Sphingomyelin metabolites inhibit sphingomyelin synthase and CTP:phosphocholine cytidylyltransferase.

Tissue injury in inflammation involves the release of several cytokines that activate sphingomyelinases and generate ceramide. In the lung, the impaired metabolism of surfactant phosphatidylcholine (PC) accompanies this acute and chronic injury. These effects are long-lived and extend beyond the time frame over which tumor necrosis factor (TNF)-alpha and interleukin-1beta are elevated. In this paper, we demonstrate that in H441 lung cells these two processes, cytokine-induced metabolism of sphingomyelin and the inhibition of PC metabolism, are directly interrelated. First, metabolites of sphingomyelin hydrolysis themselves inhibit key enzymes necessary for restoring homeostasis between sphingomyelin and its metabolites. Ceramide stimulates sphingomyelinases as effectively as TNF-alpha, thereby amplifying the sphingomyelinase activation, and TNF-alpha, ceramide, and sphingosine all inhibit PC:ceramide phosphocholine transferase (sphingomyelin synthase), the enzyme that restores homeostasis between sphingomyelin and ceramide pools. Second, ceramide inhibits PC synthesis, probably because of its effects on CTP:phosphocholine cytidylyltransferase, the rate-limiting enzymatic step in de novo PC synthesis. The data presented here suggest that TNF-alpha may be an inhibitor of phospholipid metabolism in inflammatory tissue injury. These actions may be amplified because of the ability of metabolites of sphingomyelin to inhibit the pathways that should restore the normal ceramide-sphingomyelin homeostasis.

CTP:phosphocholine cytidylyltransferase inhibition by ceramide via PKC-alpha, p38 MAPK, cPLA2, and 5-lipoxygenase.

In a companion paper (Vivekananda J, Smith D, and King RJ. Am J Physiol Lung Cell Mol Physiol 281: L98-L107, 2001), we demonstrated that tumor necrosis factor (TNF)-alpha inhibited the activity of CTP:phosphocholine cytidylyltransferase (CT), the rate-limiting enzyme in the de novo synthesis of phosphatidylcholine (PC), and that its actions were likely exerted through a metabolite of sphingomyelin. In this paper, we explore the signaling pathway employed by TNF-alpha using C2 ceramide as a cell-penetrating sphingolipid representative of the metabolites induced by TNF-alpha. We found that in H441 cells, as reported in other cell types, cytosolic phospholipase A2 (cPLA2) is activated by TNF-alpha. We also observed that the inhibiting action of C2 ceramide on CT requires protein kinase C-alpha, p38 mitogen-activated protein kinase, and cPLA2. The actions of C2 ceramide on CT activity can be duplicated by adding 2 microM lysoPC to these cells. Furthermore, we found that the effects of C2 ceramide are dependent on 5-lipoxygenase but that cyclooxygenase II is unimportant. We hypothesize that CT activity is inhibited by the lysoPC generated as a consequence of the activation of cPLA2 by protein kinase C-alpha and p38 mitogen-activated protein kinase. The other product of the activation of cPLA2, arachidonic acid, is a substrate for the synthesis of leukotrienes, which raise intracellular Ca2+ levels and complete the activation of cPLA2.

Hepatocyte growth factor is elevated in chronic lung injury and inhibits surfactant metabolism.

Adult respiratory distress syndrome may incorporate in its pathogenesis the hyperplastic proliferation of alveolar epithelial type II cells and derangement in synthesis of pulmonary surfactant. Previous studies have demonstrated that hepatocyte growth factor (HGF) in the presence of serum is a potential mitogen for adult type II cells (R. J. Panos, J. S. Rubin, S. A. Aaronson, and R. J. Mason. J. Clin. Invest. 92: 969-977, 1993) and that it is produced by fetal mesenchymal lung cells (J. S. Rubin, A. M.-L. Chan, D. P. Botarro, W. H. Burgess, W. G. Taylor, A. C. Cech, D. W. Hirschfield, J. Wong, T. Miki, P. W. Finch, and S. A. Aaronson. Proc. Natl. Acad. Sci. USA 88: 415-419, 1991). In these studies, we expand on this possible involvement of HGF in chronic lung injury by showing the following. First, normal adult lung fibroblasts transcribe only small amounts of HGF mRNA, but the steady-state levels of this message rise substantially in lung fibroblasts obtained from animals exposed to oxidative stress. Second, inflammatory cytokines produced early in the injury stimulate the transcription of HGF in isolated fibroblasts, providing a plausible mechanism for the increased amounts of HGF seen in vivo. Third, HGF is capable of significantly inhibiting the synthesis and secretion of the phosphatidylcholines of pulmonary surfactant. Fourth, HGF inhibits the rate-limiting enzyme in de novo phosphatidylcholine synthesis, CTP:choline-phosphate cytidylyltransferase (EC 2.7.7.15). Our data indicate that fibroblast-derived HGF could be partially responsible for the changes in surfactant dysfunction seen in adult respiratory distress syndrome, including the decreases seen in surfactant phosphatidylcholines.

Transcriptional and posttranscriptional control of tyrosine hydroxylase gene expression during persistent stimulation of pituitary adenylate cyclase-activating polypeptide receptors on PC12 cells: regulation by protein kinase A-dependent and protein kinase A-independent pathways.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates catecholamine release and biosynthesis in sympathetic postganglionic cells. Moreover, PACAP receptor activation in cultured adrenal chromaffin and superior cervical ganglion cells has been reported to increase the expression of the gene coding for tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis. However, the relative contribution of transcriptional and posttranscriptional mechanisms to the effects of PACAP on TH gene expression has not been evaluated. Therefore, in this study we compared the temporal effects of PACAP on TH gene transcription with the duration of its effects on TH mRNA levels. We had previously shown that vasoactive intestinal polypeptide, peptide histidine isoleucine, and secretin, peptides closely related to PACAP, induce TH gene expression through a cyclic AMP (cAMP)-dependent pathway. Therefore, using a mutant PC12 cell line deficient in cAMP-dependent protein kinase II (PKA), we also evaluated the role of the cAMP pathway in the effect of PACAP on TH gene expression. Continuous treatment of wild-type PC12 cells with PACAP (1 nM) increased TH mRNA levels maximally by 12 h and maintained TH mRNA at near maximal levels for at least 2 days. In contrast, the rate of TH gene transcription, as measured by a nuclear run-on assay, was maximal by 1 h and returned to basal levels by 3 h. The fact that a new steady-state level of TH mRNA was achieved and maintained for days in the absence of a sustained increase in TH gene transcription supports the involvement of posttranscriptional mechanisms. Removal of PACAP after 12 h, a time at which TH gene transcription was at basal levels, resulted in a subsequent return of TH mRNA to unstimulated levels within 36 h. Thus, continuous PACAP stimulation is required to maintain sustained increases in TH mRNA levels in the absence of a sustained elevation of transcription. To examine the role of the cAMP pathway in these effects, we compared the effects of PACAP in wild-type PC12 cells and in a mutant PC12 cell line (A126-1B2) that is deficient in PKA. PACAP failed to stimulate either TH mRNA levels or TH gene transcription in the mutant cells. In contrast to the effects of PACAP, dexamethasone increased TH mRNA levels by the same magnitude in both cell lines. It is noteworthy that stimulation of the PKA-deficient mutant cells with a combination of PACAP and dexamethasone (1 microM) produced a synergistic increase in TH mRNA levels, which was nearly twice that induced by dexamethasone stimulation alone. This synergistic effect was not transcriptionally mediated. The effect of the combined treatment on TH gene transcription was identical to the effect of dexamethasone alone. Taken together, these data indicate that PACAP regulates TH gene expression through a transcriptional mechanism requiring an intact cAMP pathway and through posttranscriptional mechanisms under the control of a cAMP-independent pathway(s).

Acute inflammatory injury in the lung precipitated by oxidant stress induces fibroblasts to synthesize and release transforming growth factor-alpha.

Although transforming growth factor-alpha (TGF-alpha) is widely distributed in transformed cells and in some normal cells and much is known about its structure and metabolism, there is little information about its physiological actions. TGF-alpha is not thought to be synthesized by nontransformed fibroblasts, but it is thought to be a mitogen for these and epithelial cells (Derynck, R. (1986) J. Cell. Biochem. 32, 293-304). We report here that fibroblasts obtained from hamsters with oxidant-induced lung injury release TGF-alpha at levels comparable with those reported for transformed cells. In conditioned media, one isoform of 18 kDa was recognized by a monoclonal antibody to mature TGF-alpha; five isoforms ranging from 18 to 42 kDa were recognized in cell lysates. Conditioned media from these fibroblasts stimulated tyrosine phosphorylation of the epidermal growth factor (EGF)/TGF-alpha receptor, competed with radioactive EGF for binding sites on A431 cells, and were mitogenic for mesenchymal and epithelial cells. This mitogenic activity could be almost completely blocked by anti-TGF-alpha. Conditioned media from normal lung fibroblasts exhibited none of these activities. Using normal lung fibroblasts, we found that TGF-alpha synthesis could be induced in vitro with 25 nmol/ml EGF, suggesting that the induction in vivo may have been due, in part, to a stimulation by EGF (or TGF-alpha) released by other cell types such as alveolar macrophages recruited to the injury site. TGF-alpha is, in general, a mitogen for epithelial cells (Derynck, 1986); more specific to acute injury in the lung, it may affect the proliferation (Ryan, R. M., Mineo-Kuhn, M. M., Kromer, C. M., and Finkelstein, J. N. (1994) Am. J. Physiol. 266, L17-L23) and metabolic activities (Whitsett, J. A., Weaver, T. E., Lieberman, M. A., Clark, J. G., and Daugherty, C. (1987) J. Biol. Chem. 262, 7908-7913) of alveolar epithelial type II cells. This is, we believe, the first report of a fibroblast-derived TGF-alpha induced with oxidant injury. If this response was ubiquitously manifested in other tissues, then fibroblast-derived TGF-alpha might be an important determinant of the epithelial and mesenchymal hyperplasia commonly observed in tissue repair.

Developmental and hormonal regulation of sunflower helianthinin genes: proximal promoter sequences confer regionalized seed expression.

DNA elements involved in the regulation of two sunflower helianthinin genes were identified by analysis of beta-glucuronidase (GUS) expression in transgenic tobacco driven by sequences derived from the 5' upstream regions of these genes. A 2.4-kb upstream region of the helianthinin gene HaG3-A conferred rigorous developmental GUS expression in transgenic tobacco seeds with no significant GUS activity in nonembryonic tissues. Regions of the helianthinin upstream regulatory ensemble (URE) conferred ectopic expression in nonembryonic tissues when analyzed outside of the context of the complete helianthinin regulatory complex. A proximal promoter region was identified that conferred significant GUS expression in seeds but not in leaves of transgenic tobacco. Three sequence motifs that bind to seed nuclear proteins were identified in the proximal promoter region; mutations in these motifs significantly reduced the level of nuclear protein binding. Another important class of cis-regulatory elements was identified in the helianthinin URE that conferred abscisic acid-responsive GUS expression. In the full-length helianthinin URE, these elements only responded to abscisic acid in the developing seed, suggesting that the helianthinin gene contains additional regulatory elements, possibly in the proximal promoter region, that ensure hierarchical control in the developing seed.

Hormonal and Environmental Regulation of the Carrot lea-Class Gene Dc3.

Dc3 is a carrot lea-class gene belonging to a small gene family that encodes Dc3 and Dc3-like RNA sequences. We have examined the responsiveness to water deficit and abscisic acid (ABA) of the promoter/enhancer complex of Dc3 fused to a beta-glucuronidase (GUS) reporter gene in vegetative cells of transgenic tobacco. In 56-d tobacco, GUS expression in leaves increased about 200-fold during a 3-d drying cycle, during which there were small decreases (3 atmospheres or less) in leaf water potential and a 16-fold increase in free ABA. These effects were reversed by rewatering. Changes in GUS activity were closely paralleled by changes in GUS transcript levels during the desiccation/watering cycle, indicating transcriptional regulation of GUS gene expression. The Dc3 promoter responds to exogenous ABA; the effect is time and concentration dependent, with greater than 10-fold induction in 8 h with 10 mum ABA. Histochemical visualization of GUS activity in seedlings induced by water deficit or exogenous ABA revealed Dc3-driven GUS expression in all organs of transgenic tobacco seedlings. We suggest that the Dc3/GUS reporter system is a sensitive analytical tool to study various environmental effects on plant growth and development.

Transient foreign gene expression in chloroplasts of cultured tobacco cells after biolistic delivery of chloroplast vectors.

Expression of chloramphenicol acetyltransferase (cat) by suitable vectors in chloroplasts of cultured tobacco cells, delivered by high-velocity microprojectiles, is reported here. Several chloroplast expression vectors containing bacterial cat genes, placed under the control of either psbA promoter region from pea (pHD series) or rbcL promoter region from maize (pAC series) have been used in this study. In addition, chloroplast expression vectors containing replicon fragments from pea, tobacco, or maize chloroplast DNA have also been tested for efficiency and duration of cat expression in chloroplasts of tobacco cells. Cultured NT1 tobacco cells collected on filter papers were bombarded with tungsten particles coated with pUC118 (negative control), 35S-CAT (nuclear expression vector), pHD312 (repliconless chloroplast expression vector), and pHD407, pACp18, and pACp19 (chloroplast expression vectors with replicon). Sonic extracts of cells bombarded with pUC118 showed no detectable cat activity in the autoradiograms. Nuclear expression of cat reached two-thirds of the maximal 48 hr after bombardment and the maximal at 72 hr. Cells bombarded with chloroplast expression vectors showed a low level of expression until 48 hr of incubation. A dramatic increase in the expression of cat was observed 24 hr after the addition of fresh medium to cultured cells in samples bombarded with pHD407; the repliconless vector pHD312 showed about 50% of this maximal activity. The expression of nuclear cat and the repliconless chloroplast vector decreased after 72 hr, but a high level of chloroplast cat expression was maintained in cells bombarded with pHD407. Organelle-specific expression of cat in appropriate compartments was checked by introducing various plasmid constructions into tobacco protoplasts by electroporation. Although the nuclear expression vector 35S-CAT showed expression of cat, no activity was observed with any chloroplast vectors.

Isolation and partial characterization of the glutamate/aspartate transporter from pea leaf mitochondria using a specific monoclonal antibody.

A library of monoclonal antibodies directed against the proteins of the inner mitochondrial membrane was screened for antibodies that could bind to the glutamate/aspartate transporter of pea mitochondria and thereby inhibit its activity. One antibody, 2C7, had the property of inhibiting glutamate and aspartate-dependent oxaloacetate metabolism by pea mitochondria without affecting the metabolism of other substrates. The antibody specifically recognized a 21,000 dalton protein, which was tentatively identified as the glutamate/aspartate transporter. The antibody was used to follow the extraction of this protein by Triton X-114 and cardiolipin and the partial purification of the protein by centrifugation and chromatography on hydroxylapatite. The partially purified preparation was reconstituted into azolectin vesicles and shown to catalyze glutamate/glutamate and glutamate/aspartate exchange in an apparently nonelectrogenic manner. The antibody was shown to specifically bind to the glutamate/aspartate exchanger by its ability to inhibit this reconstituted exchange reaction.

Monoclonal antibodies as tools in membrane biochemistry. Identification and partial characterization of the dicarboxylate transporter from pea leaf mitochondria.

Monoclonal antibodies specific for the dicarboxylate transporter of pea mitochondria were prepared and used to identify this substrate carrier. The hybridoma library was derived from mice that had been immunized with total mitochondrial membranes. The monoclonal antibodies specific for the dicarboxylate transporter were selected by screening hybridoma supernatants for their ability to inhibit malate- and succinate-dependent oxalacetate reduction by osmotically shocked pea leaf mitochondria. Three monoclonal antibodies were shown to be specific for the dicarboxylate transporter by their ability to 1) inhibit malate and succinate metabolism without affecting alpha-ketoglutarate, citrate, pyruvate, glycine, glutamate, or aspartate metabolism by mitochondria and 2) inhibit malate uptake by a partially purified transporter fraction reconstituted into asolectin vesicles. The dicarboxylate transporter was identified by Western blotting and immunoprecipitation and had an apparent molecular mass of 26,000 Da. The techniques described should prove useful for identifying a number of membrane proteins that can be assayed in situ but are difficult to assay following dissolution of the membrane.