Protein Kinase A and 5′ AMP-Activated Protein Kinase Signaling Pathways Exert Opposite Effects on Induction of Autophagy in Luteal Cells

Abstract

In the absence of pregnancy the ovarian corpus luteum undergoes regression, a process characterized by decreased production of progesterone and structural luteolysis involving apoptosis. Autophagy has been observed in the corpus luteum during luteal regression. Autophagy is a self-degradative process important for balancing sources of cellular energy at critical times in development and in response to nutrient stress, but it can also lead to apoptosis. Mechanistic target of rapamycin (MTOR) and 5′ AMP-activated protein kinase (AMPK), key players in autophagy, are known to inhibit or activate autophagy, respectively. Here, we analyzed the signaling pathways regulating the initiation of autophagy in bovine luteal cells. In vivo studies showed increased activating phosphorylation of AMPKα (Thr172) and elevated content of LC3B, a known marker of autophagy, in luteal tissue during PGF2α-induced luteolysis. In vitro, AMPK activators 1) stimulated phosphorylation of regulatory associated protein of MTOR (RPTOR) leading to decreased activity of MTOR, 2) increased phosphorylation of Unc-51-Like Kinase 1 (ULK1) and Beclin 1 (BECN1), at sites specific for AMPK and required for autophagy initiation, 3) increased levels of LC3B, and 4) enhanced colocalization of autophagosomes with lysosomes indicating elevated autophagy. In contrast, LH/PKA signaling in luteal cells 1) reduced activation of AMPKα and phosphorylation of RPTOR, 2) elevated MTOR activity, 3) stimulated phosphorylation of ULK1 at site required for ULK1 inactivation, and 4) inhibited autophagosome formation as reflected by reduced content of LC3B-II. Pretreatment with AICAR, a pharmacological activator of AMPK, inhibited LH-mediated effects on RPTOR, ULK1 and BECN1. Our results indicate that luteotrophic signaling via LH/PKA/MTOR inhibits, while luteolytic signaling via PGF2α/Ca2+/AMPK activates key signaling pathways involved in luteal cell autophagy.

Prostaglandin F2α regulates mitochondrial dynamics and mitophagy in the bovine corpus luteum

“…Mitochondrial fission and mitophagy are two cellular mechanisms that synchronously regulate mitochondrial quality control systems to protect cells from cytotoxic ROS production ( 50 ). AMPK is an enzyme that plays a role in energy homeostasis and is a downstream target of PGF2α in bovine luteal cells ( 51 ). AMPK is also a regulator of autophagy and mitophagy through activation of the protein kinase, Unc-51-like autophagy activating kinase (ULK1) ( 32 ).…”

A matter of new life and cell death: programmed cell death in the mammalian ovary

“…Luteal cells treated with PGF2α in vitro and in vivo display prominent signs of both autophagy and apoptosis [ 248 , 283 , 286 ]. While PGF2α-dependent apoptosis is supposedly associated with the PLC/PKC and the CD95/Fas/Apo-1–FasL pathways, autophagy stimulation may be facilitated through AMPK signaling activation in bovine [ 286 ], ER stress in goat [ 248 ], or HIF1α-BNIP3-dependent ROS generation in the rat CL [ 287 ]. The last observation is especially interesting as activation of the same pathway in granulosa follicle cells results in their survival and luteinization instead of PCD [ 200 , 201 ].…”

Central Role for Glycolysis and Fatty Acids in LH-responsive Progesterone Synthesis

“…These events are also responsible for the post-transcriptional regulation of proteins involved in the maintenance of luteal function like enhanced lipolysis required to sustain cholesterol supply for progesterone synthesis or changes in mitochondrial dynamics (21)(22)(23). Considerable effort has been placed in the past five decades to understand how gonadotropins control ovarian gene transcription and post-translational modifications of various signaling proteins that positively or negatively impact transcription (21,22,24,25). Less is known regarding the metabolic events driven by LH that contribute to progesterone secretion by the corpus luteum.…”