Zinc Modulates Several Transcription-Factor Regulated Pathways in Mouse Skeletal Muscle Cells.

Zinc is an essential metal ion involved in many biological processes. Studies have shown that zinc can activate several molecules in the insulin signalling pathway and the concomitant uptake of glucose in skeletal muscle cells. However, there is limited information on other potential pathways that zinc can activate in skeletal muscle. Accordingly, this study aimed to identify other zinc-activating pathways in skeletal muscle cells to further delineate the role of this metal ion in cellular processes. Mouse C2C12 skeletal muscle cells were treated with insulin (10 nM), zinc (20 µM), and the zinc chelator TPEN (various concentrations) over 60 min. Western blots were performed for the zinc-activation of pAkt, pErk, and pCreb. A Cignal 45-Reporter Array that targets 45 signalling pathways was utilised to test the ability of zinc to activate pathways that have not yet been described. Zinc and insulin activated pAkt over 60 min as expected. Moreover, the treatment of C2C12 skeletal muscle cells with TPEN reduced the ability of zinc to activate pAkt and pErk. Zinc also activated several associated novel transcription factor pathways including Nrf1/Nrf2, ATF6, CREB, EGR1, STAT1, AP-1, PPAR, and TCF/LEF, and pCREB protein over 120 min of zinc treatment. These studies have shown that zinc's activity extends beyond that of insulin signalling and plays a role in modulating novel transcription factor activated pathways. Further studies to determine the exact role of zinc in the activation of transcription factor pathways will provide novel insights into this metal ion actions.

The Zinc Transporter Zip7 Is Downregulated in Skeletal Muscle of Insulin-Resistant Cells and in Mice Fed a High-Fat Diet.

BACKGROUND: The zinc transporter Zip7 modulates zinc flux and controls cell signaling molecules associated with glucose metabolism in skeletal muscle. The present study evaluated the role of Zip7 in cell signaling pathways involved in insulin-resistant skeletal muscle and mice fed a high-fat diet. METHODS: Insulin-resistant skeletal muscle cells were prepared by treatment with an inhibitor of the insulin receptor, HNMPA-(AM)3 or palmitate, and Zip7 was analyzed along with pAkt, pTyrosine and Glut4. Similarly, mice fed normal chow (NC) or a high-fat diet (HFD) were also analyzed for protein expression of Glut4 and Zip7. An overexpression system for Zip7 was utilized to determine the action of this zinc transporter on several genes implicated in insulin signaling and glucose control. RESULTS: We identified that Zip7 is upregulated by glucose in normal skeletal muscle cells and downregulated in insulin-resistant skeletal muscle. We also observed (as expected) a decrease in pAkt and Glut4 in the insulin-resistant skeletal muscle cells. The overexpression of Zip7 in skeletal muscle cells led to the modulation of key genes involved in the insulin signaling axis and glucose metabolism including Akt3, Dok2, Fos, Hras, Kras, Nos2, Pck2, and Pparg. In an in vivo mouse model, we identified a reduction in Glut4 and Zip7 in the skeletal muscle of mice fed a HFD compared to NC controls. CONCLUSIONS: These data suggest that Zip7 plays a role in skeletal muscle insulin signaling and is downregulated in an insulin-resistant, and HFD state. Understanding the molecular mechanisms of Zip7 action will provide novel opportunities to target this transporter therapeutically for the treatment of insulin resistance and type 2 diabetes.

Targeting the Zinc Transporter ZIP7 in the Treatment of Insulin Resistance and Type 2 Diabetes.

Type 2 diabetes mellitus (T2DM) is a disease associated with dysfunctional metabolic processes that lead to abnormally high levels of blood glucose. Preceding the development of T2DM is insulin resistance (IR), a disorder associated with suppressed or delayed responses to insulin. The effects of this response are predominately mediated through aberrant cell signalling processes and compromised glucose uptake into peripheral tissue including adipose, liver and skeletal muscle. Moreover, a major factor considered to be the cause of IR is endoplasmic reticulum (ER) stress. This subcellular organelle plays a pivotal role in protein folding and processes that increase ER stress, leads to maladaptive responses that result in cell death. Recently, zinc and the proteins that transport this metal ion have been implicated in the ER stress response. Specifically, the ER-specific zinc transporter ZIP7, coined the "gate-keeper" of zinc release from the ER into the cytosol, was shown to be essential for maintaining ER homeostasis in intestinal epithelium and myeloid leukaemia cells. Moreover, ZIP7 controls essential cell signalling pathways similar to insulin and activates glucose uptake in skeletal muscle. Accordingly, ZIP7 may be essential for the control of ER localized zinc and mechanisms that disrupt this process may lead to ER-stress and contribute to IR. Accordingly, understanding the mechanisms of ZIP7 action in the context of IR may provide opportunities to develop novel therapeutic options to target this transporter in the treatment of IR and subsequent T2DM.

The zinc transporter SLC39A7 (ZIP7) harbours a highly-conserved histidine-rich N-terminal region that potentially contributes to zinc homeostasis in the endoplasmic reticulum.

The zinc transporter SLC39A7 (ZIP7)1 is a resident endoplasmic reticulum (ER) protein that is involved in controlling the release of zinc from this organelle into the cytosol. Subsequently, zinc plays a major role in processes that preserve cellular homeostasis. The ER contains a high concentration of zinc, and under normal physiological responses, maintains ER function. Disturbances in the concentration and distribution of zinc in the ER leads to abnormal processes that typify many disease states. ZIP7 is protective against ER stress and is a critical 'gate-keeper' of zinc release from the ER during processes that require cellular maintenance. However, it is not known how ZIP7 achieves this protective activity while maintaining cellular function. Bioinformatics analysis was utilised to determine the relationship between ZIP7 and other zinc transporters across humans and the animal and plant kingdom to determine the structure of this transporter in binding zinc in the ER. Analysis of the amino acid sequence of ZIP7 revealed several potential histidine binding sites for zinc in the N-terminal region that were significantly different in comparison to the other members of this family. Moreover, this histidine-rich region in the N-terminal of ZIP7 was highly conserved across the animal and plant kingdom. Accordingly, the highly conserved histidine-rich region in the N-termini of ZIP7 across the animal and plant kingdom suggests that this domain has critical function(s). We hypothesise that ER-localized ZIP7 can potentially sequester zinc to these histidine-rich regions and therefore provides a mechanism that is protective of this cellular structure.

Zinc stimulates glucose oxidation and glycemic control by modulating the insulin signaling pathway in human and mouse skeletal muscle cell lines.

Zinc is a metal ion that is an essential cell signaling molecule. Highlighting this, zinc is an insulin mimetic, activating cellular pathways that regulate cellular homeostasis and physiological responses. Previous studies have linked dysfunctional zinc signaling with several disease states including cancer, obesity, cardiovascular disease and type 2 diabetes. The present study evaluated the insulin-like effects of zinc on cell signaling molecules including tyrosine, PRSA40, Akt, ERK1/2, SHP-2, GSK-3ß and p38, and glucose oxidation in human and mouse skeletal muscle cells. Insulin and zinc independently led to the phosphorylation of these proteins over a 60-minute time course in both mouse and human skeletal muscle cells. Similarly, utilizing a protein array we identified that zinc could active the phosphorylation of p38, ERK1/2 and GSK-3B in human and ERK1/2 and GSK-3B in mouse skeletal muscle cells. Glucose oxidation assays were performed on skeletal muscle cells treated with insulin, zinc, or a combination of both and resulted in a significant induction of glucose consumption in mouse (p<0.01) and human (p<0.05) skeletal muscle cells when treated with zinc alone. Insulin, as expected, increased glucose oxidation in mouse (p<0.001) and human (0.001) skeletal muscle cells, however the combination of zinc and insulin did not augment glucose consumption in these cells. Zinc acts as an insulin mimetic, activating key molecules implicated in cell signaling to maintain glucose homeostasis in mouse and human skeletal muscle cells. Zinc is an important metal ion implicated in several biological processes. The role of zinc as an insulin memetic in activating key signaling molecules involved in glucose homeostasis could provide opportunities to utilize this ion therapeutically in treating disorders associated with dysfunctional zinc signaling.

Zinc transporters and insulin resistance: therapeutic implications for type 2 diabetes and metabolic disease.

BACKGROUND: Zinc is a metal ion that is essential for growth and development, immunity, and metabolism, and therefore vital for life. Recent studies have highlighted zinc's dynamic role as an insulin mimetic and a cellular second messenger that controls many processes associated with insulin signaling and other downstream pathways that are amendable to glycemic control. MAIN BODY: Mechanisms that contribute to the decompartmentalization of zinc and dysfunctional zinc transporter mechanisms, including zinc signaling are associated with metabolic disease, including type 2 diabetes. The actions of the proteins involved in the uptake, storage, compartmentalization and distribution of zinc in cells is under intense investigation. Of these, emerging research has highlighted a role for several zinc transporters in the initiation of zinc signaling events in cells that lead to metabolic processes associated with maintaining insulin sensitivity and thus glycemic homeostasis. CONCLUSION: This raises the possibility that zinc transporters could provide novel utility to be targeted experimentally and in a clinical setting to treat patients with insulin resistance and thus introduce a new class of drug target with utility for diabetes pharmacotherapy.

Zinc and Gastrointestinal Disorders: A Role for the Zinc Transporters Zips and ZnTs.

BACKGROUND: Zinc is a critical metal ion essential for life. The biological significance of zinc is highlighted by the enormous number of proteins (approximately 10% of the human proteome) that have zinc-binding capacity. Accordingly, zinc concentrations in cells are tightly regulated by two families of zinc transporter proteins: Slc30a (ZnT) and Slc39a (Zip). ZnT and Zip are known to decrease and increase cytosolic zinc concentrations respectively. Both zinc transporters are suggested to be implicated in a number of disorders and disease states through dysfunctional zinc transport including cancer, diabetes and gastrointestinal (GI) disease. METHOD: The goal of this work was to identify the role of zinc transporters in GI disease/disorders. Where possible, reference will be made in the context of the function of zinc transporters and their potential role in GI disorders/ diseases with a view towards their possible therapeutic utility in the treatment of these ailments. PubMed was utilized to search for articles with the terms "zinc and GI disease", "zinc transporters and GI disease", "zinc and gut", zinc transporters and gut", and "zinc transporters and intestinal disorders". RESULTS: We identified a number of reviews on GI disorders/disease states associated with zinc deficiencies, but the very proteins that transport this metal ion in these systems are not well-defined. From a systematic review, we identified the following zinc transporters (Zip1, 2, 4-7, 10, 11 and 14; and ZnT1, 8 and 10) as having some functional role in the GI system and potentially could have a therapeutic role in the treatment of GI disease/disorders. CONCLUSION: An increasingly common health issue in our communities is disorder/disease of the GI system. Although therapies targeting these disorders are somewhat beneficial, there is a need to develop better, more efficient treatments. Despite that many gut-related disorders/disease states have been analyzed in the context of zinc deficiencies, the transport proteins that move zinc across cells are not well-defined. Zinc transporters are expressed in a range of GI tissue and cells, and their roles in the maintenance, integrity and disease processes of the GI system need to be addressed. This might open a whole new avenue of opportunities for the development of novel therapies targeting these receptors in GI disease states.