Postpartum levels of 8-iso-prostaglandin F2α in plasma and milk phospholipid fractions as biomarker of oxidative stress in first-lactating dairy cows.

F2-isoprostanes such as 8-iso-prostaglandin F2 (8-iso-PGF2α) are formed by free radical-catalyzed mechanisms from membrane phospholipids and from low density lipoproteins through peroxidation of arachidonic acid. Esterified 8-iso-PGF2α is cleaved by phospholipases, circulates in blood and is excreted as putatively harmful oxidatively modified lipid via the kidney into urine. In this study we demonstrate that 8-iso-PGF2α concentrations in plasma samples from heifers are higher (p<0.005) compared to those from first-lactating dairy cows at 71 days postpartum. Furthermore, plasma 8-iso-PGF2α concentrations vary with ovarian activity and differ in response to luteolytic initiation as well as activation of the hypothalamic-pituitary-gonadal axis between heifers and first-lactating cows. Sustainable concentrations of 8-iso-PGF2α (50-150 pg/ml) are detectable in the phospholipid fraction of milk, suggesting milk as an additional excretion route for 8-isoprostanes. Plasma levels largely paralleled levels in milk (p<0.001). Plasma phospholipid 8-iso-PGF2α concentrations in cyclic cows decreased (p<0.05) from day 38 to day 71 postpartum, whereas milk phospholipid 8-iso-PGF2α rather increased (p<0.05). Cyclic cows tend to have higher 8-isoprostane levels compared to acyclic animals. In contrast to lipohydroperoxides, concentration of 8-iso-PGF2α were not correlated with milk yield (p>0.05). Our data indicate 8-iso-PGF2α may be a novel biomarker of oxidative stress in dairy cow, which is detectable in blood as well as in milk.

Inactivation of the LOX-1 pathway promotes the Golgi apparatus during cell differentiation of mural granulosa cells.

In female mammals, granulosa cells of the ovarian follicle differentiate into the corpus luteum after ovulation of the pregnable oocyte into the fallopian tube. During these differentiation processes several morphological alterations have to occur and the molecular basis is not fully understood. As an endpoint estradiol production from granulosa cells has to switch off in favor for progesterone production from the proceeding corpus luteum to sustain the developing embryo. Previously, we demonstrated that the multiligand receptor LOX-1 plays a critical role in steroid hormone synthesis of granulosa cells via intracellular calcium release from endoplasmic (ER)-dependent and ER-independent calcium pools. In the present study, we show that inhibition of LOX-1 leads to a rearrangement of ceramide from the basal membrane toward the Golgi apparatus. This activity is accomplished by a calcium-dependent phosphorylation of aromatase, the key step in estradiol production. Phosphorylated aromatase increased estradiol production in a dose-dependent manner. Our data indicate that the ceramide cascade is essential for proper granulosa cell function and ceramide redistribution serves as a first step in order to proceed with the prosperous differentiation into a corpus luteum.

LOX-1 regulates estrogenesis via intracellular calcium release from bovine granulosa cells.

Estradiol produced by ovarian granulosa cells triggers the luteinizing hormone surge which in turn initiates ovulation in female mammals. Disturbances in estradiol production from granulosa cells are a major reason for reproductive dysfunctions in dairy cows. Endogenous estradiol production might be altered by reactive oxygen species (ROS) such as oxidized low-density lipoprotein (ox-LDL). Inhibition of lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), a receptor of ox-LDL, leads to increased estrogenesis in granulosa cells. This activity is mediated by calcium release from endoplasmic reticulum (ER)-dependent and ER-independent calcium pools. Inhibition of the LOX-1 signal transduction pathway is followed by mitochondrial alterations. The membrane potential ΔΨ increases and the ROS production decreases in mitochondria after blocking LOX-1. Our data indicate that blocking the LOX-1 receptor signal pathway might be a promising way to improve steroid hormone concentrations in metabolically highly active female mammals and, therefore, to defend against reproductive dysfunctions in humans and animals.