Sodium Fluoride Mimics the Effect of Prostaglandin E2 on Catecholamine Release from Bovine Adrenal Chromaffin Cells

We have reported recently that prostaglandin E2 (PGE2) stimulated phosphoinositide metabolism in bovine adrenal chromaffin cells and that PGE2 and ouabain, an inhibitor of Na+, K(+)-ATPase, synergistically induced a gradual secretion of catecholamines from the cells. Here we examined the involvement of a GTP-binding protein(s) in PGE receptor-induced responses by using NaF. In the presence of Ca2+ in the medium, NaF stimulated the formation of all three inositol phosphates, i.e., inositol monophosphate, bisphosphate, and trisphosphate, linearly over 30 min in a dose-dependent manner (15-30 mM). This effect on phosphoinositide metabolism was accompanied by an increase in cytosolic free Ca2+. NaF also induced catecholamine release from chromaffin cells, and the dependency of stimulation of the release on NaF concentration was well correlated with those of NaF-enhanced inositol phosphate formation and increase in cytosolic free Ca2+. Although the effect of NaF on PGE2-induced catecholamine release in the presence of ouabain was additive at concentrations below 20 mM, there was no additive effect at 25 mM NaF. Furthermore, the time course of catecholamine release stimulated by 20 mM NaF in the presence of ouabain was quite similar to that by 1 microM PGE2, and both stimulations were markedly inhibited by amiloride, with half-maximal inhibition at 10 microM. Pretreatment of the cells with pertussis toxin did not prevent, but rather enhanced, PGE2-induced catecholamine release over the range of concentrations examined. These results demonstrate that NaF mimics the effect of PGE2 on catecholamine release from chromaffin cells and suggest that PGE2-evoked catecholamine release may be mediated by the stimulation of phosphoinositide metabolism through a putative GTP-binding protein insensitive to pertussis toxin.     

A second Warburg-like effect in cancer metabolism: The metabolic shift of glutamine-derived nitrogen

Carbon and nitrogen are essential elements for life. Glucose as a carbon source and glutamine as a nitrogen source are important nutrients for cell proliferation. About 100 years ago, it was discovered that cancer cells that have acquired unlimited proliferative capacity and undergone malignant evolution in their host manifest a cancer-specific remodeling of glucose metabolism (the Warburg effect). Only recently, however, was it shown that the metabolism of glutamine-derived nitrogen is substantially shifted from glutaminolysis to nucleotide biosynthesis during malignant progression of cancer—which might be referred to as a “second” Warburg effect. In this review, address the mechanism and relevance of this metabolic shift of glutamine-derived nitrogen in human cancer. We also examine the clinical potential of anticancer therapies that modulate the metabolic pathways of glutamine-derived nitrogen. This shift may be as important as the shift in carbon metabolism, which has long been known as the Warburg effect.     

The Aryl Hydrocarbon Receptor-interacting Protein (AIP) Is Required for Dioxin-induced Hepatotoxicity but Not for the Induction of the Cyp1a1 and Cyp1a2 Genes

The aryl hydrocarbon receptor (AHR) plays an essential role in the toxic response to environmental pollutants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin), in the adaptive up-regulation of xenobiotic metabolizing enzymes, and in hepatic vascular development. In our model of AHR signaling, the receptor is found in a cytosolic complex with a number of molecular chaperones, including Hsp90, p23, and the aryl hydrocarbon receptor-interacting protein (AIP), also known as ARA9 and XAP2. To understand the role of AIP in adaptive and toxic aspects of AHR signaling, we generated a conditional mouse model where the Aip locus can be deleted in hepatocytes. Using this model, we demonstrate two important roles for the AIP protein in AHR biology. (i) The expression of AIP in hepatocytes is essential to maintain high levels of functional cytosolic AHR protein in the mammalian liver. (ii) Expression of the AIP protein is essential for dioxin-induced hepatotoxicity. Interestingly, classical AHR-driven genes show differential dependence on AIP expression. The Cyp1b1 and Ahrr genes require AIP expression for normal up-regulation by dioxin, whereas Cyp1a1 and Cyp1a2 do not. This differential dependence on AIP provides evidence that the mammalian genome contains more than one class of AHR-responsive genes and suggests that a search for AIP-dependent, AHR-responsive genes may guide us to the targets of the dioxin-induced hepatotoxicity.     

The S-layer protein from Campylobacter rectus: sequence determination and function of the recombinant protein

The gene encoding the crystalline surface layer (S-layer) protein from Campylobacter rectus , designated slp , was sequenced and the recombinant gene product was expressed in Escherichia coli . The gene consisted of 4086 nucleotides encoding a protein with 1361 amino acids. The N-terminal amino acid sequence revealed that Slp did not contain a signal sequence, but that the initial methionine residue was processed. The deduced amino acid sequence displayed some common characteristic features of S-layer proteins previously reported. A homology search showed a high similarity to the Campylobacter fetus S-layer proteins, especially in their N-terminus. The C-terminal third of Slp exhibited homology with the RTX toxins from Gram-negative bacteria via the region including the glycine-rich repeats. The Slp protein had the same N-terminal sequence as a 104-kDa cytotoxin isolated from the culture supernatants of C. rectus . However, neither native nor recombinant Slp showed cytotoxicity against HL-60 cells or human peripheral white blood cells. These data support the idea that the N-terminus acts as an anchor to the cell surface components and that the C-terminus is involved in the assembly and/or transport of the protein.