The production of arachidonic acid metabolites in rat testis

PUFAs and Their Derivatives as Emerging Players in Diagnostics and Treatment of Male Fertility Disorders

About 15% of couples worldwide are affected by infertility, with the male factor responsible for approximately 50% of reproductive failures. Male fertility can be influenced by various factors, including an unhealthy lifestyle and diet, often associated with oxidative stress. These changes are frequently the reason for spermatozoan dysfunction, malformations, and lowered count. However, sometimes even with proper semen parameters, fertilization does not occur, and this is referred to as idiopathic infertility. Of particular importance may be molecules contained in the spermatozoan membrane or seminal plasma, such as polyunsaturated fatty acids, including omega-3 (docosahexaenoic and eicosapentaenoic acids) and omega-6 (arachidonic acid) fatty acids and their derivatives (prostaglandins, leukotrienes, thromboxanes, endocannabinoids, isoprostanes), which are vulnerable to the effects of oxidative stress. In the present review, we discuss the influence of these molecules on human male reproductive health and its possible causes, including disrupted oxidative-antioxidative balance. The review also discusses the potential use of these molecules in the diagnostics and treatment of male infertility, with a particular focus on the innovative approach to isoprostanes as biomarkers for male infertility. Given the high occurrence of idiopathic male infertility, there is a need to explore new solutions for the diagnosis and treatment of this condition.

Unbalanced Expression of Glutathione Peroxidase 4 and Arachidonate 15-Lipoxygenase Affects Acrosome Reaction and In Vitro Fertilization

Glutathione peroxidase 4 (Gpx4) and arachidonic acid 15 lipoxygenase (Alox15) are counterplayers in oxidative lipid metabolism and both enzymes have been implicated in spermatogenesis. However, the roles of the two proteins in acrosomal exocytosis have not been explored in detail. Here we characterized Gpx4 distribution in mouse sperm and detected the enzyme not only in the midpiece of the resting sperm but also at the anterior region of the head, where the acrosome is localized. During sperm capacitation, Gpx4 translocated to the post-acrosomal compartment. Sperm from Gpx4^+/Sec46Ala mice heterozygously expressing a catalytically silent enzyme displayed an increased expression of phosphotyrosyl proteins, impaired acrosomal exocytosis after in vitro capacitation and were not suitable for in vitro fertilization. Alox15-deficient sperm showed normal acrosome reactions but when crossed into a Gpx4-deficient background spontaneous acrosomal exocytosis was observed during capacitation and these cells were even less suitable for in vitro fertilization. Taken together, our data indicate that heterozygous expression of a catalytically silent Gpx4 variant impairs acrosomal exocytosis and in vitro fertilization. Alox15 deficiency hardly impacted the acrosome reaction but when crossed into the Gpx4-deficient background spontaneous acrosomal exocytosis was induced. The detailed molecular mechanisms for the observed effects may be related to the compromised redox homeostasis.

Male Subfertility Induced by Heterozygous Expression of Catalytically Inactive Glutathione Peroxidase 4 Is Rescued in Vivo by Systemic Inactivation of the Alox15 Gene

Glutathione peroxidase 4 (GPX4) and arachidonic acid 15-lipoxygenase (ALOX15) are antagonizing enzymes in the metabolism of hydroperoxy lipids. In spermatoid cells and/or in the male reproductive system both enzymes are apparently expressed, and GPX4 serves as anti-oxidative enzyme but also as a structural protein. In this study we explored whether germ line inactivation of the Alox15 gene might rescue male subfertility induced by heterozygous expression of catalytically silent Gpx4. To address this question we employed Gpx4 knock-in mice expressing the Sec46Ala-Gpx4 mutant, in which the catalytic selenocysteine was replaced by a redox inactive alanine. Because homozygous Gpx4 knock-in mice (Sec46Ala-Gpx4^+/+) are not viable we created heterozygous animals (Sec46Ala-Gpx4^+/-) and crossed them with Alox15 knock-out mice (Alox15^-/-). Male Sec46Ala-Gpx4^+/- mice, but not their female littermates, were subfertile. Sperm extracted from the epididymal cauda showed strongly impaired motility characteristics and severe structural midpiece alterations (swollen mitochondria, intramitochondrial vacuoles, disordered mitochondrial capsule). Despite these structural alterations, they exhibited similar respiration characteristics than wild-type sperm. When Sec46Ala-Gpx4^+/- mice were crossed with Alox15-deficient animals, the resulting males (Sec46Ala-Gpx4^+/-+Alox15^-/-) showed normalized fertility, and sperm motility was reimproved to wild-type levels. Taken together these data suggest that systemic inactivation of the Alox15 gene normalizes the reduced fertility of male Sec46Ala-Gpx4^+/- mice by improving the motility of their sperm. If these data can be confirmed in humans, ALOX15 inhibitors might counteract male infertility related to GPX4 deficiency.

OxeR1 regulates angiotensin II and cAMP-stimulated steroid production in human H295R adrenocortical cells

Hormone-regulated steroidogenesis and StAR protein induction involve the action of lipoxygenated products. The products of 5-lipoxygenase act on inflammation and immunity by stimulation of a membrane receptor called OxeR1. The presence of OxeR1 in other systems has not been described up to date and little is known about its mechanism of action and other functions. In this context, the aim of this study was the identification and characterization of OxeR1 as a mediator of cAMP-dependent and independent pathways. Overexpression of OxeR1 in MA-10 Leydig cells increased cAMP-dependent progesterone production. Angiotensin II and cAMP stimulation of adrenocortical human H295R cells produced an increase in StAR protein induction and steroidogenesis in cells overexpressing OxeR1 as compared to mock-transfected cells. Additionally, activation of OxeR1 caused a time-dependent increase in ERK1/2 phosphorylation. In summary, membrane receptor OxeR1 is involved in StAR protein induction and activation of steroidogenesis triggered by cAMP or angiotensin II, acting, at least in part, through ERK1/2 activation.

Expression and function of OXE receptor, an eicosanoid receptor, in steroidogenic cells

Hormonal regulation of steroidogenesis involves arachidonic acid (AA) metabolism through the 5-lipoxygenase pathway. One of the products, 5-hydroperoxy-eicosatetraenoic acid (5-HpETE), acts as a modulator of the activity of the steroidogenic acute regulatory (StAR) protein promoter. Besides, an oxoeicosanoid receptor of the leukotriene receptor family named OXE-R is a membrane protein with high affinity and response to 5-HpETE, among other AA derivatives. The aim of our work was to elucidate whether this receptor may be involved in steroidogenesis. RT-PCR and western blot analysis demonstrated the presence of the mRNA and protein of the receptor in human H295R adrenocortical cells. The treatment of H295R or MA-10 cells (murine Leydig cell line) with 8Br-cAMP together with docosahexaenoic acid (DHA, an antagonist of the receptor) partially reduced StAR induction and steroidogenesis. On the contrary, 5-oxo-ETE - the prototypical agonist, with higher affinity and potency on the receptor - increased cAMP-dependent steroid production, StAR mRNA and protein levels. These results lead us to conclude that AA might modulate StAR induction and steroidogenesis, at least in part, through 5-HpETE production and activation of a membrane receptor, such as the OXE-R.

Constitutive expression of prostaglandin-endoperoxide synthase 2 by somatic and spermatogenic cells is responsible for prostaglandin E2 production in the adult rat testis

Prostaglandins (PGs), particularly PGE(2), have been implicated in the control of testicular steroidogenesis, spermatogenesis, and local immunity. However, virtually nothing is known about the expression or activity of the prostaglandin-endoperoxide synthases (PTGSs; also referred to as the cyclooxygenases), the specific rate-limiting enzymes responsible for PG production, in the adult testis. This activity was investigated in rats under normal conditions and during lipopolysaccharide-induced inflammation using quantitative real-time PCR, in situ hybridization, Western blotting, and PGE(2) measurements by ELISA. The mRNA for both the "constitutive" Ptgs1 and the "inducible" Ptgs2 forms was detected in multiple testicular cell types. Testicular Ptgs2 expression was substantially higher than that of Ptgs1, and testicular production of PGE(2) in vitro was found to be suppressed by a specific PTGS2 inhibitor (NS-398), but not by an inhibitor of PTGS1. Further investigation indicated that 1) PGE(2) production in the adult testis is attributable to constitutive expression of PTGS2 by somatic (Leydig cells and Sertoli cells) and spermatogenic cells; 2) testicular macrophages constitutively produce relatively low levels of PTGS2 and PGE(2) but are the only cell type to respond significantly to an inflammatory stimulus by increasing production of PGE(2); and 3) testicular PTGS2 expression and intratesticular PGE(2) levels are only marginally affected by acute inflammation. These data point toward a previously unanticipated maintenance role for the "inducible" PTGS2 enzyme in normal testicular function, as well as an anomalous response of testicular PTGS2 to inflammatory stimuli. Both observations are consistent with the reduced capacity of the testis to initiate and support inflammatory reactions.

Alterations of Activities of Cytosolic Phospholipase A2 and Arachidonic Acid-Metabolizing Enzymes in Di-(2-Ethylhexyl)Phthalate-Induced Testicular Atrophy

Di-(2-ethylhexyl) phthalate (DEHP), a peroxisome proliferator-activated receptor alpha (PPARalpha) ligand, alters the lipid composition of rat testis, yet the mechanism is unclear. In this study, we investigated the effect of DEHP on the synthesis and metabolism of arachidonic acid (AA), a precursor of eicosanoids, in the testis of prepubertal rats. DEHP (100 and 1,000 mg/kg, 5 days) administration caused a significant reduction in activity of cytosolic phospholipase A2 (cPLA2), the rate-limiting enzyme in the AA and eicosanoid synthesis pathways. DEHP increased the expression of 12-lipoxygenase (12-LOX) in rat testis, whereas cyclooxygenase-2 (COX-2) expression was not altered. Cytochrome P450 4A1 (CYP4A1), a product of a PPARalpha-regulated gene, was markedly increased in the testis by DEHP administration. Taken together, DEHP suppresses cPLA2 activity and induces the AA metabolizing enzymes such as 12-LOX and CYP4A1, resulting in the reduction of AA level. These data suggest that altered AA metabolic cascades may be related to the decrease of testosterone concentration in DEHP-induced testicular atrophy.

Expression of cyclooxygenase-2 in rat testis

Cyclooxygenase, the rate-limiting enzyme in the production of prostaglandins, exists in two isoforms, the constitutive cyclooxygenase 1 (Cox-1) and the inducible cyclooxygenase 2 (Cox-2). Cox-1 is involved in homeostatic functions while Cox-2 is implicated in various pathological processes such as inflammation and cancer. The present study describes the constitutive expression of Cox-2 in immature and mature rat testis, primarily localized in the spermatogonial cells. An interesting observation is the presence of Cox-2 on the chromatin and also in cytosol, apart from nuclear and endoplasmic reticular membranes. The significance of this observation is not yet clear, though its presence in the nucleus raises the possibility of Cox-2 having a more direct role in gene regulation than was thought earlier. In addition, the Cox-2 mRNA in testis is the smaller (2.8 kb) of the two isoform transcripts reported for Cox-2. Further hormone treatment regimes (testosterone/follicle stimulating hormone) increased the levels of Cox-2 protein within 6 h after treatment, suggesting that the sustained levels of Cox-2 protein in testis can be further influenced by gonadotrophins and androgens.

Signal transduction involving cyclic AMP-dependent and cyclic AMP-independent mechanisms in the control of steroidogenesis

The control of steroidogenesis via signal transduction mechanisms involving cAMP-dependent and cAMP-independent mechanisms is reviewed. Several structurally unrelated factors that are potent stimulators of steroidogenesis whose actions do not require cAMP and/or synthesis of proteins have been identified. These include various interleukins, a lipophilic factor from macrophages, a steroidogenic inducing protein from follicular fluid and an imidazole compound, calmidazolium. All of these factors are capable of inducing maximum steroidogenesis. Calcium is required for steroidogenesis in all steroidogenic cells. With the exception of the effects of angiotensin II, there is little evidence for a role of IP3 in the stimulation of the release of calcium from intracellular stores in steroidogenic cells under physiological conditions. There may however, be a cAMP-mediated activation of a plasma membrane calcium channel. Chloride channels that can be regulated by cAMP-dependent and -independent mechanisms, are present in steroidogenic cells. Chloride ions exert a negative effect on steroidogenesis because exclusion of chloride from the extracellular medium markedly enhances cAMP-stimulated steroidogenesis. Arachidonic acid and its lipoxygenase products are involved in the control of steroidogenesis via cAMP mediated processes. An arachidonic acid related thioesterase has been isolated that is activated by ACTH and which may be involved in the release of arachidonic acid. It is concluded that while cAMP is a second messenger for LH/ACTH in the control of steroidogenesis, other signalling systems exist which are potentially equally effective in controlling steroidogenesis. In addition, the action of cAMP requires other signalling pathways involving calcium and chloride ions, as well as arachidonic acid and its lipoxygenase products.

Lipid dismetabolism in Leydig and Sertoli cells isolated from streptozotocin-diabetic rats

This work evaluates how the abnormalities in fatty acid biosynthesis, already described in diabetic rats, were extended to Sertoli and Leydig cells isolated from testes of streptozotocin-treated rats. Both kinds of cells were incubated in the presence of labeled eicosa-8,11,14-trienoic acid. The uptake of the substrate and its conversion to arachidonic acid were significantly depressed in both cell types compared to the non-diabetic controls. The indirect evidence of this inhibition was observed in the total fatty acid pattern of the cells where the 20:4 n-6/18:2 n-6 ratio was significantly decreased. The addition of insulin to the incubation medium produced no changes in the uptake of the substrate by the cells. Under similar experimental conditions the synthesis of arachidonic acid was partially recovered, however, the values obtained were still below those ones of cells isolated from control animals. These results were correlated with the fatty acid profile of different lipid fractions of plasma and with the activity of enzymes involved in polyunsaturated fatty acids biosynthesis measured in the testicular microsomes of diabetic rats. We conclude that Sertoli and Leydig cells evidenced similar lipid disorders than those observed in the whole testis or in other tissues of diabetic rats, and that the biosynthesis of arachidonic acid is under insulin regulation.

Arachidonic acid metabolites as intratesticular factors controlling androgen production

The effect of inhibitors and products of arachidonic acid metabolism on rat testicular steroidogenesis has been investigated. In the presence of indomethacin (inhibitor of cyclooxygenase) and nordihydroguaiaretic acid (NDGA) (inhibitor of lipoxygenase), the activity of 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) and 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) were both inhibited. The LH-stimulated increase in secretion of testosterone and progesterone was also inhibited by indomethacin and NDGA. On the other hand, vitamin E (antioxidant and inhibitor of lipoxygenase), stimulated the activity of both 3 beta-HSD and 17 beta-HSD and enhanced LH-stimulated androgen production. The metabolites of lipoxygenase (15-HPETE, 15-HETE, 5-HPETE and 5-HETE) and cyclooxygenase (PGF2 alpha) pathways stimulated 3 beta-HSD and 17 beta-HSD activity and enhanced the secretion of progesterone and testosterone. It is concluded that arachidonic acid metabolites are intratesticular factors which can regulate LH-stimulated testicular steroidogenesis.