The sertoli cells and leydig cells of the rats were collected. The expression levels of BAP18 and AR mRNA in the cells were detected by real time fluorescence quantitative PCR （RT- qPCR） method， the expression levels of BAP18 and AR proteins in the cells were detected by Western blotting method， and the interaction between BAP18 and AR in the testicular R2C cells was detected by protein immunoprecipitation assay （co-IP）. The BAP18 over-expression plasmid with FLAG tag was used to over-express the BAP18 in the R2C cells（BAP18 over-expression group）.Luciferase double gene report test was used to detect the luciferase activity in the cells. In control group， the pcDNA3 empty plasmid was used to transfect the R2C cells. The expression levels of BAP18， AR， STAR， HSD3B1， CYP17A1， and DDX25 mRNA in the R2C cells treated with testosterone（T） and dihydrotestosterone（DHT） before and after BAP18 over-expression were detected by RT-qPCR method. The expression levels of CYP17A1 protein in the R2C cells treated with T and DHT before and after BAP18 over-expression were detected by Western blotting method.

BAP18 was expressed in the leydig cells and sertoli cells， but AR was mainly expressed in the leydig cells； BAP18 interacted with AR and up-regulated the AR-mediated gene transcription in the testicular R2C cells. Compared with control group， after over-expression of BAP18 under the action of T and DHT， the mRNA and protein expression levels of CYP17A1 in the R2C cells in BAP18 over-expression group were increased （P<0.01）. After exogenous transfection of quantitative BAP18 expression plasmid in R2C cells at the same time， the luciferase activity in R2C cells was increased nearly 3 times under the action of DHT， while the luciferase activity in R2C cells was increased 1.5 times under the action of T. After BAP18 over-expression of in the R2C cells， the expression levels of AR， STAR， HSD3B1，and DDX25 mRNA in the R2C cells in BAP18 over-expression group were not significantly different from those before BAP18 over-expression（P>0.05）， while the expression level of CYP17A1 mRNA was higher than that before BAP18 over-expression（P<0.05）； after T or DHT treatment， the expression levels of CYP17A1 mRNA in the R2C cells were higher than before T or DHT treatment（P<0.05）. After BAP18 over-expression in the cells， the expression levels of CYP17A1 protein in the cells after T or DHT treatment were significantly higher than before T or DHT treatment（P<0.05）， but there was no significant difference in the expression level of DDX25 protein before and after T or DHT treatment（P>0.05）.

BAP18 can interact with AR in the Leydig cells and regulate the important target gene CYP17A1 of AR signaling pathway， which may be involved in the physiological process of Leydig cells.