Effects of the monovalent ionophore monensin on the intracellular transport and processing of pro-opiomelanocortin in cultured intermediate lobe cells of the rat pituitary

Homophilic ATP1A1 binding induces activin A secretion to promote EMT of tumor cells and myofibroblast activation

Tumor cells with diverse phenotypes and biological behaviors are influenced by stromal cells through secretory factors or direct cell-cell contact. Pancreatic ductal adenocarcinoma (PDAC) is characterized by extensive desmoplasia with fibroblasts as the major cell type. In the present study, we observe enrichment of myofibroblasts in a juxta-tumoral position with tumor cells undergoing epithelial-mesenchymal transition (EMT) that facilitates invasion and correlates with a worse clinical prognosis in PDAC patients. Direct cell-cell contacts forming heterocellular aggregates between fibroblasts and tumor cells are detected in primary pancreatic tumors and circulating tumor microemboli (CTM). Mechanistically, ATP1A1 overexpressed in tumor cells binds to and reorganizes ATP1A1 of fibroblasts that induces calcium oscillations, NF-κB activation, and activin A secretion. Silencing ATP1A1 expression or neutralizing activin A secretion suppress tumor invasion and colonization. Taken together, these results elucidate the direct interplay between tumor cells and bound fibroblasts in PDAC progression, thereby providing potential therapeutic opportunities for inhibiting metastasis by interfering with these cell-cell interactions.

Drug Library Screening for the Identification of Ionophores That Correct the Mistrafficking Disorder Associated with Oxalosis Kidney Disease

Primary hyperoxaluria is the underlying cause of oxalosis and is a life-threatening autosomal recessive disease, for which treatment may require dialysis or dual liver-kidney transplantation. The most common primary hyperoxaluria type 1 (PH1) is caused by genetic mutations of a liver-specific enzyme alanine:glyoxylate aminotransferase (AGT), which results in the misrouting of AGT from the peroxisomes to the mitochondria. Pharmacoperones are small molecules with the ability to modify misfolded proteins and route them correctly within the cells, which may present an effective strategy to treat AGT misrouting in PH1 disorders. We miniaturized a cell-based high-content assay into 1536-well plate format and screened ~4200 pharmacologically relevant compounds including Food and Drug Administration, European Union, and Japanese-approved drugs. This assay employs CHO cells stably expressing AGT-170, a mutant that predominantly resides in the mitochondria, where we monitor for its relocation to the peroxisomes through automated image acquisition and analysis. The miniaturized 1536-well assay yielded a Z' averaging 0.70 ± 0.07. Three drugs were identified as potential pharmacoperones from this pilot screen, demonstrating the applicability of this assay for large-scale high-throughput screening.

Effect of monensin on secretory granules and basal beta-endorphin secretion in the melanotroph of the rat pituitary

The effect of monensin on the Golgi complex, formation of secretory granules and basal beta-endorphin secretion in cultured melanotrophs from the rat pituitary was studied. Earlier studies on the effect of monensin on regulated secretion have generally showed only minor effects on secretory granules. The initial (within 5 min) effect of monensin on the melanotroph was the appearance of large vacuoles at the trans-side of the thiamine pyrophosphatase-positive trans-most Golgi cisternae. This was associated with a dose-dependent inhibition of the condensation of electron-dense secretory products. After 1 h of treatment with 1 microM monensin the Golgi stack was completely vacuolized. At the same time mature secretory granules were enlarged to severalfold their original size, and after 4 h of treatment secretory granules were no longer observed. Despite the marked effects on granule formation and mature secretory granules monensin did not affect the basal release of beta-endorphin-immunoreactive material during continued incubation for up to 4 h, indicating that basal peptide secretion can bypass the monensin block.

Secretogranin III is a sulfated protein undergoing proteolytic processing in the regulated secretory pathway

Secretogranin III (SgIII) is an acidic protein of unknown function that is present in the storage vesicles of many neuroendocrine cells. It is coexpressed with the prohormone proopiomelanocortin in the intermediate pituitary of Xenopus laevis. We developed an antiserum to investigate the biosynthesis of SgIII in pulse-chase incubated Xenopus neurointermediate lobes. SgIII was synthesized as a 61- or 63-kDa (N-glycosylated) protein and processed to a 48-kDa form which, in turn, was partially cleaved to fragments of 28 and 20 kDa. The 48-, 28-, and 20-kDa cleavage products, but not their precursors, were secreted. This secretion is regulated and can be blocked in parallel with that of proopiomelanocortin-derived peptides by the hypothalamic factors dopamine, gamma-aminobutyric acid, and neuropeptide Y. Coexpression of Xenopus SgIII with prohormone convertase (PC)1 or PC2 in transfected fibroblasts was sufficient to reconstitute the processing events observed in the neurointermediate lobes. Site-directed mutagenesis revealed that Xenopus SgIII is cleaved at two dibasic sites, namely Lys68-Arg69 and Arg237-Arg238. Pulse-chase incubations of lobes with Na2[35S]SO4 showed that SgIII is sulfated in the trans-Golgi network before it is processed. Finally, SgIII processing was found in several neuroendocrine cell types from various species. We conclude that SgIII is a precursor protein and that the intact molecule can only have an intracellular function, whereas an extracellular role can only be attributed to its cleavage products.

Effect of some potassium selective crown ethers on the permeability and structure of a phospholipid membrane

The effect of some new crown ethers on the cation efflux and phase transition parameters of dipalmitoyl phosphatidylcholine liposomes was studied. The effects were correlated with the lipophilicity of the crown ethers. The results indicate that the presence of two crown ring structures in one crown either molecule is a prerequisite for the increase of ion permeability of liposomes. The effective crown ethers decrease the temperature, enthalpy and cooperativity of the gel-to-liquid crystalline phase transition. The crown ethers increase membrane permeability for potassium and, to a lesser extent, for rubidium and sodium. The ratio of permeability increase for potassium/rubidium significantly correlates with the lipophilicity of the crown ethers.

Monensin influences basal and human growth hormone-releasing hormone 44-induced release of stored and new rat growth hormone and prolactin

When previous data suggested a growth hormone-releasing factor (GRF)-sensitive branch in intracellular hormone processing, the monensin-sensitive Golgi apparatus seemed a likely candidate. We examined monensin's effect on basal and GRF-stimulated release of newly synthesized and stored rat growth hormone (rGH) and rat prolactin (rPRL). 14C-Pre-labeled, perifused rat pituitary fragments were exposed to [3H]leucine in 0-10 microM monensin; a pulse of 3 nM GRF assessed subsequent secretory responsivity. Monensin dose-dependently reduced basal release of stored [14C]rGH and [14C]rPRL. GRF-stimulated release of stored [14C]hormone was doubled after 0.03 microM and 0.1 microM monensin; higher concentrations diminished stored hormone release. Low concentrations of monensin accelerated basal (0.03 microM and 0.1 microM) and GRF-stimulated (0.03 microM) [3H]rGH and [3H]rPRL release without altering recovery; higher monensin concentrations (greater than or equal to 1 microM) reduced basal, and abolished GRF-stimulated, new hormone release and reduced total [3H]rGH and [3H]rPRL recovery. These data are consistent with a GRF-sensitive and monensin-influenced branch in intracellular hormone processing that regulates the fraction of new hormone exiting the cell without prior immersion in storage compartments.

Monensin inhibition of corticotropin releasing factor mediated ACTH release

Monensin is a sodium selective carboxylic ionophore that has been helpful in studying the intracellular mechanisms of protein secretion by its ability to inhibit transport of secretory proteins, particularly through the Golgi apparatus, and by its capacity to block intracellular posttranslational processing events. We studied in rat anterior pituitary cell culture the effects of monensin on: CRF stimulated ACTH release; presynthesized (stored) ACTH release; and on forskolin- (activator of adenylate cyclase) and KCl- (a membrane depolarizer which does not stimulate ACTH synthesis) induced ACTH release. Monensin inhibited CRF stimulated ACTH release in a dose-dependent fashion. The ED50 was 2.7 x 10(-8) M and maximal inhibition was 52% at 1.5 x 10(-7) M. Inhibition at 40 minutes of CRF incubation was similar to the percent inhibition noted at 1 hr 40 min and 2 hr 40 min. Monensin (1.5 x 10(-6) M) decreased the amount of ACTH release from cells incubated with cycloheximide plus CRF by 32% (p less than 0.01). Monensin individually inhibited forskolin (2 x 10(-6) M) and dibutyryl cyclic AMP (3 x 10(-3) M) mediated ACTH release in a dose-dependent fashion. The inhibition of forskolin and dibutyryl cyclic AMP mediated ACTH release by 1.5 x 10(-6) M monensin was 48% and 46% respectively. Monensin (1.5 x 10(-6) M) also reduced KCl (50 mM) stimulated ACTH release by 48%. This study demonstrates that monensin inhibits CRF mediated ACTH release.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations of the posttranslational processing of a lysosomal enzyme in C6 glioma cells

Cathepsin D was assessed in C6 glioma cells grown in medium with an intermediate- or low-percent composition of serum. The amount, form, and subcellular location of cathepsin D differed after treatment with cyanate or monensin in cells grown in a low-serum, growth-factor-supplemented medium. Immunoblotting showed that cathepsin D in the lysosomal fraction of the C6 cell line had a molecular weight (Mr) of 42 kD, whereas that in the microsomal fraction had Mr's of 42, 47, and 78 kD. After treatment for 1 to 16 hr with 4 mmol/L cyanate and subcellular fractionation, the molecular weight of lysosomal cathepsin D was the same in treated and untreated cells, but more enzyme was found in lysosomes of treated cells at 8 and 16 hr. In the microsomal fraction, the amounts of both the 42 and 47 kD forms were increased after 1 to 16 hr of treatment. When exposed to 20 mmol/L cyanate, C6 cells remained viable, but compared with untreated cells, they showed 25% less lysosomal cathepsin D, with increased amounts found in the microsomal fraction. The 78 kD protein detected by immunoblotting was present in both the lysosomal and microsomal fractions but was predominant in the latter. The apparent molecular weight of this protein was the same after cyanate but differed with monensin, where Mr's of 39, 42, and 73 kD were found. Monensin-treated cells had less lysosomal cathepsin D and relatively more microsomal enzyme. The differing molecular weights of cathepsin D from cyanate- and monensin-treated cells suggest that their inhibitions occur at different processing loci in distal elements of the Golgi stacks. The differences in the pI of cathepsin D and the number of its forms from cyanate- and monensin-treated cells are also consistent with interference in the late stages of glycoprotein maturation. In this paper we show that the amount, molecular form, and consequent intracellular location of cathepsin D in cells of the C6 line can be affected by agents that selectively disrupt stages in Golgi-related protein modification and transport.

Intracellular processing of growth hormone receptors by adipocytes in primary culture

Rat adipocytes in primary culture have been used to study the intracellular processing of growth hormone (GH) receptors. Pretreatment of adipocytes with 20 micrograms/ml cycloheximide resulted in a rapid decline (t1/2 approximately 45 min) of the 125I-human growth hormone (hGH) binding capacity of the cells. This decline occurred at a faster rate in the presence of extracellular unlabeled hGH (400 ng/ml) and was not due to receptor occupancy. These data suggest that GH receptors turn over rapidly and constitutively on the plasma membrane and in the absence of protein synthesis are not replaced. Dissociation of GH-receptor complexes was shown not to occur at pH 5.5, the pH encountered in the acidic pre-lysosomal compartments (endosomes) where intracellular dissociation of many hormone-receptor complexes takes place. These data, together, suggest that the majority of GH receptors are not recycled but instead suffer the same fate as the majority of GH, i.e. degradation. To determine the rate of appearance of GH receptors at the cell surface, adipocytes were first treated with trypsin and then incubated at 37 degrees C to permit incorporation of any available GH receptors into the plasma membrane. Binding of 125I-hGH recovered to pre-trypsin levels by 2 h. This recovery was completely blocked by concomitant treatment with monensin, cytochalasin B, colchicine and 2,4-dinitrophenol. NH4Cl had no effect on receptor recovery. These data suggest that once GH receptors are synthesized in the rough endoplasmic reticulum, they travel via the Golgi apparatus to the plasma membrane (by processes involving both microfilaments and microtubules) and are then inserted into the plasma membrane in an energy-dependent step.

Effects of temperature, pH elevators, and energy production inhibitors on horseradish peroxidase transport through endocytic vesicles

We investigated the effects of reduced temperature, the pH elevators NH4Cl, monensin, and HEPES (N-2-hydroxy-ethylpiperazine-N'-2-ethanesulfonic acid) buffer, as well as the metabolic poisons NaF/KCN on transport of the fluid phase pinocytic marker, horseradish peroxidase (HRP), to lysosomes in Chinese hamster ovary (CHO) cells. In cell fractionation experiments, these agents appeared to block HRP transit at specific point(s) from "early" to "late" (i.e., low to high density) prelysosomal vesicles and lysosomes. Reduced temperature (17 degrees C) most strongly inhibited HRP transport from low density, early endosomes to lysosomes. In long-term HRP uptakes at 17 degrees C, marked peroxidase accumulation occurred both in early endosomes and in lysosomes. Loss (reversible pinocytosis) of HRP from "very early" endosomes occurred at 17 degrees C. All three pH elevators including the common media supplement HEPES buffer inhibited transit of internalized HRP into lysosomes. For all three pH elevators, inhibition was most pronounced at the "early" endosome stage. The respiratory inhibitors NaF/KCN also inhibited transport most strongly at the early endosome stage. Together these results suggest that "early" steps in the endocytic transport of HRP are the most sensitive and that the conditions tested may exert direct effects on the processing of endocytic vesicles.

Biphasic effect of thyrotropin-releasing factor (TRH) on alpha-melanotropin secretion from frog intermediate lobe in vitro

The kinetics of alpha-MSH secretion induced by prolonged TRH infusion were studied using perfused frog neurointermediate lobe (NIL). During a 2 h administration of TRH (10(-8) M), the secretion rate of alpha-MSH displayed two phases. During the first phase, secretion of alpha-MSH increased rapidly reaching a maximum within 20 min and then, despite continued TRH infusion, this secretion slowly declined. The second phase was characterized as plateau of elevated release (relative to basal secretion); within this second phase there was often a small peak of released alpha-MSH occurring at about 100 min. Exposure of NIL to another TRH (10(-8) M) pulse 90 min later induced a normal stimulation of alpha-MSH secretion, thus demonstrating the viability of tissue in perifusion. Continuous infusion of cycloheximide (10(-5) M) during a 5 h period totally inhibited the biosynthetic activity of NIL but did not influence TRH-induced alpha-MSH secretion. In particular, cycloheximide had no effect on the second phase of the response to prolonged infusion of TRH. Similarly, during continuous infusion of the monovalent carboxylic ionophore monensin (10(-6) M), the biphasic response to prolonged infusion of TRH (10(-8) M) was still observed. Administration of a short pulse of TRH (10(-7) M) during the declining part of the first phase or during the second phase of prolonged TRH (10(-8) M) infusion induced a significant enhancement of alpha-MSH stimulation. From these results we conclude that prolonged TRH infusion causes alpha-MSH release in a biphasic manner; attenuation of the secretory response to continuous TRH administration does not result from exhaustion of the releasable pool of alpha-MSH.(ABSTRACT TRUNCATED AT 250 WORDS)

Different transforming growth factor-alpha species are derived from a glycosylated and palmitoylated transmembrane precursor

cDNA analysis has revealed that the 50 amino acid transforming growth factor-alpha (TGF-alpha) is derived from a 160 amino acid precursor. Antibodies to TGF-alpha and to a C-terminal portion of the precursor were used to study the biosynthesis and processing of the precursor. CHO cells transfected with a TGF-alpha expression vector secrete high levels of TGF-alpha; a mixture of species of about 18 kd is secreted in addition to the 50 amino acid form. These larger species are N-glycosylated and are derived from the same precursor as the smaller form. The C-terminal segment of the precursor remains anchored in the membrane and has covalently attached palmitate. The newly synthesized TGF-alpha precursor is thus a transmembrane protein that subsequently undergoes external proteolytic cleavages, releasing several TGF-alpha species.

Characterization of sulfated forms of the pro-opiomelanocortin amino-terminal glycopeptide in rat intermediate lobe cells

Explants of rat neurointermediate lobes were incubated in the presence of radioactive amino acids, sugars or sulfate and the labeled proteins were separated by two-dimensional gel electrophoresis. A double series of acidic peptides (Mr = 16,000-21,500) were identified as variant forms of the amino-terminal glycopeptide of pro-opiomelanocortin (N-POMC). The series of peptides with the higher molecular weights (Mr = 18,000-21,500) contain a tryptic fragment (tentatively identified as the tryptic peptide of the "joining peptide": sequence 77 to 93 of rat POMC) which is absent from the forms of the lower molecular weight series (Mr = 16,000 to 18,000). Pulse-chase studies further showed that the high molecular weight forms of N-POMC could be post-translationally cleaved albeit slowly into the species of Mr = 16,000-18,000 which constitute, at least in part, the final maturation products of the N-terminal region of the precursor molecule. All the variant forms of the N-POMC glycopeptide could be labeled with [35S]sulfate. Our results strongly suggest that most of the sulfate groups are attached to N-linked oligosaccharide side chains of N-POMC. We therefore propose that one of the final maturation products of the N-terminal portion of POMC in rat intermediate lobes is a sulfated glycopeptide (Mr = 16,000-18,000) composed of the 1-74 sequence of rat POMC.

Maturation and secretion of lipoprotein lipase in cultured adipose cells. II. Effects of tunicamycin on activation and secretion of the enzyme

The effects of N-linked glycosylation on the activation and secretion of lipoprotein lipase were studied in Ob17 cells. The cells were first depleted of any activity and enzyme content by cycloheximide treatment and of precursors of oligosaccharide chains by tunicamycin. The repletion of lipoprotein lipase content was studied in these cells maintained in the presence of tunicamycin after cycloheximide removal. During the repletion phase, the EC50 values of inhibition by tunicamycin (approx. 0.2 microgram/ml) of the incorporation of labeled glucose, mannose or galactose into trichloroacetic acid-insoluble material were found to be identical. Under these conditions, the rate of protein synthesis was maximally decreased by 30%. The results showed clearly that the recovery in lipoprotein lipase activity was parallel to the recovery in hexose incorporation, no activity being recovered in the absence of glycosylation. An inactive form of lipoprotein lipase from tunicamycin-treated cells was detected by competition experiments with mature active lipoprotein lipase for the binding to immobilized antilipoprotein lipase antibodies, as well as by immunofluorescence staining. SDS-polyacrylamide gel electrophoresis and Western blots of cellular extracts and of extracellular media, obtained after tunicamycin-treated cells were exposed to heparin, revealed a single immunodetectable Mr 52 000 protein, whereas a single Mr 57 000 protein was detected in control cells. Therefore, the results indicate that the acquisition by lipoprotein lipase of a catalytically active conformation is linked directly or indirectly to glycosylation. Despite this lack of activation, the lipoprotein lipase molecule was able to migrate intracellularily and to undergo secretion after heparin stimulation of the tunicamycin-treated cells.

Human transforming growth factor-beta complementary DNA sequence and expression in normal and transformed cells

The partial amino-acid sequence of purified human transforming growth factor-beta (TGF-beta) was used to identify a series of cDNA clones encoding the protein. The cDNA sequence indicates that the 112-amino acid monomeric form of the natural TGF-beta homodimer is derived proteolytically from a much longer precursor polypeptide which may be secreted. TGF-beta messenger RNA is synthesized in various normal and transformed cells.