Temperature dependence of prolactin endocytosis and casein exocytosis in epithelial mammary cells.

As previously reported in epithelial mammary cells of lactating rabbit, prolactin exerts a stimulatory effect on casein secretion. After binding to a membrane receptor, the complex hormone-receptor is internalized in mammary cells. Peptide hormone action involves the generation of second messengers. These second messengers can be emitted as soon as hormone is linked to the membrane receptor. However, it is not excluded that endocytosis and transfer of prolactin inside the cell take part in the emission of second messenger and related secretory response. In order to precise intracellular transport pathways in the lactating mammary cell, we have examined the effects of reduced temperature on the one hand on prolactin endocytosis, on the other hand on casein secretion and on the stimulating effect of prolactin on casein secretion. Endocytosed prolactin was cytochemically localized mainly on the plasma membrane at 4 degrees C. At 25 degrees C, the hormone accumulated, during 60 min, in endosomes and multivesicular bodies. At 37 degrees C, prolactin was detectable after 15 and 30 min inside the cells and disappeared after 60 min. Transport and exocytosis of secretory proteins were only partly inhibited at 25 degrees C as attested by autoradiography localization and biochemical assays of newly synthesized caseins. However, at 25 degrees C, prolactin was no more able to stimulate casein exocytosis. These results show that intracellular transport of prolactin and secretagogue effect of the hormone does not proceed at 25 degrees C. However, secretory mechanisms of the cell are always able to be stimulated by exogenous arachidonic acid at this temperature. Low temperature appears as a good means to study intracellular transport in the mammary cell.

[Early action of colchicine, ammonium chloride and prolactin, on secretion of milk lipids in the lactating mammary gland (author's transl)]. / Effet rapide in vitro de la colchicine, du chlorure d'ammonium et de la prolactine, sur la sécrétion des lipides du lait dans la glande mammaire.

The secretion of milk lipids was investigated by incubating mammary tissue fragments from lactating ewes and rabbits. After the lipids were pulse-labelled (3 min with 14C-sodium acetate or 14C-sodium butyrate), the intracellular and extracellular lipid globules, localized in the acinar lumen, were labelled (as shown by electron microscopy), and the labelled lipids were released into the medium. Colchicine, a microtubule modifier inhibiting protein secretion, did not modify lipid secretion. NH4Cl, a lysosomotropic agent, did not decrease lipid secretion. When prolactin was added immediately after the pulse, the amount of labelled lipids secreted for two hours increases significantly. These results show that prolactin stimulates lipid secretion in the same way as protein secretion. However, microtubular integrity, necessary for protein secretion, is not required for lipid secretion.

Brefeldin A differently affects basal and prolactin-stimulated milk protein secretion in lactating rabbit mammary epithelial cells.

When lactating mammary epithelial cells were treated with prolactin in vitro, numerous small vesicles rapidly accumulated in the Golgi area, and secretion of milk proteins increased. The effects of brefeldin A on these intracellular events were investigated. As observed by electron microscopy, stacks of the median Golgi were not altered after incubation in the presence of 50 nM brefeldin A but were dissociated when the drug concentration was > or = 500 nM. Small vesicles did not accumulate in the Golgi area when mammary cells were incubated in medium containing both prolactin and brefeldin A, whatever the concentration of the latter. Immunofluorescence experiments showed that 50 nM brefeldin A did not modify the localization of the CTR 433 median Golgi protein, but it induced redistribution of trans-Golgi network-associated proteins such as TGN38, AP-1 adaptor and clathrin. These effects occurred in the presence of brefeldin A plus prolactin. Pulse-chase experiments showed that brefeldin A concentrations > or = 100 nM induced the intracellular accumulation of milk proteins, provoked the appearance of immature forms of caseins, and inhibited milk protein secretion. In contrast, concentrations of brefeldin A of < or = 50 nM did not affect basal casein secretion but inhibited the secretagogue effect of prolactin. These data show not only that several biochemical events in the transport of milk proteins which are sensitive to different brefeldin A concentrations occur in lactating mammary epithelial cells, but also that it is possible to inhibit a hormonal stimulus in a selective manner, while the machinery responsible for basal secretion is still active.

[Prolactin internalisation by the epithelial mammary cell: effects of lysosomotropic agents and transglutaminase inhibitors]. / Endocytose de la prolactine dans la cellule épithéliale mammaire: effets des agents lysosomotropes, et des inhibiteurs de la transglutaminase.

Prolactin endocytosis was studied by electron microscopy with 125I-prolactin 125I-hGH (human growth hormone) and prolactin-ferritin. Endocytosis and intracellular transit of the labelled hormone proceeded identically in epithelial cells isolated from the mammary glands of pseudopregnant rabbits and in surviving fragments from mammary glands of lactating rabbits. After binding of the hormone to its receptor, the labelled material was rapidly detectable in vesicles showing an homogeneous aspect; 15 min later part of the labelled material was still localized within the same kind of vesicles, but in addition it appeared to have migrated into microvesicles of the Golgi region and into vesicles of heterogeneous aspect tentatively identified with lysosomes. Endocytosis of bovine serum albumin, labelled with ferritin followed the same intracellular pathway. Native ferritin accumulated in vesicles of various sizes, but seemed excluded from the microvesicles of the Golgi zone. In the presence of lysosomotropic agents labelled prolactin accumulated in cytoplasmic vesicles. In the presence of dansylcadaverine, endocytosis of the labelled material proceeded unimpaired. Conversely, in the presence of bacitracin, the internalisation of labelled prolactin seemed to be reduced. These observations show that the endocytosis of the hormone/receptor complex is linked to membrane movements, which eventually lead to its location within both the Golgi apparatus and the lysosomes.

Rat prolactin synthesis by lactating mammary epithelial cells.

It has previously been suggested that the mammary cell could produce prolactin (PRL). This hypothesis was investigated by incubation with [35S]methionine-cysteine followed by SDS-PAGE, immunoblotting and autoradiography of immunoprecipitated PRL, and by electron microscopic analysis after incubation without or with cycloheximide. Immunoreactive 14-, 23-, 25-, 32- and 36-kDa PRL forms were radioactive. By two-dimensional electrophoresis analysis, immunoreactive and radioactive spots, of about 25 kDa and high molecular weight, were also detected. After incubation of mammary epithelial cells with cycloheximide, immunogold electron microscopy showed a drastic decrease of labelling in organelles involved in synthesis and secretion, compared to those incubated in control medium. These results make it possible to conclude that lactating mammary tissue is able to synthesize PRL.

Role of glucocorticoids and progesterone in the development of rough endoplasmic reticulum involved in casein biosynthesis.

Hydrocortisone acetate injected into pseudopregnant rabbits induced casein synthesis and a parallel accumulation of casein mRNA. These effects were not accompanied by any enrichment of total RNA in the mammary cell. Hydrocortisone acetate did not favour the attachment of polysomes to endoplasmic reticulum. Casein mRNA concentration was enhanced in free and membrane-bound polysomes. After long treatments, the concentration of casein mRNA reached a plateau in membrane bound polysomes whereas it continued to be accumulated in free polysomes, suggesting that a substantial part of casein synthesis is then carried out by free polysomes. Progesterone injected with high doses of prolactin was unable to prevent the stimulatory action of prolactin on the synthesis of casein, the accumulation of casein mRNA and mammary gland growth, as judged by DNA content. By contrast, the increase in the total RNA content of mammary gland was still significantly reduced by progesterone. In addition, progesterone inhibited almost completely the formation of membrane-bound polysomes and the anchorage of casein mRNA to endoplasmic reticulum. From these data, it was concluded that the formation of the endoplasmic reticulum is not a prerequisite for the initiation of casein synthesis. Glucocorticoids do not play a major role in the formation of the endoplasmic reticulum and the Golai apparatus and in the binding of casein synthesizing polysomes to membranes. Progesteronne is capable of inhibiting preferentially and gradually the stimulation of cellular functions requiring the most potent prolactin stimulation.

[Role of membrane colchicine binding proteins in the transmission of prolactin message to casein genes in the rabbit mammary gland]. / Rôle des protéines membranaires liant la colchicine dans la transmission du message prolactinique aux gènes des caséines dans la glande mammaire de lapine.

Previous work demonstrated that tubulin binding drugs specifically inhibit the capacity of prolactin to initiate casein and DNA synthesis in the mammary cell. It was concluded that microtubules or other tubulin containing cellular structures were involved in the transmission of the prolactin message to genes. In the present work, it is shown that griseofulvin, an antimitotic drug which alters microtubule structure and function, does not prevent prolactin actions. Autoradiographic studies showed that [3H]colchicine binds preferentially to plasma and Golgi membranes in the mammary cell. Short term cultures of mammary explants with [3H]colchicine demonstrated that the labelled drug binds to membranous cellular structures which were isolated from explants at the end of the culture. Fractions containing plasma and Golgi membranes contained the highest amount of radioactivity. Solubilisation of the membranes by Triton X-100 dissociated the [3H]colchicine from the prolactin receptors as judged by a chromatography of the soluble fraction on a Sepharose 6 B column. On the column, the labelled colchicine remains associated with a molecular entity which may be free tubulin. In all cases, the binding of [3H]colchicine was greatly attenuated by an excess of unlabelled colchicine but was only slightly affected by the competition with lumicolchicine. These results suggest that mammary membranes contain tubulin and that binding of drugs to this molecule inhibits the generation of the prolactin second messengers eliciting the hormonal actions in the mammary cell. This also suggests that microtubules are probably not involved in the mechanism of prolactin action.

Endocytosis and intracellular transport of transferrin across the lactating rabbit mammary epithelial cell.

To study the transcytosis and segregation of ligand in the mammary epithelial cell, endocytosis and intracellular transit of human blood transferrin were followed in lactating rabbit mammary epithelial cells. Human transferrin labeled with biotin added to an incubation medium was bound to the basal membrane of mammary epithelial cells and carried across the cell to the lumen of the acini within 5-60 min. At the same time, biotinylated human transferrin accumulated at the apex of the cell. After incubation with human transferrin labeled with colloidal gold, label was detected inside endosome-like structures, vesicles and saccules of the Golgi apparatus, and inside the lumen within 2-5 min. A significant label accumulated at the apex of the cell after 30-60 min. Biotin labeling did not modify the time of transit of human transferrin, as attested by comparison with the time of transit of native transferrin. Human transferrin was never detected inside vesicles containing casein micelles. In contrast, rabbit milk transferrin was immunocytochemically detected inside vesicles containing casein micelles. These results indicate that transcytosis of human transferrin follows a pathway different from vesicles that carry casein micelles.

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.

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.

Casein secretion in mammary tissue: tonic regulation of basal secretion by protein kinase A.

Despite its quantitative importance in the secretion of lactoproteins, little is known about the triggering and control mechanisms that initiate, regulate and terminate the operation of the basal pathway of lactoprotein secretion throughout the lactation cycle. This study investigated the possible modulation by cAMP-mediated mechanisms, of cellular transit of newly-synthesised caseins and their basal secretion in explants of mammary tissue from lactating rats and rabbits. Enhancement of the rate of secretion of newly-synthesised caseins occurs when mammary explants are challenged in vitro with agents that activate protein kinase A (PKA). Inhibition of PKA slows casein secretion. The PKA-sensitive step(s) in casein secretion is early in the exocytosis pathway but inhibition of PKA does not impair casein maturation. Ultrastructural, immunochemical and biochemical methods locate PKA on membranes of vesicles situated in the Golgi region. Exposure of tissue to a cell-permeant PKA inhibitor results in morphological modification of these vesicular structures. We conclude that PKA mediates tonic positive regulation of the basal secretory pathway for lactoproteins in the mammary epithelial cell.

Targeting of PKA in mammary epithelial cells. Mechanisms and functional consequences.

Targeting of protein kinases, promoting association with specific partner-molecules and localisation to particular sites within the cell, has come to be recognised as a key mechanism for attributing specificity to these enzymes. In mammary epithelial cells, the repertoire of acute regulatory roles played by cyclic AMP-dependent protein kinase (PKA) differs from that in other lipogenic cell-types. Furthermore, PKA is implicated in the regulation of mammary-specific function, mediating a tonic stimulation of the flux of newly-synthesised casein through its basal secretory pathway. Both these observations imply mammary-specific properties of either PKA targeting systems or of PKA itself. Evidence for the latter is currently lacking. Pulse-chase labelling experiments in the presence and absence of selective effectors of PKA have enabled the site(s) of action of this protein kinase on casein secretion to be localised to the early stages of the secretory pathway. Possible mechanisms are considered for the physical targeting of PKA to the membrane-enclosed components of the secretory pathway and evidence for their occurrence in mammary epithelial cells is presented.

Mammary gland secretion: hormonal coordination of endocytosis and exocytosis.

The mammary epithelium coordinates the uptake of milk precursors and the transport of milk components in order to produce milk of relatively constant composition at a particular stage of lactation, as long as the mammary gland is healthy. The mammary epithelial cell controls the uptake of blood-borne molecules at its basal side and the release of products into milk at its apical side, through mechanisms of internalization (endocytosis) and mechanisms of release (exocytosis). These events are strictly dependent on the physiological stage of the mammary gland. This review addresses the mechanisms responsible for these processes and points out new questions that remain to be answered concerning possible interconnections between them, for an optimal milk secretion.

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.

[In vitro effect of oxytocin on intracellular transit and secretion of milk proteins]. / Effet de l'ocytocine in vitro sur le transit intracellulaire et la sécrétion des protéines du lait

Mammary tissue slices are pulse-labelled for 3 mn in 3H leucine, then incubated in a chase-medium. When oxytocin (10(-6) UI/ml) is added 20 mn after the beginning of the pulse and for 5 mn, intracellular transit of radioactive milk proteins is accelerated and discharge in the incubation medium is increased. Hence it appears that oxytocin can stimulate secretion of milk proteins at the cellular level.

[Role of Ca2+ in the secretion of milk caseins in lactating rabbit mammary epithelial cells]. / Role du Ca2+ dans la sécrétion des caséines du lait par la cellule épithéliale mammaire de lapine en lactation.

Prolactin and arachidonic acid increase milk casein secretion in mammary gland slices. These effects do not necessitate Ca2+ in the incubation medium. Prolactin does not modify the influx or the efflux of 45Ca2+. The Ca2+ channel blocking agent D600 (6 micrograms/ml) decreases the stimulatory effect of prolactin on casein secretion, but does not interfere in the stimulatory effect of arachidonic acid. The calmodulin inhibitor trifluoperazine (100 microM) inhibits stimulation of casein secretion by both prolactin and arachidonic acid. From these data, it is concluded that a flow of Ca2+ from the outside into the cell is not a requisite for the stimulation of casein secretion. However, stimulation by prolactin, but not stimulation by arachidonic acid, requires Ca2+ movement through calcium pathways. Intracellular transport of Ca2+ seems necessary for the stimulation of secretion.

Early effects of prolactin on lactating rabbit mammary gland. Ultrastructural changes and stimulation of casein secretion.

Effects of prolactin on the secretion of milk proteins have been investigated by incubating mammary tissue fragments from lactating rabbits. Within 15 min of adding the hormone to the incubation medium, cell morphology is modified: the relative volume occupied by the Golgi region is greatly increased. When prolactin is added immediately after a pulse labelling of proteins (3 min with 3H-L-leucine), the amount of labelled caseins secreted during one hour is significantly increased. This increase proceeds neither from an acceleration of intracellular transit of caseins (as shown by electron microscopic autoradiography) nor by an enhancement of amino acid uptake (as measured by incorporation of non-metabolizable amino acids) nor by an increase of overall protein synthesis, during the first hour. The action of prolactin on the morphology of such subcellular organelles as the Golgi apparatus and its influence on casein secretion are discussed.

[Effect of arachidonic acid on the secretion of milk caseins in vitro]. / Effet de l'acide arachidonique sur la sécrétion des caséines du lait in vitro.

Arachidonic acid increases milk casein secretion in mammary gland slices. This stimulation is similar to prolactin stimulation. Aspirin, a prostaglandin-synthetase inhibitor, has no effect on prolactin or on arachidonic acid stimulation. Prostaglandin PGF2 alpha does not modify casein secretion. Therefore arachidonic acid stimulates casein secretion by metabolic products different from prostaglandins.