Luteoprotective roles of luteinizing hormone are mediated by not only progesterone production but also glucocorticoid conversion in bovine corpus luteum

Loss of luteal sensitivity to luteinizing hormone underlies luteolysis in cattle: A hypothesis

By virtue of the secretion of progesterone (P4), corpus luteum (CL) is important not only for normal cyclicity but also for conception and continuation of pregnancy in female mammals. Luteolysis (also called luteal regression) is defined as loss of the capacity to synthesize and secrete P4 followed by the demise of the CL. There is strong evidence that sequential pulses of prostaglandin F2α (PGF) secreted from the uterus near the end of luteal phase induces luteolysis in farm animals. Loss of luteal sensitivity to luteinizing hormone (LH) at the end of menstrual cycle has been reported to be critical for initiation of luteolysis in primates, however this has not been investigated in farm animals. A closer observation of the published real-time profiles of circulating hormones (P4, LH, and PGF) and their inter-relationships around the time of the beginning of spontaneous luteolysis in cattle revealed- 1) A natural pulse of PGF causes a transient P4 suppression lasting a couple of hours followed by a rebound in P4 concentration, 2) The P4 secretions that occur in response to LH pulses before the beginning of luteolysis (i.e., preluteolysis) either fail or do so to a lesser extent during luteolysis indicating a loss of sensitivity to LH, and 3) The loss of sensitivity coincides with the beginning of luteolysis (i.e., transition), and apparently luteolysis does not initiate until there is loss of sensitivity to LH. The CL is sensitive to LH during preluteolysis, and the LH-stimulated P4-dependent and/or independent local survival mechanisms maintain the steroidogenic capability and viability of the CL until the very end of preluteolysis. Luteolysis does not appear to initiate with the PGF pulse(s) that occur during this period. With the loss of sensitivity to LH at the transition, however, a progressive decline in P4 begins initiating luteolysis. Also, the survival mechanisms become compromised making the CL less viable. The uterine PGF pulses that occur after the beginning of luteolysis induces increase in the local luteolytic factors, which contribute to further luteolysis, more importantly, structural luteolysis with ultimate demise of the CL. Therefore, I hypothesize that the loss of luteal sensitivity to LH underlies luteolysis in cattle. The hypothesis not only unifies the basic mechanism of luteolysis in a farm animal and primates but also provides a perspective to view luteolysis as a process rather than a factor-mediated event. A novel unified working model for luteolysis in a farm animal and primates is described. A better understanding of the luteal physiology including how responsiveness to LH diminishes in aging CL would help in the development of novel strategies in modulating CL structure-function to improve and/or control fertility in humans as well as in animals.

The effects of luteinizing hormone as a suppression factor for apoptosis in bovine luteal cells in vitro

The fate of the corpus luteum, a transient endocrine gland formed and degraded during an oestrous cycle, is decided by various physiological factors, such as luteinizing hormone (LH). As a stimulator of progesterone, LH is known to maintain corpus luteum functional and structural integrity by inhibiting apoptosis, a programmed cell death. Therefore, we aim to investigate its action during the mid-luteal phase hypothesized that LH suppresses the death mechanism of bovine luteal steroidogenic cells (LSC) by analysing the expression of proteins involved. Cultured bovine LSC obtained from corpus luteum were treated for 24 hr with recombinant TNF and IFNG in the presence or absence of LH. The result showed that LH proved to have a protective effect by increased cell viability (p < .05) and prevented DNA fragmentation (p < .05), as demonstrated by the WST-1 colorimetric assay and TUNEL assay. Expression analysis of mRNA and protein levels showed that LH altered the expression of BCL2 (p < .05), CASP3 (p < .05), FAS (p < .05), and BAX (p < .05) to support cell survival. In conclusion, our study suggests that LH prolongs the corpus luteum life span through the anti-apoptotic mechanism by increasing cell viability and suppressing apoptosis-related genes and protein expression.

Role of oxidant–antioxidant balance in reproduction of domestic animals

Reproductive process leads to dynamic changes in metabolism and energy consumption, which may be responsible for the excessive production of free radicals (oxidants) that are generated during the physiological process of oxygen consumption. As the ovary is a metabolically active organ, it produces oxidants. Growing follicles, granulose cells of Graffian follicles and ovulated follicles all produce both enzymatic and non-enzymatic antioxidants to preserve themselves from the oxidative damage of oxidants. Oxidants and antioxidants are involved in several reproductive functions such as the regulation of follicular fluid environment, folliculogenesis, steroidogenesis, corpus luteum function, and luteolysis. In this article, the currently available literature is reviewed in relation to the roles of oxidants and oxidative stress in both normal and abnormal reproductive physiological processes.

The luteotrophic function of galectin-1 by binding to the glycans on vascular endothelial growth factor receptor-2 in bovine luteal cells

The corpus luteum (CL) is a temporary endocrine gland producing a large amount of progesterone, which is essential for the establishment and maintenance of pregnancy. Galectin-1 is a β-galactose-binding protein that can modify functions of membrane glycoproteins and is expressed in the CL of mice and women. However, the physiological role of galectin-1 in the CL is unclear. In the present study, we investigated the expression and localization of galectin-1 in the bovine CL and the effect of galectin-1 on cultured luteal steroidogenic cells (LSCs) with special reference to its binding to the glycans on vascular endothelial growth factor receptor-2 (VEGFR-2). Galectin-1 protein was highly expressed at the mid and late luteal stages in the membrane fraction of bovine CL tissue and was localized to the surface of LSCs in a carbohydrate-dependent manner. Galectin-1 increased the viability in cultured LSCs. However, the viability of LSCs was decreased by addition of β-lactose, a competitive carbohydrate inhibitor of galectin-1 binding activity. VEGFR-2 protein, like galectin-1, is also highly expressed in the mid CL, and it was modified by multi-antennary glycans, which can be recognized by galectin-1. An overlay assay using biotinylated galectin-1 revealed that galectin-1 directly binds to asparagine-linked glycans (N-glycans) on VEGFR-2. Enhancement of LSC viability by galectin-1 was suppressed by a selective inhibitor of VEGFR-2. The overall findings suggest that galectin-1 plays a role as a survival factor in the bovine CL, possibly by binding to N-glycans on VEGFR-2.

The janus face of stress on reproduction: from health to disease

Parenthood is a fundamental feature of all known life. However, infertility has been recognized as a public health issue worldwide. But even when the offspring are conceived, in utero problems can lead to immediate (abortion), early (birth), and late (adulthood) consequences. One of the most studied factors is stress. However, stress response is, per se, of adaptive nature allowing the organism to cope with challenges. Stressors lead to deterioration if one is faced with too long lasting, too many, and seemingly unsolvable situations. In stress adaptation the hypothalamus-pituitary-adrenocortical axis and the resulting glucocorticoid elevation are one of the most important mechanisms. At cellular level stress can be defined as an unbalance between production of free radicals and antioxidant defenses. Oxidative stress is widely accepted as an important pathogenic mechanism in different diseases including infertility. On the other hand, the goal of free radical production is to protect the cells from infectious entities. This review aims to summarize the negative and positive influence of stress on reproduction as a process leading to healthy progeny. Special emphasis was given to the balance at the level of the organism and cells.