3.1 NaAsO2 induced viability changes of human bronchial epithelial cells.
Cell viability was analyzed by CCK-8 kit after Beas-2B cells were treated with NaAsO2 (0, 1, 2, 4, 8, 10 µM) for 24 hours, and our results showed that 1µM NaAsO2 promoted the viability of Beas-2B cells, while high-dose exposure induced a decrease of cell viability in a dose-dependent manner (Fig. 1A) and the cell morphology gradually changed from spindle to round (Fig. 1B), compared to the Control. When the concentration of NaAsO2 was controlled at 4 µM, the cell viability decreased to 70% of the control group; and when the concentration increased to 10 µΜ, the cell viability was only 42% of that of the control group, indicating that a low dose (1 µM) NaAsO2 treatment promoted the proliferation, while high dose (2 µM) NaAsO2 could induce a dose-dependent viability decrease and change the cell morphology in Beas-2B cells.
3.2 Genome-wide expression differences of mRNA in Beas-2B cells induced by NaAsO2 detected by RNA-seq assay.
By RNA-seq assay, we detected the genome-wide mRNA changes induced by NaAsO2 and analyzed the changes of genes or pathways related to cell cycle and mitochondrial damage of Beas-2B cells. Based on the cell viability results, cells were treated with 4 µM NaAsO2 for 24 hours and then collected for RNA-seq sequencing. The differential gene (DEG) was determined by |Log2FoldChange| > 1 and Padj < 0.05 and results were shown in Figure.2.
Box diagram of gene expression level distribution of As-Beas-2B and Beas-2B cells was displayed in Fig. 2A, showing that the distribution of log10 (Fragments Per Kilo base of exon model per Million mapped fragments+1, FPKM+1) between As-Beas-2B and Beas-2B was similar, indicating that the standardized gene expression data could truly reflect the biological differences of samples. Hierarchical cluster analysis could cluster samples with similar expression patterns and showed the expression of the same gene among different samples, and as Fig. 2B showed, the three duplicate samples of the same treatment were gathered together, suggesting that the biological repeatability of the sample was qualified. Fig. 2C showed the volcanic diagram of co-expressed genes between As-Beas-2B and Beas-2B cells: there were a total of 1699 differential genes between the two groups, of which.855 genes were up-regulated while 844 genes were down regulated.
The DEGs were analyzed by gene ontology (GO) function enrichment analysis and KEGG pathway analysis. Results of GO function enrichment analysis showed main enrichment items of up and down regulation,and enrichment items of up regulation mainly included pathway of targeting endoplasmic reticulum protein, pathway of apoptosis signal transduction, release of mitochondrial cytochrome C and activity of growth factor (Fig. 3A); items of down regulation mainly included ATP activity, cell adhesion factor, cell growth and cell migration et central (Fig. 3B).
A total of 244 up and 273 down-regulated pathways were enriched in KEGG annotation information, of which the up-regulated pathways in Figure 3 were most enriched in seven pathways, including glycine threonine metabolism, iron death and transcriptional imbalance in cancer et central (Figure 3C); and down-regulated pathways were enriched in small cell lung cancer, cell cycle and response of ECM receptor, et central (Fig. 3D).
3.3 NaAsO2 induced changes of G1/S transition.
To further verify the difference of expression of genes relating to G1/S transition in RNA-seq results of Beas-2B cells treated with NaAsO2, we used real-time quantitative polymerase chain reaction (qRT-RCR) to verify the expression of some differential genes (Fig. 4A). Our data of exposed cells (1µM NaAsO2 treatment for 24h) showed that the gene expression levels of cyclin D1, cyclin D2, cyclin E1, cyclin dependent kinase 2 (CDK2) and cyclin dependent kinase inhibitors p15 and p21 increased significantly (Fig. 4B), while the mRNA levels of cyclin dependent kinase 4/6 (CDK4/6) and cyclin dependent kinase inhibitors p16 and p27 decreased slightly. And we also found that there was a decrease of gene expression level of ubiquitin ligase E3, like F-box and WD repeat containing protein 8 (FBXW8) and F-box only protein 4 (FBXO4) that mediated ubiquitination of cyclin D1, F-box and Leucine-rich repeat protein 2 (FBXL2) that mediated ubiquitination of cyclin D2 and F-box and WD repeat containing protein 7 (FBXW7) that mediated ubiquitination of cyclin E1, while gene expression of F-box only protein 31 (FBXO31) that mediated ubiquitination of cyclin D1 was up-regulated under the treatment. In addition, our previous studies had shown that AKAP95 promoted cell cycle by increasing expression of cyclin Ds/Es, and, here, our RNA-seq results also showed that the expression level of AKAP95 gene was increased (about doubled).
After 24 hours exposure to NaAsO2 with different concentrations, expression levels of AKAP95 and cyclin Ds/Es were detected by WB assay, and results showed that, compared with the control group, expression levels of AKAP95, cyclin D1, cyclin D2 and cyclin E1 were significantly up-regulated when cells were exposed to 2 µΜ NaAsO2 (Fig. 4C, 4D). With the dose increase of NaAsO2, expression of each protein mentioned above showed an obvious upward trend and a dose-dependent relationship. Meanwhile, changes of cell cycle of exposed Beas-2B under treatments with different doses of NaAsO2 were measured by flow cytometry, and the results were shown in Figure 4E. Compared with the control group, the proportion of cells at G1 phase and G2 phase treated with different doses all decreased with the dose increase of NaAsO2, while the proportion of S phase increased under the same situation.
To further discuss influence of AKAP95 on cell cycle under NaAsO2 treatment, we silenced AKAP95 by transfecting siAKAP95 and analyzed how interaction between AKAP95 and NaAsO2 influenced protein expressions of cyclin D1, cyclin D2 and cyclin E1. Results were displayed in Fig. 4F: compared with the control group (WT cells) in the first column, bands of the second column showed that the expression of AKAP95, cyclin D1, cyclin D2 and cyclin E1 in WT cells treated with NaAsO2 was higher; bands of the third column showed that the expression of cyclin E1 in AKAP95 silenced WT cells decreased and no significant changes of expressions of cyclin D1 and cyclin D2 were observed; bands of the forth column showed expression levels of AKAP95, cyclin D1, cyclin D2 and cyclin E1 recovered and increased compared to those in the third column. These data indicated that, when AKAP95 and NaAsO2 both existed in cells (column 2), expressions of AKAP95, cyclin D1, cyclin D2 and cyclin E1 all peaked, while their expressions were the lowest among all groups when none of them existed (column 3). The statistical results of their interaction were displayed in Figure G, which showed an additive role in promoting the expression levels of cyclin D1, cyclin D2 and cyclin E1 played AKAP95 and NaAsO2. In addition, by detecting cell cycle of treated cells under the interaction of AKAP95 and NaAsO2, we found that NaAsO2 treatment decreased the proportion of G0/G1 phase cells and increased the proportion of S phase cells, while siAKAP95 treatment significantly decreased the proportion of S phase cells and increased the proportion of G2 phase cells (Figure 4H).
3.4 Long term exposure to low dose NaAsO2 induced carcinogenesis of human bronchial epithelial cells.
Beas-2B cells were continuously cultured in a medium containing 1 µΜ NaAsO2 for 20 generations (about 3 months), and detected whether the proliferation and migration rate of cells changed. Our results in Fig. 5A showed that, 48h after the scratching on the plate, the more cell generations, the farther the healing distance of cell scratch, suggesting that the proliferation rate of As-Beas-2B cells was accelerated and the migration ability was strengthened. And the cell cycle of 0, 5, 10, 15 and 20 generations As-Beas-2B cells was detected by flow cytometry, and results were displayed in Fig. 5B, showing that the more the cell generation, the lower the proportion of G1 while the higher the proportion of S phase cells.
As-Beas-2B cells at different stages were inoculated subcutaneously into the left hindlimb of mice, and the growth of volume of tumor tissue was monitored for 24 days. Compared with normal Beas-2B cell groups, no significant changes of volume of bear tumor tissue in the 5th generation As-Beas-2B cells group was detected; volume of tumor tissue grew significantly in 16 days after the injection, in in the 10th and 15th generation As-Beas-2B cells groups; volume of tumor tissue of the 20th generation As-Beas-2B group was similar to that of A549 group, and the growth rate of volume of tumor tissue increased significantly from the 10th day after injection and significantly higher than that of A549 group. The tumor was removed from mice on the 24th day after injection, then photographed and measured (Fig. 5D, 5E). Some tissues were taken for HE staining to further verify that continuous low-dose NaAsO2 exposure induced the canceration of Beas-2B cells. As shown in Fig. 5F, after low-dose NaAsO2 exposure, the cell morphology of tumor tissue changed with different cell sizes, significantly increased nucleus and widened cell gap, from 5th generation.
In order to further investigate whether continuous low-dose NaAsO2 exposure induced cell cycle changes, we then detected expressions of some cell cycle related proteins in tumor tissues, and our data showed that the expression of AKAP95, cyclin D1, cyclin D2 and cyclin E1 were all up-regulated (Fig. 5G).
In general, our results suggested that continuous low-dose NaAsO2 exposure could promote the cell growth and migration, accelerate cell cycle transition, and induce cell carcinogenesis.
3.5 NaAsO2 exposure triggered mitochondrial damage.
Our early Go function enrichment and KEGG pathway analysis results of RNA-seq enriched mitochondrial cytochrome C release, ATP activity, ECM receptor response etc. pathways related to mitochondrial function, suggesting that NaAsO2 exposure might induce changes in mitochondrial structure and function. RNA-seq results of Beas-2B cells treated with 4 µΜ NaAsO2 24 hours showed that the mRNA expression levels of mitochondrial damage related genes Tim 22, Tim 23 and Tom 40 increased (Fig. 6A), while mRNA expression level of OPA1, which mediates mitochondrial fusion, was down regulated. After verification, results of RNA-seq and qRT-PCR were consistent (Fig. 6B).
By TEM assay, we detected the cells exposed in 4 µΜ NaAsO2 for 24h and found that the volume of mitochondria in cells increased significantly, compared with untreated cells, and the dual-mode structure of both inner and outer membrane disappeared into a single membrane structure and the mitochondrial ridge shrank into fragments (Fig. 6C). And by detecting the expression of mitochondrial outer membrane channel forming protein Tom 40, mitochondrial inner membrane carrier protein Tim 22, mitochondrial inner membrane leading sequence transposase Tim 23 and mitochondrial fusion protein OPA1, we found that after Beas-2B cells were treated with different concentrations (0, 2, 4, 8, 10 µΜ) NaAsO2 for 24 hours, the protein expression levels of Tim 22 and Tim 23 decreased significantly with the increase of NaAsO2 concentration, showing a significant dose-dependent manner; when the concentration reaches 10 µΜ, the expression of Tim 22 and Tim 23 decreased to 0.5 and 0.33 times that of the control group, suggesting that NaAsO2 could significantly reduce the expression of Tim22 and Tim 23, and, in addition, expression of OPA1 also showed a gentle downward trend when concentration of NaAsO2 was 0, 2, 4 or 8 µΜ and significantly decreased when the concentration was 10 µΜ; however, Tom40 showed an obvious upward trend with the increase of NaAsO2 concentration and also showed a significant dose-dependent manner (Fig. 6D).
To investigate whether continuous low-dose NaAsO2 exposure could induce mitochondrial dysfunction, we detected expressions of mitochondrial damage associated proteins in bare tumor tissues (different generations cells exposed continuously to 1 µΜ NaAsO2), and results showed that expressions of Tim22, Tim23 and OPA1 decreased significantly from the 15th generation cells' group, while that of Tom40 increased (Fig. 6E), suggesting that continuous low-dose NaAsO2 exposure induced cell carcinogenesis, and accompanied by mitochondrial dysfunction.
Mitochondrial membrane potential is considered as an important index to indicate changes of membrane permeability. In our study, the mitochondrial membrane potential of Beas-2B cells decreased significantly after NaAsO2 treatment, and cell proportion in Q3 increased (Fig. 7A). Reduced glutathione (GSH), as an important antioxidant in cells, can cause the accumulation of oxygen free radicals and induce oxidative stress. And after NaAsO2 exposure, we found that GSH levels were effectively reduced in cells, as shown in Fig. 7B. LDH release is regarded as an important indicator of cell membrane integrity, and in NaAsO2 treated cells, the LDH level in the culture supernatant of BEAS-2B cells decreased, in a dose-dependent manner (Fig. 7C), suggesting that the damage of NaAsO2 to cell membrane permeability increased gradually with the increase of NaAsO2 dose. All these results indicated that effects of NaAsO2 on BEAS-2B cells induces changes in mitochondrial structure and functions.