Constitutive steroidogenesis in ovine large luteal cells may be mediated by tonically active protein kinase A

Review: Maintenance of the ruminant corpus luteum during pregnancy: interferon-tau and beyond

This manuscript reviews the mechanisms that maintain the corpus luteum (CL) of pregnancy in ruminants. In mammals, ovulation and luteinization of the remaining cells in the CL are due to a surge in Luteinizing Hormone (LH). In cattle, continued secretion of pulses of LH is essential for full development and function of the CL during the estrous cycle (LH pulses), however, the few studies on the CL after d20 of pregnancy do not indicate that LH is essential for maintaining the CL of pregnancy. The first essential step in maintaining the CL of pregnancy in ruminants is overcoming the mechanisms that cause regression of the CL in non-pregnant ruminants (d18-25 in cattle; d13-21 in sheep). These mechanisms have a uterine component involving oxytocin-induced prostaglandin F2α (PGF2A) pulses and a luteal component involving decreased progesterone production and luteal cell death. There is a critical role for embryonic interferon-tau (IFNT) in suppressing the uterine secretion of PGF2A during early pregnancy (d13-21 in sheep; d16-25 in cattle) and preventing luteolysis. There are also effects of IFNT on the expression of interferon-stimulated genes in other tissues including the CL but the physiologic role of these interferon-stimulated genes is not yet clear. After the IFNT period, there is another mechanism that maintains the CL of pregnancy in ruminants since embryonic IFNT is inhibited as attachment occurs and trophoblastic binucleate/giant cells begin secretion of pregnancy-associated glycoproteins. The second mechanism for luteal maintenance has not yet been defined but acts in a local manner (ipsilateral to pregnancy), and remains functional from d25 until just before parturition. The most likely mechanisms mediating later maintenance of the CL of pregnancy are increased uterine blood flow or decreased prostaglandin transporter expression in the utero-ovarian vasculature, preventing PGF2A reaching the CL. Finally, implications of these ideas on pregnancy loss in cattle are explored, highlighting the importance of inappropriate regression of the CL of pregnancy as a mechanism for pregnancy loss in cattle.

History, insights, and future perspectives on studies into luteal function in cattle

The corpus luteum (CL) forms following ovulation from the remnant of the Graafian follicle. This transient tissue produces critical hormones to maintain pregnancy, including the steroid progesterone. In cattle and other ruminants, the presence of an embryo determines if the lifespan of the CL will be prolonged to ensure successful implantation and gestation, or if the tissue will undergo destruction in the process known as luteolysis. Infertility and subfertility in dairy and beef cattle results in substantial economic loss to producers each year. In addition, this has the potential to exacerbate climate change because more animals are needed to produce high-quality protein to feed the growing world population. Successful pregnancies require coordinated regulation of uterine and ovarian function by the developing embryo. These processes are often collectively termed "maternal recognition of pregnancy." Research into the formation, function, and destruction of the bovine CL by the Northeast Multistate Project, one of the oldest continuously funded Hatch projects by the USDA, has produced a large body of evidence increasing our knowledge of the contribution of ovarian processes to fertility in ruminants. This review presents some of the seminal research into the regulation of the ruminant CL, as well as identifying mechanisms that remain to be completely validated in the bovine CL. This review also contains a broad discussion of the roles of prostaglandins, immune cells, as well as mechanisms contributing to steroidogenesis in the ruminant CL. A triadic model of luteolysis is discussed wherein the interactions among immune cells, endothelial cells, and luteal cells dictate the ability of the ruminant CL to respond to a luteolytic stimulus, along with other novel hypotheses for future research.

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.

PKA tightly bound to human placental mitochondria participates in steroidogenesis and is not modified by cAMP

INTRODUCTION

Protein phosphorylation plays an important role in the modulation of steroidogenesis and it depends on the activation of different signaling cascades. Previous data showed that PKA activity is related to steroidogenesis in mitochondria from syncytiotrophoblast of human placenta (HPM). PKA localization and contribution in progesterone synthesis and protein phosphorylation of HPM was assessed in this work.

METHODS

Placental mitochondria and submitochondrial fractions were used. Catalytic and regulatory PKA subunits were identified by Western blot. PKA activity was determined by the incorporation of (32)P into proteins in the presence or absence of specific inhibitors. The effect of PKA activators and inhibitors on steroidogenesis and protein phosphorylation in HPM was tested by radioimmunoassay and autoradiography.

RESULTS

The PKAα catalytic subunit was distributed in all the submitochondrial fractions whereas βII regulatory subunit was the main isoform observed in both the outer and inner membranes of HPM. PKA located in the inner membrane showed the highest activity. Progesterone synthesis and mitochondrial protein phosphorylation are modified by inhibitors of PKA catalytic subunit but are neither sensitive to inhibitors of the regulatory subunit nor to activators of the holoenzyme.

DISCUSSION

The lack of response in the presence of PKA activators and inhibitors of the regulatory subunit suggests that the activation of intramitochondrial PKA cannot be prevented or further activated.

CONCLUSIONS

The phosphorylating activity of PKA inside HPM could be an important component of the steroidogenesis transduction cascade, probably exerting its effects by direct phosphorylation of its substrates or by modulating other kinases and phosphatases.

Effects of prostaglandin E and F receptor agonists in vivo on luteal function in ewes

Loss of progesterone secretion at the end of the estrous cycle is via uterine PGF(2alpha) secretion; however, uterine PGF(2alpha) is not decreased during early pregnancy in ewes to prevent luteolysis. Instead the embryo imparts resistance to PGF(2alpha)-induced luteolysis, which is via the 2-fold increase in prostaglandins E(1) and E(2) (PGE(1), PGE(2); PGE) in the endometrium during early pregnancy. Chronic intrauterine infusion of PGE(1) or PGE(2) prevents spontaneous or an estradiol-17beta, IUD, or PGF(2alpha)-induced luteolysis. Four PGE receptor subtypes (EP(1), EP(2), EP(3), and EP(4)) and an FP receptor specific for PGF(2alpha) have been identified. The objective of this experiment was to determine the effects of EP(1), EP(2), EP(3), or FP receptor agonists in vivo on luteal mRNA for LH receptors, occupied and unoccupied LH receptors, and circulating progesterone in ewes. Ewes received a single treatment of 17-phenyl-tri-Nor-PGE(2) (EP(1), EP(3)), butaprost (EP(2)), 19-(R)-OH-PGE(2) (EP(2)), sulprostone (EP(1), EP(3)), or PGF(2alpha) (FP) receptor agonists into the interstitial tissue of the ovarian vascular pedicle adjacent to the luteal-containing ovary. 17-Phenlyl-tri-Nor-PGE(2) had no effect (P> or =0.05) on any parameter analyzed. Butaprost and 19-(R)-OH-PGE(2) increased (P< or =0.05) mRNA for LH receptors, occupied and unoccupied LH receptors, and circulating progesterone. Both sulprostone and PGF(2alpha) decreased (P< or =0.05) mRNA for LH receptors, occupied and unoccupied LH receptors, and circulating progesterone. It is concluded that both EP(3) and FP receptors may be involved in luteolysis. In addition, EP(2) receptors may mediate prevention of luteolysis via regulation of luteal mRNA for LH receptors to prevent loss of occupied and unoccupied LH receptors and therefore to sustaining luteal function.

Regulation of sterol regulatory element-binding transcription factor 1a by human chorionic gonadotropin and insulin in cultured rat theca-interstitial cells

Theca-interstitial (T-I) cells of the ovary synthesize androgens in response to luteinizing hormone (LH). In pathological conditions such as polycystic ovarian syndrome, T-I cells are hyperactive in androgen production in response to LH and insulin. Because cholesterol is an essential substrate for androgen production, we examined the effect of human chorionic gonadotropin (hCG) and insulin on signaling pathways that are known to increase cholesterol accumulation in steroidogenic cells. Specifically, the effect of hCG and insulin on sterol regulatory element-binding transcription factor 1a (SREBF1a) required for cholesterol biosynthesis and uptake was examined. Primary cultures of T-I cells isolated from 25-day-old rat ovaries responded to hCG and insulin to increase the active/processed form of SREBF1a. The hCG and insulin significantly reduced insulin-induced gene 1 (INSIG1) protein, a negative regulator of SREBF processing. Furthermore, an increase in the expression of selected SREBF target genes, 3-hydroxy-3-methylglutaryl-coenzyme A reductase (Hmgcr) and mevalonate kinase (Mvk), was also observed. Protein kinase A (PRKA) inhibitor completely abolished the hCG-induced increase in SREBF1a, while increasing INSIG1. Although the hCG-induced depletion of total and free cholesterol was abolished by aminoglutethimide, the stimulatory effect on SREBF1a was not totally suppressed. Treatment with 25-hydroxycholesterol abrogated the effect of hCG on SREBF1a. Inhibition of the phosphatidylinositol 3-kinase pathway did not block the insulin-induced increase in SREBF1a, whereas mitogen-activated protein kinase inhibition reduced the insulin response. These results suggest that the increased androgen biosynthesis by T-I cells in response to hCG and insulin is regulated, at least in part, by increasing the expression of sterol response element-responsive genes by increasing SREBF1a.

Dynamic changes in gene expression that occur during the period of spontaneous functional regression in the rhesus macaque corpus luteum

Luteolysis of the corpus luteum (CL) during nonfertile cycles involves a cessation of progesterone (P4) synthesis (functional regression) and subsequent structural remodeling. The molecular processes responsible for initiation of luteal regression in the primate CL are poorly defined. Therefore, a genomic approach was used to systematically identify differentially expressed genes in the rhesus macaque CL during spontaneous luteolysis. CL were collected before [d 10-11 after LH surge, mid-late (ML) stage] or during (d 14-16, late stage) functional regression. Based on P4 levels, late-stage CL were subdivided into functional-late (serum P4 > 1.5 ng/ml) and functionally regressed late (FRL) (serum P4 < 0.5 ng/ml) groups (n = 4 CL per group). Total RNA was isolated, labeled, and hybridized to Affymetrix genome microarrays that contain elements representing the entire rhesus macaque transcriptome. With the ML stage serving as the baseline, there were 681 differentially expressed transcripts (>2-fold change; P < 0.05) that could be categorized into three primary patterns of expression: 1) increasing from ML through FRL; 2) decreasing from ML through FRL; and 3) increasing ML to functional late, followed by a decrease in FRL. Ontology analysis revealed potential mechanisms and pathways associated with functional and/or structural regression of the macaque CL. Quantitative real-time PCR was used to validate microarray expression patterns of 13 genes with the results being consistent between the two methodologies. Protein levels were found to parallel mRNA profiles in four of five differentially expressed genes analyzed by Western blot. Thus, this database will facilitate the identification of mechanisms involved in primate luteal regression.

Intracellular mechanisms underlying the nicotinic enhancement of LTP in the rat dentate gyrus

We have previously shown that activation of nicotinic acetylcholine receptors (nAChRs) enhanced long-term potentiation (LTP) in the rat dentate gyrus in vitro via activation of alpha7 nAChR. In the present studies, mechanisms underlying the acute and chronic nicotinic enhancement of LTP were examined. In particular, the involvement of activation of intracellular kinases was examined using selective kinase antagonists, and the effects of enhancing cholinergic function with positive allosteric modulators of the alpha7 nAChR and with acetylcholinesterase (AChE) inhibitors were also investigated. Activation of extracellular signal-regulated kinase (ERK) and cAMP-dependent protein kinase (PKA) was found to be involved in the induction of the acute nicotinic enhancement of LTP, although not control LTP. In contrast, activation of the tyrosine kinase Src, Ca(2+)-calmodulin-dependent protein kinase II, Janus kinase 2 and p38 mitogen-activated protein kinase was not involved in the acute nicotinic enhancement of LTP, although Src activation was necessary for control LTP. Moreover, activation of phosphoinositide 3-kinase was involved in the acute nicotinic enhancement of LTP to a much lesser extent than in control LTP. Chronic nicotine enhancement of LTP was found to be dependent on PKA, ERK and Src kinases. Acute nicotinic enhancement of LTP was occluded by chronic nicotine treatment. The positive allosteric modulator PNU-120596 was found to strongly reduce the threshold for nicotinic enhancement of LTP, an affect mediated via the alpha7 nAChR as it was blocked by the selective antagonist methyllycaconitine. The AChE inhibitors tacrine and physostigmine enhanced control LTP.