Identification of Sec23ip, part of 14-3-3γ protein network, as a regulator of acute steroidogenesis in MA-10 Leydig cells.

Testosterone production occurs in the Leydig cells of the testes and is essential for virilization, development, reproduction and quality of life. Although the steroidogenic proteins involved in cholesterol conversion to testosterone (T) are well-characterized, the causes of reduced T during fetal, neonatal and adult life remain uncertain. It is well established that normal cellular function is achieved through fine-tuning of multiple rather than single protein networks. Our objective was to use mass spectrometry (MS) -based proteomics to identify which cellular pathways, other than the steroidogenic machinery, influences testosterone production in MA-10 mouse tumor Leydig cells. The 14-3-3 family of scaffolds mediate protein-protein interactions facilitating the crosstalk between protein networks. We previously showed that in MA-10 cells, 14-3-3γ is a critical regulator of steroidogenesis. Therefore, identifying proteins that interact with 14-3-3γ during steroidogenesis could provide clues into the other networks involved. Using liquid chromatography (LC)-MS, we identified 688 proteins that interact with 14-3-3γ and thus potentially impact MA-10 cell steroidogenesis. The identified proteins belong to multiple protein networks, including ER-Golgi cargo sorting and vesicle biogenesis, microRNA-induced gene silencing, inflammation, and vesicle trafficking, to name a few. We found that silencing one of the candidates, Sec23ip, a protein known to be involved in vesicle trafficking, resulted in decreased steroidogenesis. We further showed that in Sec23ip-silenced MA-10 cells, cholesterol mobilization from the cytoplasmic membrane to mitochondria is impaired. Taken together these data suggest that Sec23ip is involved in cholesterol trafficking to supply cholesterol for acute steroidogenesis through its interactions with 14-3-3γ.