Phospholipase D-dependent and -independent mechanisms are involved in milk protein secretion in rabbit mammary epithelial cells

Phospholipase D has been implicated in membrane traffic in the secretory pathway of yeast and of some mammalian cell lines. Here we investigated the involvement of phospholipase D in protein transport at various steps of the secretory pathway of mammary epithelial cells. Treatment of rabbit mammary explants with butanol, which blocks the formation of phosphatidic acid, decreased the secretion of caseins and to a lesser extent that of whey acidic protein. Butanol interfered with both the endoplasmic reticulum to Golgi complex transport of the caseins and secretory vesicle formation from the trans-Golgi network. In contrast, the transport of whey acidic protein to the Golgi was less affected. Activation of protein kinase C enhanced the overall secretion of both markers and interestingly, this stimulation of secretion was maintained for whey acidic protein in the presence of butanol. Transphosphatidylation assays demonstrated the existence of a constitutive phospholipase D activity which was stimulated by the activation of protein kinase C. We conclude that phospholipase D plays a role in casein transport from the endoplasmic reticulum to the Golgi and in the secretory vesicle formation from the trans-Golgi network. Moreover, our results suggest a differential requirement for phospholipase D in the secretion of caseins and that of whey acidic protein.

Polymeric IgA are sulfated proteins

The main sulfated proteins secreted by rabbit mammary gland tissue had M(r) of approximately 67 000, 63 000 and 23 000, and one component which most likely corresponded to proteoglycans had a diffuse electrophoretic mobility (M(r)200 000). The sulfate groups in the 67-63 kDa proteins were mostly linked to carbohydrates. These proteins and the 23 kDa protein were co-purified and identified to heavy chains of immunoglobulin A (IgA) and J chain, respectively. Sulfation of alpha-chains also occurred in rat mammary and rabbit lacrimal glands. We conclude that polymeric IgA which are produced by plasma cells and released in secretion fluids after transcytosis through epithelia are sulfated.

Alpha(S1)-casein is required for the efficient transport of beta- and kappa-casein from the endoplasmic reticulum to the Golgi apparatus of mammary epithelial cells

In lactating mammary epithelial cells, interaction between caseins is believed to occur after their transport out of the endoplasmic reticulum. We show here that, in alpha(S1)-casein-deficient goats, the rate of transport of the other caseins to the Golgi apparatus is highly reduced whereas secretion of whey proteins is not significantly affected. This leads to accumulation of immature caseins in distended rough endoplasmic reticulum cisternae. Casein micelles, nevertheless, were still observed in secretory vesicles. In contrast, no accumulation was found in mammary epithelial cells which lack beta-casein. In mammary epithelial cells secreting an intermediate amount of alpha(S1)-casein, less casein accumulated in the rough endoplasmic reticulum, and the transport of alpha(S1)-casein to the Golgi occurred with kinetics similar to that of control cells. In prolactin-treated mouse mammary epithelial HC11 cells, which do not express alpha(S)-caseins, endoplasmic reticulum accumulation of beta-casein was also observed. The amount of several endoplasmic reticulum-resident proteins increased in conjunction with casein accumulation. Finally, the permeabilization of rough endoplasmic reticulum vesicles allowed the recovery of the accumulated caseins in soluble form. We conclude that optimal export of the caseins out of the endoplasmic reticulum is dependent upon alpha(S1)-casein. Our data suggest that alpha(S1)-casein interacts with the other caseins in the rough endoplasmic reticulum and that the formation of this complex is required for their efficient export to the Golgi.

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.

The disulfide bond in chromogranin B, which is essential for its sorting to secretory granules, is not required for its aggregation in the trans-Golgi network

Chromogranin B (secretogranin I), a protein sorted to secretory granules in many endocrine cells and neurons, undergoes selective aggregation during the sorting process in the trans-Golgi network. Reduction of the single, highly conserved intramolecular disulfide bond of chromogranin B by exposure of intact PC12 cells to the thiol reducing agent dithiothreitol has previously been shown to cause its missorting to the constitutive pathway of secretion. Using saponin perforation of membrane vesicles in aggregative buffer mimicking the milieu in the lumen of the trans-Golgi network (pH 6.4, 10 mM calcium), we show here that treatment with dithiothreitol does not prevent the aggregation of chromogranin B in this compartment. This implies that the loop in the chromogranin B polypeptide that is formed by the disulfide bond has a critical role in the membrane recognition of aggregated chromogranin B during secretory granule formation.

Reduction of the disulfide bond of chromogranin B (secretogranin I) in the trans-Golgi network causes its missorting to the constitutive secretory pathways

The role of the single, highly conserved disulfide bond in chromogranin B (secretogranin I) on the sorting of this regulated secretory protein to secretory granules was investigated in the neuroendocrine cell line PC12. Treatment of PC12 cells with dithiothreitol (DTT), a membrane permeable thiol reducing agent known to prevent disulfide bond formation in intact cells, resulted in the secretion of newly synthesized chromogranin B, but only slightly decreased the intracellular storage of newly synthesized secretogranin II, a regulated secretory protein devoid of cysteines. The secretion of newly synthesized chromogranin B in the presence of DTT occurred with similar kinetics to those of a heparan sulfate proteoglycan, a known marker of the constitutive secretory pathway in PC12 cells. Analysis of the various secretory vesicles derived from the trans-Golgi network (TGN) indicated that DTT treatment diverted newly synthesized chromogranin B to constitutive secretory vesicles, whereas the packaging of secretogranin II into immature secretory granules was unaffected by the reducing agent. The chromogranin B molecules diverted to constitutive secretory vesicles, in contrast to those stored in secretory granules, were found to contain free sulfhydryl residues. The effect of DTT on chromogranin B occurred in the TGN rather than in the endoplasmic reticulum. We conclude that the sorting of CgB in the TGN to secretory granules is dependent upon the integrity of its single disulfide bond.

Milieu-induced, selective aggregation of regulated secretory proteins in the trans-Golgi network

Regulated secretory proteins are thought to be sorted in the trans-Golgi network (TGN) via selective aggregation. The factors responsible for this aggregation are unknown. We show here that two widespread regulated secretory proteins, chromogranin B and secretogranin II (granins), remain in an aggregated state when TGN vesicles from neuroendocrine cells (PC12) are permeabilized at pH 6.4 in 1-10 mM calcium, conditions believed to exist in this compartment. Permeabilization of immature secretory granules under these conditions allowed the recovery of electron dense cores. The granin aggregates in the TGN largely excluded glycosaminoglycan chains which served as constitutively secreted bulk flow markers. The low pH, high calcium milieu was sufficient to induce granin aggregation in the RER. In the TGN of pituitary GH4C1 cells, the proportion of granins conserved as aggregates was higher upon hormonal treatment known to increase secretory granule formation. Our data suggest that a decrease in pH and an increase in calcium are sufficient to trigger the selective aggregation of the granins in the TGN, segregating them from constitutive secretory proteins.

What the granins tell us about the formation of secretory granules in neuroendocrine cells

The biochemical properties of the granins, studied in vitro and in a perforated TGN system, support the concept that the Sciective aggregation of regulated secretory proteins, promoted by the specific lumenal milieu of the TGN, is a key step in their segregation from constitutive secretory proteins in this compartment. A recently identified membrane-associated form of the granins is likely to also be involved in this aggregation, as well as in the membrane envelopment of the aggregate during the formation of an immature secretory granule.

Peptides co-released with luteinizing hormone by perifused pituitary cell aggregates

The proteins co-released with gonadotropins were analyzed using perifusion of pituitary cell aggregates from 14-day-old female rats, after a pre-labeling period with [35S]methionine. Radioimmunoassays of hormones and electrophoretic analysis were performed on each 4 min effluent. Gonadotropin-releasing hormone (GnRH) pulses increased significantly (P less than 0.01) the release of several proteins (Mr range from 140,000 to 28,000). The main stimulation appeared for -1, a 87 kDa species, previously characterized as gonadotrope polypeptide 87 (GP87) in monolayer cultures and identified as a secretogranin II (SgII) form; -2, a second species of 80 kDa designated as B2. Secretory patterns of radiolabeled GP87 and B2 paralleled the luteinizing hormone (LH) ones. The release of these species was -1, GnRH dose dependent; -2, monophasic for short pulses but complex when the duration of GnRH pulses increased to 16 min, suggesting different pools of GP87 and B2 as for LH; -3, induced by thyrotropin-releasing hormone (TRH). A slight output was also elicited by corticotropin-releasing factor (CRF) and growth hormone-releasing hormone (GHRH), but this release was partly impaired in the presence of a potent anti-GnRH ([Ac-D-(2)-NAL1,pF-D-Phe2,D-Trp3,D-Arg6]-LAF) suggesting a non-specific effect of these two factors. GP87/SgII thus appeared mainly associated with the release of hormonal glycoproteins. In conclusion, perifusion of pituitary cell aggregates allows a precise minute-to-minute kinetic analysis of the various proteins co-released with hormones. The similar timing in output of LH, GP87 and B2 suggests that these three proteins co-exist in the same secretory granules inside gonadotropes.

The gonadotrope polypeptide (GP 87) released from pituitary cells under luteinizing hormone-releasing hormone stimulation is a secretogranin II form

Cultured gonadotrope cells from 14 day old female rat pituitaries have been shown to release a highly acidic protein when incubated with LHRH: the gonadotrope polypeptide (GP 87). Moreover, a new tyrosine-sulfated acidic protein, secretogranin II (Sg II), clearly distinct from the chromogranin species, was described in the secretory granule matrix of endocrine cells secreting peptide hormones by the regulated pathway. Recently, the release of Sg II from female rat pituitary stimulated by LHRH was demonstrated in vitro. Several physicochemical (Mr; pI) and biological (cellular localization in the pituitary; LHRH-stimulated release) properties are common to Sg II and GP 87. To verify if these 2 polypeptides are similar or distinct components, other physicochemical characteristics (heat-stability, sulfation, phosphorylation) were compared using isotope incorporation followed by either 1- or 2-dimensional polyacrylamide gel electrophoresis and autoradiography. Furthermore, the similarity of GP 87 to Sg II was studied by immunoblotting on nitrocellulose sheets following electrophoresis of intracellular and secreted proteins. Antisera raised against bovine Sg II (extracted from whole pituitaries) and against rat GP 87 (released into the medium of cultured pituitary cells stimulated by LHRH) were used. The overall data presented here suggest that GP 87 is the Sg II form contained in, and released by, gonadotrope cells.

LHRH-stimulated release of stored and newly synthesized gonadotrop polypeptide (GP-87) by cultured rat gonadotrophs

Enriched gonadotrophs from rat pituitaries were used to analyze the kinetics of release in vitro of the Gonadotrop Polypeptide (GP-87) under LHRH stimulation. Proteins in cultured cells were labeled with [35S]methionine. Labeling of the intracellular GP-87 pool was effected during 16 hours prior to LHRH (10(-7) M) stimulation. Proteins were analyzed either by one-dimensional SDS-PAGE (Medium content) or by two-dimensional PAGE (Cell content). An apparent half-life (intracellular catabolism + release) of 31 h for GP-87 was estimated from control cells; it dropped to 2.5 h in stimulated cells due to intense release (26% after 1 h and about 80% after 8 h of stimulation). When cells were simultaneously labeled and stimulated, the newly synthesized species appeared in the medium after a lag phase of 30 minutes, time required for synthesis and full subsequent processing. From both series of experiments, it is concluded that the hypothalamic decapeptide promotes exocytosis of the newly synthesized GP-87 well before the endogenous GP-87 pool is exhausted. Furthermore, the release of another discrete protein (B2, Mr: 81 kDa) is also stimulated by LHRH. These proteins being co-released with LH could be part of the sorting and/or routing process of hormones towards exocytosis.

LHRH promotes the synthesis and release of a 87,000 Da protein (GP-87) by enriched gonadotrophs

Protein secretion by cultured pituitary cells from 14-day-old female rats was estimated using [35S]methionine incorporation followed by either one- or two-dimensional electrophoresis and autoradiography. Stimulation of total cells or gonadotrophs by LHRH promoted the synthesis and release of a specific polypeptide (apparent molecular weight 87,000, pI = 4.6). Silver staining of cellular proteins from both gonadotroph-enriched and gonadotroph-depleted populations prepared by centrifugal elutriation revealed a high concentration of this polypeptide in the gonadotrophs and a very low level in the other cell population. This species was thus called Gonadotrope Polypeptide GP-87. Release of labeled GP-87 by gonadotrophs was both time dependent (up to 4 h) and LHRH dose dependent (from 10(-9) M to 10(-7) M) as was the release of LH. Attempts to precipitate GP-87 from the incubation medium with anti-LH antiserum were unsuccessful suggesting that GP-87 is not a 'big' form of LH. TRH neither stimulated the release of GP-87 from gonadotrophs nor from lactotrophs though it did stimulated PRL release. From these data, we conclude that gonadotrophs in culture synthesize a specific polypeptide (GP-87), LHRH stimulates both the synthesis and release of GP-87, and the pituitary cell response is peptide specific. The LHRH-induced synthesis and release of GP-87 could be an important step in the molecular processes that regulate gonadotrophin secretion.