that is overexpressed in various kinds of tumors and is essential for cancer cell survival and malignancy [47]. High expression levels of CDCA8 are associated with the development and poor survival of malignancies such as breast cancer, osteosarcoma, and melanoma [48-50]. Furthermore, Bidkhori G et al. indicated that CDCA8 is linked with the cell cycle progression of lung adenocarcinoma [51]. Hayama S et al. demonstrated that CDCA8 is phosphorylated and coactivated by AURKB in NSCLC cells and that phosphorylated CDCA8 contributes to the survival and growth of NSCLC cells [52]. Thus, CDCA8 is considered a novel diagnostic and therapeutic target of great promise.
Although there have been several studies on CDCA8, the role of CDCA8 in SCLC has not been explored. The present study revealed that although CDCA8 was overexpressed in SCLC, the expression of miR-133b, which regulates CDCA8 expression, was significantly decreased.
MiR-133b, as a particular member of myomiRs, was originally thought to be muscle-specific and played a key regulatory role in muscle development and remodeling [53]. However, more recent work shows that miR-133b is downregulated in various cancers, indicating that it is closely linked to oncogenesis [54]. In the field of lung cancer research, Crawford M et al. first reported miR-133b underexpression in lung adenocarcinoma and found that it targeted the Bcl-2 family to inhibit tumors [55]. Furthermore, Liu et al. reported that the expression of miR-133b was decreased in NSCLC tissues and that miR-133b suppressed the development of NSCLC through targeting EGFR [56]. Lin et al. showed that miR-133b reduces cisplatin resistance in NSCLC by targeting GSTP1, and its overexpression suppresses the invasion and malignant growth of cisplatin-resistant NSCLC cells [57]. Nevertheless, the expression level and carcinogenic mechanism of miR-133b associated with SCLC remains unclear. We can infer that miRNA-133b regulating CDCA8 may be another underlying mechanism of the pathogenesis of SCLC.
However, our work has limitations. As a bioinformatics analysis, the results are based on publically available data instead of laboratory experiments. Although this study verified the expression at the gene level and protein level, we did not conduct functional verification of the genes in vitro. In addition, due to the lack of publicly available data on the clinical information of SCLC patients, we did not further analyze the association between the identified genes and patient survival, tumor recurrence, tumor stage, sex, age, etc.