Ceramide generated by acidic sphingomyelinase contributes to tumor necrosis factor-alpha-mediated apoptosis in human colon HT-29 cells through glycosphingolipids formation. Possible role of ganglioside GD3

In the present study we assessed the contribution of acidic sphingomyelinase (ASMase), a ceramide generating enzyme, in tumor necrosis factor (TNF)-mediated apoptosis in human colon HT-29 cells. TNF induced apoptosis in HT-29 cells in a time- and dose-dependent fashion. Downregulation of the active endogenous ASMase form prevented TNF-stimulated ASMase activity and apoptosis. Furthermore, inhibition of glucosylceramide synthase, which blunted TNF-stimulated GD3 levels, abolished TNF-mediated cell death. Immunocytochemical staining revealed the co-localization of GD3 with mitochondria induced by TNF. The knockdown of targeted GD3 synthase by antisense expression vector protected HT-29 cells against TNF-induced cell death. Thus, ASMase plays a key role in TNF-induced cell death in human colon epithelial cells possibly through GD3 generation.     

Route of jejunal mucosa absorption of trypsin demonstrated by immunofluorescence

Previous studies have demonstrated the absorption of porcine trypsin in isolated jejunal loops from male Wistar rats by open-loop perfusion. The possible routes of absorption were examined in the study reported here. Trypsin (0.5 mg/ml) was dissolved in tyrode solution and perfused at a rate of 0.5 ml/min, at 37 degrees C, for 40 min. Using immunoperoxidase and immunofluorescence techniques, strong reactivity towards anti-TLCK-trypsin antibody was demonstrated through out the enterocyte cytosol. The present data indicate that trypsin was absorbed by enterocytes, probably through a transcellular route.     

Stabilization of the biotinoyl domain of Escherichia coli acetyl-CoA carboxylase by interactions between the attached biotin and the protruding "thumb" structure

We previously reported (Chapman-Smith, A., Forbes, B. E., Wallace, J. C., and Cronan, J. E., Jr. (1997) J. Biol. Chem. 272, 26017-26022) that the biotinylated (holo) species of the biotin carboxyl carrier protein (BCCP) biotinoyl domain is much more resistant to chemical modification and proteolysis than the unbiotinylated (apo) form. We hypothesized that the increased stability was due to a conformational change engendered by interaction of the domain with biotin protein ligase, the enzyme that attaches the biotin moiety. We now report that a BCCP-87 species to which the biotin moiety was attached by chemical acylation rather than by biotin protein ligase showed the characteristically greater stability of the holo biotinoyl domain. This result demonstrates that our hypothesis was incorrect; the attached biotin is solely responsible for the increased stability. The bacterial and chloroplast multisubunit acetyl-CoA carboxylases are unusual in that the highly symmetrical and conserved structure of the biotinoyl domain of the BCCP subunit is disrupted by a structured loop called the "thumb" that protrudes from body of the domain. Prior structural work showed that the thumb interacts with uriedo ring of the attached biotin moiety. We have tested whether the thumb-biotin interactions are responsible for the greater holo form stability by examination of two BCCP-87 species that lack the thumb. These BCCP species were produced in both the apo and holo forms, and their sensitivities to trypsin digestion were compared. The holo forms of these proteins were found to be only marginally more stable than their apo forms and much more sensitive to trypsin digestion than the wild type holo-BCCP-87. Therefore, removal of the thumb has an effect similar to lack of biotinylation, indicating that thumb-biotin interactions are responsible for most (but not all) of the increased stability of the holo biotinoyl domain. In the course of these experiments we demonstrated that treatment of Escherichia coli with the peptide deformylase inhibitor, actinonin, results in the expected (but previously unreported) accumulation of an N-formylated protein species.     

A novel connection between the yeast Cdc42 GTPase and the Slt2-mediated cell integrity pathway identified through the effect of secreted Salmonella GTPase modulators

Modulation of host cellular GTPases through the injection of the effector proteins SopE2 and SptP is essential for Salmonella typhimurium to enter into non-phagocytic cells. Here we show that expression of the guanine nucleotide exchange factor for Cdc42 SopE2 in Saccharomyces cerevisiae leads to the activation of Fus3 and Kss1 MAPKs, which operate in the mating and filamentation pathways, causing filamentous growth in haploid yeast cells. Furthermore, it promotes the activation of the cell integrity MAPK Slt2. Cdc42 activation by removal of its putative intrinsic GTPase-activating proteins (GAPs), Rga1, Rga2, and Bem3, also results in the phosphorylation of Kss1, Fus3, and Slt2 MAPKs. These data support the role of these GAP proteins as negative regulators of Cdc42, confirm the modulating effect of this GTPase on the filamentation and mating pathways and point to a novel connection between Cdc42 and the cell integrity pathway. Cdc42-induced activation of Slt2 occurs in a mating and filamentation pathway-dependent manner, but it does not require the function of Rho1, which is the GTPase that operates in the cell integrity pathway. Moreover, we report that Salmonella SptP can act as a GAP for Cdc42 in S. cerevisiae, down-regulating MAPK-mediated signaling. Thus, yeast provides a useful system to study the interaction of bacterial pathogenic proteins with eukaryotic signaling pathways. Furthermore, these proteins can be used as a tool to gain insight into the mechanisms that regulate MAPK-mediated signaling in eukaryotes.     

Elucidation of the metabolic fate of glucose in the filamentous fungus Trichoderma reesei using expressed sequence tag (EST) analysis and cDNA microarrays

Despite the intense interest in the metabolic regulation and evolution of the ATP-producing pathways, the long standing question of why most multicellular microorganisms metabolize glucose by respiration rather than fermentation remains unanswered. One such microorganism is the cellulolytic fungus Trichoderma reesei (Hypocrea jecorina). Using EST analysis and cDNA microarrays, we find that in T. reesei expression of the genes encoding the enzymes of the tricarboxylic acid cycle and the proteins of the electron transport chain is programmed in a way that favors the oxidation of pyruvate via the tricarboxylic acid cycle rather than its reduction to ethanol by fermentation. Moreover, the results indicate that acetaldehyde may be channeled into acetate rather than ethanol, thus preventing the regeneration of NAD(+), a pivotal product required for anaerobic metabolism. The studies also point out that the regulatory machinery controlled by glucose was most probably the target of evolutionary pressure that directed the flow of metabolites into respiratory metabolism rather than fermentation. This finding has significant implications for the development of metabolically engineered cellulolytic microorganisms for fuel production from cellulose biomass.     

The inhibition of glutamate release by metabotropic glutamate receptor 7 affects both [Ca2+]c and cAMP: evidence for a strong reduction of Ca2+ entry in single nerve terminals

Metabotropic glutamate receptors (mGluRs) from group III reduce glutamate release. Because these receptors reduce cAMP levels, we explored whether this signaling pathway contributes to release inhibition caused by mGluRs with low affinity for L-2-amino-4-phosphonobutyrate (L-AP4). In biochemical experiments with the population of cerebrocortical nerve terminals we find that L-AP4 (1 mm) inhibited the Ca(2+)-dependent-evoked release of glutamate by 25%. This inhibitory effect was largely prevented by the pertussis toxin but was insensitive to inhibitors of protein kinase C bisindolylmaleimide and protein kinase A H-89. Furthermore, this inhibition was associated with reduction in N-type Ca(2+) channel activity in the absence of any detectable change in cAMP levels. In the presence of forskolin, however, L-AP4 decreased the levels of cAMP. The activation of this additional signaling pathway was very efficient in counteracting the facilitation of glutamate release induced either by forskolin or the beta-adrenergic receptor agonist isoproterenol. Imaging experiments to measure Ca(2+) dynamics in single nerve terminals showed that L-AP4 strongly reduced the Ca(2+) response in 28% of the nerve terminals. Moreover, immunochemical experiments showed that 25-35% of the nerve terminals that were immunopositive to synaptophysin were also immunoreactive to the low affinity L-AP4-sensitive mGluR7. Then, mGluR7 mediates the inhibition of glutamate release caused by 1 mm L-AP4, primarily by a strong inhibition of Ca(2+) channels, although high cAMP uncovers the receptor ability to decrease cAMP.     

Reactions of the class II peroxidases, lignin peroxidase and Arthromyces ramosus peroxidase, with hydrogen peroxide. Catalase-like activity, compound III formation, and enzyme inactivation

The reactions of the fungal enzymes Arthromyces ramosus peroxidase (ARP) and Phanerochaete chrysosporium lignin peroxidase (LiP) with hydrogen peroxide (H(2)O(2)) have been studied. Both enzymes exhibited catalase activity with hyperbolic H(2)O(2) concentration dependence (K(m) approximately 8-10 mm, k(cat) approximately 1-3 s(-1)). The catalase and peroxidase activities of LiP were inhibited within 10 min and those of ARP in 1 h. The inactivation constants were calculated using two independent methods; LiP, k(i) approximately 19 x 10(-3) s(-1); ARP, k(i) approximately 1.6 x 10(-3) s(-1). Compound III (oxyperoxidase) was detected as the majority species after the addition of H(2)O(2) to LiP or ARP, and its formation was accompanied by loss of enzyme activity. A reaction scheme is presented which rationalizes the turnover and inactivation of LiP and ARP with H(2)O(2). A similar model is applicable to horseradish peroxidase. The scheme links catalase and compound III forming catalytic pathways and inactivation at the level of the [compound I.H(2)O(2)] complex. Inactivation does not occur from compound III. All peroxidases studied to date are sensitive to inactivation by H(2)O(2), and it is suggested that the model will be generally applicable to peroxidases of the plant, fungal, and prokaryotic superfamily.     

Ethnicity and type 2 diabetes in Rio de Janeiro,Brazil, with a review of the prevalence of the disease in Amerindians

To what extent can ethnic factors contribute to the prevalence of type 2 diabetes and impaired glucose tolerance (IGT) in an urban Brazilian population? Conversely, how can environmental factors such as diet change these prevalences in a given ethnic group, in this case Brazilian Indians? To answer these questions estimates of ethnic admixture in Afro- and Euro-Brazilians from Rio de Janeiro, Brazil, were established using eight genetic systems and compared with the prevalences of these conditions obtained previously. This information was integrated with results obtained inside and outside of Brazil. The similarity of prevalences for type 2 diabetes and IGT in Afro- and Euro-Brazilians may be related to the extensive gene flow that occurred between them and to similar socioeconomic levels in the samples investigated. On the other hand, changes in the traditional diet are probably conditioning the appearance of diabetes among Brazilian and other South American Indians.     

Trypanosoma cruzi reinfections in mice determine the severity of cardiac damage

In two murine models we studied Trypanosoma cruzi reinfection in the acute and chronic phase of experimental Chagas' disease in order to elucidate the relevance of reinfections in determining the variability of cardiac symptoms and the irreversible cardiac damage. They were followed for 120 and 600 days post infection (p.i.) for the acute and chronic model, respectively. Reinfected mice reached higher parasitaemia levels than infected mice. The survival was 33 and 21% in the chronic phase for mice reinfected in the acute phase and 13% for mice reinfected in the chronic stage at the end of the experiments. Sixty-six percent of the infected group presented electrocardiographic abnormalities (heart frequency, prolonged PQ segment or QRS complex) in the chronic stage whereas 100% of the reinfected animals exhibited electric cardiac dysfunction since 90 and 390 days p.i. for the acute and chronic reinfected model, respectively (P<0.01). Heart histopathological studies showed fibrosis and necrosis areas and mononuclear infiltrates supporting the view that parasite persistence is a major factor in continuing the tissue inflammation. This work shows that T. cruzi reinfections could be related to the variability and severity of the clinical course of Chagas' disease and that parasite persistence is involved in exacerbation of the disease.