Fatty acid and amino acid modulation of glucose cycling in isolated rat hepatocytes.

We studied the influence of glucose/glucose 6-phosphate cycling on glycogen deposition from glucose in fasted-rat hepatocytes using S4048 and CP320626, specific inhibitors of glucose-6-phosphate translocase and glycogen phosphorylase respectively. The effect of amino acids and oleate was also examined. The following observations were made: (1) with glucose alone, net glycogen production was low. Inhibition of glucose-6-phosphate translocase increased intracellular glucose 6-phosphate (3-fold), glycogen accumulation (5-fold) without change in active (dephosphorylated) glycogen synthase (GSa) activity, and lactate production (4-fold). With both glucose 6-phosphate translocase and glycogen phosphorylase inhibited, glycogen deposition increased 8-fold and approached reported in vivo rates of glycogen deposition during the fasted-->fed transition. Addition of a physiological mixture of amino acids in the presence of glucose increased glycogen accumulation (4-fold) through activation of GS and inhibition of glucose-6-phosphatase flux. Addition of oleate with glucose present decreased glycolytic flux and increased the flux through glucose 6-phosphatase with no change in glycogen deposition. With glucose 6-phosphate translocase inhibited by S4048, oleate increased intracellular glucose 6-phosphate (3-fold) and net glycogen production (1.5-fold), without a major change in GSa activity. It is concluded that glucose cycling in hepatocytes prevents the net accumulation of glycogen from glucose. Amino acids activate GS and inhibit flux through glucose-6-phosphatase, while oleate inhibits glycolysis and stimulates glucose-6-phosphatase flux. Variation in glucose 6-phosphate does not always result in activity changes of GSa. Activation of glucose 6-phosphatase flux by fatty acids may contribute to the increased hepatic glucose production as seen in Type 2 diabetes.

The immune dysregulatory compound mercuric chloride induces integrin-mediated T-lymphocyte adhesion.

Exposure of Brown Norway rats to mercuric chloride induces systemic autoimmunity, involving T- and B-lymphocyte activation, (auto-)antibody production and multiorgan inflammation. Several divalent metal ions, such as Mg2+ and Mn2+, can activate binding of integrins to their ligands, thus causing lymphocyte adhesion. To test the hypothesis that Hg2+ acts in a similar way, we studied the effect of HgCl2 on integrin-mediated T-cell adhesion. HgCl2 induced cell-cell aggregation of human T lymphoblasts. Exposure of a human T-cell clone to HgCl2 for 1 hr enhanced, in a dose-dependent way, cell binding to fibronectin (FN) and to intercellular adhesion molecules (ICAM) -1, -2 and -3. Furthermore, HgCl2 induced strong binding of Jurkat T cells to FN. These effects of HgCl2 were of similar magnitude as the effects of phorbol 12-myristate 13-acetate (PMA) or MnCl2. Studies using blocking antibodies indicated the involvement of CD11a in binding to ICAMs, and of CD49d, CD49e, and CD29 in binding to FN. Adhesion to FN induced by HgCl2 or by PMA, but not by MnCl2, was dependent on temperature and on extracellular Ca2+ or Mg2+. Addition of cytochalasin B enhanced synergistically the FN adhesion induced by MnCl2, whereas the effects of PMA and HgCl2 were not modified. These results indicate that Hg2+ is a potent activator of T-cell adhesion, mediated by several integrins and ligands. In contrast to the effect of MnCl2, HgCl2-induced cell adhesion probably involves an intracellular pathway. Activation of integrins by HgCl2 may play an important role in activation and migration of leucocytes involved in HgCl2-induced immune dysregulation in vivo.