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Smad ubiquitylation regulatory factor 1 promotes LIM‐homeobox gene 9 degradation and represses testosterone production in Leydig cells

Testosterone is essential for spermatogenesis and the maintenance of secondary sexual characteristics in males. An important transcription factor, LIM‐homeobox gene 9 (Lhx9) is indispensable for testis development and testosterone production; however, post‐translational modifications of Lhx9 are largely unknown. Here, for the first time to our knowledge, we demonstrate that the level of Lhx9 protein increases in human chorionic gonadotropin‐exposed Leydig cells and can be polyubiquitylated. We found that Smad ubiquitylation regulatory factor 1 (Smurf1), an E3 ubiquitin ligase, targets Lhx9 for ubiquitin‐mediated proteasome degradation, thereby negatively modulating its function. Increasing Smurf1 decreases the level of Lhx9 and inhibits the Lhx9 trans‐activation capacity of steroidogenic factor 1 [nuclear receptor subfamily 5, group A, member 1 (NR5A1)]. In contrast, the depletion of Smurf1 leads to increased expression of Lhx9 protein and enhances testosterone biosynthesis‐related gene transcripts [NR5A1, steroidogenic acute regulatory protein, CYP17A1, hydroxy‐δ‐5‐steroid dehydrogenase, hydroxy‐δ‐5‐steroid dehydrogenase isomerase 6, and hydroxysteroid (17‐β) dehydrogenase 3] and testosterone production in Leydig cells. Furthermore, we found that Smurf1 knockout mice exhibit higher levels of Lhx9 protein and steroidogenesis, which leads to increased serum testosterone concentration. These findings reveal that Smurf1 promotes Lhx9 ubiquitylation and is involved in testosterone production in Leydig cells directly. Our results provide new insights into the molecular events that play a role in the homeostasis of testosterone levels and may provide a new target for testosterone regulation.—Hu, F., Zhu, Q., Sun, B., Cui, C., Li, C., Zhang, L. Smad ubiquitylation regulatory factor 1 promotes LIM‐homeobox gene 9 degradation and represses testosterone production in Leydig cells. FASEB J. 32, 4627–4640 (2018). www.fasebj.org