ZIP14 (SLC39A14) is Required for Suppression of Apoptosis and Hepatic Steatosis Induced by ER stress in Mouse Liver
Authors: Kim Min‐Hyun , Aydemir Tolunay Beker , Kim Jinhee , Cousins Robert J 
Published: November 2023

Perturbation of the protein folding function of the endoplasmic reticulum (ER) activates special pathways to restore ER homeostasis, termed the unfolded protein response (UPR). UPR enhances protein folding and reduces the translation of mRNA, which leads cells to adaptation against ER stress. When ER stress is too severe, ER stress‐mediated apoptosis can lead to further insults such as hepatic steatosis. Zinc, an essential trace element, is required for normal ER function to provide zinc for biosynthesis of zinc metalloproteins. Zinc deficiency has been demonstrated to trigger ER stress in a number of cell models. We recently have shown in mice that zinc deficiency mediated by low zinc diet exacerbates ER stress‐induced apoptosis in liver. Functional zinc transporter activity and zinc homeostasis are important to ER functions. Zinc transporter ZIP14 (SLC39A14) is abundantly expressed in liver maintaining hepatic zinc homeostasis by transporting extracellular and organellar zinc into the cytosol of cells. We found highly increased ZIP14 expression in mouse liver by administration of tunicamycin (TM), a potent ER stress inducer. However, the exact roles of ZIP14 during ER stress and UPR activation are unknown. Here we report that liver of wild type (WT) mice accumulated extra zinc following TM administration, whereas this response was not observed in Zip14−/− (KO) mice. Impaired zinc uptake of Zip14 KO during ER stress coincided with greater expression of pro‐apoptotic proteins in the UPR pathway including phosphorylated eukaryotic translation initiation factor 2α (eIF2α), activating transcription factor 4 (ATF4), and C/EBP homologous protein (CHOP). This indicates Zip14 KO mice had a greater level of hepatic apoptosis. In addition, we found that Zip14 KO mice had higher levels of liver tissue damage and hepatic steatosis after TM, which are hallmarks of severe ER stress. Gene expression profiling revealed that the hepatic steatosis in Zip14 KO coincided with higher expression of genes inducing de novo fatty acid synthesis, which were suppressed in WT during ER stress. Next, we fed mice with a high fat diet (HFD) for 16 weeks to induce ER stress with greater physiological relevance. After HFD, ZIP14 expression was elevated in livers of WT mice producing extra zinc uptake, which was not observed in Zip14 KO mice. Similar to TM‐challenge, Zip14 KO exhibited higher expression of pro‐apoptotic UPR proteins, indicating the genotype had greater apoptosis during HFD‐induced ER stress. Additionally, we found that Zip14 gene induction was transcriptionally regulated during TM. Transcription factor studies, including ChIP‐PCR, revealed that both ATF4 and activating transcription factor 6 (ATF6) bound to the Zip14 promoter to induce Zip14 transcripts during ER stress. In conclusion, these experiments using the whole‐body Zip14 KO mouse model demonstrated that functional ZIP14 is essential for suppression of ER stress‐induced apoptosis and hepatic steatosis by influencing the expression of proteins involved in the UPR pathway.Support or Funding InformationSupported by grant DK94244 from the National Institutes of Health