Arachidonic acid metabolism and casein secretion in lactating rabbit mammary epithelial cells: effects of inhibitors of prostaglandins and leukotrienes synthesis

Mammary uptake and excretion of prostanoids in relation to mammary blood flow and milk yield during pregnancy-lactation and somatotropin treatment in dairy goats

Mammary arterious-venous differences (A-V) and excretion into milk of four prostanoids were related to changes in milk yield and milk vein blood velocity (MBV) in goats at different stages of pregnancy and lactation, and during somatotropin (ST) treatment in mid-lactation. Arterial concentrations and mammary A-V for the vasodilators prostacyclin (PGI(2)) and prostaglandin (PG) E(2) (measured as 6-keto-PGF(1 alpha) and bicyclic PGE(2), respectively) decreased from late pregnancy to lactation. A-V were negatively correlated to MBV (r = -0.32 to -0.34). Arterial concentrations of the vasoconstrictors PGF(2 alpha) and TXA(2) (measured as TXB(2)) changed similarly, but no A-V across the mammary gland were found. The vasodilator to vasoconstrictor ratio in plasma was around 1:1, and in skimmed milk around 0.29-0.49 due to significantly higher TXB(2) levels in milk compared to plasma. Close linear correlations were established between milk yield and excretion of TXB(2) into milk (r = 0.80, P < 0.001), and between MBV and PGE(2) excretion into milk (r = 0.69, P < 0.001). ST treatment stimulated MBV and mammary prostanoid supply, and decreased prostanoid concentration in milk vein plasma. The high arterial levels of prostaglandins during pregnancy most likely reflected uterine synthesis. Our results support a role for PGI(2) and PGE(2) in local mammary blood flow regulation during lactation. Increased mammary uptake of these two prostanoids may be involved in the mammary blood flow response to ST. TXA(2) may be synthesized by mammary epithelial as well as vascular cells, and TXA(2) may be an important factor in regulation of mammary function.

Mammary physiology and milk secretion

The presence of drugs or other potentially toxic materials in milk is an obvious public health risk, especially to infants and neonates. There is also increasing concern that human breast cancer is principally epigenetic in origin and results from environmentally produced lesions. Little is known about the mechanisms by which toxic substances enter milk or mammary tissue but knowledge of these processes is important to toxicologists and researchers involved in drug design and metabolism. Five general pathways have been described for transport of proteins, lipids, ions, nutrients and water into milk. Four of these pathways are transcellular, involving transport across at least two membrane barriers; the fifth is paracellular and allows direct exchange of interstitial and milk components. Solute transport by these pathways is mediated by a diverse, and complex array of transport and secretory processes that are regulated by hormonal, developmental, and physiological factors. Current research is beginning to define the mechanisms underlying some of these processes, however the regulation and coordination of solute transport mechanisms remains poorly understood. In this article we review our current understanding of the normal solute transport and secretory processes involved in milk production, and discuss potential regulatory mechanisms.

Prolactin signalling to milk protein secretion but not to gene expression depends on the integrity of the Golgi region

Prolactin added to the incubation medium of lactating mammary epithelial cells is transported from the basal to the apical region of cells through the Golgi region and concomitantly stimulates arachidonic acid release and protein milk secretion. We report that when PRL is added after disorganisation of the Golgi apparatus by brefeldin A treatment, prolactin signalling to expression of genes for milk proteins and prolactin endocytosis are not affected. However, prolactin transport to the apical region of cells (transcytosis), as well as prolactin-induced arachidonic acid release and subsequent stimulation of the secretion of caseins, which are located in a post-Golgi compartment, are inhibited. This inhibition was not a consequence of damage to the secretory machinery, as under the same conditions, protein secretion could be stimulated by the addition of arachidonic acid to the incubation medium. Thus, it is possible to discriminate between prolactin-induced actions that are dependent (signalling to milk protein secretion) or independent (signalling to milk gene expression) on the integrity of the Golgi apparatus. These results suggest that these two biological actions may be transduced via distinct intracellular pathways, and support the hypothesis that prolactin signals may be emitted at various cellular sites.

Transferrin and prolactin transcytosis in the lactating mammary epithelial cell

The mammary epithelial cell ferries constituents originating from blood and from stromal cells, into milk, by transcytosis. Morphological analysis of a membrane marker of transcytosis in the lactating mammary epithelial cell showed that very rapid endocytosis of surface membrane occurs from both the basolateral and the apical side of the cell. In both cases, membrane trafficking between endosomes and the Golgi complex allows communication between the endocytic and the biosynthetic pathways. Transferrin and prolactin are internalized in mammary cells and transported through multivesicular bodies and Golgi stacks. They are released into milk via different types of secretory vesicles, prolactin being carried in secretory vesicles containing casein micelles. Consequences of the intracellular transport of these proteins and physiological benefits for cell function are discussed.

Effect of reproductive states on lipid mobilization and linoleic acid metabolism in mammary glands

Effects of pregnancy and lactation on lipid metabolism in mouse mammary fat pads and nonmammary adipose tissues have been studied. In order to address the question whether the influence of hormonal milieu on lipid metabolism in mammary epithelial cells during pregnancy and lactation is the same as in fat cells, we have studied the mobilization of lipids and metabolism of fatty acids in the intact mammary glands, parenchyma-free mammary fat pads and in the perimetrial fat tissues of virgin, pregnant and lactating mice. Compared to parenchyma-free mammary fat pads, the perimetrial adipose tissues accumulated 5-fold higher levels of triglycerides during pregnancy. Mammary fat cells maintained overall lipid levels during pregnancy and lactation (16-20 micrograms/fat pad). In contrast, lactation depleted total lipid stores from 108 +/- 5 to 24 +/- 4.5 micrograms/fat pad in perimetrial fat pads. Results of comparative analysis of fatty acid composition of mammary fat pads, with and without epithelial tissue, from virgin and lactating mice showed stimulation of 18:2 omega 6 metabolism leading to 130% increase in the ratio 20:4 omega 6 to 18:2 omega 6 in the epithelial compartment. Pregnancy and lactation resulted in the elevation of 20:4 omega 6 levels probably due to a 4-fold increase in delta 5 desaturase activity and a decrease in oxidative degradation of 18:2 omega 6. These results suggest that, unlike other adipose tissues, the metabolic pathways in mammary fat cells are not dedicated to sequestration and accumulation of dietary lipids during pregnancy. Lactation favors mammary epithelial cell-stimulated production of precursors of eicosanoids which are known to have agonist-like effect on mammary epithelial cells.

Crotoxin, a phospholipase A2 neurotoxin from snake venom, interacts with epithelial mammary cells, is internalized and induces secretion

Prolactin (PRL) induces liberation of arachidonic acid (AA) from phospholipids of lactating mammary epithelial cells and stimulates casein secretion. In order to investigate the possible involvement of phospholipase A2 (PLA2) activity in the hormonal control of casein secretion by PRL, we examined the effects of crotoxin, a PLA2 neurotoxin from snake venom, on mammary epithelial cells. Crotoxin is made of two subunits: a basic PLA2 with low toxicity (component B, CB) and an acidic, non-toxic and enzymatically inactive component A (CA) which enhances the pharmacological action of CB. While CA is inactive, the PLA2 subunit (CB) induces an accumulation of secretory products in the lumen of mammary acini, an extensive development of the Golgi apparatus. The secretion of newly synthesized casein is increased in the presence of CB and this effect is inhibited by nordihydroguaiaretic acid (NDGA) and caffeic acid, two inhibitors of the lipoxygenase pathway which also prevent stimulation of secretion by PRL. Further, CB transiently induces the release of radiolabelled AA from mammary tissues previously labelled with [14C]AA, the highest release being observed between 15 s and 5 min of contact with CB and CA. Immunofluorescence labelling by anti-CB antibodies of epithelial mammary tissues previously incubated with CA, CB or a combination of CA and CB indicates that CB binds to epithelial cells and is internalized, at least in part, and that CA enhances both CB binding and its internalization. These observations emphasize the involvement of PLA2 in the control of casein secretion and suggest that PLA2 acts intracellularly.

The actions of forskolin, cholera toxin and iloprost on casein secretion by lactating doe mammary glands

The secretagogue effects of prolactin (PRL) and of various agents acting on cAMP levels, forskolin, cholera toxin and iloprost (a stable analogue of prostaglandin I2) have been assessed in lactating doe mammary gland fragments in vitro. Forskolin (10 microM), cholera toxin (1 microgram/ml) and iloprost (10 mM) stimulated milk casein secretion. The effects of forskolin (10 microM) and cholera toxin (1 microgram/ml) were potentiated by PRL (10 micrograms/ml). Conversely, the action of iloprost (10 microM) was not amplified by PRL (10 micrograms/ml). Forskolin (10 microM) and cholera toxin (1 microgram/ml) stimulated the intracellular accumulation of cAMP. Neither PRL nor iloprost, at concentrations which stimulated casein secretion, modified the accumulation of cAMP. These results demonstrate that PRL does not act directly by any increase in intracellular cAMP levels. However, stimulating effects of forskolin and cholera toxin on casein secretion and intracellular cAMP levels suggest that various transduction signals are effective in the mammary cells.