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Janus Kinase 3 phosphorylation and the JAK/STAT pathway are positively modulated by follicle‐stimulating hormone (FSH) in bovine granulosa cells

Janus kinase 3 (JAK3) is a member of the JAK family of tyrosine kinase proteins involved in cytokine receptor‐mediated intracellular signal transduction through the JAK/STAT signaling pathway. JAK3 was shown as differentially expressed in granulosa cells (GC) of bovine preovulatory follicles and downregulated by the luteinizing hormone. These observations suggested JAK3 regulation could modulate GC proliferation, steroidogenic activity and activation/inhibition of downstream targets. To investigate the mechanisms of JAK3 actions in GC, we used JANEX‐1, a pharmacological JAK3 inhibitor, and FSH treatments and analyzed proliferation markers, steroidogenic enzymes and phosphorylation of target proteins including STAT3 and previously identified JAK3 partners CDKN1B/p27Kip1 and MAPK8IP3/JIP3. Cultured GCs were treated with or without FSH in the presence or not of JANEX‐1. Total RNA and proteins were extracted and analyzed by RT‐qPCR, western blotting and UHPLC/MS‐MS. Expression of steroidogenic enzyme CYP11A1, but not CYP19A1, was significantly upregulated in GC treated with FSH and both were significantly decreased when JAK3 was inhibited as compared to control. Proliferation markers CCND2 and PCNA were significantly reduced in JANEX‐1‐treated GC and upregulated by FSH. Western blots analyses showed that JANEX‐1 treatment significantly reduced pSTAT3 amounts while JAK3 overexpression increased pSTAT3. Similarly, FSH treatment increased pSTAT3 even in JANEX‐1‐treated GC. UHPLC/MS‐MS analyses showed phosphorylation and additional modifications of specific amino acid residues within JAK3 as well as its binding partners CDKN1B and MAPK8IP3 revealing possible activation or inhibition of JAK3 following FSH or JANEX‐1 treatments, respectively. Abundance of JAK3 total protein was increased post‐FSH treatment and significantly decreased, along with MAPK8IP3, in JANEX‐1‐treated GC while CDKN1B total abundance was altered post‐FSH and increased post‐JANEX‐1. We show that JAK3 influences GC activity through phosphorylation of target proteins in response to stimulations such as FSH, which leads to the activation of JAK/STAT and likely modulating other signaling pathways involving CDKN1B and MAPK8IP3.