Luteolysis in ruminants: Past concepts, new insights, and persisting challenges

It is well established that in ruminants, and in other species with estrous cycles, luteal regression is stimulated by the episodic release of prostaglandin F2α (PGF2α) from the uterus, which reaches the corpus luteum (CL) through a countercurrent system between the uterine vein and the ovarian artery. Because of their luteolytic properties, PGF2α and its analogues are routinely administered to induce CL regression and synchronization of estrus, and as such, it is the basis of protocols for synchronizing ovulation. Luteal regression is defi ned as the loss of steroidogenic function (functional luteolysis) and the subsequent involution of the CL (structural luteolysis). During luteolysis, the CL undergoes dramatic changes in its steroidogenic capacity, vascularization, immune cell activation, ECM composition, and cell viability. Functional genomics and many other studies during the past 20 years elucidated the mechanism underlying PGF2α actions, substantially revising old concepts. PGF2α acts directly on luteal steroidogenic and endothelial cells, which express PGF2α receptors (PTGFR), or indirectly on immune cells lacking PTGFR, which can be activated by other cells within the CL. Accumulating evidence now indicates that the diverse processes initiated by uterine or exogenous PGF2α, ranging from reduction of steroid production to apoptotic cell death, are mediated by locally produced factors. Data summarized here show that PGF2α stimulates luteal steroidogenic and endothelial cells to produce factors such as endothelin-1, angiopoietins, nitric oxide, fi broblast growth factor 2, thrombospondins, transforming growth factor-B1, and plasminogen activator inhibitor-B1, which act sequentially to inhibit progesterone production, angiogenic support, cell survival, and ECM remodeling to accomplish CL regression.