Interaction of odorous lactones with phospholipids: implications in toxicity towards producing yeast cells

Some lactonic aroma compounds, that can be produced industrially by microorganisms, become toxic towards the producing cells as these compounds reach high concentrations in the culture medium. To determine the manner by which these metabolites may influence yeast physiology, the effects of four lactones (concentration range of 100 to 300 mg l^–1) on the growth of Yarrowia lipolytica and on the phase behaviour of deuterated dimyristoylphosphatidylcholine (DMPC-d27) were studied. The results showed that the hydrophobic lactones decrease the phase transition temperature (Tm) of DMPC-d27 bilayers and that Tm decrease (Tm) was related to the inhibitory action of the lactones on yeast growth (evaluated by the lag time). These results suggest that whatever the lactone, a Tm of at least 2.5 °C resulted in a total growth inhibition: this implicates the lactone-phospholipids interaction in the mechanism of yeast growth inhibition. The test used in the present study may be a predicting method to assess the in vivo action of potential membrane active compounds.     

Human endothelial cells selectively express large amounts of pancreatic-type ribonuclease (RNase 1)

Pyrimidine-specific ribonucleases are a superfamily of structurally related enzymes with distinct catalytic and biological properties. We used a combination of enzymatic and non-enzymatic assays to investigate the release of such enzymes by isolated cells in serum-free and serum-containing media. We found that human endothelial cells typically expressed large amounts of a pancreatic-type RNase that is related to, if not identical to, human pancreatic RNase. This enzyme exhibits pyrimidine-specific catalytic activity, with a marked preference for poly(C) substrate over poly(U) substrate. It was potently inhibited by placental RNase inhibitor, the selective pancreatic-type RNase inhibitor Inhibit-Ace, and a polyclonal antibody against human pancreatic RNase. The enzyme isolated from medium conditioned by immortalized umbilical vein endothelial cells (EA.hy926) possesses an amino-terminal sequence identical to that of pancreatic RNase, and shows molecular heterogeneity (molecular weights 18,000-26,000) due to different degrees of N-glycosylation. Endothelial cells from arteries, veins, and capillaries secreted up to 100 ng of this RNase daily per million cells, whereas levels were low or undetectable in media conditioned by other cell types examined. The corresponding messenger RNA was detected by RT-PCR in most cell types tested so far, and level of its expression was in keeping with the amounts of protein. The selective strong release of pancreatic-type RNase by endothelial cells suggests that it is endowed with non-digestive functions and involved in vascular homeostasis.     

Critical evaluation of the role of the Toll-like receptor 18-Wheeler in the host defense of Drosophila

Essential aspects of innate immune responses to microbial infections appear to be conserved between insects and mammals. In particular, in both groups, transmembrane receptors of the Toll superfamily play a crucial role in activating immune defenses. The Drosophila Toll family member 18-Wheeler had been proposed to sense Gram-negative infection and direct selective expression of peptides active against Gram-negative bacteria. Here we re-examine the role of 18-Wheeler and show that in adults it is dispensable for immune responses. In larvae, 18wheeler is required for normal fat body development, and in mutant larvae induction of all antimicrobial peptide genes, and not only of those directed against Gram-negative bacteria, is compromised. 18-Wheeler does not qualify as a pattern recognition receptor of Gram-negative bacteria.     

Insulin-like growth factor-binding protein-3 activates a phosphotyrosine phosphatase. Effects on the insulin-like growth factor signaling pathway

The proliferative action of insulin-like growth factors (IGF-I and -II) is mediated via the type I IGF receptor (IGF-IR) and is modulated by their association with high affinity binding proteins, IGFBP-1 to -6. We recently found that, in addition to its ability to bind IGFs, IGFBP-3 also inhibits IGF-IR activation independently of IGF binding and without interacting directly with IGF-IR. Here, we show that IGFBP-3 is capable of blocking the signal triggered by IGFs. Breast carcinoma-derived cells (MCF-7) were stimulated by des(1-3)IGF-I or [Gln(3),Ala(4),Tyr(15),Leu(16)]IGF-I, two IGF analogues with intact affinity for IGF-IR, but with weak or virtually no affinity for IGFBPs, then incubated with IGFBP-3. The activated IGF-IR was desensitized through reversal of its autophosphorylation, following which both phosphatidylinositol 3-kinase and p42(MAPK) activities were depressed. Direct measurement of phosphotyrosine phosphatase activity and reconstitution experiments using tyrosine-phosphorylated insulin receptor substrate-1 (IRS-1) indicated that IGFBP-3 activated a phosphotyrosine phosphatase (PTPase). This action appeared to be peculiar to IGFBP-3 among the IGFBPs, since neither IGFBP-1 nor IGFBP-5 (structurally the closest to IGFBP-3), had any such effect. Several cell lines derived from normal or tumor cells responsive to IGF-I were used to show that IGFBP-3-stimulated PTPase is cell type-specific. Although the precise nature of the phosphatase remains to be determined, the results of this study demonstrate that IGFBP-3 stimulates a phosphotyrosine phosphatase activity that down-regulates the IGF-I signaling pathway, suggesting a major role for IGFBP-3 in regulating cell proliferation.     

The amino-terminal region of insulin-like growth factor binding protein-3, (1-95)IGFBP-3, induces apoptosis of MCF-7 breast carcinoma cells

In an earlier study, we reported that an N-terminal proteolytic fragment ((1-95)IGFBP-3) corresponding to the first 95 residues of human insulin-like growth factor binding protein-3 (IGFBP-3) inhibits proliferation in a variety of fibroblasts. With a view to investigating its cytostatic capacity in carcinoma cells, we transiently transfected MCF-7 breast adenocarcinoma cells with an expression vector containing (1-95)IGFBP-3 cDNA. The transfected cells secreted a hyper-glycosylated form of (1-95)IGFBP-3. Twenty-four hours after transfection, cell morphology and viability were similar in control and (1-95)IGFBP-3-secreting cells. However, after 48 h, (1-95)IGFBP-3-secreting cells were apoptotic, with marked cytoplasmic vacuolation and increased free histones in the cytoplasm. Culture media conditioned by (1-95)IGFBP-3-secreting cells also induced morphological changes and apoptosis in wild-type MCF-7 cells, indicating that (1-95)IGFBP-3 was responsible for the effects observed. These results provide further evidence that the N-terminal proteolytic fragment of IGFBP-3 has a functional role.     

The Arabidopsis TUBULIN-FOLDING COFACTOR A gene is involved in the control of the alpha/beta-tubulin monomer balance

The control of the stoichiometric balance of alpha- and beta-tubulin is important during microtubule biogenesis. This process involves several tubulin-folding cofactors (TFCs), of which only TFC A is not essential in mammalian in vitro systems or in vivo in yeast. Here, we show that the TFC A gene is important in vivo in plants. The Arabidopsis gene KIESEL (KIS) shows sequence similarity to the TFC A gene. Expression of the mouse TFC A gene under the control of the 35S promoter rescues the kis mutation, indicating that KIS is the Arabidopsis ortholog of TFC A. kis plants exhibit a range of defects similar to the phenotypes associated with impaired microtubule function: plants are reduced in size and show meiotic defects, cell division is impaired, and trichomes are bulged and less branched. Microtubule density was indistinguishable from that of the wild type, but microtubule organization was affected in trichomes and hypocotyl cells of dark-grown kis plants. The kis phenotype was rescued by overexpression of an alpha-tubulin, indicating that KIS is involved in the control of the correct balance of alpha- and beta-tubulin monomers.     

Does mutual interference always stabilize predator–prey dynamics? A comparison of models

Based on a qualitative analysis of ODE systems, the dynamic properties of alternative predator-prey models with predator-dependent functional response have been compared in order to study the role that predator interference plays in the stabilisation of trophic systems. The models considered for interference have different mathematical expressions and different conceptual foundations. Despite these differences, they give essentially the same qualitative results: when interference is low, increasing it has a positive effect on asymptotic stability and thus on the resilience of the biological system. When it is high, it is the contrary (with logistic prey growth, increasing the interference parameter ensures stability but leads to very small predator densities). Possible consequences on the evolution of the interference level in real ecosystems are discussed.