Macromolecular accessibility of fluorescent taxoids bound at a paclitaxel binding site in the microtubule surface

The macromolecular accessibility of the paclitaxel binding site in microtubules has been investigated using a fluorescent taxoid and antibodies against fluorescein, which cannot diffuse into the microtubule lumen. The formation of a specific ternary complex of microtubules, Hexaflutax (7-O-{N-[6-(fluorescein-4'-carboxamido)-n-hexanoyl]-l-alanyl}paclitaxel) and 4-4-20 IgG (a monoclonal antibody against fluorescein) has been observed by means of sedimentation and electron microscopy methods. The kinetics of binding of the antibody to microtubule-bound Hexaflutax has been measured. The quenching of the observed fluorescence is fast (k+ 2.26 +/- 0.25 x 10(6) m(-1) s(-1) at 37 degrees C), indicating that the fluorescein groups of Hexaflutax are exposed to the outer solvent. The velocity of the reaction is linearly dependent on the antibody concentration, indicating that a bimolecular reaction is being observed. Another fluorescent taxoid (Flutax-2) bound to microtubules has also been shown to be rapidly accessible to polyclonal antibodies directed against fluorescein. A reduced rate of Hexaflutax quenching by the antibody is observed in microtubule-associated proteins containing microtubules or in native cellular cytoskeletons. It can be concluded that the fluorescent taxoids bind to an outer site on the microtubules that is shared with paclitaxel. Paclitaxel would be internalized in a further step of binding to reach the known luminal site, this step being blocked in the case of the fluorescent taxoids. Because the fluorescent ligands are able to induce microtubule assembly, binding to the outer site should be enough to induce assembly by a preferential binding mechanism.     

Mutations of the PC2 substrate binding pocket alter enzyme specificity

By taking advantage of the recently published furin structure, whose catalytic domain shares high homology with other proprotein convertases, we designed mutations in the catalytic domain of PC2, altering residues Ser206, Thr271, Asp278, ArgGlu282, AlaSer323, Leu341, Asn365, and Ser380, which are both conserved and specific to this convertase, and substituting residues specific to PC1 and/or furin. In order to investigate the determinants of PC2 specificity, we have tested the mutated enzymes against a set of proenkephalin-derived substrates, as well as substrates representing Arg, Ala, Leu, Phe, and Glu positional scanning variants of a peptide B-derived substrate. We found that the exchange of the Ser206 residue with Arg or Lys led to a total loss of activity. Increased positive charge of the substrate generally resulted in an increased specificity constant. Most intriguingly, the RE281GR mutation, corresponding to a residue placed distantly in the S6 pocket, evoked the largest changes in the specificity pattern. The D278E and N356S mutations resulted in distinct alterations in PC2 substrate preferences. However, when other residues that distinguish PC2 from other convertases were substituted with PC1-like or furin-like equivalents, there was no significant alteration of the PC2 specificity pattern, suggesting that the overall structure of the substrate binding cleft rather than individual residues specifies substrate binding.     

Nuclear factor-kappaB activation leads to down-regulation of fatty acid oxidation during cardiac hypertrophy

Little is known about the mechanisms responsible for the fall in fatty acid oxidation during the development of cardiac hypertrophy. We focused on the effects of nuclear factor (NF)-kappaB activation during cardiac hypertrophy on the activity of peroxisome proliferator-activated receptor (PPAR) beta/delta, which is the predominant PPAR subtype in cardiac cells and plays a prominent role in the regulation of cardiac lipid metabolism. Phenylephrine-induced cardiac hypertrophy in neonatal rat cardiomyocytes caused a reduction in the expression of pyruvate dehydrogenase kinase 4 (Pdk4), a target gene of PPARbeta/delta involved in fatty acid utilization, and a fall in palmitate oxidation that was reversed by NF-kappaB inhibitors. Lipopolysaccharide stimulation of NF-kappaB in embryonic rat heart-derived H9c2 myotubes, which only express PPARbeta/delta, caused both a reduction in Pdk4 expression and DNA binding activity of PPARbeta/delta to its response element, effects that were reversed by NF-kappaB inhibitors. Coimmunoprecipitation studies demonstrated that lipopolysaccharide strongly stimulated the physical interaction between the p65 subunit of NF-kappaB and PPARbeta/delta, providing an explanation for the reduced activity of PPARbeta/delta. Finally, we assessed whether this mechanism was present in vivo in pressure overload-induced cardiac hypertrophy. In hypertrophied hearts of banded rats the reduction in the expression of Pdk4 was accompanied by activation of NF-kappaB and enhanced interaction between p65 and PPARbeta/delta. These results indicate that NF-kappaB activation during cardiac hypertrophy down-regulates PPARbeta/delta activity, leading to a fall in fatty acid oxidation, through a mechanism that involves enhanced protein-protein interaction between the p65 subunit of NF-kappaB and PPARbeta/delta.     

Na+/Ca2+ exchanger activity modulates connective tissue growth factor mRNA expression in transforming growth factor beta1- and Des-Arg10-kallidin-stimulated myofibroblasts

Transforming growth factor (TGF)-beta and des-Arg(10)-kallidin stimulate the expression of connective tissue growth factor (CTGF), a matrix signaling molecule that is frequently overexpressed in fibrotic disorders. Because the early signal transduction events regulating CTGF expression are unclear, we investigated the role of Ca(2+) homeostasis in CTGF mRNA expression in TGF-beta1- and des-Arg(10)-kallidin-stimulated human lung myofibroblasts. Activation of the kinin B1 receptor with des-Arg(10)-kallidin stimulated a rise in cytosolic Ca(2+) that was extracellular Na(+)-dependent and extracellular Ca(2+)-dependent. The des-Arg(10)-kallidin-stimulated increase of cytosolic Ca(2+) was blocked by KB-R7943, a specific inhibitor of Ca(2+) entry mode operation of the plasma membrane Na(+)/Ca(2+) exchanger. TGF-beta1 similarly stimulated a KB-R7943-sensitive increase of cytosolic Ca(2+) with kinetics distinct from the des-Arg(10)-kallidin-stimulated Ca(2+) response. We also found that KB-R7943 or 2',4'-dichlorobenzamil, an amiloride analog that inhibits the Na(+)/Ca(2+) exchanger activity, blocked the TGF-beta1- and des-Arg(10)-kallidin-stimulated increases of CTGF mRNA. Pretreatment with KB-R7943 also reduced the basal and TGF-beta1-stimulated levels of alpha1(I) collagen and alpha smooth muscle actin mRNAs. These data suggest that, in addition to regulating ion homeostasis, Na(+)/Ca(2+) exchanger acts as a signal transducer regulating CTGF, alpha1(I) collagen, and alpha smooth muscle actin expression. Consistent with a more widespread role for Na(+)/Ca(2+) exchanger in fibrogenesis, we also observed that KB-R7943 likewise blocked TGF-beta1-stimulated levels of CTGF mRNA in human microvascular endothelial and human osteoblast-like cells. We conclude that Ca(2+) entry mode operation of the Na(+)/Ca(2+) exchanger is required for des-Arg(10)-kallidin- and TGF-beta1-stimulated fibrogenesis and participates in the maintenance of the myofibroblast phenotype.     

Functional characterization of KlHEM13, a hypoxic gene of Kluyveromyces lactis

The KlHEM13 gene of Kluyveromyces lactis encoding the coproporphyrinogen oxidase (EC 1.3.3.3), an oxygen-requiring enzyme that catalyzes the sixth step of heme biosynthesis, was cloned and functionally characterized. The coding and upstream regions of KlHEM13 were analyzed and the putative cis regulatory elements were discussed in relation to the mechanisms of regulation of this hypoxic gene in K. lactis.     

Genomic DNA macroarrays as a tool for analysis of gene expression in Leishmania

Gene-array technologies have been applied in a wide number of organisms to study gene expression profiling under several physiological and experimental conditions. Gene-array implementations combined with the information arising from emerging genome sequencing projects are expected to be in the near future a major tool to characterize genes involved in different processes. So far, gene expression profile studies in trypanosomatids have been performed in microarrays that use a glass support to immobilize fragments of genomic DNA followed by fluorescent detection. Here, we wanted to test the potential of genomic DNA macroarrays of Leishmania infantum using nylon membranes and radioactive detection. Nylon macroarrays present a number of advantages since the processing of the membranes is based on standard Southern blotting protocols familiar to molecular biologists, and the data acquisition equipment is available to most research institutions. Nylon macroarrays were employed to search for genes showing increased mRNA abundance during an axenic differentiation of L. infantum promastigotes to amastigotes. Several clones were rescued and, after validation by Northern blot assays, these L. infantum sequences were used to screen the Leishmania major gene database. The L. major contigs with high homology to the L. infantum sequences allowed a consistent identification of the regulated genes.     

Charge substitution shows that repulsive electrostatic interactions impede the oligomerization of Alzheimer amyloid peptides

The strong pH dependence of A beta oligomerization could arise from favorable intermolecular charge-charge interactions between His and carboxylate groups, or, alternatively, by mutual electrostatic repulsion of peptide molecules. To test between these two possibilities, the pH dependence of the oligomerization of A beta and three charge substitution variants with Asp, Glu and His substituted by Ala is measured. All four peptides oligomerize, as detected by thioflavin T fluorescence, turbidity, and amyloid fibril formation; therefore, specific charge-charge interactions are nonessential for oligomerization. The strong negative correlation between net charge and oligomerization indicates that electrostatic repulsion between A beta monomers impedes their association.     

Seasonal environmental changes regulate the expression of the histone variant macroH2A in an eurythermal fish

Adaptation to cold and warm conditions requires dramatic change in gene expression. The acclimatization process of the common carp Cyprinus carpio L. in its natural habitat has been used to study how organisms respond to natural environmental changes. At the cellular level, adaptation to cold condition is accompanied by a dramatic alteration in nucleolar structure and a down regulation of the expression of ribosomal genes. We show that the enrichment of condensed chromatin in winter adapted cells is not correlated with an increase of the heterochromatin marker trimethyl and monomethyl K20H4. However, the expression of the tri methyl K4 H3 and of the variant histone macroH2A is significantly increased during the winter season together with a hypermethylation of CpG residues. Taking into account the properties of macroH2A toward chromatin structure and dynamics and its role in gene repression our data suggest that the increased expression of macroH2A and the hypermethylation of DNA which occurs upon winter-acclimatization plays a major role for the reorganization of chromatin structure and the regulation of gene expression during the physiological adaptation to a colder environment.     

CD45 expression on rat acinar cells: involvement in pro-inflammatory cytokine production

CD45 transduces activation signals in inflammatory cells. We investigate CD45 expression on pancreatic acinar cells and examine its role in the inflammatory response which these cells have also shown under certain circumstances. Similar CD45 mRNA levels were found in acinar cells and leukocytes (positive control). Flow cytometric and immunohistochemical analysis showed a heterogeneous CD45 distribution on acinar cells. Activation of acinar cells by incubation with pancreatitis-associated ascitic fluid as evidencied by TNF-alpha production resulted in a decreased CD45 expression, suggesting that CD45 acts as a negative regulator of cytokine production. As a validation of this finding in vivo, a decrease in the acinar CD45 expression in parallel with an increased ability to produce TNF-alpha was found in rats with acute pancreatitis. Our data show that CD45 is constitutively expressed in acinar cells and suggest that it plays an important role in negatively regulating cytokine production.