Sensitization effect of ATO in cholangiocarcinoma cells
We treated cholangiocarcinoma cells RBE and HCCC-9810 with ATO according to the concentration gradient, and observed the effect of ATO on the activity of RBE and HCCC-9810 cells. In RBE, when the concentration of ATO was 2 µM, the cell viability was not affected evidently. The results in HCCC-9810 were similar (Fig. 1A). As a result, 2 µM was supposed to be the maximum non-toxic concentration of ATO on cholangiocarcinoma cells. We found that 2 µM ATO had no inhibitory effect on the activity of normal bile duct epithelial cell (HIBEpic), RBE and HCCC-9810 compared with the control group without ATO (Fig. 1B). Then, RBE and HCCC-9810 were treated with 2 µM ATO and 5-FU according to the concentration gradient. IC50 in RBE and HCCC-9810 decreased significantly, compared with 5-Fu treated alone. Similarly, the IC50 of the group treated with ATO combined with CDDP (0-160 µM) was significantly lower than that treated with CDDP alone (Fig. 1C). These results confirmed that low-dose ATO can significantly enhance the effect of common chemotherapy drugs for cholangiocarcinoma, such as 5-Fu, CDDP.
Effect of miR-885-5p on chemosensitivity of cholangiocarcinoma cell lines
Based on our previous study [20], thousands of microRNAs were expressed differently, and these microRNAs were considered to be "useful microRNAs" that affect the occurrence and development of CCA (Fig. S1A). Six differently expressed microRNAs were selected for PCR verification, two were up-regulated and four were down-regulated (Fig. S1B). We detected the expression of six miRNAs in HIBEpic, RBE and HCCC-9810. We found that there was distinct difference between the expression level of miR-885-5p in HIBEpic, RBE and HCCC-9810. Meanwhile, compared with HIBEpic, the expression level of miR-196a-5p was significantly higher. However, the expression level of the other four microRNAs was not significantly different (Fig. 2A). Consistent with the results in cells, the expression level of miR-885-5p in 35 CCA tissues was lower than that in normal tissues, while miR-196a-5p was opposite (Fig. 2B). Furthermore, the expression of miR-885-5p and miR-196a-5p of samples in The Cancer Genome Atlas (TCGA) supported our findings. We can speculate that miR-885-5p plays an anti-cancer role in CCA, while miR-196a-5p is an oncogene. To investigate the effect of miR-885-5p and miR-196a-5p on the chemoresistance of cholangiocarcinoma, we successfully transfected miRNA-885-5p mimic in RBE and HCCC-9810. After 24 hours of transfection, the two cells were treated with 5-Fu or CDDP according to the concentration gradient, the inhibiting effect on cell viability of the experimental group overexpressing miR-885-5p was significantly lower than that of the control group (Fig. 2C). However, after transfection of miR-196a-5p inhibitor, the inhibiting effect on cell viability of the experimental group was not significantly reduced, whether treated with 5-Fu or CDDP (Fig. S2). Consequently, we believe that miRNA-885-5p is a potential microRNA to improve the chemosensitivity of CCA.
ATO enhanced the chemosensitivity of cholangiocarcinoma cells through miR-885-5p
Due to the significant enhancement of chemosensitivity of ATO to cholangiocarcinoma cells and the particularity of miR-885-5p in the effect of chemotherapy, we speculated whether the enhancement of chemosensitivity by ATO was related to miR-885-5p. After treating RBE and HCCC-9810 with ATO (0–2 µM), we detected the expression of miR-885-5p. It was found that with the increase of ATO concentration, the expression level of miR-885-5p increased gradually, and it was the highest under 2 µM ATO (Fig. 3A). In order to further confirm the conjecture, we carried out rescue experiments. The rescue experiment also showed that ATO could reverse the reduced sensitivity of 5-Fu and CDDP in RBE and HCCC-9810 caused by miR-885-5p knock down (Fig. 3B). Therefore, the above results prove that ATO can promote chemosensitivity by inducing the expression of miR-885-5p.
MTPN was a target gene of miR-885-5p in CCA
According to the sequencing results, of the 16968 detected genes, each of them may be a target gene for one or even several microRNAs. These genes were shown by Heatmap (Fig. 4A). We found that 382 genes were differentially expressed. The differential multiple of 9 genes were greater than 5, 5 of which were up-regulated genes and 4 of which were down-regulated genes (Fig. S3). In order to identify the target gene of miRNA-885-5p, we predicted it in database starBase 3.0 and Target Scan 7.2. There were 155 target genes of miR-885-5p in starBase and 3082 in Targetscan. MTPN was the only one gene as the target gene of miRNA-885-5p, based on the predicted results and the results of second generation sequencing (Fig. 4B). MTPN was first found in hypertrophic myocardium of mice and its role in promoting myocardial hypertrophy was also discovered [18]. However, in cancer, what role MTPN plays is poorly understood. To verify the possible effect of MTPN in cholangiocarcinoma, first of all, we found that the expression of MTPN in RBE and HCCC-9810 were significantly higher than that in HIBEpic (Fig. 4C). Then we carried out PCR experiments in 35 cholangiocarcinoma tissues and found that compared with the adjacent tissues, the expression of MTPN in 28 tissues were significantly higher. Besides, our findings concurred with the analysis of TCGA samples. (Fig. 4D). Additionally, the results of 36 specimens in starBase 3.0 database showed that MTPN expression was negatively correlated with miRNA-885-5p (Fig. 4E). It follows that there was a strong possibility that MTPN may involve in the multidrug resistance of CCA.
ATO regulated the expression of MTPN through miR-885-5p
As a result of the high expression of MTPN both in cholangiocarcinoma cells and tissues and the prediction results of database and second-generation sequencing, we used luciferase reporter gene experiment to confirmed the relationship between miRNA-885-5p and MTPN. First, the 2978–2985 base fragment of MTPN may be the binding region of miR-885-5p according to the prediction results. We constructed luciferase plasmids containing wild-type MTPN and mutant MTPN 3'UTR respectively. Then, miR-885-5p mimic or control was co-transfected with plasmid. The results showed that miR-885-5p only combined with 3'UTR of wild-type MTPN, and significantly down regulated the expression of luciferase gene in sea pansy where the 3'UTR sequence of MTPN was located (Fig. 5A). Because miR-885-5p can directly bind to MTPN, we observed that MTPN both in RNA and protein in the cholangiocarcinoma cells lines of which miR-885-5p was overexpressed was significantly down-regulated (Fig. 5B).Moreover, in RBE and HCCC-9810 treated with ATO, MTPN decreased both in RNA and protein, compared with the control group (Fig. 5C). We conducted rescue experiments and found that ATO could block the increase of MTPN caused by miR-885-5p knockdown in RBE and HCCC-9810 (Fig. 5D). Our findings showed that MTPN was a novel direct target gene of ATO-mediated miR-885-5p in cholangiocarcinoma.
ATO/miR-885-5p/MTPN axis improved the anti-drug-resistance of cholangiocarcinoma cells
In order to explore whether MTPN affects the drug resistance of CCA cells, we successfully know-downed MTPN in RBE and HCCC-9810, and then added 5-Fu to the cells according to the concentration gradient. Compared with the control group, the IC50 of the experimental group was markedly lower, similarly, IC50 of CDDP in RBE and HCCC-9810 decreased significantly after MTPN knock-out (Fig. 6A). The rescue experiment showed that IC50 of 5-Fu and CDDP in RBE and HCCC-9810 induced by MTPN overexpression was decreased by ATO treatment (Fig. 6B). Above results indicated that ATO can reverse the primary chemotherapy resistance of cholangiocarcinoma by regulating MTPN.
Validation of prognostic values of miR-885-5p and MTPN in CCA
To explore the relationship between miR-885-5p/MTPN and clinical characteristics in CCA, we divided 35 patients into “low miR-885-5p expression/high MTPN expression”,“high miR-885-5p expression/high MTPN expression or low miR-885-5p expression/low MTPN expression” and “high miR-885-5p expression/low MTPN expression” groups. A Kaplan-Meier analysis showed that the patients in the“low miR-885-5p expression/high MTPN expression” group exhibited worse survival than those in the “high miR-885-5p expression/low MTPN expression” group. (Fig. 7). Apparently, miR-885-5p and MTPN could serve as biomarkers in the field of CCA. Collectively, these results indicated that the upregulation of miR-885-5p induced by ATO treatment may contribute to the decrease of MTPN and thereby lead to anti-drug resistance to 5-Fu and CDDP.