Search

Extensive endoplasmic reticulum (ER) stress damages the liver, causing apoptosis and steatosis despite the activation of the unfolded protein response (UPR). Restriction of zinc from cells can induce ER stress, indicating that zinc is essential to maintain normal ER function. However, a role for zinc during hepatic ER stress is largely unknown despite important roles in metabolic disorders, including obesity and nonalcoholic liver disease. We have explored a role for the metal transporter ZIP14 during pharmacologically and high-fat diet-induced ER stress using Zip14 -/- (KO) mice, which exhibit impaired hepatic zinc uptake. Here, we report that ZIP14-mediated hepatic zinc uptake is critical for adaptation to ER stress, preventing sustained apoptosis and steatosis. Impaired hepatic zinc uptake in Zip14 KO mice during ER stress coincides with greater expression of proapoptotic proteins. ER stress-induced Zip14 KO mice show greater levels of hepatic steatosis due to higher expression of genes involved in de novo fatty acid synthesis, which are suppressed in ER stress-induced WT mice. During ER stress, the UPR-activated transcription factors ATF4 and ATF6α transcriptionally up-regulate Zip14 expression. We propose ZIP14 mediates zinc transport into hepatocytes to inhibit protein-tyrosine phosphatase 1B (PTP1B) activity, which acts to suppress apoptosis and steatosis associated with hepatic ER stress. Zip14 KO mice showed greater hepatic PTP1B activity during ER stress. These results show the importance of zinc trafficking and functional ZIP14 transporter activity for adaptation to ER stress associated with chronic metabolic disorders., Competing Interests: The authors declare no conflict of interest.

An effective measure to assess zinc status of humans has remained elusive, in contrast to iron, where a number of indicators of metabolism/function are available. Using monocytes, T lymphocytes, and granulocytes isolated by magnetic sorting and dried blood spots (DBS) derived from 50 mul of peripheral blood, we evaluated the response of metallothionein (MT), zinc transporter, and cytokine genes to a modest (15 mg of Zn per day) dietary zinc supplement in human subjects. Transcript abundance was measured by quantitative real-time RT-PCR (QRT-PCR). Zinc supplementation increased MT mRNA abundance by up to 2-fold in RNA from leukocyte subsets, and 4-fold in RNA from DBS. Transcript levels for the zinc transporter genes ZnT1 and Zip3 were increased and decreased, respectively, by zinc supplementation. Expression of the ZnT and Zip genes among leukocyte subsets differ by up to 270-fold. Monocytes and granulocytes from supplemented subjects were activated by LPS, whereas T lymphocytes were activated by mimicking antigen presentation. With zinc consumption, TNF-alpha and IL-1beta expression was greater in activated monocytes and granulocytes, and IFN-gamma mRNA levels were higher in activated T lymphocytes. These studies show that QRT-PCR is a tool to reliably measure transcript abundance for nutritionally responsive genes in human subjects, and that a small sample of whole dried blood, when appropriately collected, can be used as the source of total RNA for QRT-PCR analysis. The results obtained also show that zinc supplementation of human subjects programs specific leukocytic subsets to show enhanced cytokine expression upon activation by stimulators of immunity.

Infection and inflammation produce systemic responses that include hypozincemia and hypoferremia. The latter involves regulation of the iron transporter ferroportin 1 by hepcidin. The mechanism of reduced plasma zinc is not known. Transcripts of the two zinc transporter gene families (ZnT and Zip) were screened for regulation in mouse liver after turpentine-induced inflammation and LPS administration. Zip14 mRNA was the transporter transcript most up-regulated by inflammation and LPS. IL-6 knockout (IL-6(-/-)) mice did not exhibit either hypozincemia or the induction of Zip14 with turpentine inflammation. However, in IL-6(-/-) mice, LPS produced a milder hypozincemic response but no Zip14 induction. Northern analysis showed Zip14 up-regulation was specific for the liver, with one major transcript. Immunohistochemistry, using an antibody to an extracellular Zip14 epitope, showed both LPS and turpentine increased abundance of Zip14 at the plasma membrane of hepatocytes. IL-6 produced increased expression of Zip14 in primary hepatocytes cultures and localization of the protein to the plasma membrane. Transfection of mZip14 cDNA into human embryonic kidney cells increased zinc uptake as measured by both a fluorescent probe for free Zn(2+) and (65)Zn accumulation, as well as by metallothionein mRNA induction, all indicating that Zip14 functions as a zinc importer. Zip14 was localized in plasma membrane of the transfected cells. These in vivo and in vitro experiments demonstrate that Zip14 expression is up-regulated through IL-6, and that this zinc transporter most likely plays a major role in the mechanism responsible for hypozincemia that accompanies the acute-phase response to inflammation and infection.