Radioimmunoassay measurement of prostaglandins E, A, and F in human plasma.

The details of a radioimmunoassay capable of measuring as 5 pg of prostaglandin A, E, and F (PGA, PGE, and PGF) in human and rat plasma are described. Plasma samples are extracted (with 4000 cpm [(3)H] PGE(1) added for calculation of recovery) with an organic solvent system at an apparent pH of 5.8 and then chromatographed on silicic acid columns with increasing concentrations of methanol to separate PGA, PGE, and PGF. Each chromatographed sample is measured by radioimmunoassay, using the homologous antibody and tritiated marker. 40 normal individuals had mean plasma concentrations of PGA, PGE, and PGF of 1062+/-107 pg/ml, 385+/-30 pg/ml, and 141+/-15 pg/ml, respectively. Elevated PGE levels were measured in the plasma of patients with medullary carcinoma of the thyroid, carcinoid, and neuroblastoma. Treatment of rats with indomethacin decreased serum PGE levels by 67%. The radioimmunoassay appears to be of considerable experimental as well as clinical interest.

Heat shock protein-70 induction mediates luteal regression in the rat.

Induction of heat shock protein 70 (HSP-70) is associated with inhibition of hormone-sensitive steroidogenesis and interruption of cholesterol translocation to or into the mitochondria. A number of pharmacological and physiological inhibitors of luteal cell function stimulate HSP-70 synthesis via activation of the heat shock transcription factor (HSF). In the present study we address the following questions: 1) is HSP-70 synthesis increased during prostaglandin F2 alpha (PGF2 alpha)-induced luteolysis or natural luteal regression? 2) Does blocking HSP-70 synthesis reverse the inhibition of hormone-sensitive steroidogenesis induced by heat stress and PGF2 alpha? Gel-retardation assays demonstrated HSF activation within 7.5 min after PGF2 alpha (400 micrograms) administered in vivo. Western blotting revealed HSP-70 synthesis by 1 h with higher levels seen at 2 h. The stress response was similarly activated during natural regression of the corpus luteum in rats. Gel retardation assays demonstrated maximal HSF activation 10 days after ovulation. Western blotting showed that HSP-70 levels increased dramatically on this same day and were sustained for several days after the initiation of luteal regression. Inhibition of HSP-70 synthesis was investigated in isolated luteal cells using a cholesteryl-conjugated phosphorothioate antisense oligodeoxynucleotide. The control was an oligodeoxynucleotide with the same base composition, but with scrambled base sequence. Incubation with 3 microM antisense oligodeoxynucleotide for 2 h before a 42 C heat shock prevented synthesis of HSP-70 while up to 4.5 microM control oligodeoxynucleotide had no effect on HSP-70 synthesis in response to heat shock. Preincubation with antisense oligodeoxynucleotide partially reversed heat stress-induced inhibition of LH-stimulated steroidogenesis. More importantly, preincubation with antisense oligodeoxynucleotide also significantly reversed inhibition of cAMP-stimulated steroidogenesis induced by PGF2 alpha. Treatment with control oligodeoxynucleotide did not reverse heat shock or PGF2 alpha inhibition of hormone-dependent steroidogenesis. The findings that the synthesis of HSP-70 coincides with the loss of luteal function and that blocking its synthesis reverses inhibition of hormone-dependent steroidogenesis strongly suggest a role for HSPs as physiological mediators of luteal regression.

The calcium-mobilizing agent, thapsigargin, inhibits progesterone production in rat luteal cells by a calcium-independent mechanism.

In rat luteal cells, an increase in intracellular [Ca]i impairs luteal function similar to that of prostaglandin F2a (PGF2a). However, calcium per se is not the mediator of the antigonadotropic action of PGF2a. Thapsigargin, a plant sesquiterpene lactone, increases intracellular calcium concentration concentration ([Ca]i) in several cell types by a mechanism that involves specific inhibition of the endoplasmic reticulum Ca2(+)-ATPase. To further investigate the antigonadotropic role of [Ca]i and the mechanism of action of PGF2a in rat luteal cells, the action of thapsigargin on cellular functional responses was examined in the absence and presence of PGF2a. Thapsigargin dose dependently increased [Ca]i and inhibited cAMP accumulation and progesterone production in response to LH. The inhibitory effect of thapsigargin on cAMP accumulation was calcium dependent but in contrast, inhibition of LH-stimulated progesterone production was independent of calcium mobilization by thapsigargin. Steroidogenesis stimulated by (Bu)2cAMP was also inhibited by thapsigargin. Thus, thapsigargin mimicked some effects of PGF2a with inhibitory sites of action on both cAMP accumulation and progesterone production. Thapsigargin also blocked the mobilization of [Ca]i by PGF2a, but when coincubated with PGF2a an additive effect on inhibition of LH-stimulated progesterone production occurred. However, no additive effects of thapsigargin and PGF2a on gonadotropin-sensitive cAMP accumulation were evident. In conclusion, although thapsigargin and PGF2a may share some similar actions, their antigonadotropic effects are mediated differently.

Mitochondria mediate amplification of luteinizing hormone action by adenosine in luteal cells.

Adenosine markedly amplifies the response of isolated rat and human luteal cells to LH via an intracellular site of action that is associated with an increase in cell ATP levels. This effect of adenosine is maximal in midstage cells and minimal at the onset of functional regression in late stage luteal cells. The objective of the present studies was to evaluate the role of mitochondria in mediating the action of adenosine in isolated rat luteal cells and to assess whether mitochondrial function may be compromised in regressing luteal cells. The present studies show that adenosine produced a significant increase in luteal cell levels of ADP and ATP, but had no effect on cell levels of GTP. Since ADP stimulates oxidative phosphorylation, we evaluated the role of mitochondria in mediating the amplification of LH action by adenosine in luteal cells with two mitochondrial inhibitors, oligomycin and dinitrophenol. Both inhibitors markedly reduced, in a dose-dependent manner, LH-stimulated cAMP accumulation in the presence or absence of adenosine. In parallel, both inhibitors decreased basal and adenosine-elevated ATP levels in a dose-related manner. Although late stage luteal cells showed a marked reduction in adenosine amplification of LH-stimulated cAMP accumulation, no change in adenosine-dependent elevation of cell levels of ATP was seen. We conclude that amplification of LH action and elevation of ATP levels in midstage cells by adenosine requires an increase in oxidative phosphorylation that is stimulated by an increase in cell levels of ADP. However, attenuation of adenosine amplification of LH action in late stage luteal cells is not due to impaired ATP production.

Luteolytic actions of peroxide in rat ovarian cells.

A hallmark of luteolysis is leukocyte infiltration. Phagocytic leukocytes are well known to evoke a burst release of hydrogen peroxide (H2O2) of sufficient magnitude to injure cells. We, therefore, evaluated the effect of H2O2 in isolated rat luteal cells. Peroxide (100 microM; the near-half-maximal dose) markedly inhibited both LH-sensitive cAMP accumulation and progesterone production within 5 min of treatment. Cell levels of ATP were also reduced by H2O2, but not until 10 min after exposure, with more than 50% depletion within 60 min. This depletion of ATP by H2O2 was prevented by nicotinamide or 3-aminobenzamide, inhibitors of DNA repair, but these inhibitors did not prevent the antigonadotropic action of H2O2. cAMP accumulation in response to forskolin was not inhibited by H2O2 when ATP depletion was blocked with 3-aminobenzamide. The specific binding of radiolabeled hCG to isolated cells or the biological activity of LH was not affected by H2O2 pretreatment of the cells. Isobutylmethylxanthine had no effect on abrogation of LH-sensitive cAMP accumulation by H2O2. The actions of H2O2 were not mediated by prostaglandin, since indomethacin was without effect and an increase in prostaglandin F2 alpha production was not seen with H2O2 treatment. The acute luteolytic actions of H2O2 thus appear to be due to a very rapid desensitization of the LH-receptor complex, followed by depletion of ATP. The abrogation of luteotropic support and the injurious consequences of ATP depletion raise the possibility that H2O2 may be a physiological mediator of luteolysis.

Alteration of transmembrane sodium and potassium gradients inhibits the action of luteinizing hormone in the luteal cell.

To evaluate the role of transmembrane ion flux on the acute response of luteal cells to LH, the effects of two drugs, ouabain and monensin, and changes in ion composition of the medium were examined. Ouabain, an inhibitor of the Na+ extrusion pump, and monensin, a selective Na+ ionophore, would be expected to increase the intracellular level of Na+. Both drugs produced a highly significant, dose-related and Na+-dependent inhibition of LH-stimulated cAMP accumulation and progesterone secretion. The IC50 values for ouabain and monensin for both responses to LH were about 50 and 0.1 microM, respectively, and inhibition of cAMP accumulation was competitive [inhibitory constant (Ki) = 20 and 0.06 microM, respectively]. Both drugs showed inhibition only in the intact cell, and no nonspecific cytotoxic effects were evident. No effect on the specific binding of gonadotropin or on LH-stimulated adenylate cyclase activity in membranes was seen, nor was inhibition reduced by coincubation of the cells with isobutyl methylxanthine, a cAMP phosphodiesterase inhibitor. Both drugs inhibited (Bu)2cAMP-stimulated progesterone accumulation. No effect of ouabain was seen on cholera toxin- or forskolin-stimulated cAMP accumulation, whereas monensin significantly inhibited this response to both agonists. Preincubation of cells with LH protected against inhibition of cAMP accumulation by ouabain and monensin. Removal of extracellular Na+ completely prevented inhibition by both drugs and slightly blunted the response to LH alone. However, reduction of extracellular Na+ from 128 to 32 mM, treatment of cells with tetrodotoxin (a sodium channel blocker), or increasing extracellular K+ from 5.4 to 66 mM had no effect on the stimulation of cAMP accumulation by LH. On the other hand, removal of extracellular Ca2+ completely blocked inhibition of LH-stimulated cAMP accumulation by ouabain or monensin. It is concluded that ouabain and monensin inhibit the acute stimulation of cAMP accumulation by LH probably as a result of an influx of Na+ into the luteal cell. The increase in intracellular Na+ does not appear to directly inhibit LH-sensitive adenylate cyclase activity, but induces a secondary influx of extracellular Ca2+ which inhibits activation of adenylate cyclase by LH at a site involved in coupling of the receptor to the enzyme. Luteal regression may be initiated by mechanisms similar to that caused by ouabain and monensin, because the characteristics of inhibition by these drugs is identical to that caused by prostaglandin F2 alpha, a physiological luteolysin.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo generation of hydrogen peroxide in the rat corpus luteum during luteolysis.

The hypothesis that hydrogen peroxide generation occurs in the corpora lutea of superovulated rats during luteolysis was tested using a peroxide-dependent inhibitor of catalase, 3-amino-1,2,4-triazole (AT). Luteal regression was induced during midpseudopregnancy by injection of 500 micrograms prostaglandin F2 alpha (PGF2 alpha) 1 h before administration of AT (0.1 g/kg, ip) and was confirmed by progesterone analysis of peripheral blood serum. Within groups of both PGF2 alpha-treated and untreated control rats, other rats also received ethanol (0.2 g/kg, ip), which prevents hydrogen peroxide-mediated inhibition of catalase by AT. Diluted homogenates of ovaries removed 1 h after AT administration were assayed for catalase activity by measuring the decrease in absorbance at 240 nm for 30 sec after the addition of hydrogen peroxide (10 mM). Ethanol-sensitive catalase inhibition by AT was significantly higher (47.9 +/- 3.38%) in samples from PGF2 alpha-treated groups than in controls (23.1 +/- 4.82%; P less than 0.01; n = 9). Similar increases in catalase inhibition by AT were found in luteal tissue of rats treated with PGF2 alpha 24 h earlier and in rats in which luteolysis was allowed to occur spontaneously in late pseudopregnancy. Hemoglobin an AT assays revealed that the changes in catalase activity were not the result of altered blood contamination or AT concentration in the luteal homogenates. Since catalase inhibition by AT is only seen in the presence of hydrogen peroxide, these results support the conclusion that an early and sustained component of corpus luteum regression is the generation of hydrogen peroxide in luteal tissue.

Evidence that hydrogen peroxide blocks hormone-sensitive cholesterol transport into mitochondria of rat luteal cells.

In luteal and granulosa cells, hydrogen peroxide abruptly inhibits activation of adenylate cyclase by receptor-bound gonadotropin and blocks steroidogenesis. In the present studies a post-cAMP site of peroxide action on inhibition of steroidogenesis was investigated. Steroidogenesis, stimulated by dibutyryl or 8-bromo-cAMP, was inhibited by hydrogen peroxide. Yet, cAMP-dependent protein kinase activation in cytosol or intact cells was unaffected by peroxide treatment. Hydrogen peroxide also did not inhibit the activity of cholesterol esterase and acyl coenzyme-A:acyltransferase. Progesterone synthesis was maximally increased 5- to 50-fold with 25- and 22-hydroxycholesterol, respectively. Unlike that seen with cAMP analogs and LH, however, progestin synthesis stimulated by these cell- and mitochondria-permeant cholesterol analogs was not inhibited by hydrogen peroxide. Treatment of animals with amino-glutethimide produces a marked accumulation of steroidogenic cholesterol substrate and a large increase in hormone-independent steroidogenesis in subsequently isolated and washed luteal tissue. In this paradigm, hydrogen peroxide did not inhibit elevated basal progesterone synthesis in luteal cells produced by in vivo aminoglutethimide treatment, yet LH-stimulated steroidogenesis was blocked. However, treatment of luteal cells with hydrogen peroxide inhibited pregnenolone synthesis in isolated mitochondria, an effect partially reversed by the addition of luteal cell cytosol. In summary, while peroxide inhibited cAMP-dependent steroidogenesis, it did not appear to inhibit protein kinase activation or mobilization of cholesterol from intracellular esterified stores. Although peroxide inhibited pregnenolone synthesis, it had no effect on steroidogenesis when substrate was made available by either addition of cholesterol analogs or prior treatment with aminoglutethimide in vivo. We conclude, therefore, that hydrogen peroxide inhibits steroidogenesis by blocking intracellular transport of cholesterol to mitochondria or translocation of cholesterol across the outer mitochondrial membrane.

Endocrine-regulated and protein kinase C-dependent generation of superoxide by rat preovulatory follicles.

The ovulatory LH surge results in follicular inflammation with an increase in cytokines and PGs. Reactive oxygen species (ROS) are also produced during inflammatory processes. To study ROS generation during the ovulatory cascade, preovulatory follicles were dissected from immature female rats primed with PMSG. Follicles were isolated, and ROS generation was assessed by luminol-amplified chemiluminescence. Immature rat granulosa cells were also subjected to luminometry after isolation from immature rats treated with diethylstilbestrol. Phorbol ester-stimulated ROS generation by follicular cells was completely suppressed by superoxide dismutase and the NADPH/NADH oxidase inhibitor diphenylene iodonium bisulfate, whereas catalase was without effect. Fractionation of granulosa cells with an antibody against leukocyte common antigen-1 showed that leukocyte-enriched cells produced more than 95% of the superoxide measured. In vivo treatment with LH produced a 5-fold increase in phorbol-stimulated superoxide production by isolated follicles. This response was maximal within 4 h and was blocked by indomethacin. In vivo administration of PGE(2) and PGF(2alpha) did not reverse the blockade by indomethacin; however, isolated follicles incubated with PGE(2) produced a time-dependent increase in phorbol-stimulated superoxide generation. Thus, a superoxide generator is present in the preovulatory follicle that is leukocytic in origin, hormone regulated, and activated by a protein kinase C-dependent pathway. The regulated generation of superoxide by preovulatory follicles may indicate a role for ROS in the periovulatory period.

Hormone-regulated and glucose-sensitive transport of dehydroascorbic acid in immature rat granulosa cells.

Ascorbic acid is concentrated in granulosa cells of the follicle, and ascorbate deficiency causes follicular atresia. Dehydroascorbic acid (DHAA), the oxidized form of ascorbic acid, serves as an important source for the recycling of ascorbate. As we previously demonstrated endocrine up-regulation of ascorbic acid transport by granulosa cells, we investigated DHAA as an alternate source of ascorbate in the follicle. Granulosa cells were cultured for 24 h, and DHAA uptake was initiated by the addition of 14C-labeled ascorbic acid (300 microM) in the presence of ascorbic acid oxidase (2 U/ml), which catalyzes DHAA production. Almost 90% of accumulated DHAA was present as ascorbic acid within 2 h. Preculture of cells for 24 h with FSH (50 ng/ml) and IGF-I (30 ng/ml) significantly stimulated DHAA uptake compared with the control (158 +/- 16 vs. 43 +/- 8 pmol/10(6) cells, respectively). DHAA uptake by granulosa cells was inhibited by D-glucose (ID50, approximately 2.5 mM) and by the glucose transport inhibitors phloretin (200 microM) and cytochalasin B (10 microM), which reduced uptake to 13 +/- 2% and 8 +/- 3% of the control, respectively. Northern and Western analysis of GLUT1 in granulosa cells following 24 h coincubation with FSH and IGF-I revealed up-regulation of GLUT1 at both the messenger RNA and protein levels (1.6- and 1.3-fold of control, respectively), suggesting that the stimulatory effects of FSH and IGF-I on DHAA transport are mediated by the induction of GLUT1. GLUT4 protein was not detectable by Western analysis. Endocrine-regulated DHAA transport may represent an important mechanism for maintaining adequate antioxidant tone within the developing follicle.

Antigonadotropic and antisteroidogenic actions of peroxide in rat granulosa cells.

Reactive oxygen species are produced in the ovary. In luteal cells, peroxide abruptly inhibits LH-sensitive cAMP and progesterone production, and may serve a role as a mediator of luteolysis by such mechanisms. The objective of the present studies was to evaluate the acute actions of peroxide in rat granulosa cells. Peroxide at concentrations in the low micromolar range produced a marked and dose-dependent inhibition of FSH-sensitive cAMP accumulation and progesterone production, and depleted cell levels of ATP within 1 min. Longer treatment with peroxide (60 min) caused complete abrogation of the actions of FSH. Peroxide-induced depletion of ATP was prevented by 3-aminobenzamide, an inhibitor of DNA repair, but maintenance of cell levels of ATP did not prevent the anti-FSH effects of peroxide. Peroxide also abrogated cAMP accumulation and progesterone production in response to LH in granulosa cells. Unlike that seen with LH, inhibition of FSH-sensitive cyclic AMP accumulation by peroxide was partially reversed with isobutylmethyl xanthine, an inhibitor of cyclic AMP phosphodiesterase. Although peroxide inhibited cAMP accumulation in response to cholera toxin, it did not inhibit this same response to forskolin, which indicates that peroxide may interfere with G-protein-dependent activation of adenylate cyclase. Peroxide inhibited steroidogenesis in response to cholera toxin, forskolin, and 8-bromo-cAMP. The marked inhibitory actions of peroxide on gonadotropic hormone action and steroidogenesis in granulosa cells raise the possibility that peroxide may mediate events associated with loss of follicular function.

The antigonadotropic actions of prostaglandin F2 alpha and phorbol ester are mediated by separate processes in rat luteal cells.

Protein kinase-C (PKC) has been suggested as a possible mediator of the antigonadotropic action of prostaglandin F2 alpha (PGF2 alpha) in luteal cells. To examine this possibility, we evaluated the effects of phorbol ester [12-O-tetradecanoylphorbol-13-acetate (TPA)] in relation to those of PGF2 alpha on cAMP accumulation and ATP levels as well as on the subcellular distribution of PKC activity in rat luteal cell cultures. Treatment of luteal cells for 1 h with TPA or PGF2 alpha produced a dose-dependent inhibition of LH-stimulated cAMP accumulation. Maximal inhibition produced by PGF2 alpha was about 35% greater than that produced by TPA. Moreover, PGF2 alpha produced a further inhibition of LH action when the cells were maximally inhibited by TPA. Staurosporine, a PKC inhibitor, reversed inhibition of LH-dependent cAMP accumulation produced by TPA, but had no effect on the response to PGF2 alpha. Furthermore, cells in which PKC was persistently activated by prolonged TPA treatment lost their responsiveness to additional TPA, but continued to show inhibition of cAMP accumulation by PGF2 alpha. TPA also produced a dose-dependent decrease in cell levels of ATP in contrast to PGF2 alpha. Finally, TPA produced a rapid redistribution of PKC activity from the cytosolic to the particulate fraction, whereas PGF2 alpha produced only a slight redistribution. We conclude that the acute antigonadotropic action of PGF2 alpha in rat luteal cells occurs via mechanisms other than phorbol ester-sensitive PKC activation.

Adenosine differentially amplifies luteinizing hormone- over follicle-stimulating hormone-mediated effects in acute cultures of rat granulosa cells.

Adenosine has been shown to acutely amplify LH-dependent events in luteal cells and FSH-dependent events in granulosa cells. In this study, the specificity of purines on mature rat granulosa cell ATP levels in short term culture was assessed, and a comparison of the relative effect of adenosine on amplification of FSH- and LH-stimulated cAMP accumulation was made. Adenosine rapidly and significantly increased ATP levels in granulosa cells. This effect was maximal (approximately 2-fold) within 60 min of culture and occurred in the absence or presence of FSH or LH. The increase in granulosa cell ATP levels by adenosine was dose dependent, with half-maximal and maximum responses of 10 and 30 microM adenosine, respectively. Dipyridamole (10 microM), a purine transport inhibitor, blocked the adenosine-dependent increase in granulosa cell ATP levels. Adenosine and 5'-AMP were equipotent in increasing cell ATP levels; adenine also increased ATP levels, but was significantly less active (approximately 50% of adenosine), whereas hypoxanthine, inosine, and xanthine were inactive. FSH was consistently found to decrease granulosa cell ATP levels by about 30% in the absence or presence of adenosine, whereas LH had no effect on cell levels of ATP. Both FSH and LH significantly stimulated cAMP accumulation in granulosa cells, but the maximal response to FSH was substantially greater than that to LH. Adenosine significantly amplified cAMP accumulation in response to both FSH and LH, but the effect of adenosine on this response to FSH was modest. Amplification by adenosine of cAMP accumulation in response to LH was substantial and about 2- to 3-fold greater than that seen with FSH. These studies show that purines acutely and specifically increase ATP levels in rat granulosa cells. Since adenosine augments LH-dependent cAMP accumulation to a greater extent than FSH-stimulated cAMP production, we suggest that adenosine may favor premature follicular luteinization and, perhaps, function as a mediator of atresia in the developing follicle.

Effects of neutrophils in rat luteal cells.

Ovarian function may be modulated by cells of the immune system. We have investigated the role of neutrophils (polymorphonuclear leukocytes) on rat luteal cell function. Activated neutrophils inhibited LH-sensitive cAMP accumulation, which was dependent on neutrophil cell number. At a concentration of 10(6) neutrophils/ml and 10(5) luteal cells/ml, LH-stimulated cAMP accumulation was inhibited by 50%. The inhibitory effect of activated neutrophils was reversed by superoxide dismutase (SOD) and catalase. LH-stimulated progesterone production was also inhibited by activated neutrophils. Progesterone production by 10(5) luteal cells was inhibited approximately 20% in the presence of 10(6) activated neutrophils, and this inhibition was blocked by SOD and catalase. Conditioned medium from activated neutrophils also produced inhibitory effects on LH-stimulated cAMP accumulation and progesterone production, which could be reversed by SOD and catalase. The phosphodiesterase inhibitor isobutylmethylxanthine had no significant effect on the inhibition of cAMP accumulation by conditioned medium from activated neutrophils. Luteal cells loaded with a fluorescent indicator for determining intracellular reactive oxygen species (dichlorofluorescein diacetate) showed increased fluorescence in the presence of activated neutrophils. No increase in fluorescence occurred in the absence of neutrophils or in the presence of SOD and catalase. These studies demonstrate that reactive oxygen species produced by activated neutrophils can enter the luteal cell and cause antigonadotropic effects. Although the experimental model used in the present studies may not be truly physiological, the data demonstrate that neutrophils may play a role in functional and structural regression of the corpus luteum in the rat.

Inhibition of gonadotropin action and progesterone synthesis by xanthine oxidase in rat luteal cells.

Hydrogen peroxide produces marked antigonadotropic and lytic actions in luteal cells, but the effects of superoxide, the archetypal oxygen radical, are unknown. Xanthine oxidase generates superoxide, and the activity of this enzyme, and purine substrate, are increased under ischemia, such as that seen at luteal regression. We therefore examined the actions of xanthine oxidase on luteal cells to assess the effects of this enzyme and the superoxide anion on luteal function. Xanthine oxidase, in the presence of hypoxanthine (50 microM), produced marked inhibition of LH-sensitive cAMP and progesterone production with complete inhibition at 25 mU/ml and half-maximal inhibition at about 5 mU/ml. These antigonadotropic actions of xanthine oxidase were rapid with maximal effects within 5 min, followed several minutes later by substantial depletion of ATP. Heat, superoxide dismutase, and catalase or catalase alone abolished the actions of xanthine oxidase. While depletion of ATP by xanthine oxidase was prevented by 3-amino-benzamide, an inhibitor of DNA repair, inhibition of cAMP and progesterone production was still evident. Xanthine oxidase also inhibited progesterone synthesis stimulated by 8-bromo-cAMP. Isobutylmethylxanthine, a cAMP phosphodiesterase inhibitor, did not reverse the inhibition of cAMP accumulation by xanthine oxidase, and the enzyme had no effect on LH receptor binding activity. Since catalase reversed the effects of xanthine oxidase, we conclude that superoxide was rapidly dismuted to hydrogen peroxide and mediated the antigonadotropic and antisteroidogenic actions of xanthine oxidase in luteal cells. The sensitivity of luteal cells to xanthine oxidase raises the possibility that this enzyme may serve as a significant source of hydrogen peroxide in the corpus luteum.

Physiological and pharmacological inhibitors of luteinizing hormone-dependent steroidogenesis induce heat shock protein-70 in rat luteal cells.

Heat shock protein (HSP) synthesis increases in cells with a broad range of stress conditions. We recently showed that induction of HSP-70 is associated with inhibition of hormone-sensitive steroidogenesis, but not hormone-sensitive cAMP accumulation, in rat luteal cells by a mechanism associated with interruption of cholesterol translocation in mitochondria. As HSP induction may be an early mediator of luteal regression, we investigated whether physiological and pharmacological inhibitors of luteal function would induce HSP-70 in rat luteal cells. Both [35S]methionine labeling and Western blotting with antibodies against the inducible form of HSP-70 revealed HSP induction in rat luteal cells by 1 microM prostaglandin F2 alpha (PGF2 alpha) coincident with inhibition of progesterone synthesis. In contrast, PGE2 (1 microM) failed to increase HSP-70 synthesis. Phorbol 12-myristate 13 acetate (3 microM), tumor necrosis factor-alpha (100 ng/ml), and ionomycin (1 microM) also induced HSP synthesis. Induction of HSP-70 was preceded by the rapid activation of heat shock transcription factor, which binds to the heat shock transcriptional control element. Gel retardation assays demonstrated heat shock transcription factor activation within 15 min of PGF2 alpha treatment. Northern analysis with an oligonucleotide probe specific for inducible HSP-70 showed induction at the transcriptional level by the above agents within 30 min. As functional luteal regression is known to display elements of a stress response, the finding that a number of factors that inhibit hormone-sensitive progesterone synthesis rapidly activate the heat shock response further implicates HSPs as possible mediators of luteolysis.

Thymosin gene expression is modulated by pregnant mare's serum gonadotropin, human chorionic gonadotropin, and prostaglandin F2 alpha in the immature rat ovary.

We have investigated thymosin beta 10 mRNA levels in the PMSG/hCG-treated immature rat ovary. Thymosin beta 10 mRNA was constitutively expressed as a single (greater than 600-nucleotide) abundant transcript in the immature rat ovary. Administration of a single dose (50 IU/rat) of PMSG to immature rats resulted in a gradual increase in steady state ovarian thymosin beta 10 mRNA content detectable as early as 12 h and maximal (2-to 3-fold stimulation above preinjection levels) 48 h after PMSG treatment. Ovarian thymosin beta 10 mRNA levels declined thereafter. In separate experiments treatment of PMSG (50 IU)-primed rats with hCG (25 IU) precipitated a dramatic (80%) inhibition of ambient ovarian thymosin beta 10 protein and thymosin beta 10 mRNA; both parameters remained suppressed for the duration of the hormone-induced pseudopregnancy (15 days). HPLC analysis also indicated the presence in the ovary of thymosin beta 4, a variant member of the same protein family; in general, ovarian thymosin beta 4 levels fluctuated in a manner reciprocal to that exhibited by thymosin beta 10. A luteolytic dose of prostaglandin F2 alpha (500 micrograms/rat) had little impact on ovarian thymosin beta 10 gene expression. These findings show that 1) PMSG stimulation of ovarian thymosin beta 10 biosynthesis involves increased expression of the thymosin beta 10 gene; 2) decreased expression of thymosin beta 10 is associated with luteinization, while increased thymosin beta 4 levels characterize this process; and that 3) thymosin beta 4 and beta 10 are coexpressed in this tissue and may play a significant but, as of yet, undefined role(s) in the ovary.

Inhibition of protein synthesis and hormone-sensitive steroidogenesis in response to hydrogen peroxide in rat luteal cells.

Hydrogen peroxide (H2O2) is generated in the corpus luteum at functional luteal regression and produces rapid antigonadotropic effects in rat luteal cells. However, the mechanism by which peroxide interrupts LH- and cAMP-sensitive progesterone synthesis is unknown. The post-cAMP site of H2O2 action is due to the reduced cholesterol availability in mitochondria, and this process is well known to be dependent on protein synthesis. Therefore, we examined whether H2O2 may interfere with protein and RNA synthesis, and whether such responses may be associated with inhibition of steroidogenesis. Incorporation of radiolabeled amino acids into luteal proteins was inhibited in response to H2O2 in a time- and dose-dependent manner, and these doses are similar to those that inhibit progesterone synthesis, shown earlier in the identical paradigm. The inhibitory effect of H2O2 on amino acid incorporation was not due to increased protein degradation, impaired transport of amino acids, or depletion of cellular ATP levels. H2O2 also inhibited RNA synthesis, increased RNA degradation, and impaired the efficiency of mRNA as a translation template. The time course for the inhibitory effect of H2O2 on protein and RNA synthesis was very rapid and coincident with inhibition of steroidogenesis. Inhibition of protein and RNA synthesis and steroidogenesis were reversed by preincubation of cells with the cell-permeable metal chelator o-phenanthroline, which implicates metal-dependent radical generation as the probable mediator of these actions of H2O2. We conclude that the target of the post-cAMP site of peroxide-induced inhibition of cAMP-dependent steroidogenesis is the inhibition of rapidly inducible proteins that are known to mediate translocation of cholesterol within mitochondria, where it is used as a substrate for pregnenolone synthesis.

Selective amplification of luteinizing hormone by adenosine in rat luteal cells.

Adenosine amplification of LH-stimulated cAMP accumulation in rat luteal cells is rapid and dependent on mitochondrial ATP production. The objective of the present studies was to determine if this effect of adenosine is specific for LH and to gain information on the mechanism of the ATP-dependent amplification of LH action in rat luteal cells. Adenosine significantly amplified maximum cAMP accumulation in response to LH, isoproterenol, forskolin, and cholera toxin. However, amplification of this response by adenosine was significantly greater for LH than for the other agonists. The relative order of amplification by adenosine was LH greater than isoproterenol greater than forskolin greater than cholera toxin; the relative magnitudes of amplification by adenosine were 1, 0.6, 0.2, and 0.2, respectively. Neither LH, isoproterenol, forskolin, nor cholera toxin had any effect on cellular levels of ATP, and adenosine produced a similar rate of increase and maximal levels of ATP in the presence of all agonists. Ionomycin, a calcium ionophore, inhibited LH- and cholera toxin-stimulated cAMP accumulation and produced a dose-dependent depletion of ATP. Adenosine reversed the inhibitory effect of ionomycin on LH-stimulated cAMP accumulation and cellular levels of ATP. However, adenosine did not reverse the inhibitory effect of ionomycin on cholera toxin-stimulated cAMP accumulation, although its effects on cellular ATP levels were identical to those on LH. Thus, the selective amplification of LH by adenosine is not merely a substrate effect on adenylate cyclase activity. The nature of adenylate cyclase activation by cholera toxin and forskolin and the weak amplification by adenosine of these agonists compared to that of LH indicate that the site of the ATP-dependent action of adenosine appears to be before or on the G-protein of adenylate cyclase. We suggest that adenosine, by an ATP-dependent process, either increases the availability of functional LH receptors or increases coupling between the LH receptor and adenylate cyclase.

Ovarian gonadotropin-releasing hormone-like protein(s): demonstration and characterization.

Both GnRH and prostaglandin F2 alpha inhibit LH-stimulated cAMP accumulation and progesterone secretion in isolated luteal cells. Moreover, since ovarian GnRH receptors have been demonstrated, this study was conducted to determine if GnRH-like substances were present in the luteinized rat ovary. Detection of GnRH-like activity was based on a sensitive and specific ovarian membrane GnRH radioreceptor assay and an immunoassay specific for GnRH. Ovaries were extracted with an aqueous medium containing formic acid, HCl, trifluoroacetic acid, and NaCl. Material present in the supernatant fraction which adsorbed to Waters C18 Sep-Paks was subsequently eluted with acetonitrile and lyophilized. The redissolved ovarian extract showed substantial radioreceptor activity, but very little immunoassayable activity. The GnRH-like activity of the ovary was sensitive to proteolytic enzyme digestion and to incubations at 50 C for as little as 5 min, had an apparent mol wt greater than 1,000 but less than 10,000, and was not soluble in ether. Extracts of plasma did not exhibit radioreceptor or immunoreactive activity, whereas hypothalamic extracts exhibited both radioreceptor and immunoreactive activities. Liver and kidney extracts showed less radioreceptor activity than ovarian extracts and very little immunoreactive activity. Two peaks of radioreceptor activity appeared when the ovarian extract was further fractionated by reverse phase HPLC. The two peaks of GnRH-like activity were clearly separated from GnRH or [D-Ala6, des-Gly10] GnRH ethylamide, an analog used in this study, when these were included in ovarian extracts. It is concluded that rat ovaries contain a GnRH-like protein(s) with membrane binding properties similar to those of GnRH but with other characteristics distinctly different from those of GnRH. The ovarian GnRH-like protein is immunologically different from GnRH, sensitive to elevated temperatures which do not effect GnRH, and chromatographically different from GnRH during reverse phase HPLC.