Janus kinase 3 (JAK3) increases phosphorylation of signal transducers and activators of transcription 3 (STAT3) in reproductive cells

Abstract

JAK3 is a member of the membrane‐associated non‐receptor tyrosine kinase protein family and is considered predominantly expressed in hematopoietic cells. We previously identified JAK3 as a differentially expressed gene in granulosa cells (GC) of bovine preovulatory follicles. This study aimed to further investigate JAK3 mRNA and protein regulation and to identify its binding partners in GC of bovine dominant follicles. GC were obtained from small follicles (SF), dominant follicles at day 5 of the estrous cycle (DF), and ovulatory follicles, 24 hours following hCG injection (OF). RT‐PCR analyses showed greatest expression of JAK3 in GC of DF, while the weakest expression was in OF (P < 0.0001). There was a 5‐ and 20‐fold reduction of JAK3 steady‐state mRNA levels in follicular walls, respectively at 12 and 24 hours post‐hCG as compared to 0 hour (P < 0.05). These results were confirmed in western blot analysis showing weakest JAK3 protein levels in OF as compared to DF. Yeast two‐hybrid screening of a DF‐cDNA library resulted in the identification of 23 JAK3 partners in GC. Moreover, the amount of phosphorylated STAT3 in GC was greatest in SF and DF as compared to OF while the opposite was observed for non‐phosphorylated STAT3. In functional studies using bovine endometrial cells, we discovered that phosphorylation of STAT3 (pSTAT3) in a bovine endometrial cell line was increased by JAK3 while inhibition of JAK3 by JANEX‐1 decreased the amount of pSTAT3. In the same study, we showed that JAK3 increased endometrial cell proliferation as well. We found a similar effect of JAK3 on STAT3 phosphorylation using an immortalized murine granulosa cell line. These results support a physiologically relevant role of JAK3 in follicular development and provide insights into the mode of action and function of JAK3 in reproductive tissues.Support or Funding InformationThis work was supported in part by internal funds from Université de Montréal to KN and by a Discovery Grant from Natural Sciences and Engineering Research Council of Canada (NSERC) to JGL.