2-Chloroadenosine decreases tyrosylprotein sulfotransferase activity in the Golgi apparatus in PC12 cells. Evidence for a novel receptor

Exogenous cyclic AMP, cholera toxin, and endotoxin induce expression of the lipopolysaccharide receptor CD14 in murine bone marrow cells: role of purinoreceptors

Little is known about the mechanisms of lipopolysaccharide (LPS) signaling in immature cells that do not express the LPS receptor CD14 yet. Bone marrow granulocytes do not constitutively express CD14 but can be stimulated by low doses of LPS in the absence of serum and then express an inducible form of LPS receptor (iLpsR). We show that in addition to LPS, cholera toxin (CT) and various cyclic AMP (cAMP) analogs can also induce the expression of iLpsR, which was identified as CD14. Induction was independent of intracellular cAMP. The hypothesis that cAMP analogs act via a cell surface receptor was suggested by the unresponsiveness of trypsin-treated cells to these inducers and by the specific binding of [(3)H]cAMP to the cells. This binding was not inhibited by LPS or CT but was inhibited by various purine derivatives. However, the receptor involved is not a conventional purinoreceptor since both an agonist and an antagonist of such receptors were able to induce iLpsR expression. The results suggest that cAMP analogs and other purine derivatives induce iLpsR after interaction with an unconventional, trypsin-sensitive, purinoreceptor distinct from LPS and CT receptors.

Vitellogenin is glycosylated in the fat body of the stick insect Carausius morosus and not further modified upon transfer to the ovarian follicle

Synthesis and secretion of vitellogenin (Vg) polypeptides were studied in egg-laying females of the stick insect Carausius morosus following in vivo exposure to [35S]-methionine and acetyl-N-[3H]-glucosamine. The specificity of radioisotope incorporation was assessed by in vitro inhibition with tunicamycin and carbohydrate extraction with endo-glycosidase H. Vg polypeptides change in molecular weight during synthesis in the fat body and are not further modified upon transfer to the haemolymph or to the oocyte, suggesting that they are already fully glycosylated prior to secretion. Radioactivity in the fat body was initially distributed over cisternae of the rough endoplasmic reticulum and gradually transferred to the Golgi apparatus. Within an hour of exposure, electron-dense granules budding from the trans-Golgi network became preferentially labeled. Radioactivity in the ovarian follicle was restricted to the yolk granules of the cortical ooplasm and to the amorphous material lying within the intercellular channels of the follicular epithelium. This amorphous material was also shown to react positively when tested with a monoclonal antibody raised specifically against a Vg polypeptide.

2-chloroadenosine stimulates granule exocytosis from mouse natural killer cells: evidence for signal transduction through a novel extracellular receptor

The effect of 2-chloroadenosine (2CA), an adenosine receptor agonist, on the activation status of mouse natural killer (NK) cells was determined. Splenic lymphocytes incubated with 2CA exocytosed an NK cell-associated granzyme with N alpha-CBZ-L-lysine thiobenzyl ester (BLT) esterase activity in a dose- and time-dependent manner. Selective depletion of NK cells by anti-asialoGM1 antibody plus complement pretreatment confirmed that NK cells were the source of the BLT esterase activity. 2CA-induced granule exocytosis was not reduced in the presence of the nucleoside uptake blockers NBTI, dilazep, or dipyridamole, indicating the involvement of an extracellular receptor. However, adenosine or other A1, A2, or A3 cell-surface adenosine receptor agonists failed to trigger the exocytotic process. Furthermore, the nonselective adenosine receptor antagonist theophylline, as well as the selective A1 receptor antagonist DPCPX and the selective A2 receptor antagonist DMPX, did not interfere with 2CA-induced BLT esterase secretion. These data suggest that 2CA acts on NK cells via a novel (non-A1/A2/A3) cell-surface receptor. Genistein, a protein tyrosine kinase inhibitor, and calphostin C, a protein kinase C inhibitor, both interfered with 2CA-induced granule exocytosis. Pertussis toxin, an ADP-ribosylating toxin to which certain GTP-binding proteins are sensitive, also inhibited 2CA-stimulated BLT esterase release. In addition, 2CA-induced granule exocytosis was reduced in the presence of cyclosporin A, an inhibitor of Ca(2+)-dependent signaling pathways, and the Ca(2+)-chelating agent EGTA. We conclude that 2CA, acting through a novel extracellular receptor on mouse NK cells, triggers granule exocytosis via a Ca(2+)-dependent signal transduction pathway that is coupled to GTP-binding proteins and involves protein tyrosine kinase and protein kinase C activation.

Solubilization of full-length amyloid precursor proteins from PC12 cell membranes

The amyloid beta protein (A beta) of Alzheimer disease (AD) is derived from the proteolytic processing of the amyloid precursor proteins (APP), which are considered type I transmembrane proteins. Production of A beta from a transmembrane precursor predicts a proteolytic cleavage within the lipid bilayer, a site relatively inaccessible to proteases. Here we show that incubation of a membrane fraction of PC12 cells at 37 degrees C results in the solubilization of an APP species which migrates on SDS-PAGE as full-length APP. The release of this full-length APP was pH-dependent with a peak of activity of pH 9.0. At this pH about 19% of the membrane APP was released from the active subcellular fraction. Under the same conditions other transmembrane proteins remained insoluble. Very little APP was solubilized at 4 degrees C. APP solubilization was specifically inhibited by the serine protease inhibitors aprotinin and pefabloc. Other protease inhibitors, including leupeptin and alpha 1-antitrypsin, had no effect. Several metal cations, including Ca++ and Zn++, also inhibited release of soluble full-length APP. Low levels of full-length APP were also detected in both the soluble fraction of PC12 cell extracts and in the media of PC12 cell cultures. These data suggest the involvement of a serine protease in the solubilization of membrane, full-length APP. The release of this APP could provide a soluble substrate for the proteolytic enzymes involved in the production of A beta.

Protein tyrosine sulfation, 1993--an update

Sulfation is the most abundant post-translational modification of tyrosine residues and occurs in many soluble and membrane proteins passing through the secretory pathway of metazoan cells. The sulfation reaction is catalysed by tyrosylprotein sulfotransferase, a membrane-bound enzyme of the trans-Golgi-network. Tyrosylprotein sulfotransferase has been purified and its substrate specificity characterized. Tyrosine sulfation has been shown to be important for protein-protein interactions occurring during the intracellular transport of proteins and upon their secretion.

Effect of manganese on tyrosylprotein sulfotransferase activity in PC12 cells

Recent studies in our laboratory have revealed that Mn2+ is capable of promoting cell spreading and neurite outgrowth in PC12 cells, a process which is dependent on Mn2+ stimulation of the interaction between extracellular matrix (ECM) components and their corresponding integrin receptors. Since the major ECM proteins implicated in the Mn(2+)-induced morphogenesis, fibronectin and vitronectin, are both tyrosine sulfated, it was of interest to determine whether Mn2+ can regulate the activity of the enzyme responsible for tyrosine sulfation, tyrosylprotein sulfotransferase (TPST). Results of the present studies demonstrated that Mn2+ can suppress TPST activity in PC12 cells in both a time and concentration-dependent manner at concentrations of Mn2+ (0.1 to 1 mM) that promote morphological changes in PC12 cells. Since uptake of Mn2+ may occur via the Ca2+ channel, LaCl3, an inhibitor of Ca2+ transport, was examined and found not to prevent the suppression of TPST activity induced by Mn2+, suggesting that Mn2+ may function at an extracellular site. Suppression of TPST activity occurred with cells plated in serum-free medium on a substrata consisting of either serum or polylysine, suggesting that attachment to extracellular matrix was not an absolute requirement for regulation of activity. Consistent with this is the fact that an RGD-containing pentapeptide did not prevent suppression of TPST activity. Results from the present study demonstrated that Mn2+ is capable of promoting the suppression of TPST activity in PC12 cells at concentrations that have been shown to induce cell spreading and neurite outgrowth. However, unlike the Mn(2+)-induced morphological changes, the presence of ECM proteins is not an absolute requirement for suppression of TPST activity.

Manganese induces spreading and process outgrowth in rat pheochromocytoma (PC12) cells

Mn2+ has been shown to promote cell-substrate adhesion and cell spreading in many cell culture systems. In this study, we present data demonstrating that Mn2+ not only promotes spreading, but also induces process outgrowth in rat pheochromocytoma (PC12) cells. In the presence of 1.0 mM MnCl2, cell spreading was apparent by 6 hr, and nearly 50% of the exposed cells extended neurite-like processes. These morphological effects of Mn2+ were both time- and dose-dependent. In the presence of cycloheximide, a protein synthesis inhibitor, both Mn(2+)-induced spreading and neurite outgrowth were prevented, indicating that de novo protein synthesis is required for the effects of Mn2+ to take place. Of the other divalent cations tested, Mg2+, Cd2+, Cu2+, Ni2+, and Zn2+ were ineffective, and only Co2+ partially mimicked the effects of Mn2+. Although Mn(2+)-induced cell adhesion and spreading have been extensively studied, this is the first report that this divalent cation can cause neurite outgrowth. The neurite outgrowth-promoting effects of Mn2+ were distinct from those of nerve growth factor in that the response to Mn2+ was considerably more rapid, but apparently lacked the ability to sustain continuous outgrowth and networking of neurites. Mn2+ also induced the levels of GAP-43 and peripherin, two proteins associated with neuronal differentiation of PC-12 cells. In cells grown in serum-free defined medium, Mn2+ was capable of promoting neurite outgrowth when the cells were plated on surfaces pretreated with normal growth medium, vitronectin, or fibronectin, while it failed to cause these morphological changes in cells plated on untreated or poly-D-lysine-coated substrata. Similarly, Mn2+ also promoted neurite outgrowth from rat sympathetic neurons attached to laminin-treated substrate, but had no effect on neurons maintained on substrate with polylysine only. The pentapeptide Gly-Arg-Gly-Asp-Ser nearly completely prevented the morphological effects of Mn2+ on PC12 cells. These findings are consistent with a hypothesis that Mn(2+)-mediated alteration of an RGD-dependent extracellular matrix-integrin interaction is responsible for the neuritogenic effects.

Role of extracellular adenosine in ethanol-induced desensitization of cyclic AMP production

The decrease in receptor-stimulated cyclic AMP production after chronic ethanol exposure was suggested previously to be secondary to an ethanol-induced increase in extracellular adenosine. The present study was undertaken to ascertain whether a similar mechanism was responsible for the ethanol-induced desensitization of cyclic AMP production in PC12 pheochromocytoma cells. The acute addition of ethanol in vitro significantly increased both basal cyclic AMP content and extracellular levels of adenosine. A 4-day exposure to ethanol decreased basal as well as 2-chloroadenosine- and forskolin-stimulated cyclic AMP contents. No change in cyclic AMP content was observed after a 2-day exposure of PC12 cells to ethanol. Inclusion of adenosine deaminase during the chronic ethanol treatment significantly decreased extracellular levels of adenosine, yet the percentage decrease in 2-chloroadenosine- and forskolin-stimulated cyclic AMP levels after chronic ethanol exposure was not changed by the inclusion of the adenosine deaminase. Similar results were obtained when the chronic treatment was carried out with serum-free defined media. The ethanol-induced desensitization could not be mimicked by chronic exposure of PC12 cells to adenosine analogues. A 24-h exposure of PC12 cells to 2-chloroadenosine resulted in a decrease in the subsequent ability of this adenosine analogue to stimulate cyclic AMP content, but basal and forskolin-stimulated cyclic AMP levels were increased. Similar results were obtained after a 4-day exposure of PC12 cells to 2-chloroadenosine or 5'-N-ethylcarboxamido-adenosine. The present results indicate that the ethanol-induced decrease in receptor-stimulated cyclic AMP content in PC12 cells is not due to an increase in extracellular adenosine.