Chemerin enhances insulin signaling and potentiates insulin-stimulated glucose uptake in 3T3-L1 adipocytes

To explore a novel adipokine, we screened adipocyte differentiation-related gene and found that TIG2/chemerin was strongly induced during the adipocyte differentiation. Chemerin was secreted by the mature 3T3-L1 adipocytes and expressed abundantly in adipose tissue in vivo as recently described. Intriguingly, the expression of chemerin was differently regulated in the liver and adipose tissue in db/db mice. In addition, serum chemerin concentration was decreased in db/db mice. Chemerin and its receptor/ChemR23 were expressed in mature adipocytes, suggesting its function in autocrine/paracrine fashion. Finally, chemerin potentiated insulin-stimulated glucose uptake concomitant with enhanced insulin signaling in the 3T3-L1 adipocytes. These data establish that chemerin is a novel adipokine that regulates adipocyte function.     

The swinging movement of the distal histidine residue and the autoxidation reaction for midge larval hemoglobins.

Some insects have a globin exclusively in their fast-growing larval stage. This is the case in the 4th-instar larva of Tokunagayusurika akamusi, a common midge found in Japan. In the polymorphic hemoglobin comprised of 11 separable components, hemoglobin VII (Ta-VII Hb) was of particular interest. When its ferric met-form was exposed to pH 5.0 from 7.2, the distal histidine was found to swing away from the E7 position. As a result, the iron(III) was converted from a hexacoordinate to a pentacoordinate form by a concomitant loss of the axial water ligand. The corresponding spectral changes in the Soret band were therefore followed by stopped-flow and rapid-scan techniques, and the observed first-order rate constants of k(out) = 25 s(-1) and kin = 128 s(-1) were obtained for the outward and inward movements, respectively, of the distal histidine residue in 0.1 m buffer at 25 degrees C. For O2 affinity, Ta-VII Hb showed a value of P50 = 1.7 Torr at pH 7.4, accompanied with a remarkable Bohr effect (deltaH+ = -0.58) almost equal to that of mammalian hemoglobins. We have also investigated the stability property of Ta-VII HbO2 in terms of the autoxidation rate over a wide range of pH from 4 to 11. The resulting pH-dependence curve was compared with those of another component Ta-V HbO2 and sperm whale MbO2, and described based on a nucleophilic displacement mechanism. In light of the O2 binding affinity, Bohr effect and considerable stability of the bound O2 against acidic autoxidation, we conclude that T. akamusi Hb VII can play an important role in O2 transport and storage as the major component in the larval hemolymph.     

The alpha 1 beta 1 contact of human hemoglobin plays a key role in stabilizing the bound dioxygen.

When the alpha and beta chains were separated from human oxyhemoglobin (HbO(2)), each individual chain was oxidized easily to the ferric form, their rates being almost the same with a very strong acid-catalysis. In the HbO(2) tetramer, on the other hand, both chains become considerably resistant to autoxidation over a wide range of pH values (pH 5-11). Moreover, HbA showed a biphasic autoxidation curve containing the two rate constants, i.e. k(f) for the fast oxidation due to the alpha chains, and k(s) for the slow oxidation to the beta chains. The k(f)/k(s) ratio increased from 3.2 at pH 7.5-7.3 at pH 5.8, but became 1 : 1 at pH values higher than 8.5. In the present work, we used the valency hybrid tetramers such as (alpha(3+))2(beta O(2))(2) and (alpha O(2)(2)(beta(3+))(2), and demonstrated that the autoxidation rate of either the alpha or beta chains (when O2- ligated) is independent of the valency state of the corresponding counterpart chains. From these results, we have concluded that the formation of the alpha 1 beta 1 or alpha 2 beta 2 contact suppresses remarkably the autoxidation rate of the beta chain and thus plays a key role in stabilizing the HbO(2) tetramer. Its mechanistic details were also given in terms of a nucleophilic displacement of O(2)(-) from the FeO(2) center, and the emphasis was placed on the proton-catalyzed process performed by the distal histidine residue.     

Ghrelin modulates the downstream molecules of insulin signaling in hepatoma cells.

Ghrelin was identified in the stomach as an endogenous ligand specific for the growth hormone secretagogue receptor (GHS-R). GHS-R is found in various tissues, but its function is unknown. Here we show that GHS-R is found in hepatoma cells. Exposure of these cells to ghrelin caused up-regulation of several insulin-induced activities including tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1), association of the adapter molecule growth factor receptor-bound protein 2 with IRS-1, mitogen-activated protein kinase activity, and cell proliferation. Unlike insulin, ghrelin inhibited Akt kinase activity as well as up-regulated gluconeogenesis. These findings raise the possibility that ghrelin modulates insulin activities in humans.     

Frizzled receptors activate a novel JNK-dependent pathway that may lead to apoptosis.

Extracellular Wnt ligands and their receptors of the Frizzled family control cell fate, proliferation, and polarity during metazoan development. Frizzled signaling modulates target gene expression through a beta-catenin-dependent pathway, functions to establish planar cell polarity in Drosophila epithelia, and activates convergent extension movements and intracellular Ca(2+) signaling in frog and fish embryos. Here, we report that a Frizzled receptor, Xenopus Frizzled 8 (Xfz8), activates c-Jun N-terminal kinases (JNK) and triggers rapid apoptotic cell death in gastrulating Xenopus embryos. This activity of Xfz8 required the cytoplasmic tail of the receptor and was blocked by a dominant inhibitor of JNK. Moreover, the cytoplasmic tail of Xfz8 targeted to the membrane was sufficient for activation of JNK and apoptosis. The apoptotic signaling was shared by a specific subset of Frizzled receptors, was inhibited by Wnt5a, and occurred in a Dishevelled- and T cell factor (TCF)-independent manner. Thus, our experiments identify a novel Frizzled-dependent signaling pathway, which involves JNK and differs from the beta-catenin-dependent and convergent extension pathways.     

Identification of novel proteins in isolated polyphosphate vacuoles in the primitive red alga Cyanidioschyzon merolae

Plant vacuoles are organelles bound by a single membrane, and involved in various functions such as intracellular digestion, metabolite storage, and secretion. To understand their evolution and fundamental mechanisms, characterization of vacuoles in primitive plants would be invaluable. Algal cells often contain polyphosphate‐rich compartments, which are thought to be the counterparts of seed plant vacuoles. Here, we developed a method for isolating these vacuoles from Cyanidioschyzon merolae, and identified their proteins by MALDI TOF‐MS. The vacuoles were of unexpectedly high density, and were highly enriched at the boundary between 62 and 80% w/v iodixanol by density‐gradient ultracentrifugation. The vacuole‐containing fraction was subjected to SDS–PAGE, and a total of 46 proteins were identified, including six lytic enzymes, 13 transporters, six proteins for membrane fusion or vesicle trafficking, five non‐lytic enzymes, 13 proteins of unknown function, and three miscellaneous proteins. Fourteen proteins were homologous to known vacuolar or lysosomal proteins from seed plants, yeasts or mammals, suggesting functional and evolutionary relationships between C. merolae vacuoles and these compartments. The vacuolar localization of four novel proteins, namely CMP249C (metallopeptidase), CMJ260C (prenylated Rab receptor), CMS401C (ABC transporter) and CMT369C (o‐methyltransferase), was confirmed by labeling with specific antibodies or transient expression of hemagglutinin‐tagged proteins. The results presented here provide insights into the proteome of C. merolae vacuoles and shed light on their functions, as well as indicating new features.