Biosynthesis of prostaglandins in human ovarian tissues

Ovulation as a tissue remodelling process. Proteolysis and cumulus expansion

Ovulation, recurring every midcycle of the mammalian female and triggered by a surge of luteinizing hormone (LH) released from the pituitary, is an essential prerequisite for fertilization and subsequent embryonic development. Here we shall describe two of the biological components of the ovulatory response, cumulus expansion (frequently denoted as cumulus maturation) and the rupture of follicular wall, both crucial for the release of a fertilizable ovum. The role of a proteolytic cascade and its regulation by eicosanoids will be emphasized in relation to follicle rupture. The new data implicating cumulus maturation as an essential step for the release of the ovum and the apparent mediatory role of interleukin-1 in this process will be presented. LH/hCG stimulates, in the preovulatory follicles, a cascade of proteolytic enzymes, including plasminogen activator (PA), plasmin and matrix metalloproteinase 1 (MMP-1). These enzymes bring about the degradation of perifollicular matrix and, most notably, the decomposition of the meshwork of collagen fibers which provides the strength to follicular wall. Furthermore, pharmacological blockage of any of these enzymes resulted in inhibition of follicle rupture. LH/hCG stimulates, in addition, an increase in ovarian production of eicosanoids. These include prostaglandins, obtained from arachidonic acid via the cyclooxygenase pathway and leukotrienes, the products of lipoxygenase. Previous studies from our and other laboratories have demonstrated the ability of inhibitors of cyclooxygenase and of lipoxygenases to suppress ovulation in several mammalian species. MK-886, which inhibits the translocation of 5-lipoxygenase (5-LO) from the cytosol and its binding to the membranal 5-LO activating enzyme, suppressed dose-dependently follicular rupture from the treated ovary. Zymographic analysis of ovarian extracts from PMSG/hCG-stimulated rats revealed a band of collagenolytic activity at 52kD, corresponding to human MMP-1 and at 72kD, corresponding to human MMP-2. Both activities were markedly stimulated by administration of hCG and were significantly inhibited by indomethacin, NDGA or MK-886. Thus, eicosanoids seem to mediate LH stimulation of follicular collagenase. Interleukin-1 (IL-1) has been recently implicated in ovulation. The ability of an IL-1 receptor antagonist (ra) to block ovulation in vivo and in vitro has been demonstrated recently. Morphological examination of the ovulatory follicles failing to ovulate suggests that this effect is exerted by inhibiting cumulus oophorus expansion and detachment from mural granulosa cells. In vitro, IL-1ra attenuated the action of hCG and FSH on cumulus expansion and follicular hyaluronic acid synthesis. Thus, IL-1 seems to mediate and/or facilitate gonadotropin action on cumulus expansion, and hence on ovulation.

Clinical significance of prostacyclin and thromboxane in cancer of the female breast and genital tract

Studies investigating the role of thromboxane A2 and prostacyclin in cancer of the female breast and genital tract are reviewed. Whereas thromboxane A2 was found to promote the tumour growth and metastasis, prostacyclin exerted a protective effect in maintaining vascular and platelet homeostasis. Thus, monitoring of prostacyclin and thromboxane levels in plasma and urine of cancer patients may be essential for the evaluation of tumour growth and metastasis. Of all modulators of thromboxane and prostacyclin biosynthesis, nafazatrom was found to exhibit promising results for the treatment of advanced breast cancer, although its use in the routine therapy is questionable at this stage.

Prostacyclin and steroidogenesis in goat ovarian cell types in vitro

Granulosa, theca and corpus luteum cells of the goat ovary were isolated and incubated separately for 6 hours, with or without various modulators. Arachidonic acid (AA, 10 ng to 100 micrograms/ml), the precursor for prostaglandin synthesis, produced a dose-dependent increase in progesterone (P4) and estradiol-17 beta (E2) production by all the cell types. Prostaglandin synthetase inhibitors, aspirin (10(-6)-10(-3)M) and indomethacin (100 ng-1 mg/ml), produced a dose-dependent decrease in arachidonic acid-stimulated (100 micrograms/ml) steroid production. Prostacyclin synthetase stimulators, trapidil (1.6 micrograms- 1 mg/ml) and dipyridamole (10(-6)-10(-3)M), when added alone or along with AA, did not affect steroid production. Up to 100 micrograms/ml of U-51605 (9,11-azoprosta-5,13-dienoic acid), a prostacyclin synthetase inhibitor, did not inhibit basal or AA-stimulated steroid production. Prostacyclin (PGI2) and its stable analog 6 beta PGI1 (0.01-10 micrograms/ml) produced a dose-dependent increase in P4 and E2 production in all the three cell types. Increase at 1 and 10 micrograms/ml was significant in all cases. 6-keto-PGE1 (an active metabolite of PGI2 in certain systems) produced an increase in steroid production which was significant in theca at greater than or equal to 1 microgram/ml concentrations but had no significant effect on granulosa and corpus luteum cells at any dose level. 6-keto-PGF1 alpha (stable metabolite of PGI2) was without effect in the present system. The lack of effect of PGI2 at lower concentrations was not altered by either differentiation of the cells with FSH and testosterone or addition of steroid precursors, testosterone and pregnenolone. The present results indicate that AA-stimulated steroid production in the goat ovarian cell type is mediated by prostaglandins other than PGI2 though PGI2 itself can positively modulate the steroid production.

Production of prostacyclin by different cell types of the goat ovary

A sensitive platelet aggregation-inhibition assay was used to quantitate the production of prostacyclin by different cell types of the goat ovary. The assay could detect as low as 0.16 ng in the test sample. Different cell types i.e. granulosa, theca and corpus luteum or the total ovarian homogenate were incubated at 37 degrees C for 10 minutes with or without 0.2mM arachidonic acid. Rat aortic strips were incubated under similar conditions as a positive control. Under basal conditions the amount of prostacyclin produced by corpus luteum cells was higher compared to that by granulosa cells. When the precursor of prostaglandins (arachidonic acid) was provided the production markedly increased in corpus luteum, granulosa, and ovarian homogenate as well as in aortic strips. Theca cells did not produce detectable levels of prostacyclin even when the precursor was provided. Trapidil did not alter the basal but enhanced the arachidonic acid-stimulated prostacyclin production in homogenate and granulosa cells with no further increase in corpus luteum cells. U-51605 decreased basal as well as arachidonic acid-stimulated prostacyclin production in all the cell types. The prostacyclin production in ovaries is compartmentalized suggesting a possible role in ovarian physiology.

Follicular fluid prostaglandins in endometriosis and ovarian hyperstimulation

To study the presence of prostaglandin F2 alpha (PGF2 alpha), prostacyclin (PGI2), and thromboxane A2 (TxA2) in the human ovary, follicular fluid samples were collected with puncture at laparoscopy in 17 women with pelvic endometriosis, 17 women with tubal occlusion and healthy ovaries, and 5 women with tubal occlusion and induced ovarian hyperstimulation between menstrual cycle days 8 and 18. The concentrations of the metabolites of PGF2 alpha, PGI2, and TxA2, i.e., 13,14-dihydro-15-keto PGF2 alpha (M-PGF2 alpha), 6-keto PGF1 alpha, and TxB2, respectively, were measured with radioimmunoassays. Each prostanoid was detected in follicular fluid, but their concentrations were unrelated to the menstrual cycle day at collection. Moreover, these concentrations were similar in various groups of patients. The data suggest that the human ovary produces PGF2 alpha, PGI2, and TxA2 in vivo and that these prostanoids, as measured from follicular fluid, may not be of primary significance in ovulation or endometriosis.

Luteinizing hormone stimulates the production of prostacyclin by isolated ovarian cell in vitro

Granulosa cells isolated from mature Graafian follicles of swine produced significant quantities of immunoreactive 6-keto-PGF1 alpha under chemically defined conditions in vitro. Luteinizing hormone elicited a dose-dependent stimulation of 6-keto-PGF1 alpha accumulation, but follicle stimulating hormone, prolactin, L-epinephrine, estradiol-17B, or PGE2 were devoid of effect. The time-dependent in vitro production of 6-keto-PGF1 alpha by ovarian cells was susceptible to inhibition by indomethacin, U-51506, cycloheximide, and actinomycin D. These observations implicate granulosa cells in the specific and hormonally regulated production of prostacyclin.

Bioconversion of arachidonic acid to prostaglandins by Graafian follicles and stroma from the human ovary

In order to study production of prostaglandins (PGs) by human pre-ovulatory ovarian tissue, bioconversion of 14C-arachidonic acid to PGs was examined using homogenates of human Graafian follicles and stromal tissues fully stimulated by endogenous follicle-stimulating hormone in the regular menstrual cycle. Among the radioactive metabolites extracted, PGE2 and 6-keto PGF1 alpha were purified and identified by silicic acid column-, thin layer-, reversed phase partition chromatographies and radio-gas chromatography. The follicular tissue was shown to convert 14C-arachidonic acid to PGE2 at a conversion rate of 0.34% but not to 6-keto PGF1 alpha. On the other hand, the stromal tissue was demonstrated to produce not only 14C-labeled PGE2 at a conversion of 0.09% but mainly 14C-labelled 6-keto PGF1 alpha at 0.26%.

Biosynthesis of prostaglandins from arachidonic acid by the rat ovary

Biosynthesis of prostaglandins (PGs) from 14C-arachidonic acid was studied using homogenates of the ovaries from immature rats. In ascending order of metabolizing potency were, the ovaries from untreated rats, from rats treated with pregnant mare's serum gonadotropin (PMS), and from PMS-human chorionic gonadotropin (hCG) treated rats. Among the radioactive metabolites extracted, PGE2 and 6-keto PGF1 were purified and identified by silicic acid column-, thin layer-, reversed phase partition chromatographies, and radiogaschromatography. Production of PGE2 and 6-keto PGF1 was observed in homogenates of the ovaries of intact and PMS-hCG treated rats at conversion rates of 0.72; 0.43% and 7.62; 2.31%, but not by PMS treated rat ovaries. Treatment with PMS-hCG activated metabolism of arachidonic acid into radioactive metabolites including PGE2 and 6-keto PGF1 to a large extent. Accordingly, it is concluded that luteinizing hormone and hCG play a significant role in the biosynthesis of PGs by the rat ovarian follicle.