Zinc and the Innovative Zinc-α2-Glycoprotein Adipokine Play an Important Role in Lipid Metabolism: A Critical Review.

Numerous studies indicate that zinc and the new zinc-related adipokine, zinc-α2-glycoprotein (ZAG), are involved in lipid metabolism. Excess body fat lowers blood concentrations of Zn and ZAG, leading not only to the development of obesity but also to other components of the metabolic syndrome. Zinc homeostasis disorders in the body negatively affect the lipid profile and cytokine secretion. Zinc appears to be a very important ZAG homeostasis regulator. The physiological effects of ZAG are related to lipid metabolism, but studies show that ZAG also affects glucose metabolism and is linked to insulin resistance. ZAG has a zinc binding site in its structure, which may indicate that ZAG mediates the effect of zinc on lipid metabolism. The review aimed to verify the available studies on the effects of zinc and ZAG on lipid metabolism. A literature review within the scope of this research area was conducted using articles available in PubMed (including MEDLINE), Web of Science and Cochrane Library databases. An analysis of available studies has shown that zinc improves hepatic lipid metabolism and has an impact on the lipid profile. Numerous studies have found that zinc supplementation in overweight individuals significantly reduced blood levels of total cholesterol, LDL (Low-density lipoprotein)cholesterol and triglycerides, potentially reducing cardiovascular morbidity and mortality. Some results also indicate that it increases HDL-C (High-density lipoprotein) cholesterol levels. ZAG has been shown to play a significant role in reducing obesity and improving insulin sensitivity, both in experimental animal model studies and in human studies. Furthermore, ZAG at physiologically relevant concentrations increases the release of adiponectin from human adipocytes. In addition, ZAG has been shown to inhibit in vitro leptin production. Further studies are needed to provide more data on the role of zinc and zinc-α2-glycoprotein.

Th17 and IL-17 Cause Acceleration of Inflammation and Fat Loss by Inducing α2-Glycoprotein 1 (AZGP1) in Rheumatoid Arthritis with High-Fat Diet.

Rheumatoid arthritis (RA) is a chronic autoimmune disorder that affects the joints. High-fat diet (HFD) is a risk factor for RA and is related to inflammation but responds minimally to medication. Given the association between HFD and inflammation, it is important to understand the function of inflammation-related T cells in RA with HFD. Collagen-induced arthritis (CIA), a model of RA, was induced in HFD mice by injection of collagen II, and metabolic markers and T cells were analyzed. The metabolic index and IgG assay results were higher in HFD-CIA mice than in nonfat diet-CIA mice. Numbers of inflammation-related T cells and macrophages, such as Th1 and Th17 cells and M1 macrophages, were higher in spleens of HFD-CIA mice. HFD-CIA mice had a high level of α2-glycoprotein 1 (Azgp1), a soluble protein that stimulates lipolysis. To examine the association between Azgp1 and Th17 cells, the reciprocal effects of Azgp1 and IL-17 on Th17 differentiation and lipid metabolism were measured. Interestingly, Azgp1 increased the Th17 population of splenocytes. Taken together, our data suggest that the acceleration of fat loss caused by Azgp1 in RA with metabolic syndrome is related to the increase of IL-17. Mice injected with the Azgp1-overexpression vector exhibited more severe CIA compared with the mock vector-injected mice.

Effect of curcumin on visfatin and zinc-α2-glycoprotein in a rat model of non-alcoholic fatty liver disease.

PURPOSE:: To investigate the effect of curcumin on visfatin and zinc-α2-glycoprotein (ZAG) expression levels in rats with non-alcoholic fatty liver disease (NAFLD). METHODS:: Fifty-six male rats were randomly divided into a control group (n=16) and model group (n=40) and were fed on a normal diet or a high-fat diet, respectively. Equal volumes of sodium carboxymethyl cellulose (CMC) were intragastrically administered to the control group for 4 weeks. At the end of the 12th week, visfatin and ZAG protein expression levels were examined by immunohistochemistry. Visfatin mRNA levels were measured by semi-quantitative reverse transcription polymerase chain reaction. RESULTS:: Compared with the control group, the model group showed significantly increased expression of visfatin in liver tissue (P < 0.01) and significantly decreased expression of ZAG (P < 0.01). These effects were ameliorated by curcumin treatment. CONCLUSIONS:: Visfatin and zinc-α2-glycoprotein may be involved in the pathogenesis of NAFLD. Treatment of NAFLD in rats by curcumin may be mediated by the decrease of visfatin and the increase of non-alcoholic fatty liver disease.

Adipokine zinc-α2-glycoprotein regulated by growth hormone and linked to insulin sensitivity.

OBJECTIVE: Hypertrophic obesity is associated with impaired insulin sensitivity and lipid-mobilizing activity of zinc-α2-glycoprotein. Adipose tissue (AT) of growth hormone (GH) -deficient patients is characterized by extreme adipocyte hypertrophy due to defects in AT lipid metabolism. It was hypothesized that zinc-α2-glycoprotein is regulated by GH and mediates some of its beneficial effects in AT. METHODS: AT from patients with GH deficiency and individuals with obesity-related GH deficit was obtained before and after 5-year and 24-month GH supplementation therapy. GH action was tested in primary human adipocytes. Relationships of GH and zinc-α2-glycoprotein with adipocyte size and insulin sensitivity were evaluated in nondiabetic patients with noncancerous cachexia and hypertrophic obesity. RESULTS: AT in GH-deficient adults displayed a substantial reduction of zinc-α2-glycoprotein. GH therapy normalized AT zinc-α2-glycoprotein. Obesity-related relative GH deficit was associated with almost 80% reduction of zinc-α2-glycoprotein mRNA in AT. GH increased zinc-α2-glycoprotein mRNA in both AT of obese men and primary human adipocytes. Interdependence of GH and zinc-α2-glycoprotein in regulating AT morphology and metabolic phenotype was evident from their relationship with adipocyte size and AT-specific and whole-body insulin sensitivity. CONCLUSIONS: The results demonstrate that GH is involved in regulation of AT zinc-α2-glycoprotein; however, the molecular mechanism linking GH and zinc-α2-glycoprotein in AT is yet unknown.

Mechanism of attenuation of skeletal muscle atrophy by zinc-alpha2-glycoprotein.

The mechanism by which the adipokine zinc-α2-glycoprotein (ZAG) increases the mass of gastrocnemius, but not soleus muscle of diabetic mice, has been evaluated both in vivo and in vitro. There was an increased phosphorylation of both double-stranded RNA-dependent protein kinase and its substrate, eukaryotic initiation factor-2α, which was attenuated by about two-thirds in gastrocnemius but not soleus muscle of ob/ob mice treated with ZAG (50 µg, iv daily) for 5 d. ZAG also reduced the expression of the phospho forms of p38MAPK and phospholipase A2, as well as expression of the ubiquitin ligases (E3) muscle atrophy F-box/atrogin-1 and muscle RING finger protein, and the increased activity of both caspase-3 and casapse-8 to values found in nonobese controls. ZAG also increased the levels of phospho serine-threonine kinase and mammalian target of rapamycin in gastrocnemius muscle and reduced the phosphorylation of insulin receptor substrate-1 (Ser307) associated with insulin resistance. Similar changes were seen with ZAG when murine myotubes were incubated with high glucose concentrations (10 and 25 mm), showing that the effect of ZAG was direct. ZAG produced an increase in cAMP in murine myotubes, and the effects of ZAG on protein synthesis and degradation in vitro could be replicated by dibutyryl cAMP. ZAG increased cAMP levels of gastrocnemius but not soleus muscle. These results suggest that protein accretion in skeletal muscle in response to ZAG may be due to changes in intracellular cAMP and also that ZAG may have a therapeutic application in the treatment of muscle wasting conditions.

Identification of potential serum biomarkers of inflammation and lipid modulation that are altered by fish oil supplementation in healthy volunteers.

Long chain n-3 polyunsaturated fatty acids (n-3 LCPUFA) lower risk of coronary heart disease (CHD), but mechanisms are not well understood. We used proteomics to identify human serum proteins that are altered by n-3 LCPUFA. Such proteins could identify pathways whereby they affect CHD. Eighty-one healthy volunteers entered a double blind randomised trial to receive 3.5 g of fish oil or 3.5 g of high oleic sunflower oil daily. Serum was collected before and after 6 wk of intervention. Serum was analysed by proteomics using 2-DE. Proteins that were differentially regulated were identified by MS. We also analysed serum apolipoprotein A1 (apo A1), high-density lipoprotein (HDL) particle size and haptoglobin. Serum levels of apo A1, apo L1, zinc-alpha-2-glycoprotein, haptoglobin precursor, alpha-1-antitrypsin precursor, antithrombin III-like protein, serum amyloid P component and haemopexin were significantly downregulated (all p<0.05) by fish oil compared with high oleic sunflower oil supplementation. Fish oil supplementation caused a significant shift towards the larger, more cholesterol-rich HDL(2) particle. The alterations in serum proteins and HDL size imply that fish oil activates anti-inflammatory and lipid modulating mechanisms believed to impede the early onset of CHD. These proteins are potential diagnostic biomarkers to assess the mechanisms whereby fish oil protects against CHD in humans.

Adipose tissue loss in adjuvant arthritis is associated with a decrease in lipogenesis, but not with an increase in lipolysis.

Adjuvant-induced arthritis is a model of rheumatoid arthritis that induces cachexia. In other cachectic situations, there is an increase in lipolysis resulting in a loss of adipose tissue mass. The aim of this work was to analyse the effect of chronic arthritis, induced by adjuvant injection, on white adipose tissue (WAT). For this purpose, rats were killed 10 days after adjuvant injection, when the first external symptoms appeared, on days 15 and 22 when the external signs of the illness reach their severest level. As arthritis decreases food intake, a pair-fed group was also included. Serum concentrations of insulin, leptin, adiponectin, glycerol and nitrites, as well as gene expression of leptin, adiponectin, hormone-sensitive lipase (HSL), fatty acid synthase (FAS), tumour necrosis factor alpha and zinc-alpha(2)-glycoprotein (ZAG) were determined. Arthritis decreased food intake between days 5 and 16, but not during the last 5 days of the experiment. There was a marked decrease in relative adipose tissue weight and in serum leptin and adiponectin as well as in their gene expression in WAT in arthritic rats. Arthritis decreased the gene expression of FAS in the WAT. However, none of these effects was found in pair-fed rats. Arthritis did not increase lipolysis, since arthritic rats have lower serum concentrations of glycerol, HSL mRNA in WAT, as well as liver ZAG mRNA than the pair-fed or control rats. These data suggest that in chronic arthritis the decrease in white adipose mass is secondary to a reduced adipose lipogenesis, and this effect is not mainly due to the decrease in food intake.

Detection and cloning of epidermal zinc-alpha 2-glycoprotein cDNA and expression in normal human skin and in tumors.

Zinc-alpha 2-glycoprotein (Zn alpha 2gp) is almost ubiquitous in body fluids, and its antibody labels the corresponding secretory epithelia. We have found that Zn alpha 2gp is also expressed in human epidermis. We cloned the Zn alpha 2gp cDNA by screening our cDNA library, derived from epidermal keratinocytes, with a probe for prostate Zn alpha 2gp. It had complete nucleic acid sequence homology with that from prostate, including the signal peptide. Just as Zn alpha 2gp expression is higher in more differentiated breast tumors, so in skin tumors the highest mRNA levels occurred in the normal controls, the lowest in basal cell carcinomas (the least differentiated epidermal tumor type), and intermediate levels in squamous cell carcinomas and Merkel cell carcinomas. A similar increase in Zn alpha 2gp gene expression with differentiation was observed when epidermal keratinocytes were cultured in media that varied in cellular maturation potential.

Effects of chronic administration of N-(4-hydroxyphenyl)retinamide (4-HPR) in rats on vitamin A metabolism in the eye.

The retinoid N-(4-hydroxyphenyl)retinamide (4-HPR) effectively inhibits cancer in a variety of tissues. In contrast to many other retinoids, the toxicity problems associated with administration of 4-HPR have been found to be minimal or absent. However, the effects of 4-HPR upon normal metabolism of native physiological forms of vitamin A in vivo have not been adequately investigated. To understand better the interaction between 4-HPR and the native physiological forms of vitamin A, the present study examines the effects of long-term administration of 4-HPR upon normal vitamin A metabolism in the eyes. Male Sprague-Dawley rats were fed either a control diet sufficient in vitamin A (CON group; 0.8 retinol equivalents [RE]/g diet; n = 28) or a CON diet supplemented with 4-HPR (CON + 4-HPR group; 1173 micrograms 4-HPR/g diet; n = 28). Following an i.v. dose of physiologically radiolabelled retinol, associated with its normal plasma transport complex, the vitamin A content and radioactivity of the plasma and eyes were examined at different times over a 41 day period. Mean plasma retinol levels measured during the study period were significantly reduced in the CON + 4-HPR group as compared with the CON group (23.5 +/- 7.0 and 50.3 +/- 5.3 [mean +/- S.D.] micrograms/dl, respectively). From approximately 7 days post-dosing, vitamin A levels in the eyes of the 4-HPR-treated group steadily decreased such that by the end of the study, they were only approximately one-fifth those of the CON group (0.098 +/- 0.075 and 0.50 +/- 0.053 RE, respectively). Kinetic analysis of vitamin A turnover in the eyes indicated that there was no apparent down-regulation of the fraction of vitamin A leaving this tissue on a daily basis; these values were found to be similar in both groups, averaging 0.104 +/- 0.0393 and 0.113 +/- 0.0373 per day (mean +/- fractional standard deviation [F.S.D.]) for the CON and CON + 4-HPR groups, respectively. At the same time, the flow of vitamin A through the eyes was significantly decreased in the CON + 4-HPR group eyes (0.0162 +/- 0.101 microgram/day) as compared with the CON group (0.0604 +/- 0.0672 micrograms/day). Our results suggest that compensatory mechanisms that would normally function to conserve depleting ocular vitamin A stores may be blocked in the 4-HPR-treated animals and further, that the 4-HPR itself appears to be interfering with the normal uptake and/or metabolism of vitamin A in the eye. These findings may help to provide at least a partial explanation for the visual impairment problems that have been reported in human trials that include long-term administration of 4-HPR.

Alternative splicing gives rise to two novel long isoforms of Zn-alpha 2-glycoprotein, a member of the immunoglobulin superfamily.

We have isolated, from a rat liver cDNA library, two cDNAs encoding novel long isoforms of Zn-alpha2-glycoprotein (Zn-alpha2-gp), a member of the immunoglobulin superfamily with a high degree of sequence similarity to class-I major histocompatibility complex (MHC) antigens. Nucleotide (nt) sequence analysis of these two novel cDNAs has revealed that they contain insertions of 138 and 123 nt between the second and third exons of Zn-alpha2-gp, resulting in in-frame insertions of 46 and 41 amino acids (aa), respectively. Analysis of the mechanism of generation of both isoforms, named Zn-alpha2-gpA and Zn-alpha2-gpB, has shown that they result from a series of alternative splicing events, including alternative use of two additional exons, and of two different 3'-splice sites present in the first of these novel exons. The occurrence of these alternative splicing events in Zn-alpha2-gp could contribute to increasing the diversity of this nonpolymorphic and soluble class-I MHC antigen.

Structure and expression of rat and mouse mRNAs for Zn-alpha 2-glycoprotein.

Rat and mouse cDNAs for Zn-alpha 2-glycoprotein (Zn alpha 2gp) were isolated from liver libraries (lambda gt11) and compared with the human one. The lengths of cDNA inserts analyzed were 1,233 and 1,273 nucleotides for rat and mouse, respectively. The deduced amino acid sequences suggested that rat and mouse Zn alpha 2gp proteins consist of 279 and 290 amino acid residues in the mature form, respectively. They have 59.4% (rat) and 58.6% (mouse) identities in amino acid sequence with the human counterpart, and between rat and mouse the identity is 88.5%. Among the three domains, domain B is best conserved; the identities are 74.7, 73.6, and 95.6% between human and rat, human and mouse, and rat and mouse, respectively. Four cysteine and eight tryptophan residues are all conserved, and two of the three asparagine residues that carry a glycan in the human protein are conserved. Analysis of rat tissues by Northern blot suggested that its mRNA is expressed in liver, and, to a much lesser extent in submandibular gland, lung, kidney, and stomach. A more detailed study by in situ hybridization demonstrated that some epithelial cells of renal tubules and the isthmus and the neck zone cells of gastric fundic glands express Zn alpha 2gp mRNA.

Cloning and expression analysis of the cDNA encoding rat Zn-alpha 2-glycoprotein.

We report here the nucleotide (nt) sequence of a rat liver cDNA encoding Zn-alpha 2-glycoprotein (Zn-alpha 2-gp), a plasma protein with a high degree of sequence similarity to class-I major histocompatibility complex (MHC) antigens. The deduced amino acid (aa) sequence contains the coding information for 293 aa residues and shows 60% identity with the aa sequence of human Zn-alpha 2-gp and 35% identity with that corresponding to the extracellular domains of RT1, a rat class-I MHC antigen. Northern blot analysis showed that rat Zn-alpha 2-gp is expressed in liver, but not in a wide number of tissues, including prostate, mammary gland, kidney, intestine, lung, pancreas, ovary, uterus, thyroid, placenta, spleen, brain and heart.