ERK1/2 is involved in luteal cell autophagy regulation during corpus luteum regression via an mTOR-independent pathway

Autophagy is known to be regulated by the phosphoinositide-3 kinase (PI3K)-protein kinase B (AKT) and/or mitogen-activated protein kinase 1/2 (MEK1/2)-extracellular signal-regulated kinase 1/2 (ERK1/2) pathways, leading to activation of mammalian target of rapamycin (mTOR), a major negative regulator of autophagy. However, some reports have also suggested that autophagic regulation by the PI3K-AKT and/or MEK1/2-ERK1/2 pathways may not be mediated by mTOR activity, and there is no direct evidence of the involvement of these pathways in luteal cell autophagy regulation. To elucidate the luteal cell-specific regulatory mechanisms of autophagy induction during corpus luteum (CL) regression, we evaluated whether luteal cell autophagy is regulated by the PI3K-AKT pathway and/or MEK1/2-ERK1/2 pathway and if this regulation is mediated by mTOR. We found that autophagy induction increased despite mTOR activation in luteal cells cultured with prostaglandin F2α (PGF2α), an important mediator of CL regression, suggesting that PGF2α-induced autophagy is independent of mTOR regulation. We also found that PGF2α-induced autophagy was not mediated by AKT activity, because AKT inhibition using a PI3K inhibitor (wortmannin) did not change autophagy induction or mTOR activity. In contrast, ERK1/2 activity increased in PGF2α-treated luteal cells, as did the levels of autophagy induction despite increased mTOR activity. Furthermore, PGF2α-mediated up-regulation of luteal cell autophagy was reversed by addition of ERK1/2 inhibitors, despite a decrease in mTOR activity. These in vitro results suggest that luteal cell autophagy is induced by increased ERK1/2 activity during CL regression, and is independent of mTOR activity. This finding was further supported by in vivo experiments in a pseudo-pregnant rat model, which showed that induction of luteal cell autophagy increased during luteal stage progression and that this was accompanied by increased ERK1/2 and mTOR activity. Taken together, our findings indicate that activation of ERK1/2 is a key event in the induction of luteal cell autophagy during CL regression which is not associated with mTOR regulation.