Liver hepatocellular carcinoma (LIHC) is the most common type of lethal cancer and is the fourth leading cause of death among cancer patients worldwide (Bray et al., 2018; Villanueva, 2019). The World Health Organization estimates that more than 1 million patients will die from LIHC in 2030 (Villanueva, 2019). Several risk agents, such as chronic infection with hepatitis B virus (HBV) or hepatitis C virus (HCV), and exposure to alcohol and aflatoxins are significantly involved in the intrinsic mechanisms (Wang et al., 2002; El-Serag and Rudolph 2007). Furthermore, recent studies have established that accumulated genetic alterations, such as somatic mutations, and chromosomal aberrations may be involved in this process (Villanueva, 2019). Somatic mutations in the TERT promoter, which is a recurrent insertion site for the genome of HBV, are the most frequent genetic alterations (approximately 60% of cases) (Schulze et al., 2016). Other mutated genes could affect the cell cycle (TP53, approximately 30% of cases) and WNT signaling (CTNNB1 and AXIN1 occurring in approximately 30 and 10% of cases, respectively) (Villanueva, 2019). In addition, chromatin remodeling (ARID1A and ARID2) may also account for approximately 10 and 5% of LIHC patients, respectively (Villanueva, 2019). Though the molecular mechanisms of LIHC remain far from being fully understood, the survival rate of LIHC patients could be improved by more than 50% with early detection of hepatocellular carcinoma (Kim et al., 2016; Lu et al., 2020). Conversely, the early diagnosis of LIHC is far from satisfactory, hence the exploration of novel molecular markers for early diagnosis and therapies is of great value for LIHC patients.

DEP domain-containing protein 1B (DEPDC1B), which is located at chromosome 5 (5q12.1), was initially discovered by mRNA expression profiling in MDA-MB231 human breast cancer cells (Boudreau et al., 2007). DEPDC1B contains two conserved domains: the DEP domain and the RhoGAP domain. The DEP domain is a globular domain containing about 90 amino acids, and was named from three proteins: Drosophila disheveled, Caenorhabditis elegans EGL-10, and mammalian Pleckstrin (Wong et al., 2000; Wharton, 2003). Being more than just a membrane anchor, the DEP domain could negatively interact with charged phospholipids located in membranes to activate Wnt signaling (Sokol, 2000). In addition, the DEP domain could interact directly with the G protein-coupled receptors to regulate GPCR signaling pathways (Ballon et al., 2006; Chen and Hamm, 2006). Moreover, the RhoGAP domain is a key participator in Rho GTPase signaling (Martemyanov et al., 2003). As a protein accumulating during G2 phase, Marchesi et al. have reported that the role of DEPDC1B in coordinating de-adhesion and cell-cycle progression at mitotic entry (Marchesi et al., 2014). Furthermore, increasing evidence in recent years suggests that the overexpression of DEPDC1B is associated with tumor aggressiveness and poor prognosis in cancers, such as oral cancer (Su et al., 2014), malignant melanoma (Xu et al., 2019), glioblastoma (Chen et al., 2020), non-small cell lung (Yang et al., 2014), and pancreatic cancers (Mishra et al., 2019; Liu et al., 2020). These findings strongly suggest that DEPDC1B could potentially contribute to tumorigenesis. However, the role of DEPDC1B in LIHC remains unclear.

In the present study, public databases were used to analyze the correlation between the expression of DEPDC1B and patient diagnostic and prognosis for LIHC. In addition, the results were confirmed by quantitative real-time polymerase chain reaction (qPCR) assay, and the findings from the study suggest that DEPDC1B may have a utility as a potential biomarker for the diagnosis and prognosis in LIHC patients.

Recently, systemic therapies for LIHC patients are rapidly changing (Villanueva, 2019). Compared with less than 15% of the 5-years survival rate of patients detected at later stages, the early diagnosis of LIHC could result in a survival rate of more than 50%. However, since the value of current diagnostic biomarkers in LIHC is greatly limited (Sun and Zhang, 2020), it becomes paramount to identify novel biomarkers for the treatment of LIHC patients as such biomarkers may help to improve the 5-years survival rate of LIHC patients and further help to establish personalized treatments for each patient. Herein, the current study presented the value of DEPDC1B as a potential biomarker in LIHC.

In 2007, Boudreau et al. reported that DEPDC1B was highly expressed in the placenta and testis with only little expressions in the small intestine and the heart (Boudreau et al., 2007). In the current study, using Human Protein Atlas database, we further confirmed that DEPDC1B was highly expressed in the placenta and testis, with only little expressions in the heart and the small intestine (Supplementary Figure S2). Furthermore, DEPDC1B is highly expressed in the 1) gallbladder and the thymus at the RNA level and 2) gallbladder, lymph node, and tonsil at protein level (Supplementary Figure S3) indicated that DEPDC1B might play an important role in the gallbladder, albeit the detailed mechanism remains unclear.

DEPDC1B plays an important role in the regulation of cell mitosis, transcription, and tumorigenesis (Martemyanov et al., 2003; Marchesi et al., 2014). Increasing evidence suggests that DEPDC1B is associated with various types of human cancers. However, the expression pattern and roles of DEPDC1B in LIHC remain unclear. To evaluate the role of DEPDC1B in LIHC, we used various online tools to observe the expression of DEPDC1B in LIHC. As indicated in Figure 1, the expression of DEPDC1B was higher in LIHC tissues than in normal tissues, and this is consistent with the results in non-small cell lung cancer, oral cancer, malignant melanoma (Xu et al., 2019), bladder cancer (Lai et al., 2020), glioblastoma, and pancreatic cancer (Liu et al., 2020). Using qPCR assay, we confirmed the higher DEPDC1B mRNA expression levels in LIHC than that in the normal tissues (Figure 7). These corresponding results further verified the expression of DEPDC1B mRNA is upregulated in LIHC tissues. Notably, some studies demonstrated that the overexpression of DEPDC1B could be used as a prognostic biomarker to predict the outcomes of patients with prostate and non-small cell lung cancers (Yang et al., 2014; Bai et al., 2017). Consistent with these previous reports, the present study indicated that the expression of DEPDC1B was negatively associated with patient prognosis (Figure 2), suggesting that the expression of DEPDC1B may be an invaluable prognostic biomarker for this disease. In addition, the univariate and multivariate Cox analyses further indicated the expression of DEPDC1B mRNA may be a useful biomarker in the prognosis of LIHC cancer (Figure 3). As aforementioned, this study was mainly focused on early-stage/grade LIHC and comparisons with normal tissues. As indicated in Figure 4, significant differences were observed in the DEPDC1B expression between the normal and tumor stage/grade cells. These exciting results further verified that DEPDC1B may be used as a valuable diagnostic biomarker of early-stage/grade LIHC. Generally, these multifaceted results strongly suggest that DEPDC1B may be an early diagnostic and prognostic biomarker in LIHC.

DEPDC1B either directly or indirectly affects the prognosis of patients with LIHC, however, the present understanding of the oncogenic function of DEPDC1B in LIHC progression remains unclarified. Increasing studies provide possible mechanistic explanations for the relationship between high DEPDC1B expression and poor prognosis in other types of tumors. Some studies indicated that the downregulation of DEPDC1B expression could suppress cell proliferation by promoting apoptosis in malignant melanoma (Xu et al., 2019), bladder cancer (Lai et al., 2020), and glioblastoma (Chen et al., 2020). In non-small cell lung cancer, DEPDC1B could enhance cell migration and invasion through the activation of Wnt/β-catenin signaling, and this biological effect could be inhibited by the depletion of LEF1 or TCF4 (Yang et al., 2014). In oral carcinoma, however, due to disruption of HPV E2, which is a viral tumor suppressor and is known to downregulate the expression of DEPDC1B, highly expressed DEPDC1B could interact with RAC1 and result in cell invasion/metastasis (Ahuja and Singh, 2016). RAC1 which is a major component of Rho GTPase signaling, and is known to regulate actin cytoskeleton (participate in the early stage of autophagosome formation), could regulate the cell cycle, cellular growth, and mediate cell proliferation via NF-κB activation (Ehrlich et al., 2002; Bauer et al., 2007; Bosco et al., 2010; Saci et al., 2011). Furthermore, LC3 which is a famous autophagy marker could interact with SOS1 to inhibit the GEF activity and block the activation of RAC1, which could also be inhibited by exogenous expression of DEPDC1B. Thus, the RAC1 signaling pathway is mutually regulated by autophagy and DEPDC1B. A similar study further proved that DEPDC1B may affect the prognosis of patients with prostate cancer through the regulation of autophagy (Bai et al., 2017). DEPDC1B could regulate RAC1 activity by increasing GTP loading in RAC1 instead of affecting Rho A activities in normal or cancer cells (Su et al., 2014). More recently, Liu et al. reported that long noncoding RNA lncNB1 could interact with ribosomal protein RPL35 to enhance the synthesis of E2F1 protein, leading to DEPDC1B gene transcription. The GTPase-activating protein DEPDC1B then induces ERK protein phosphorylation and the stabilization of the N-Myc protein in neuroblastoma cells. Conversely, the downregulation of lncNB1 mitigates the clonogenic capacity of neuroblastoma cells in vitro and leads to tumor regression in vivo (Liu et al., 2019). The study strongly suggests an additional pathway of DEPDC1B to induce carcinoma.

In summary, based on the Oncomine and TCGA databases, we have observed that high DEPDC1B expression is associated with poor prognosis in LIHC, suggesting that DEPDC1B could be a valuable diagnostic and prognostic marker in LIHC.