Expression of proliferating cell nuclear antigen in luminal epithelium during the growth and regression of rat uterus

Proliferating cell nuclear antigen (PCNA), a processivity factor of DNA synthesis, has often been used as a marker that reveals proliferating cells. However, it also plays a role other than in DNA replication. The aim of this study was to examine the relationship between the expression of PCNA and cell proliferation, and also its relation to cell death in the uterine epithelium under various hormonal conditions. Rats with regular estrous cycles were killed at various stages of the cycle, and their uteri were removed for the detection of PCNA and apoptosis by immunohistochemical and terminal deoxynucleotidyl transferase-mediated nick end-label staining respectively. There was an inverse relationship between the expression of PCNA and apoptosis in the uterine epithelium during the estrous cycle. From diestrus to proestrus, the expression of PCNA increased, and few apoptotic cells were detected in the luminal epithelium. However, at estrus, apoptosis occurred markedly, and the expression of PCNA disappeared. To study further the effects of estrogen on PCNA expression and cell growth in the uterus, rats were ovariectomized and then implanted s.c. with estrogen capsules 2 weeks later. In ovariectomized rats, only a few PCNA-positive cells were observed in the uterine epithelium. After estrogen treatment, PCNA was expressed strongly in the luminal and glandular epithelia. In these rats, the removal of estrogen capsules resulted in apoptotic death and surprisingly strong PCNA expression in the cells of luminal epithelium. Our results demonstrate that PCNA is expressed not only in the estrogen-stimulated uterine growth, but also in the processes of regression induced by the withdrawal of estrogen. Although the expression of PCNA has been reported to represent cell proliferation, our results implicate functions other than cell replication for PCNA in the uterus.

alpha-Difluoromethylornithine blocks thymocyte apoptosis via a reduction in tyrosine phosphorylation

The effect of alpha-difluoromethylornithine on cell apoptosis was investigated. Freshly isolated mouse thymocytes were cultured in the medium alone or with dexamethasone, and apoptotic cell death was monitored after 6 h. A correlation was seen between cell apoptosis and a reduction in the polyamine levels of thymocytes. Addition of exogenous polyamines decreased the levels of apoptosis induced spontaneously in the culture medium or by dexamethasone. However, addition of alpha-difluoromethylornithine, an irreversible inhibitor of ornithine decarboxylase, to the cultures did not enhance apoptosis but rather caused inhibition of thymocyte apoptosis. Analysis of the mechanism of alpha-difluoromethylornithine-mediated inhibition of apoptosis indicated that alpha-difluoromethylornithine treatment blocked protein tyrosine phosphorylation, which was elevated drastically during the first hour of thymocyte cultivation. Treatment with the phosphotyrosine phosphatase inhibitor phenylarsine oxide reversed this inhibitory effect of alpha-difluoromethylornithine on apoptotic cell death. Our results provide an alternative mechanism for alpha-difluoromethylornithine showing the inhibition of apoptosis via reduction of protein tyrosine phosphorylation.

Ovarian steroids reduced apoptotic death in SV40 temperature-sensitive mutant virus transformed uterine epithelial cells

Ovarian steroids have been shown to inhibit uterine cell death in vivo. In this study, we investigated whether ovarian steroids regulated cell death in an uterine epithelial cell line transformed with SV40 temperature-sensitive (ts) mutant virus. To assess cell death rate, cells were grown at permissive temperature (34 degrees C) and pulsed with 3H-thymidine. The retention of incorporated radioactivity was then examined after a temperature shift to nonpermissive temperature (40 degrees C) in the absence or presence of estradiol and progesterone. When cells were continuously cultured at 34 degrees C, cell number increased rapidly and most of radioactivity was retained in the attached cells. However, the temperature shift from 34 degrees C to 40 degrees C resulted in a decrease in cell number and radioactivity in attached cells. Estradiol and progesterone attenuated this temperature shift-induced cell death. Morphological examination with Hoechst 33258 staining revealed that the temperature shift increased the percentage of apoptotic death. The treatment of ovarian steroids reduced the extent of apoptotic death. Our studies demonstrated that ovarian steroids could act directly on uterine epithelial cells to reduce apoptotic death in culture.

Regulation of plasminogen activator inhibitor activity by plasmin in endothelial cells

The fibrinolytic activity in endothelial cells was regulated by balance of plasminogen activators and plasminogen activator inhibitors. Plasmin can specifically inhibit the biosynthesis of tissue-type plasminogen activator (t-PA), but not plasminogen activator inhibitor, type 1 (PAI-1) in endothelial cells. The PAI activity in the conditioned medium of endothelial cells was low and remained constant in 24 hours. However, the PAI activity in the conditioned medium of the plasmin-pretreated cells increased linearly in 24 hours. Pretreatment with protein kinase C inhibitors, H-7 or staurosporine, partially suppressed the PAI activity induced by plasmin. Pretreatment of endothelial cells with a G-protein inhibitor pertussis toxin resulted in an inhibition of the plasmin-induced PAI activity. The phospholipase A2 inhibitor mepacrine specifically eliminated the effect of plasmin stimulation on PAI activity. Cyclooxygenase and lipoxygenase inhibitors also partially inhibited the plasmin-stimulated PAI activity in endothelial cells. All these inhibitors did not affect the biosynthesis of the PAI-1 antigen in the presence or absence of plasmin. The results indicate that plasmin increased the PAI activity of endothelial cells via pathways in which protein kinase C, G protein, and phospholipase A2 may be involved.

Changes of protein kinase C subspecies in staphylococcal enterotoxin-B-induced thymocyte apoptosis

Our previous studies demonstrated that intravenous administration of staphylococcal enterotoxin B (SEB) to BALB/c mice resulted in thymocyte apoptosis. In the present study, we evaluated the role of protein kinase C (PKC) in thymocyte apoptosis induced by SEB. Our results showed that the level of protein phosphorylation in the thymocytes was reduced after the in vivo SEB treatment for 24 h. The activity of classical PKC subspecies was decreased in both cytosolic and membrane fractions of thymocytes following SEB administration. The lowest level of PKC activity was reached by 24 and 48 h, then was recovered gradually after 72 h. Furthermore, the mRNA expression of PKC-beta and, to a much less extent, PKC-alpha, but not PKC-sigma, in thymocytes was reduced by SEB. The decrease of mRNA level of PKC-beta showed good correlation with the pattern of PKC activity. These results provide direct evidence showing the changes in PKC subspecies mRNA expression during the process of apoptosis.

Diacylglycerol formation induced by human plasmin in cultured endothelial cells

Treatment of cultured bovine carotid artery endothelial cells with 10(-7) M plasmin increased the cellular diacylglycerol which was determined by the formation of [3H]palmitate-labeled diacylglycerol and diacylglycerol mass. Upon the stimulation with plasmin, a gradual increase in diacylglycerol formation was observed within 20 min then slightly declined. The maximal effect during the 1-h time course study was 45 and 55% increases in [3H]palmitate-labeled diacylglycerol and diacylglycerol mass, respectively, at 20 min after plasmin treatment. Formation of phosphatidylethanol was also studied in [3H]palmitate-prelabeled cells in the presence of ethanol. Treatment with plasmin for 20 min induced a significant 45% increase in phosphatidylethanol formation. The present results indicate that the plasmin-induced diacylglycerol formation in endothelial cells was at least in part mediated through the phospholipase D activation.

Regional differences in prostaglandin production rates among porcine intrathoracic vessels

To investigate the regional variability in intrathoracic vascular prostaglandin (PG) synthesis, we obtained vessel segments from porcine coronary artery (COA), thoracic aorta (AT), common carotid artery (CRA), pulmonary artery (PA), pulmonary vein (PV), and inferior vena cava (IVC). Vascular production rates of 6-keto-PGF1 alpha (an indicator for PGI2), PGF2 alpha, and PGE2 were measured both in unstimulated state and in arachidonic acid-stimulated state using immunosorbent assays. Our results indicated that PGI2 production rate in all vessel segments decayed with time after vessel dissection. In all vessel segments tested under unstimulated conditions, PGI2 production rates were about one order of magnitude higher than PGF2 alpha and PGE2 production rates of the same specimens. Results from unstimulated, 1.5 hr pre-incubated specimens indicated that i) PGI2 production rates in COA, AT, and PV were greater than those in CRA, PA, and IVC; ii) PGF2 alpha production rates from the same specimens were higher in PV than in AT, CRA, and IVC, while these in PA were higher than in IVC; and iii) PGE2 production rates from the same specimens were not significantly different from one another. Arachidonic acid added at about 1.5 hr after vessel harvest stimulated the PGI2 and PGF2 alpha synthesis rates by 3 to 15 folds. However, this arachidonic acid treatment caused 70 to 300-fold increases in PGE2 production rates, reaching levels comparable to PGI2. All three prostanoid production rates under stimulated conditions were also variable among different intrathoracic vessels. Although either physiological gas concentrations or local hemodynamic conditions alone can partially explain our results, which physiological parameter(s) actually causes these regional differences remains to be verified.

Acetylpolyamines decrease blood pressure, [Ca++]i and isometric force of vascular smooth muscle

Polyamines are polycations in cells and acetylation is the first step in their intracellular metabolism. We investigated the effects of the acetylated polyamines on arterial blood pressure and vascular reactivities in rats. Acetylspermine and acetylspermidine, administered at concentrations ranging from 12.5 to 50 mmol/kg b.wt., both induced a transient decrease in mean arterial blood pressure. The magnitudes of the hypotensive effects of these acetylpolyamines and polyamines were in the order of spermine > acetylspermine > acetylspermidine = spermidine. Pretreatment of rats with calcium diminished polyamine-induced hypotensive effects. The effects of spermine and acetylspermine on isolated vascular smooth muscle were examined in rat aortic rings and tail artery strips. Both compounds relaxed precontracted arterial preparations, and this relaxation could be counteracted by increasing extracellular calcium concentration. Tail artery strips were more sensitive to acetylspermine when compared to aortic rings. In tail artery strips preloaded with the bioluminescent protein aequorin, both spermine and acetylspermine caused a concomitant decrease in intracellular calcium and isometric force activated by 36 mM of KCl. These results demonstrate clearly that acetylspermine and spermine alike decrease intracellular calcium concentration of vascular smooth muscle, which is likely to account for the relaxation of vasculature. The relaxation of smooth muscle in the vascular wall in turn might lead to decreased arterial blood pressure.

Human plasmin induces a receptor-mediated arachidonate release coupled with G proteins in endothelial cells

Treatment of cultured bovine carotid artery endothelial cells with 10(-7) M plasmin increased arachidonate release coupled with the increase in prostacyclin production. The stimulatory effect of plasmin on arachidonate release could be divided into the early and late phases according to its calcium dependency and pertussis toxin sensitivity. The early phase of plasmin-induced arachidonate release was a calcium-dependent and pertussis toxin-sensitive response, which was observed within 20 min after plasmin treatment. The late phase was a calcium-independent and pertussis toxin-insensitive response, which was induced gradually from 20 to 60 min. Induction of the early phase of plasmin's effect required both the lysine binding and catalytic sites in plasmin molecule because it was inhibited either by the binding antagonist tranexamic acid or by the serine protease inhibitor aprotinin. Guanosine 5'-O-(2-thiotriphosphate) potentiated the effect of plasmin in permeabilized or nonpermeabilized cells, indicating that the early phase effect was mediated by a pertussis toxin-sensitive guanosine 5'-triphosphate (GTP)-binding protein. The late phase of plasmin's effect was due to the catalytic activity because it was inhibited by aprotinin but not by tranexamic acid. Microplasmin structurally having the catalytic sites induced a similar late phase effect. Plasmin did not elicit the metabolism of phosphatidyl polyphosphoinositides. These studies demonstrate that the activation of phospholipase A2, which results in arachidonate release, in the early phase of plasmin's effect is a receptor-mediation via GTP-binding protein that is not coupled through phospholipase C activation.

Plasmin and the regulation of tissue-type plasminogen activator biosynthesis in human endothelial cells

Plasmin inhibited the biosynthesis of tissue-type plasminogen activator (tPA) antigen by human umbilical vein endothelial cells (HUVEC) in a dose-dependent manner. The amount of tPA antigen found in the 24-h conditioned medium of cells treated with 100 nM plasmin for 1 h was 20-30% of that in the control group. However, in contrast to tPA, such treatment led to a 3-fold increase in plasminogen activator inhibitor (PAI) activity, whereas the amount of PAI type 1 antigen was unchanged. The effects of plasmin on HUVEC were binding- and catalytic activity-dependent and were specifically blocked by epsilon-aminocaproic acid. Microplasmin, which has no kringle domains, was less effective in reducing tPA antigen biosynthesis or enhancing PAI activity in HUVEC. Kringle domains of plasmin affected neither tPA antigen nor PAI activity of the cells. Other proteases including chymotrypsin, trypsin, and collagenase at comparable concentrations did not have a significant effect on the biosynthesis of tPA antigen or PAI activity of HUVEC. Thrombin stimulated the biosynthesis of tPA and PAI-1 antigens by HUVEC. Thrombin also stimulated an increase in the protein kinase activity in HUVEC, whereas plasmin inhibited the protein kinase activity of the cells. It is possible that plasmin regulates the biosynthesis of tPA in HUVEC through the signal transduction pathway involving protein kinase.

Antigen-specific tachycardia and hypotension in rodents

Tachycardia and hypotension, two cardiovascular responses to anaphylaxis, were specifically induced by antigen in mice and rats, respectively. Intravenous injection of poly (Glu60Ala30Tyr10) (GAT) elicited tachycardia within 30-40 sec in GAT-primed B6 mice. Moreover, a minute amount of GAT (0.2 micrograms) was enough to sensitize the mice to subsequent GAT-induced tachycardia. Challenging doses ranging from 100 ng to 500 micrograms. could elicit tachycardia. The kinetics of tachycardia induction was different from that of antibody production or delayed-type hypersensitivity. Tachycardia was induced from day 6 after immunization, while delayed-type hypersensitivity developed as early as day 4, and anti-GAT antibodies were undetectable on day 6 and would not reach a maximum until day 8. Specific antigen-induced hypotension was also observed in rats. Furthermore, cardiovascular changes in both species could be passively transferred by heat-treated (56 degrees C, 30 min) sera from immunized animals. These benchmarks of antigen-induced cardiovascular changes in mice or rats could be used as models to study the immune control of cardiovascular changes in anaphylactic responses.

Cytoprotective effect of reduced glutathione in hydrogen peroxide-induced endothelial cell injury

The kinetic effects of hydrogen peroxide (H2O2) on cultured endothelial cells isolated from bovine carotid artery were studied. The cytoprotective effects of glutathione (GSH) on H2O2-induced cell injury were also investigated. H2O2-induced a dose- and time-dependent cell injury in cultured endothelial cells. H2O2-induced cell injury was blocked by simultaneous treatment by catalase, but not by superoxide dismutase. H2O2 also induced endogenous PGI2 biosynthesis, and the maximum PGI2 production was reached after 1 h treatment. Stimulation of PGI2 production was parallel with arachidonate release from H2O2-treated cells. However the prostaglandin biosynthesis enzyme activity in cells was inhibited by H2O2 treatment. When the cells were treated with GSH, the intracellular GSH reached a plateau after 3 h treatment. Both H2O2-induced cell injury and PGI2 production were significantly inhibited by the 3 h pretreatment with GSH. The cytoprotective effect of GSH was completely inhibited by buthionine sulfoximine which is a specific inhibitor of gamma-glutamylcysteine synthetase. The results indicate that the cytoprotective effect of GSH on H2O2-induced cell injury in cultured bovine carotid artery endothelial cells depends on the increase in intracellular GSH content.

Chemotactic and mitogenic activities of granulosa cells in developing follicles

Chemotactic and mitogenic activities of granulosa cells in developing follicles were studied. Immature rats were subcutaneously injected with 20 IU of pregnant mare's serum gonadotrophin and killed at various intervals after injection. The ovaries were removed and granulosa cells were isolated and cultured in a serum-free medium supplemented with insulin, transferrin and hydrocortisone. Chemotactic and mitogenic activities in the conditioned medium were determined. Our results demonstrated that in addition to mitogenic activity, chemotactic activity was also expressed in the conditioned medium of granulosa cells. Both activities increased with the maturity of follicles. A gel filtration analysis revealed that there were two peaks showing both mitogenic and chemotactic activities with a molecular size smaller than 5000. These peaks had various sensitivities to heat and trypsin treatment. In addition, the active component of both peaks was organic solvent-extractable. A thin-layer chromatography analysis indicated that the lipid component was not prostaglandin, estradiol or hydrocortisone.

Role of polyamines in regulation of uterine blood flow of ovariectomized rats

Polyamines have been shown to relax several types of smooth muscle including vasculature. In the estrogen-treated uterus, uterine blood flow and polyamine levels in the uterus are increased. The relaxant effect of polyamines on blood vessels suggest that polyamines may act on uterine vasculature to cause uterine hyperemia. In this study, we examined the roles of polyamines in regulating uterine blood flow. Ovariectomized rats were administered with polyamines or estrogen. The uterine blood flow was then measured by employing radioactive microspheres. The direct injection of polyamines into systemic circulation caused a 3-fold increase in uterine blood flow within 30 seconds. The polyamine-induced uterine hyperemia was counteracted by increasing extracellular calcium concentration. When rats were treated with estradiol, uterine blood flow increased. However, the treatment with alpha-difluromethylornithine, a polyamine synthesis inhibitor, did not attenuate the estrogen-induced uterine blood flow. Our results suggest that polyamines may affect uterine blood flow via antagonizing the entry of extracellular calcium. However, the detailed mechanisms via which polyamines involved in estrogenic stimulation of uterine hyperemia may require further studies.

Differential effects of sex steroids on uterine and renal ODC activity in ovariectomized rats

Many hormones are known to induce the activity of ornithine decarboxylase (ODC), the first and rate-limiting enzyme for polyamine biosynthesis, in their target tissues. Using ovariectomized rats, we have compared the effects of sex hormones on ODC activity in the uterus and the kidney which contain estrogen and androgen receptors. The results show that 1) both estrogen and androgen stimulate renal ODC activity, 2) estrogen but not androgen effectively increases ODC activity in the uterus, 3) estrogen at higher dosage can stimulate renal ODC activity to an extent similar to that in the uterus, 4) daily treatment with estradiol for 5 days results in the desensitization of uterine ODC activity, but not that of renal ODC activity to the hormonal stimuli. Although both uterus and kidney are targets of sex hormones, our results indicate that estrogen and androgen have differential influences on the ODC activity in these two organs.

Differential responses of sex steroid target tissues of rats treated with 4-hydroxyandrostenedione

4-Hydroxyandrostene-3,17-dione (4-OHA) inhibits ovarian aromatase activity and causes regression of carcinogen-induced hormone-dependent mammary tumors in rats. Although estrogen levels were reduced, LH levels did not increase nor did uterine weight decline in 4-OHA-treated animals. These findings are in contrast to those in animals deprived of estrogen by ovariectomy. The possible direct action of 4-OHA on gonadotropin secretion and uterine growth was, therefore, investigated in ovariectomized rats not treated with the carcinogen. Treatment with 4-OHA for 2 weeks prevented regression of the uterus and the increase in gonadotropin secretion in ovariectomized rats in a dose-dependent manner. The effect on gonadotropin secretion of 4-OHA at 50 mg/kg.day was similar to that of dihydrotestosterone at 0.5 mg/kg.day and could be completely antagonized by administration of the antiandrogen flutamide. The stimulation of uterine growth by 4-OHA was also blocked by flutamide, but not by the antiestrogen enclomiphene. The trophic action of 4-OHA at 50 mg/kg.day was equivalent to that of 1.8 mg/kg.day dihydrotestosterone. Furthermore, treatment with 4-OHA caused a reduction in uterine estrogen receptor and progesterone receptor levels. The reduction in uterine estrogen and progesterone receptor levels was also counteracted by the concomitant injection of flutamide, but not by enclomiphene. The results suggest that in the rat 4-OHA has multiple actions on sex steroid target tissues in addition to inhibition of aromatase. The effects appear to be related to the androgenic rather than estrogenic activity of the compound. Inhibition of gonadotropins may help maintain reduced ovarian estrogen secretion and contribute to the antitumor activity of this compound.

In vitro and in vivo studies with aromatase inhibitor 4-hydroxy-androstenedione

Studies with 4-hydroxyandrostenedione (4-OHA) are described which demonstrate inhibition of aromatase in human placentra and rat ovaries. In animal experiments, the compound was compared with aminoglutethimide (AG) for antitumor activity and effects on plasma hormone levels. 4-OHA was more effective than AG in causing regression of DMBA-induced hormone dependent tumors in the rat. Although estradiol concentrations in ovarian vein blood were reduced initially by both compounds, there is a reflex rise in LH and estradiol levels during long-term treatment with AG, whereas hormone levels in 4-OHA treated animals remained suppressed. Further studies in ovariectomized rats indicated that during long-term treatment, 4-OHA acts as a weak androgen (the compound has less than 1% the activity of testosterone) to directly inhibit the post-castrational rise in gonadotropin levels. This antigonadotropin action of the steroidal aromatase inhibitor may help maintain reduced ovarian estrogen secretion and thus contribute to the antitumor activity of 4-OHA.

Aromatase inhibitors and their potential clinical significance

Estrogen biosynthesis occurs not only in reproductive tissues of the female but also in such diverse sites as testes, adipose and muscle. Our rationale for the clinical use of aromatase inhibitors is that compounds interacting with aromatase in all tissues could provide both selective and effective inhibition of estrogen production. The most potent inhibitor identified by us to date is 4-hydroxyandrostene-3,17-dione (4-OHA). This compound causes rapid competitive inhibition followed by irreversible inactivation of aromatase. Treatment of rats with 4-OHA results in inhibition of ovarian aromatase and estrogen secretion, accompanied by marked regression of carcinogen induced mammary tumors. Using rhesus monkeys, marked inhibition of peripheral aromatization by 4-OHA was also demonstrated. The first clinical study with a selective aromatase inhibitor was recently carried out using once weekly injections of 500 mg 4-OHA in 60 postmenopausal patients with advanced metastatic breast cancer and unselected for the presence of estrogen receptors. The mean serum estradiol level reduced to 36% of pretreatment values for at least 4 months. No effect of treatment on gonadotropin levels occurred indicating that the reduction in estrogen levels was due to inhibition of peripheral aromatization. In spite of the fact that all patients had relapsed from previous therapy, complete or partial tumor regression occurred in 30% of patients while 15% had static disease. Although the optimum dose of 4-OHA has not yet been established, this aromatase inhibitor appears to be of value in treating postmenopausal breast cancer and may be beneficial in other diseases associated with estrogens.

Aromatase inhibitors and the treatment of breast cancer

Inhibition of aromatase to reduce estrogen production by peripheral and ovarian tissue could be a useful approach to treating hormone-dependent breast cancer. Several C19, 17 keto steroids have been identified as aromatase inhibitors. The most potent of these cause rapid competitive inhibition followed by enzyme inactivation. Injections of the compounds caused inhibition of peripheral aromatization in monkeys. In rats, these treatments result in inhibition of ovarian aromatase and estrogen secretion, accompanied by marked regression of carcinogen(DMBA or NMU)-induced mammary tumors. To date, 60 postmenopausal patients with advanced metastatic breast cancer and unselected for the presence of estrogen receptors have been treated with once weekly injections of 4-OHA. The mean estradiol level measured in 14 patients was significantly reduced to 36% of pretreatment values after 1 month and remained at this level for up to 4 months. There was no effect of treatment on gonadotropin levels. Although all patients had relapsed from previous therapy, complete or partial tumor regression occurred in 30% of patients while 15% had static disease. The results indicate that in these patients the responses are due to inhibition of peripheral aromatization and that 4-OHA may be of value in treating postmenopausal breast cancer.

Effects of aromatase inhibitors, aminoglutethimide, and 4-hydroxyandrostenedione on cyclic rats and rats with 7,12-dimethylbenz(a)anthracene-induced mammary tumors

4-Hydroxyandrostenedione (4-OHA) is a more potent and specific inhibitor of aromatase (estrogen synthetase) than aminoglutethimide (AG). The two inhibitors were compared in rats with 7,12-dimethylbenz(a)anthracene-induced, hormone-dependent tumors and in normal cyclic rats treated for 4 and 2 weeks, respectively. Ovarian estradiol levels and aromatase activities were not consistently reduced, and tumors regressed in only two of eight rats treated with AG. In animals treated with 4-OHA or 4-OHA:AG, the total tumor volume, estradiol levels, and aromatase activity decreased by greater than 70%. Ovarian weights and plasma luteinizing hormone (LH) levels were also reduced by 4-OHA but increased by AG. Uterine weights were not altered by AG treatment but were increased by 4-OHA. Similar but more consistent results were obtained with these treatments in normal, cyclic rats. In ovariectomized rats, AG had no effect, whereas 4-OHA decreased LH levels and increased uterine weights. The results suggest that, although AG reduces ovarian estrogen secretion by aromatase inhibition, this may lead to an increase in LH secretion. Increased LH may promote ovarian growth and aromatase synthesis, counteracting the inhibitory action of AG to some extent. 4-OHA which inactivates aromatase may also prevent new enzyme synthesis by directly inhibiting gonadotropins. This would result in more effective reduction in ovarian estrogen production by 4-OHA than AG during long-term treatment.

Effects of phospholipases on ornithine decarboxylase activity in mammary gland explants from midpregnant mice

Phospholipases A2 (PLA2) and C (PLC) stimulate ornithine decarboxylase (ODC) activity in mouse mammary gland explants preincubated with insulin and cortisol. PLC concentrations of 0.1 micrograms/ml produced responses that were close in magnitude to that of PRL, whereas PLA2 at concentrations of 5-50 micrograms/ml was 35-40% as efficacious as 1 microgram/ml PRL in stimulating ODC activity. The time courses of PLC, PLA2, and PRL actions on ODC activity were not different from one another. When PRL and PLC were tested together, a response greater than the sum of the responses of each of these agents alone was observed; PLA2 (25 micrograms/ml) when tested with PRL produced a nonadditive response. The action of PRL on ODC activity was significantly attenuated by quinacrine, an inhibitor of PLC and PLA2 activities. These results suggest that PRL, PLA2, and PLC stimulate ODC activity via similar mechanisms in the mammary gland and make tenable the idea that the action of PRL on ODC activity may be carried out via an action of PRL on PL activity.

Effects of cyclic nucleotides on ornithine decarboxylase activity in mammary gland explants from mid-pregnant mice

Dibutyryl cAMP and prolactin stimulated ornithine decarboxylase activity in mouse mammary gland explants which had been preincubated with insulin and cortisol for 1 day; maximally stimulatory concentrations of dibutyryl cAMP and prolactin produced a response which was greater than the sum of the responses of prolactin and dibutyryl cAMP when tested alone. 8-Bromo-cGMP inhibited ornithine decarboxylase activity whereas other derivatives of cyclic nucleotides were without effect. Cortisol concentrations were found to be important for optimizing the dibutyryl cAMP and prolactin responses. Optimal prolactin responses were obtained with cortisol concentrations greater than 10(-7) M, whereas optimal dibutyryl cAMP responses were observed with cortisol concentrations less than 10(-7) M. Despite the differing optimal cortisol concentrations for the prolactin and dibutyryl cAMP responses, it is concluded that prolactin and dibutyryl cAMP probably stimulate ornithine decarboxylase activity in the mammary gland via the same mechanism.

Prostaglandin stimulation of ornithine decarboxylase activity in mammary gland explants from mid-pregnant mice

The effects of various prostaglandins on ornithine decarboxylase (ODC) activity in mammary gland explants from mid-pregnant mice have been tested. PGE1, E2 and I2 elicit a concentration-dependent stimulation of ODC activity. The minimally effective concentrations are 0.5 ug/ml for PGE1 and E2, and 50 ug/ml for PGF2 alpha and 6-keto-PGF1 alpha. The PGE1 effect had a time course identical to that of prolactin. The prolactin action on ODC activity was attenuated by indomethacin, an inhibitor of prostaglandin biosynthesis. Arachidonic acid stimulated ODC activity and its effect was abolished by indomethacin. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, potentiated the PGE1 effect on ODC activity. The results suggest that the prostaglandins may modulate prolactin's action on ODC activity via a cAMP dependent mechanism.

Effect of 3-isobutyl-1-methylxanthine on prolactin actions on RNA synthesis, casein synthesis, lipid synthesis and ornithine decarboxylase activity in mouse mammary gland explants

The effect of 3-isobutyl-1-methylxanthine (IBMX), an inhibitor of cyclic nucleotide phosphodiesterase, was tested on several actions of prolactin in cultured mouse mammary tissues. At concentrations of 0.5 mM and above, IBMX abolished the actions of prolactin on RNA and casein synthesis. IBMX by itself, stimulated ornithine decarboxylase (ODC) activity in a dose-response fashion; but the IBMX at concentrations up to 1 mM had no effect on the magnitude of the prolactin-stimulated ODC activity. IBMX inhibited in a dose-response fashion the rate of [14C]-acetate incorporation into lipids; however, prolactin stimulated lipid biosynthesis in the presence of IBMX concentrations of up to 1 mM.

Effect of various concentrations of prolactin and growth hormone on the magnitude of stimulation of RNA synthesis, casein synthesis and ornithine decarboxylase activity in mouse mammary gland explants

The effects of various concentrations of prolactin and growth hormone on the rates of [3H]-uridine incorporation into RNA, [3H]-leucine incorporation into casein, and ornithine decarboxylase (ODC) activity were determined in mouse mammary gland explants. The lowest concentrations of prolactin which produced significant responses were between 5 and 25 ng/ml. Growth hormone, in contrast, produced significant response at concentrations between 250 and 1,000 ng/ml. The prolactin actions on RNA and casein synthesis were essentially all-or-none type responses, i.e. the magnitude of the responses were maximal at about 10 ng/ml prolactin. The action of prolactin on ODC activity was quite different; a concentration-response relationship was observed with prolactin at concentrations from 10 t 250 ng/ml. It is apparent from these studies that different concentrations of prolactin are required to produce optimal actions on different biochemical parameters in cultured mammary tissues.