3.1.IR-induced ARv7 expression led to decrease radiosensitivity
Recent studies indicated that ADT treatment would induce the expression of ARv7 (23, 24),the potential impact of IR treatment on ARv7 expression, however, remains unclear. Here we first developed the radioresistant C4-2-IRR (C4-2-Ionizing Radiation Resistance) cell line via treating parental C4-2 cells with repeated 2 Gy radiation. Radioresistance of cells were confirmed in clonogenic assays by showing higher survival of C4-2-IRR cells than respective parental cells at indicated radiation dose (Fig. S1A). Interestingly, we found that IR could increase significantly the ARv7 expression at the protein and mRNA levels after IR in ARv7-positive 22Rv1 cells and 22Rv1 xenografts (Fig. 1A and Fig. 1B).
To further examine if IR may function via inducing the ARv7 to increase the radioresistance, we first added 5um enzalutamide to inhibit AR activity, then knocked down ARv7 with shARv7 in C4-2-IRR cells or ectopic overexpression of ARv7 in C4-2 cells. The results revealed that suppressed ARv7 in C4-2-IRR cells led to increase the PCa radiosensitivity (Fig. 1C). In contrast, increased ARv7 in C4-2 cells can then increase the radioresistance of PCa cells (Fig. 1D). As 22Rv1 cells have both ARfl and ARv7, we then applied the shAR and/or shARv7 to suppress the expression of ARfl and/or ARv7. The results revealed such suppression to ARfl alone or ARv7 alone could not led to decrease clonogenic survival following IR (Fig. 1E), and only knockdowning of both ARfl and ARv7 led to decrease the clonogenic survival following IR (Fig. 1E). Together, these results suggest that suppression both ARfl and ARv7 could lead to better effect to improve PCa radiosensitivity.
To further strengthen above conclusion, we added 5um enzalutamide to inhibit AR activity, then knocked down ARv7 with shARv7 in C4-2-IRR cells or ectopic overexpression of ARv7 in C4-2 cells. Next, we examined the ARv7 potential impact on PCa cells DDR by comet assays and the results from these assays also revealed that decreasing ARv7 in C4-2-IRR cells significantly increased DNA fragmentation following IR (Fig. 1F). Similar results from WB (Fig. 1G, left) analysis of the γ-H2AX levels showed that decreasing ARv7 in C4-2-IRR cells increased the DNA damage repair time. In contrast, increasing ARv7 resulted in less time to complete DDR in the C4-2 cells (Fig. 1G, right).
Together, results from Fig. 1 suggest that IR-induced ARv7 may decrease the continue RT efficacy.
3.2. Mechanism dissection of how ARv7 can alter the radiosensitivity: via increasing circNHS expression.
To further dissect the mechanism of how ARv7 can alter the radiosensitivity, we focused on circRNAs, as recent studies indicated that the expression of selected circRNAs could be altered after IR exposure (20, 21). From circRNA Array data in radioresistant cancer cells (25), we first chose the 10 most upregulated/downregulated circRNAs and detected their expression in C4-2-IRR cells model. The results revealed that only 7 of those 20 circRNAs had changed significantly compared with C4-2 parental cells (Fig. 2A), and PCR results revealed that decreasing ARv7 led to significantly decreased expression of circNHS (hsa_circ_0089974) in C4-2-IRR cells (Fig. 2B). In contrast, increasing ARv7 led to significantly increased expression of circNHS in C4-2 cells (Fig. 2C). Moreover, we found that IR could increase significantly circNHS levels in 22Rv1 xenografts (Fig. 2D).
Next, results from circBase database analysis indicated that circNHS is generated from back-splicing of two exons (exons 2 and 3) of the NHS gene (chrX:17705861–17710588) (Fig. 2E). To avoid trans-splicing or genomic rearrangements, including head-to-tail splicing, we then applied multiple approaches to rule out such possibilities. We first designed convergent primers to amplify NHS mRNA and divergent primers to amplify circNHS. Using cDNA and genomic DNA from C4-2-IRR and 22Rv1 cell lines as templates, circNHS was amplified from cDNA only by the divergent primers, whereas no amplification product was observed from genomic DNA (Fig. 2F). Furthermore, the results from the RNase R assay revealed that circNHS is the circular form, with better resistance to RNase R digestion than linear NHS (Fig. 2G). Finally, the results from adding actinomycin D to inhibit transcription also indicated that the half-life of circNHS is more stable than NHS mRNA in C4-2-IRR and 22Rv1 cells (Fig. 2H). Finally, the circular RNA microarrays data from Yang et al (26) showed that circNHS were highly expressed in high-grade (Gleason > 8) PCa tissues when compared with low-grade (Gleason < 6) PCa tissues (Fig. S1B).
Together, the results from Fig. 2 suggest that ARv7 may decrease radiosensitivity by increasing circNHS expression.
3.3. ARv7 increases circNHS expression via altering the transcriptional regulation
To further dissect the molecular mechanism of how ARv7 can increase circNHS expression, we examined circNHS transcriptional regulation, and our data indicated that ARv7 could increase NHS expression at both the mRNA and protein levels (Fig. S1C and Fig. S1D). We then searched for potential androgen response elements (AREs) on the NHS 5' promoter region using the JASPAR database. The results revealed that 2 putative AREs were located within 2 kb of the NHS 5' promoter region (Fig. S1E). We then performed chromatin immunoprecipitation (ChIP) in vivo binding assays in 22Rv1 cells, and the results revealed that ARv7 could bind to the first ARE (Fig. 3A), suggesting that ARv7 might be able to increase circNHS expression via direct binding to a ARE to exert its transcriptional regulation.
We then performed the luciferase reporter assay by inserting the 1 kb 5' promoter region of NHS containing ARE1 into the pGL3 luciferase backbone and also generated a version with a mutated ARE1 (Fig. S1F). The luciferase assay results revealed that decreasing ARv7 by adding ShARv7 significantly decreased luciferase activity in 22Rv1 transfected with the wild-type NHS promoter construct but not in cells with the mutant NHS promoter construct (Fig. 3B). Importantly, the results from the LinkedOmics database via TCGA data analysis also showed that NHS expression was positively correlated with AR expression in PCa patients (Fig. S1G).
Together, results from Fig. 3A-B suggest that ARv7 increases circNHS expression via altering the transcriptional regulation
Next, to prove that ARv7 may function by altering circNHS expression to decrease radiosensitivity, we then suppressed the circNHS with the shcircNHS, and results revealed that suppressed circNHS (Fig. S1H) led to increase the radiosensitivity in C4-2-IRR cells (Fig. 3C). In contrast, increasing circNHS resulted in decreasing the radiosensitivity in C4-2 cells (Fig. 3D).
The results from the γ-H2AX assay also revealed that decreasing circNHS levels could increase the DNA damage repair time in C4-2-IRR cells (Fig. 3E, left panel). In contrast, increased circNHS expression resulted in less time to complete DNA damage repair in the C4-2 cell lines (Fig. 3E, right panel). Similarly, the results from the comet assay also revealed that decreasing circNHS in C4-2-IRR cells significantly increased DNA fragmentation following IR (Fig. 3F).
Finally, the results from the silencing experiment revealed that suppressing circNHS led to a partial reversal of ectopic overexpression of ARv7-suppressed radio-sensitivity in the C4-2-IRR cells (Fig. 3G) and increased circNHS led to a partial reversal of the shARv7-increased radio-sensitivity in the C4-2 cells (Fig. 3H).
Together, results from Fig. 3 suggest that ARv7 can function via increasing the circNHS to alter the radiosensitivity.
3.4. Mechanistic dissection of how the circNHS could decrease radiosensitivity: by competing with miR-512-5p/XRCC5 axis
Next, to determine how circNHS could decrease radiosensitivity, we first determined the subcellular localization of circNHS in PCa cell lines using a nuclear mass separation assay (Fig. 4A) and FISH analysis (Fig. 4B). The results revealed that circNHS was expressed mainly in the cytoplasm of PCa cells. As early studies indicated that cytoplasmic circRNAs might compete with miRNAs to exert their function (27), we then performed an RNA immunoprecipitation (RIP) assay with an antibody against argonaute 2 (AGO2) in C4-2-IRR and 22Rv1 cells, and the results revealed that circNHS was significantly enriched by the AGO2 antibody (Fig. 4C), suggesting that circNHS might act as a binding platform for AGO2 and miRNAs. Based on these findings, we predicted that circNHS might serve as a binding platform for miRNAs. We then surveyed 3 databases (28, 29) and found that 16 potential miRNAs could bind to circNHS (Fig. 4D). Using probes specifically against circNHS to analyze these 16 candidate miRNAs in the complex, we found only a specific enrichment of miR-512-5p and none of the rest 15 miRNAs, suggesting that miR-512-5p is one of the circNHS-associated miRNAs in PCa cells.
To further prove that the circNHS can alter radiosensitivity via binding to the miR-512-5p, we then constructed a mutated binding site of miR-512-5p (circNHS-Mut, Fig. S1I). The results from colony formation assays revealed that mutation of the miR-512–5p binding site could completely abolish circNHS-induced radiosensitivity in C4-2 cells (Fig. 4E).
Recent studies identified 542 ARv7-regulated genes via RNA-seq and CHIP-seq analyses (30). Among these genes, we found 9 DDR genes that may involve in DNA damage repair, and results from targetscan database predicted that XRCC5 and SPRTN can be the potential target gene of miR-512–5p (Fig. 4F). Results from western blot analysis demonstrated that circNHS could regulate the expression of XRCC5 protein, and not SPRTN protein levels, suggesting circNHS may function via sponging miR-512–5p to increase the protein expression of XRCC5 (Fig. 4G). As expectedly, adding miR-512-5p inhibitor led to increase XRCC5 protein expression and could partly reverse the shcircNHS–decreased XRCC5 expression in C4-2 cells (Fig. 4H, left) while adding miR-141-3p mimics led to decreased XRCC5 protein expression and could effectively reverse the oe-circNHS increased XRCC5 expression in C4-2-IRR cells (Fig. 4H, right).
Results from interruption approaches using XRCC5-shRNA further revealed that suppressing XRCC5 led to partially reverse the ARv7-increased radioresistance (Fig. 4I). Similarly, adding XRCC5-cDNA led to partially reverse the shARv7-decreased the radioresistance in C4-2-IRR cells (Fig. 4J).
Finally, the results from the LinkedOmics database via TCGA data analysis also showed that XRCC5 expression was positively correlated with AR expression in PCa patients (Fig. S1J).
Together, the results from Fig. 4 suggest that circNHS might function by sponging miR-512-5p to alter XRCC5 protein expression and such regulation may then lead to alter the DDR pathway.
3.5. Preclinical studies in in vitro cell lines and in in vivo mouse model to target ARv7-mediated circNHS/miR-512-5p/XRCC5 signaling with small molecule of Que to increase radiosensitivity
All above results suggest that IR may have unwanted side effects of inducing the ARv7 expression to increase the radioresistance. We are interested to see if adding small molecules can have similar effect to suppress the ARv7 expression. We are interested to test the Que, as our previous studies indicated that Que could suppress AR mRNA and protein expression (31). Here we found that adding Que could also significantly decrease the ARv7 expression at both mRNA and protein levels in 22Rv1 and VCaP cells (Fig. 5A-C).
Furthermore, adding 40µM Que to reduce ARv7 expression can then lead to increase the radiosensitivity to better suppress the C4-2-IRR and 22Rv1 cell growth using clonogenic assay (Fig. 5D-E). Mechanism dissection revealed that adding Que, and not Enz, could decrease DNA damage repair via detecting γ-H2AX levels (Fig. 5F).
To further validate these in vitro findings in the in vivo mouse model, we established subcutaneous 22Rv1 xenografts in nude mice. Results showed that combined 2Gy IR treatment with Que treatment (75mg/kg) led to suppress the tumor growth approximately 70–75% on day 28, as compared to 2Gy IR alone or Que alone (Fig. 5G).
Together, results from Fig. 5 suggest that small molecule of Que can increase radiosensitivity to better suppress the PCa cells growth via targeting both ARfl and ARv7 levels.