Copper and zinc ions differentially block asialoglycoprotein receptor-mediated endocytosis in isolated rat hepatocytes

The diabetes-susceptible gene SLC30A8/ZnT8 regulates hepatic insulin clearance

Recent genome-wide association studies demonstrated that common variants of solute carrier family 30 member 8 gene (SLC30A8) increase susceptibility to type 2 diabetes. SLC30A8 encodes zinc transporter-8 (ZnT8), which delivers zinc ion from the cytoplasm into insulin granules. Although it is well known that insulin granules contain high amounts of zinc, the physiological role of secreted zinc remains elusive. In this study, we generated mice with β cell-specific Slc30a8 deficiency (ZnT8KO mice) and demonstrated an unexpected functional linkage between Slc30a8 deletion and hepatic insulin clearance. The ZnT8KO mice had low peripheral blood insulin levels, despite insulin hypersecretion from pancreatic β cells. We also demonstrated that a substantial amount of the hypersecreted insulin was degraded during its first passage through the liver. Consistent with these findings, ZnT8KO mice and human individuals carrying rs13266634, a major risk allele of SLC30A8, exhibited increased insulin clearance, as assessed by c-peptide/insulin ratio. Furthermore, we demonstrated that zinc secreted in concert with insulin suppressed hepatic insulin clearance by inhibiting clathrin-dependent insulin endocytosis. Our results indicate that SLC30A8 regulates hepatic insulin clearance and that genetic dysregulation of this system may play a role in the pathogenesis of type 2 diabetes.

Nodal-mediated epigenesis requires dynamin-mediated endocytosis

Nodal proteins are diffusible morphogens that drive pattern formation via short-range feedback activation coupled to long-range Lefty-mediated inhibition. In the sea urchin embryo, specification of the secondary (oral-aboral) axis occurs via zygotic expression of nodal, which is localized to the prospective oral ectoderm at early blastula stage. In mid-blastula stage embryos treated with low micromolar nickel or zinc, nodal expression expands progressively beyond the confines of this localized domain to encompass the entire equatorial circumference of the embryo, producing radialized embryos lacking an oral-aboral axis. RNAseq analysis of embryos treated with nickel, zinc, or cadmium (which does not radialize embryos) showed that several genes involved in endocytosis were similarly perturbed by nickel and zinc but not cadmium. Inhibiting dynamin, a GTPase required for receptor-mediated endocytosis, phenocopies the effects of nickel and zinc, suggesting that dynamin-mediated endocytosis is required as a sink to limit the range of Nodal signaling.

Simultaneous multiple element detection by particle beam/hollow cathode-optical emission spectroscopy as a tool for metallomic studies: determinations of metal binding with apo-transferrin

Particle beam/hollow cathode-optical emission spectroscopy (PB/HC-OES) is presented as a tool for the determination of metal ion loading in transferrin (Tf). The elemental specificity of optical emission spectroscopy provides a means of assessing metal ion concentrations as well as the relative amounts of metal per unit protein concentration (up to 2 moles of Fe per mole of protein). The PB/HC-OES method allows for the simultaneous detection of metal content (Fe (I) 371.99, Ni (I) 341.41 nm, Zn (I) 213.86 nm, and Ag (I) 338.28 nm in this case), as well as elemental carbon and sulfur (C (I) 156.14 nm and S (I) 180.73 nm) that are reflective of the protein composition and concentration. Quantification for the metal species is based on calibration functions derived from aqueous solutions, with limits of detection for the entire suite being less than 1.0 μM. Determinations in this manner eliminate much of the ambiguity inherent in UV-VIS absorbance determinations of Tf metal binding. Validation of this method is obtained by analyzing loading response of Fe(3+) into Tf using the PB/HC-OES method and comparing the results with those of the standard UV-VIS absorbance method. Maximum Fe(3+) loading of Tf (based on the number of available binding sites) was determined to be 71.2 ± 4.7% by the PB/HC-OES method and 67.5 ± 2.5% for the UV-VIS absorbance method. Element emission ratios between the dopant metals and the carbon and sulfur protein constituents allow for concentration independent determinations of metal binding into Tf. Loading percentages were determined for Ni(2+), Zn(2+), and Ag(+) into Tf with maximum loading values of 19.5 ± 0.4%, 41.0 ± 4.4%, and 141.2 ± 4.3%, respectively. While of no apparent biological significance, Ag(+) presents an interesting case as a surrogate for Pt(2+), whose binding with Tf has shown to be quite different from the other metals. A different mode from the others is indeed observed, and is consistent with conjecture on the Pt(2+) mechanisms. Competitive binding studies not easily performed using absorbance spectroscopy are easily performed by simultaneous, multielement analysis, reflective of the metals and the protein content. In this work, there is clear competition between and Fe(3+) and Zn(2+) for binding in the C-terminus lobe of Tf, while Ni(2+) binds within the N-terminus lobe. Addition of Ag(+) to this mixture does not affect the other metals' distributions, but reflects binding at other protein sites.

A Nanoparticle-Based Model Delivery System To Guide the Rational Design of Gene Delivery to the Liver. 2. In Vitro and In Vivo Uptake Results

In Part 1 of our work (1), four nanoparticles were synthesized specifically for the purpose of identifying design constraints to guide next generation gene delivery to the liver. The four nanoparticles are Gal-50 and Gal-140 (galactosylated 50 and 140 nm nanoparticles) and MeO-50 and MeO-140 (methoxy-terminated 50 and 140 nm nanoparticles). All four particles have the same surface charge, and Gal-50 and Gal-140 have the same surface galactose density (ca. 25-30 pmol/cm2). Here, the hepatocyte uptake in vitro and hepatic distribution in vivo of these four nanoparticles is investigated. With freshly isolated hepatocytes, Gal-50 nanoparticles are taken up to a greater extent than are MeO-50, and both 50 nm beads are taken up to a much greater extent than either of the 140 nm nanoparticles. In mice, about 90% of the in vivo dose of Gal-140 nanoparticles is found within the liver 20 min after tail-vein injection. TEM and immunohistochemistry images confirm that Gal-140 nanoparticles are primarily internalized by Kupffer cells, though isolated examples of a few Gal-140 in hepatocytes are also observed. Gal-50 nanoparticles are overwhelmingly found in vesicles throughout the cytoplasm of hepatocytes, with only isolated examples of Kupffer cell uptake 20 min after tail vein injections in mice. Despite similar surface charge and ligand density, 50 nm nanoparticles are primarily found in hepatocytes while 140 nm nanoparticles are primarily observed in Kupffer cells. These results clearly show that slightly anionic, galactose-PEGylated nanoparticles with 25-30 pmol/cm2 galactose should be about 50 nm in diameter to preferentially target hepatocytes while they should be about 140 nm in diameter to selectively target Kupffer cells.

Expression of a prenylation-deficient Rab4 inhibits the GLUT4 translocation induced by active phosphatidylinositol 3-kinase and protein kinase B

The small GTPase Rab4 has been shown to participate in the subcellular distribution of GLUT4 under both basal and insulin-stimulated conditions in adipocytes. In the present work, we have characterized the effect of Rab4 DeltaCT, a prenylation-deficient and thus cytosolic form of Rab4, in this process. We show that the expression of Rab4 DeltaCT in freshly isolated adipocytes inhibits insulin-induced GLUT4 translocation, but only when this protein is in its GTP-bound active form. Further, it not only blocks the effect of insulin, but also that of a hyperosmotic shock, but does not interfere with the effect of zinc ions on GLUT4 translocation. Rab4 DeltaCT was then shown to prevent GLUT4 translocation induced by the expression of an active form of phosphatidylinositol 3-kinase or of protein kinase B, without altering the activities of the enzymes. Our results are consistent with a role of Rab4 DeltaCT acting as a dominant negative protein towards Rab4, possibly by binding to Rab4 effectors.

Albumin facilitates zinc acquisition by endothelial cells

Albumin has long been observed to have a marked influence on the delivery of zinc to cells, but the mechanism of the interaction remains elusive. We examined whether albumin facilitates the acquisition of zinc by endothelial cells. Cultures of endothelial cells were used to analyze binding and acquisition of zinc and albumin to test this interaction. Our results indicate that albumin plays a role in facilitating the physiological delivery of zinc to endothelial cells. Albumin receptors that preferentially recognize albumin molecules carrying a zinc atom were demonstrated on the endothelial cell surface. Endocytosis is instrumental in albumin uptake, which was also consistently true of zinc uptake. Zinc and albumin were acquired by the cells in a 1:1 molar stoichiometry during the first 20 min of incubation in a medium with equimolar concentrations of zinc and albumin. The amount of albumin associated with the cells stabilized after 30 min, whereas the amount of zinc continued to increase. One possible explanation for this result is that a physiological route for zinc delivery into endothelial cells is by co-transport with albumin via receptor-mediated endocytosis.