DPP10-AS1 is upregulated in lung cancer and predicts poor prognosis in patients.
To confirm the expression of DPP10-AS1 in lung cancer tissues, we performed qRT-PCR to detect DPP10-AS1 in 94 pairs of lung cancer tissues and adjacent noncancerous tissues. The result showed that the expression of DPP10-AS1 in tumor tissues of lung cancer patients was significantly higher than that in corresponding normal tissues (P<0.0001, Figure 1A). More specifically, 72% (68/94) of lung cancer patients showed an increase of DPP10-AS1 in tumor tissues compared to adjacent noncancerous tissues (Figure 1B). To further investigate the association between the DPP10-AS1 expression and the clinicopathological characteristics, 94 lung cancer samples were divided into two subgroups according to the relative DPP10-AS1 expression with median ratio: high DPP10-AS1 group (n=47, DPP10-AS1 ratio ≥ median ratio) and low DPP10-AS1 group (n=47, DPP10-AS1 ratio < median ratio). Correlation regression analysis showed that high DPP10-AS1 expression in lung cancer patients was closely related to serum high CYFRA21-1 level (P=0.014), large tumor size (P=0.0079), and advanced TNM stage (P=0.0406). However, age (P=0.3279), gender (P=0.0988), tumor number (P=0.1438), microvascular invasion (P=0.4532), and smoking history (P=0.2049) were not correlated with DPP10-AS1 expression (Table 1). To further evaluate the prognostic value of DPP10-AS1 in lung cancer patients, we analyzed the association between DPP10-AS1 expression and survival duration using Kaplan-Meier analysis with the log-rank test. The results revealed that lung cancer patients with higher DPP10-AS1 expression had a significantly poor recurrence-free survival (RFS) compared to those with lower DPP10-AS1 expression (log rank=9.329, P=0.0025, Figure 1C). Similarly, the lung cancer patients with higher DPP10-AS1 expression predicted a poorer overall survival (OS) compared to those with lower DPP10-AS1 expression (log rank=9.333, P=0.036, Figure 1D). Cox proportional hazards regression analysis further showed that high DPP10-AS1 expression in lung cancer tissues was an independent predictor for worse RFS (Table 2) and OS (Table 3). These results indicated that DPP10-AS1 expression is upregulated in lung cancer tissues and predicts poor prognosis in lung cancer patients.
We also measured the DPP10-AS1 expression levels in 5 lung cancer cell lines and human normal bronchial epithelial cells (BEAS-2B) by qRT-PCR. As shown in Figure 1E, the relative DPP10-AS1 expression in lung cancer cells (A549, SPC-A1 and NCI-H1299) were significantly upregulated (5~57-fold change) compared to that in human normal bronchial epithelial cells. Thus, SPC-A1 and NCI-H1299 cell lines were selected for the subsequent assays. Collectively, these results indicate that DPP10-AS1 is elevated in vivo and in vitro, and is considered as an independent prognostic factor of outcomes in patients with lung cancer.
DPP10-AS1 promotes lung cancer cell proliferation in vitro and facilitates tumor growth in xenograft animal model.
Since the results indicated that the significant upregulation of DPP10-AS1 in lung cancer specimens was associated with larger tumor size, the effect of DPP10-AS1 on lung cancer cell growth needs to be explored. To regulate DPP10-AS1 expression in lung cancer cells, the endogenous expression of DPP10-AS1 was inhibited by transfection of siRNA and overexpressed by transfection of pcDNA3.1-DPP10-AS1, respectively. The results showed that DPP10-AS1 was downregulated up to 50~60% by siRNA knockdown (Additional File 2, Figure S1A), and was upregulated up to 250~300 fold by overexpression (Additional File 2, Figure S1B). The MTT assays indicated that knockdown of endogenous DPP10-AS1 expression dramatically inhibited the growth of SPC-A1 and NCI-H1299 cells (Figure 2A, B). In contrast, overexpression of DPP10-AS1 significantly promoted cell growth in both cell lines (Figure 2C, D). Further colony formation assays showed that downregulation of DPP10-AS1 could significantly inhibit the colony formation in both SPC-A1 and NCI-H1299 cells (Figure 2E, F). In contrast, overexpression of DPP10-AS1 promoted the colony formation in these two cell lines (Figure 2G, H). To further confirm the effect of DPP10-AS1 on the lung tumor growth in vivo, two lung cancer cell lines (SPC-A1 and NCI-H1299) stably expressing DPP10-AS1 were screened by geneticin (G418) and were injected subcutaneously into nude mice to establish xenograft tumor model (Figure 2I). The results showed that overexpression of DPP10-AS1 promoted the tumor growth in the volume (Figure 2J) and tumor weight (Figure 2K) via injection of SPC-A1 cells in the xenograft animal model. Similar effect was found in the NCI-H1299 cell-derived xenograft animal model (Figure 2L, M). In addition, compared with the negative control (pcDNA3.1), pcDNA3.1-DPP10-AS1 resulted in the increase of lncRNA DPP10-AS1 (Figure 2N) and DPP10 mRNA (Figure 2O) in SPC-A1 cell-derived tumor tissues. Similarly, the same results were obtained in NCI-H1299 cell-derived tumor tissues (Figure 2P, Q). At the protein level, overexpression of lncRNA DPP10-AS1 promoted its cognate DPP10 protein expression (Figure 2R). Thus, the data suggest that DPP10-AS1 promotes lung cancer cell growth and colony formation in vitro, and facilitates lung tumor growth via upregulation of DPP10 protein in xenograft animal model.
DPP10-AS1 promotes cell cycle progression and represses apoptosis of lung cancer cells.
To probe the potential mechanisms by which DPP10-AS1 enhanced lung cancer cell proliferation, we assessed cell cycle and apoptosis in SPC-A1 and NCI-H1299 cells after the treatment of DPP10-AS1 knockdown or overexpression. Flow cytometric cell cycle assays demonstrated that knockdown of DPP10-AS1 led to a significant accumulation at G0/G1 phase and a significant decrease at G2/M-phase in both two cell lines (Figure 3A). Conversely, overexpression of DPP10-AS1 mainly resulted in a remarkable reduction in the G0/G1 population and an increase at G2/M phase in the two cell lines (Figure 3B). Moreover, the cell apoptosis assays indicated that knockdown of DPP10-AS1 significantly increased the early and late apoptosis in both SPC-A1 and NCI-H1299 cells (Figure 3C), on the other hand, overexpression of DPP10-AS1 dramatically decreased the early and late apoptosis in both two cell lines (Figure 3D). Collectively, DPP10-AS1-induced promotion of lung cancer cell growth appears to be mediated by cell cycle arrest at G2/M-phase and repression of apoptosis.
DPP10-AS1 positively regulates DPP10 gene expression.
DPP10-AS1 is a conserved 744-nt RNA transcribed from the antisense direction of the protein-coding gene DPP10 (2q14.1) (Figure 4A). To confirm the regulatory relationship between DPP10-AS1 and DPP10, we detected the expression levels of DPP10-AS1 and its sense-cognate gene DPP10. The results showed that knockdown of DPP10-AS1 reduced DPP10 gene expression at the mRNA (Figure 4B) and protein (Figure 4C) levels in both SPC-A1 and NCI-H1299 cells. On the contrary, overexpression of DPP10-AS1 enhanced DPP10 mRNA (Figure 4D) and protein (Figure 4E) expression in both lung cancer cell lines. To explore the effect of DPP10 on DPP10-AS1 expression in lung cancer cells, we used DPP10 siRNA and pcDNA3.1-DPP10 to inhibit and overexpress DPP10, respectively. The siRNA reduced the endogenous DPP10 gene expression at the mRNA (Additional File 2, Figure S2A) and protein (Additional File 2, Figure S2B) levels in both SPC-A1 and NCI-H1299 cells. In contrast, overexpression of DPP10 mediated by transfection of pcDNA3.1-DPP10 increased DPP10 gene expression at the mRNA (Additional File 2, Figure S2C) and protein (Additional File 2, Figure S2D) levels in both lung cancer cell lines. However, neither overexpression nor knockdown of DPP10 did not affect the expression of DPP10-AS1 in lung cancer cells (Figure 4F, G). These results suggest that the expression of DPP10 gene can be positively regulated by DPP10-AS1.
DPP10-AS1 and DPP10 are coordinately upregulated in lung cancer cells and tissues.
Based on the positive regulation of DPP10 by DPP10-AS1, we also detected the expression of DPP10 mRNA in the same cohort of 94 paired lung cancer tissue samples using qRT-PCR. The results indicated that DPP10 expression in lung cancer tissues was significantly higher than that in corresponding adjacent tissues (P<0.01, Figure 5A, B). We also measured the DPP10 expression in a panel of lung cancer cell lines, and found that there was an increase of DPP10 mRNA expression of in 4 lung cancer cell lines compared to human normal bronchial epithelial cells (BEAS-2B) (Figure 5C). Similarly, the protein expression of DPP10 was confirmed by Western blotting in lung cancer cells (Figure 5D). Moreover, the relative expression of DPP10 mRNA had a good positive correlation with DPP10-AS1 in lung cancer tissues (r=0.7335, P<0.0001, Figure 5E) and lung cancer cell lines (r=0.8737, P=0.0010, Figure 5F). Together, these results indicate that the upregulation of DPP10-AS1 is coordinately correlated with DPP10 mRNA expression in lung cancer cell lines and in the tissues of lung cancer patients.
DPP10-AS1 promotes malignant processes and inhibits apoptosis through upregulating DPP10 expression.
To investigate whether the coordinate upregulation of DPP10-AS1 and DPP10 could affect malignant processes of lung cancer cells, we detected the cell behavior by overexpression of DPP-10 as well as knockdown of DPP10-AS1. In both SPC-A1 and NCI-H1299 cells, the MTT assays showed that knockdown of DPP10-AS1 inhibited lung cancer cell growth, while simultaneous overexpression of DPP10 could abolish the suppressive effect mediated by knockdown of DPP10-AS1 (Figure 6A). Conversely, overexpression of DPP10-AS1 promoted lung cancer cell growth, while simultaneous knockdown of DPP10 could abrogate the promotional effect mediated by overexpression of DPP10-AS1 (Figure 6B). Furthermore, the colony formation assays showed that the downregulated DPP10-AS1 inhibited the colony formation of lung cancer cells and overexpression of DPP 10 rescued the ability of colony formation in both SPC-A1 and NCI-H1299 cell lines (Figure 6C). In contrast, overexpression of DPP10-AS1 promoted the colony formation in SPC-A1 and NCI-H1299 cells, while knockdown of DPP10 abolished this promotional effect (Figure 6D). At the RNA level, overexpression of DPP10 could rescue the suppressive effect of downregulation of DPP10-AS1 on DPP10 mRNA, and knockdown of DPP10 could abolish the positive regulation of DPP10-AS1 on DPP10 mRNA in both two cell lines (Figure 6E). These results demonstrate that DPP10-AS1 affect cell growth and proliferation through regulating DPP10 mRNA expression.
In addition, cell cycle analysis showed that knockdown of DPP10-AS1 induced cell cycle arrest at G0/G1 phase and a decrease of cell population at G2/M phase, this effect was abolished by the overexpression of DPP10 in SPC-A1 and NCI-H1299 cells (Figure 7A). Meanwhile, overexpression of DPP10-AS1 resulted in a decrease of cell population at G0/G1 phase and cell cycle arrest at G2/M phase. However, this effect was also abolished by the depletion of DPP10 (Figure 7B). Furthermore, the apoptosis analysis showed that overexpression of DPP10 abolished the DPP10-AS1 knockdown-mediated increase of early and late apoptotic cells in both two lung cancer cell lines (Figure 7C). Conversely, the depletion of DPP10 rescued the DPP10-AS1 overexpression-mediated decrease of early and late apoptotic cells in lung cancer cells (Figure 7D). Collectively, the data suggests that DPP10-AS1 promotes cell growth and proliferation, induces cell cycle arrest and inhibits apoptosis through upregulating DPP10 gene expression in lung cancer cells.
DPP10-AS1 associates with DPP10 mRNA but does not enhance DPP10 mRNA stability.
To study whether DPP10-AS1 could enhance the stability of its sense-cognate gene DPP10, we determined the nucleoplasmic localization of DPP10-AS1. The nuclear and cytoplasmic fractionation analysis showed that DPP10-AS1 were mainly located in the nucleus (Figure 8A). Next, RNase protection assay was performed to examine the RNA duplex formation between DPP10-AS1 and DPP10 mRNA. The results showed that DPP10 mRNA was totally digested and no difference of protected DPP10 mRNA was found upon the conditions of DPP10-AS1 knockdown (Figure 8B) or DPP10-AS1 overexpression (Figure 8C), suggesting DPP10 mRNA cannot form RNA duplex with DPP10-AS1 to protect against RNase digestion. These results indicate DPP10-AS1 associates with DPP10 mRNA but does not enhance DPP10 mRNA stability by formation of RNA duplex.
Hypomethylation of DPP10-AS1 and DPP10 contributes to their coordinate upregulation in lung cancer.
To further reveal the underlying mechanism that contributes to the coordinate upregulation of DPP10-AS1 and DPP10 in lung cancer, we used the DNA methyltransferase inhibitor 5-azacytidine to determine the effect of methylation on the expression of DPP10-AS1 and DPP10. The results showed that the relative expression of DPP10-AS1 was remarkably upregulated with the increase of 5-azacytidine concentration in both SPC-A1 and NCI-H1299 cells (Figure 8D). Likewise, the relative expression of DPP10 mRNA was also significantly increased in a 5-azacytidine dose-dependent manner in the same two cell lines (Figure 8E). These results suggest that hypomethylation of DPP10-AS1 and DPP10 may contribute to the coordinate upregulation of DPP10-AS1 and DPP10 in lung cancer. Interestingly, hypomethylation of DPP10 was found in lung adenocarcinoma (LUAD) patients compared to that in normal controls according to the LUAD dataset of the TCGA database (Figure 8F). Furthermore, one CpG island for DPP10-AS1 methylation and three CpG islands for DPP10 methylation were predicted by an epigenetic algorithm (Figure 8G). Taken together, the data indicate that the coordinate upregulation of DPP10- AS1 and DPP10 is epigenetically modulated by their hypomethylations, and the upregulation of DPP 10 positively regulated by DPP10-AS1 is a key event in lung cancer progression (Figure 8H).