Expression, regulation, and multifaceted molecular and biological functions of sirtuin 6 in the porcine endometrium during early pregnancy

Sirtuin 6 (SIRT6) possesses both deacetylation and mono-ADP-ribosyltransferase activities, affecting diverse biological processes via interaction with cellular substrates. However, the role of SIRT6 in female reproductive functions is largely unknown. This study examined the expression, regulation, and role of SIRT6 in the uterus of early pregnant pigs. Endometrial tissue with or without attached trophoblast was collected from gilts on days 10 to 30 of pregnancy to analyze SIRT6 mRNA and protein expression. Endometrial explants and/or luminal epithelial (LE) cells were used to examine the regulation of SIRT6 expression, SIRT6-dependent transcriptomic changes, and the effect of SIRT6 on prostaglandin E2 (PGE2) synthesis, apoptosis, proliferation, cell cycle progression, and cell adhesion. SIRT6 is expressed at the utero-trophoblast interface during early pregnancy in pigs. RNA sequencing of peri-implantation endometrium after SIRT6 activation with UBCS039 identified 788 up- and 756 down-regulated genes (adjusted p-value < 0.05 and log2 fold change > 0.58) that significantly enriched functions attributed to metabolic processes, aminoacyl-tRNA biosynthesis, intracellular protein transport, immune response, apoptotic and cytokine-mediated signaling pathways, cell proliferation and adhesion, and extracellular matrix organization. Many genes were associated with the metabolism of nutrients, steroids, and PGE2, as well as with mitochondrial activity and cell cycle progression. Consistent with elevated expression of genes encoding PG-metabolizing enzymes (PTGR1, AKR1C1) in the UBCS039-treated endometrial samples, the concentration of PGE2 in culture media was diminished by 90% as compared with the non-treated control (p < 0.05). Moreover, the activation of SIRT6 promoted LE cell proliferation, whereas the SIRT6 inhibitor diminished the number of viable cells (p < 0.01). In support, UBCS039 accelerated cell cycle progression through the G2/M phase by reducing the levels of cyclin A2 (p < 0.05) and B1 (p = 0.07). In turn, UBCS039 inhibited cell adhesion (p < 0.05). These results suggest that SIRT6, present at the maternal-conceptus interface in pigs, may modulate endometrial gene expression and support uterine function to maintain pregnancy. Given that implantation failure is a major cause of early embryonic loss in pigs, SIRT6 could be considered a novel target for developing strategies to improve survival of the early conceptuses.