Effects of Zinc Deficiency Induced By Znt7 Inactivation On Lipid And Glucose Metabolism

Cellular zinc contents are tightly regulated by two families of zinc transporters: SLC30 or ZIP, and SLC39 or ZNT. ZIP proteins increase cytoplasmic zinc by moving zinc from the extracellular space or intracellular organelles into the cytosol. In contrast, ZNT proteins decrease cytoplasmic zinc by removing zinc from the cytosol into the extracellular space or intracellular organelles. Impaired zinc homeostasis can be caused by insufficient dietary zinc intake and/or mutations in zinc transporters. Zinc deficiency is associated with a number of metabolic diseases such as type I and II diabetes and obesity. Previous studies in animals and humans have demonstrated that zinc deficiency induces body fat loss while impairs glucose tolerance and insulin sensitivity. A zinc deficient mouse model induced by zinc transporter 7 (Znt7) inactivation displays mild zinc deficiency, decreased adiposity and impaired glucose homeostasis resembling the effect of zinc deficiency on humans. In the present study, we studied how impaired zinc homeostasis induced by Znt7 inactivation affects lipid and glucose metabolism to understand molecular mechanisms by which Znt7 inactivation decreases body fat accumulation while aggravates diabetic conditions.;In the first study, an adipocyte cell line, 3T3-L1, and a mouse model were used to investigate the effect of ZnT7 on adipocyte differentiation and body fat accumulation. ZnT7 expression was detected in mouse adipose tissues and upregulated during 3T3-L1 adipocyte differentiation suggesting the function of ZnT7 in adipocyte lipid metabolism. Znt7 knockdown (KD) in 3T3-L1 adipocytes significantly decreased neutral lipid and triglyceride contents. It was consistent with the phenotype of Znt7 knockout (KO) mice showing decreased body weight gain and body fat mass. Loss of ZnT7 had no effect on expression of adipocyte regulators, PPARgamma and C/EBPalpha. In contrast, ZnT7 inactivation impaired de novo lipogenesis which takes up extracellular glucose and converts to lipids in adipocytes. Znt7 KD adipocytes inhibited phosphorylation of signaling molecules, Erk and Akt, in the glucose uptake pathway. These findings demonstrated that disrupted zinc homeostasis by Znt7 inactivation reduced Erk and Akt phosphorylation resulting in decreased glucose uptake and lipid accumulation in adipocytes.;The null mutation of Znt7 in mice decreases body weight and body fat accumulation. The expressivity of the lean phenotype in Znt7 KO mice is dependent on mouse genetic background. Znt7 KO mice in mixed genetic background between 129P1/ReJ (129P1) and C57BL/6J (B6) promoted the lean phenotype compared to those in homogenous B6 background. The second study aimed to identify genetic modifier(s) of Znt7 inactivation on body weight gain and body fat mass. Using QTL mapping in F2 Znt7 KO mice with a mixed 129P1 and B6 background, we identified a significant QTL for body weight, adiposity index and fat pad mass on chromosome (Chr) 7 ranging from 64.3 to 78.3 Mb. As expected, the 129P1 allele enhanced decreased body weight gain, adiposity index and fat pad weight in Znt7 KO mice. This QTL was not significantly detected in F2 WT controls suggesting the presence of genetic modifier(s) of Znt7 KO mice. Analysis of mRNA expression in the QTL region showed differential expression of 13 genes between the B6 and 129P1 alleles in epididymal fat of the Znt7 KO mice. Among 13 genes, the levels of differential expression of Cpt1C and Adamts17 between B6 and 129P1 alleles were more dramatic in Znt7 KO mice compared to WT controls. These studies suggested that Cpt1C and Adamts17 were candidate modifier genes of body weight, adiposity index and fat pad mass in Znt7 KO mice.;Zinc deficiency induced by Znt7 inactivation demonstrated the inhibitory effect on lipid accumulation in adipocytes in both cell line and mouse model. Znt7 KO mice additionally exhibited insulin resistance and impaired glucose tolerance. Accumulation of lipids in skeletal muscle is known to cause insulin resistance and type II diabetes. Whether Znt7 inactivation disrupted lipid metabolism in skeletal muscle leading to development of insulin resistance remained to be elucidated. Lipid profiles in insulin sensitive peripheral tissues including subcutaneous fat, liver and femoral skeletal muscle clearly showed an increase in total long chain fatty acids and triglycerides in skeletal muscle. Increased levels of fatty acids and triglyceride contents were consistent with upregulation of genes in fatty acid transport, Slc27a1 and Slc27a4, and triglyceride synthesis, Acsl1 and Dgat2 in skeletal muscle. Palmitic acids (C16:0), oleic acids (C18:1, o-9), and linoleic acids (C18:2, ?-6) were most elevated in the skeletal muscle of Znt7 KO mice. Moreover, the levels of oxylipins derived from linoleic acids such as 9,10- and 12, 13-dihydroxy-9Z-octadecenoic acids (9,10-DiHOME and 12,13-DiHOME) were remarkably high in the skeletal muscle of Znt7 KO mice. It is known that palmitic acids and oxylipins can induce insulin resistance in skeletal muscle. These studies revealed zinc deficiency induced by Znt7 inactivation altered lipid metabolism and promotes in lipid deposition in femoral skeletal muscle which may be responsible for insulin resistance and impaired glucose tolerance in Znt7 KO mice.