Effects of a genetic variant rs13266634 in the zinc transporter 8 gene (SLC30A8) on insulin and lipid levels before and after a high-fat mixed macronutrient tolerance test in U.S. adults

BACKGROUND

The common C-allele of rs13266634 (c.973C>T or p.Arg325Trp) in SLC30A8 (ZNT8) is associated with increased risk of type 2 diabetes. While previous studies have examined the correlation of the variant with insulin and glucose metabolism, the effects of this variant on insulin and lipid responses after a lipid challenge in humans remain elusive. The goal of this study was to determine whether the C-allele had an impact on an individual's risk to metabolic syndromes in U.S. adults.

METHOD

We studied the genotypes of rs13266634 in 349 individuals aged between 18 and 65 y with BMI ranging from 18.5 to 45 kg/m². The subjects were evaluated for insulin, glucose, HbA1c, ghrelin, and lipid profiles before and after a high-fat mixed macronutrient tolerance test (MMTT).

RESULTS

We found that the effects of variants rs13266634 on glucose and lipid metabolism were sex-dimorphic, greater impact on males than on females. Insulin incremental area under the curve (AUC) after MMTT was significantly decreased in men with the CC genotype (p < 0.05). Men with the CC genotype also had the lowest fasting non-esterified fatty acid (NEFA) concentrations. On the other hand, the TT genotype was associated with a slower triglyceride removal from the circulation in men after MMTT. The reduced triglyceride removal was also observed in subjects with BMI ≥ 30 carrying either the heterozygous or homozygous T-allele. Nevertheless, the SNP had little effect on fasting or postprandial blood glucose and cholesterol concentrations.

CONCLUSION

We conclude that the CC genotype negatively affects insulin response after MMTT while the T-allele may negatively influence lipolysis during fasting and postprandial blood triglyceride removal in men and obese subjects, a novel finding in this study.

Identification of novel compound heterozygous variants in the SLC30A7 (ZNT7) gene in two French brothers with stunted growth, testicular hypoplasia, and bone marrow failure

Zinc is an essential trace mineral. Dietary zinc deficiency results in stunted growth, skin lesions, hypogonadism, and frequent infections in humans. Mice genetically lacking Slc30a7 suffer from mild zinc deficiency and are prone to development of prostate cancer and insulin resistance. Disease-causing variants or mutations in the human SLC30A7 (ZNT7) gene have not been previously reported. Here we describe two-boy siblings from a French family with stunted growth, testicular hypoplasia, and bone marrow failure. Exome sequencing revealed compound heterozygous variants in ZNT7 consisting of NM_133496.5:c.21dup; p.Asp8ArgfsTer3 and c.842 + 15 T > C inherited from their unaffected mother and father, respectively. The c.21dup variant led to a premature stop codon generated in exon 1 of the ZNT7 coding sequence. RNA-seq analysis demonstrated that the c.842 + 15 T > C variant resulted in a leaky mRNA splicing event generating a premature stop codon right after the splicing donor site of exon 8. Moreover, the expression of ZNT7 protein was remarkably reduced by 80-96% in the affected brothers compared to the control cells. These findings strongly suggest that biallelic variants in SLC30A7 should be considered as a cause of growth retardation, testicular hypoplasia, and syndromic bone marrow failure.

Trimethylamine N-Oxide Response to a Mixed Macronutrient Tolerance Test in a Cohort of Healthy United States Adults

Plasma trimethylamine n-oxide (TMAO) concentration increases in responses to feeding TMAO, choline, phosphatidylcholine, L-carnitine, and betaine but it is unknown whether concentrations change following a mixed macronutrient tolerance test (MMTT) with limited amounts of TMAO precursors. In this proof-of-concept study, we provided healthy female and male adults (n = 97) ranging in age (18-65 years) and BMI (18-44 kg/m2) a MMTT (60% fat, 25% sucrose; 42% of a standard 2000 kilo calorie diet) and recorded their metabolic response at fasting and at 30 min, 3 h, and 6 h postprandially. We quantified total exposure to TMAO (AUC-TMAO) and classified individuals by the blood draw at which they experienced their maximal TMAO concentration (TMAO-response groups). We related AUC-TMAO to the 16S rRNA microbiome, to two SNPs in the exons of the FMO3 gene (rs2266782, G>A, p.Glu158Lys; and rs2266780, A>G, p.Glu308Gly), and to a priori plasma metabolites. We observed varying TMAO responses (timing and magnitude) and identified a sex by age interaction such that AUC-TMAO increased with age in females but not in males (p-value = 0.0112). Few relationships between AUC-TMAO and the fecal microbiome and FMO3 genotype were identified. We observed a strong correlation between AUC-TMAO and TNF-α that depended on TMAO-response group. These findings promote precision nutrition and have important ramifications for the eating behavior of adults who could benefit from reducing TMAO exposure, and for understanding factors that generate plasma TMAO.

SNPs in apolipoproteins contribute to sex-dependent differences in blood lipids before and after a high-fat dietary challenge in healthy U.S. adults

Background

The effect of genetic polymorphisms on fasting blood lipid levels have been widely studied but the effects of these within the context of a high-fat meal challenge remain less characterized. The current study aimed to investigate the association of SNPs in lipoprotein-related genes with blood lipid profiles in healthy adults in the U.S.

Methods

Subjects (n = 393) between 18–66 years of age with BMIs ranging from 18.5–45 kg/m² were enrolled the cross-sectional Nutritional Phenotyping Study. Among them, 349 subjects (men: 48%; women: 52%) gave consent for genotyping. SNPs in APOA5, APOB, APOC3, APOE, and LDLR were assessed. The association between lipid markers and genotypes was tested separately for each SNP with analysis of variance (ANOVA), adjusted for sex, age, and BMI. We also examined two-factor interactions between SNPs and sex, age, or BMI.

Results

Women carrying the C allele of rs3135506 in APOA5 or men carrying the C allele of rs429358 in APOE had reduced HDL-cholesterol levels during fasting and postprandially. The C allele in APOE was also correlated to increased LDL-C levels. The TT genotype of rs2854116 in APOC3 was associated with elevated total cholesterol. Additive effect of the risk alleles of APOA5 and APOE or APOC3 and APOE was detected. Nevertheless, the tested SNPs had little impact on the postprandial triglyceride responses to the high-fat challenge meal. We found no significant effects of SNPs in APOB (rs1042034) or LDLR (rs2228671) on triglycerides, cholesterol, or free fatty acid levels.

Conclusions

In healthy adults, fasting and postprandial cholesterol levels are strongly correlated with the tested APOA5, APOE, and APOC3 genotypes. Sex contributes to the genetic impact of the tested SNPs on lipid profiles.

Trial registration

ClinicalTrials.gov, NCT02367287. Registered February 20, 2015, https://clinicaltrials.gov/ct2/show/NCT02367287.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40795-022-00592-x.

The Znt7-null mutation has sex dependent effects on the gut microbiota and goblet cell population in the mouse colon

Cellular homeostasis of zinc, an essential element for living organisms, is tightly regulated by a family of zinc transporters. The zinc transporter 7, ZnT7, is highly expressed on the membrane of the Golgi complex of intestinal epithelial cells and goblet cells. It has previously been shown that Znt7 knockout leads to zinc deficiency and decreased weight gain in C57BL/6 mice on a defined diet. However, effects within the colon are unknown. Given the expression profile of Znt7, we set out to analyze the changes in mucin density and gut microbial composition in the mouse large intestine induced by Znt7 knockout. We fed a semi-purified diet containing 30 mg Zn/kg to Znt7^-/- mice with their heterozygous and wild type littermates and found a sex specific effect on colonic mucin density, goblet cell number, and microbiome composition. In male mice Znt7 knockout led to increased goblet cell number and mucin density but had little effect on gut microbiome composition. However, in female mice Znt7 knockout was associated with decreased goblet cell number and mucin density, with increased proportions of the microbial taxa, Allobaculum, relative to wild type. The gut microbial composition was correlated with mucin density in both sexes. These findings suggest that a sex-specific relationship exists between zinc homeostasis, mucin production and the microbial community composition within the colon.

Association of Lactase Persistence Genotypes (rs4988235) and Ethnicity with Dairy Intake in a Healthy U.S. Population

Lactase persistence (LP) is a trait in which lactose can be digested throughout adulthood, while lactase non-persistence (LNP) can cause lactose intolerance and influence dairy consumption. One single nucleotide polymorphism (SNP ID: rs4988235) is often used as a predictor for dairy intake, since it is responsible for LP in people in European descent, and can occur in other ethnic groups. The objective of this study was to determine whether rs4988235 genotypes and ethnicity influence reported dairy consumption in the United States (U.S.). A food frequency questionnaire (FFQ) and multiple Automated Self-Administered 24-h recalls (ASA24®) were used to measure habitual and recent intake, respectively, of total dairy, cheese, cow's milk, plant-based alternative milk, and yogurt in a multi-ethnic U.S. cohort genotyped for rs4988235. Within Caucasian subjects, LP individuals reported consuming more recent total dairy and habitual total cow's milk intake. For subjects of all ethnicities, LP individuals consumed more cheese (FFQ p = 0.043, ASA24 p = 0.012) and recent total dairy (ASA24 p = 0.005). For both dietary assessments, Caucasians consumed more cheese than all non-Caucasians (FFQ p = 0.036, ASA24 p = 0.002) independent of genotype, as well as more recent intake of yogurt (ASA24 p = 0.042). LP subjects consumed more total cow's milk than LNP, but only when accounting for whether subjects were Caucasian or not (FFQ p = 0.015). Fluid milk and alternative plant-based milk consumption were not associated with genotypes or ethnicity. Our results show that both LP genotype and ethnicity influence the intake of some dairy products in a multi-ethnic U.S. cohort, but the ability of rs4988235 genotypes to predict intake may depend on ethnic background, the specific dairy product, and whether intake is reported on a habitual or recent basis. Therefore, ethnicity and the dietary assessment method should also be considered when determining the suitability of rs4988235 as a proxy for dairy intake.

Effects of Bacillus aryabhattai TBRC8450 on vibriosis resistance and immune enhancement in Pacific white shrimp, Litopenaeus vannamei

The use of probiotics in aquaculture is a practical alternative to promote animal health and disease prevention. Meanwhile, this practice can also reduce the use of prophylactic antibiotics. The purpose of this study was to identify candidate probiotics that could control pathogen populations in host's gastrointestinal (GI) tract and stimulate host immunity in shrimp aquaculture. Bacillus aryabhattai TBRC8450, a bacterial strain isolated from the environment in a shrimp farm, has an antimicrobial activity against many pathogenic strains of Vibrio harveyi and V. parahaemolyticus. Supplementation of B. aryabhattai to Pacific white shrimp (Litopenaeus vannamei) not only decreased the abundance of Vibrio populations, but also shifted the bacterial community in the shrimp GI tract. We found that supplementation of B. aryabhattai triggered shrimp innate immunity and antioxidant activities. mRNA expression of genes encoding microbial peptides and antioxidant enzymes, including C-type lectin, penaeidin-3, heat shock protein 60, thioredoxin, and ferritin, was significantly upregulated in the hepatopancreas of shrimp fed B. aryabhattai. Furthermore, phenoloxidase activity in the hemocytes and the total antioxidant activity in the plasma were increased, indicating enhanced immune and antioxidant responses at the systemic level. In contrast, supplementation of B. aryabhattai had no effect on the total hemocyte count and superoxide dismutase activity in the plasma and hepatopancreas. Importantly, a pathogen challenge test using V. harveyi 1562 showed a significant increase in survival rates of shrimp fed B. aryabhattai compared to the control group. Our findings suggest that B. aryabhattai TBRC8450 can likely be used as a probiotic to reduce the population of V. harveyi in the shrimp GI tract and to enhance shrimp innate immunity and antioxidant capacity for vibriosis resistance in shrimp aquaculture.

Subcongenic analysis of a quantitative trait locus affecting body weight and glucose metabolism in zinc transporter 7 (znt7)-knockout mice

Background

A genome-wide mapping study using male F₂zinc transporter 7-knockout mice (znt7-KO) and their wild type littermates in a mixed 129P1/ReJ (129P1) and C57BL/6J (B6) background identified a quantitative trait locus (QTL) on chromosome 7, which had a synergistic effect on body weight gain and fat deposit with the znt7-null mutation.

Results

The genetic segment for body weight on mouse chromosome 7 was investigated by newly created subcongenic znt7-KO mouse strains carrying different lengths of genomic segments of chromosome 7 from the 129P1 donor strain in the B6 background. We mapped the sub-QTL for body weight in the proximal region of the previously mapped QTL, ranging from 47.4 to 64.4 megabases (Mb) on chromosome 7. The 129P1 donor allele conferred lower body weight gain and better glucose handling during intraperitoneal glucose challenge than the B6 allele control. We identified four candidate genes, including Htatip2, E030018B13Rik, Nipa1, and Atp10a, in this sub-QTL using quantitative RT-PCR and cSNP detection (single nucleotide polymorphisms in the protein coding region).

Conclusions

This study dissected the genetic determinates of body weight and glucose metabolism in znt7-KO mice. The study demonstrated that a 17-Mb long 129P1 genomic region on mouse chromosome 7 conferred weight reduction and improved glucose tolerance in znt7-KO male mice. Among the four candidate genes identified, Htatip2 is the most likely candidate gene involved in the control of body weight based on its function in regulation of lipid metabolism. The candidate genes discovered in this study lay a foundation for future studies of their roles in development of metabolic diseases, such as obesity and type 2 diabetes.

Electronic supplementary material

The online version of this article (10.1186/s12863-019-0715-2) contains supplementary material, which is available to authorized users.

Monitoring of white striping and wooden breast cases and impacts on quality of breast meat collected from commercial broilers (Gallus gallus)

Objective

This study aimed at investigating white striping (WS) and wooden breast (WB) cases in breast meat collected from commercial broilers.

Methods

A total of 183 breast samples were collected from male Ross 308 broilers slaughtered at the age of 6 weeks (n = 100) and 7 weeks (n = 83). The breasts were subjected to meat defect inspection, meat quality determination and histology evaluation.

Results

Of 183, 4 breasts from 6-week-old broilers were classified as non-defective while the others exhibited the WS lesion. Among the 6-week-old birds, the defective samples from the medium size birds (carcass weight ≤2.5 kg) showed mild to moderate WS degree with no altered meat quality. Some of the breasts from the 6-week-old birds with carcass weight above 2.5 kg exhibited WB in accompanied with the WS condition. Besides of a reduction of protein content, increases in collagen matter and pH values in the defective samples (p<0.05), no other impaired quality indices were detected within this group. All 7-week-old broilers yielded carcasses weighing above 2.5 kg and showed abnormal characteristics with progressive severity. The breasts affected with severe WS and WB showed the greatest cook loss, hardness, springiness and chewiness (p<0.05). Development of WB induced significantly increased drip loss in the samples (p<0.05). Histology indicated necrotic events in the defective myofibers. Based on logistic regression, increasing percent breast weight by one unit enhanced the chance of WS and WB development with advanced severity by 50.9% and 61.0%, respectively. Delayed slaughter age from 6 to 7 weeks increased the likelihood of obtaining increased WS severity by 56.3%.

Conclusion

Cases of WS and WB defects in Southeast Asia have been revealed. Despite few cases of the severe WS and WB, such abnormal conditions significantly impaired technological properties and nutritional quality of broiler breasts.

Aberrant fatty acid metabolism in skeletal muscle contributes to insulin resistance in zinc transporter 7 (znt7)-knockout mice

ZnT7 (Slc30a7) is a widely expressed zinc transporter involved in sequestration of zinc into the Golgi apparatus and vesicular compartments. znt7-knockout (KO) mice are mildly zinc-deficient and lean. Despite their lean phenotype, adult male znt7-KO mice are prone to insulin resistance. We hypothesized that fat partitioning from adipose to nonadipose tissues causes insulin resistance in znt7-KO mice. Here, we used biological and biochemical methods, including fatty acid and oxylipin profiling, EM, immunohistochemistry, quantitative RT-PCR, and Western blot analysis, to identify the underlying mechanism of insulin resistance in znt7-KO mice. We found that insulin resistance in this model was primarily associated with increased intracellular fatty acid levels in the skeletal muscle, which promoted intracellular lipid accumulation and production of bioactive lipid mediators, such as 12,13-dihydroxyoctadecanoic acid (12,13-DiHOME) and 12-hydroxyeicosatetraenoic acid (12-HETE). The expression of fatty acid-binding protein 3 (Fabp3) was dramatically up-regulated in the znt7-KO muscle cells accompanied by increased expression of Cd36, Slc27a1, and Slc27a4, the three major fatty acid transporters in the skeletal muscle. We also demonstrated that znt7-KO muscle cells had increased fatty acid oxidative capacity, indicated by enlarged mitochondria and increased mRNA or protein expression of key enzymes involved in the fatty acid mitochondrial shuttle and β-oxidation. We conclude that increased fatty acid uptake in the znt7-KO skeletal muscle is a key factor that contributes to the excessive intracellular lipid deposit and elevated production of bioactive lipid mediators. These mediators may play pivotal roles in oxidative stress and inflammation, leading to insulin resistance.

ZNT7 binds to CD40 and influences CD154-triggered p38 MAPK activity in B lymphocytes-a possible regulatory mechanism for zinc in immune function

Zinc deficiency impairs the immune system leading to frequent infections. Although zinc is known to play critical roles in maintaining healthy immune function, the underlying molecular targets are largely unknown. In this study, we demonstrate that zinc is important for the CD154–CD40‐mediated activation of downstream signaling pathways in human B lymphocytes. CD40 is a receptor localized on the cell surface of many immune cells, including B lymphocytes. It binds to CD154, a membrane protein expressed on antigen‐activated T helper (Th) lymphocytes. This CD154‐CD40 interaction leads to B‐cell activation. We showed that cellular zinc deficiency impaired the CD154‐CD40‐mediated p38 mitogen‐activated protein kinase (p38 MAPK) phosphorylation. We also showed that zinc supplemental treatment of B lymphocytes had limited effect on this CD40‐mediated p38 MAPK signaling. Most importantly, we demonstrated that the zinc transporter protein zinc transporter 7 (ZNT7) interacted with CD40 using immunoprecipitation analyses. ZNT7 knockdown in B lymphocytes had a negative effect on the cell surface expression of CD40. Consequently, the CD40‐mediated p38 MAPK signaling transduction was down‐regulated in ZNT7KD B lymphocytes. Conversely, this p38 MAPK signaling activity was up‐regulated by overexpression (OE) of ZNT7 in B lymphocytes. Moreover, we found that ZNT7 knockdown in B lymphocytes constitutively up‐ and down‐regulated the inhibitor of i kappa B kinase and AKT serine/threonine kinase phosphorylation, respectively, which implies the activation of survival signaling in ZNT7KD B cells. We conclude that CD40 is the target molecule for ZNT7 in regulation of immune function of B lymphocytes.

Zinc transporter 7 deficiency affects lipid synthesis in adipocytes by inhibiting insulin-dependent Akt activation and glucose uptake

Mice deficient for zinc transporter 7 protein (ZnT7) are mildly zinc deficient with low body weight gain and body fat accumulation. To investigate the underlying mechanism of ZnT7 deficiency in body adiposity, we examined fatty acid composition and insulin sensitivity in visceral (epididymal) and subcutaneous fat pads from Znt7 knockout and control mice. We showed that ZnT7 deficiency had adverse effects on fatty acid metabolism and insulin action in subcutaneous fat but not in epididymal fat in mice, consistent with the ZnT7 protein expression pattern in adipose tissues. Importantly, we found that the expression of ZnT7 protein was induced by lipogenic differentiation and reached a peak when the adipocyte was fully differentiated in mouse 3T3-L1 adipocytes. We demonstrated, using Znt7 knockdown (Znt7KD) 3T3-L1 adipocytes, that reduction in Znt7 expression blunted activations of the signal transduction pathways that regulated both basal and insulin-stimulated glucose uptake in adipocytes, resulting in low glucose uptake and lipid accumulation. The expression of the signaling mediators critical for the initiation of pre-adipocyte differentiation, including Pparγ and C/Ebpα, appeared not to be affected by Znt7KD in 3T3-L1 adipocytes. These findings strongly suggest a role for ZnT7 in adipocyte lipogenesis.

Linking cellular zinc status to body weight and fat mass: mapping quantitative trait loci in Znt7 knockout mice

Zinc transporter 7 (Znt7, Slc30a7) knockout (KO) mice display abnormalities in body weight gain and body adiposity. Regulation of body weight and body fat accumulation is complex, involving multiple genetic and environmental factors. To understand how zinc homeostasis influences body weight and fat deposit and to identify quantitative trait loci (QTLs) that link zinc metabolism to growth and adiposity, we conducted a genome-wide mapping study using male F2 Znt7 KO mice and wild-type (WT) littermates with a mixed 129P1/ReJ and C57BL/6J genetic background. The mice were fed a semi-purified diet containing 30-mg Zn/kg diet at weaning. Body weights and fat pad weights including epididymal, retroperitoneal, and femoral subcutaneous fat pads were measured at 16 weeks of age. We detected two significant QTLs (p < 0.05) for body weight and fat deposit. One was in the F2 Znt7 KO population and the other in the F2 WT population. In Znt7 KO mice, the body weight and fat deposit was significantly linked to a locus on chromosome 7 ranging from 64.3 to 78.3 Mb. In WT mice, a significant linkage of retroperitoneal fat mass was found on chromosome 8 between 14.5 and 63.5 Mb. In addition, several other suggestive QTLs (p < 0.63) for body weight and fat accumulation were detected in Znt7 KO and WT mice. In conclusion, the QTLs identified in this study may provide new hints to uncover the genes linking cellular zinc status to growth and body fat accumulation.

Znt7-null mice are more susceptible to diet-induced glucose intolerance and insulin resistance

The Znt7 gene encodes a ubiquitously expressed zinc transporter that is involved in transporting cytoplasmic zinc into the Golgi apparatus and a ZnT7-containing vesicular compartment. Overexpression of ZnT7 in the pancreatic β-cell stimulates insulin synthesis and secretion through regulation of insulin gene transcription. In this study, we demonstrate that ZnT7 is expressed in the mouse skeletal muscle. The activity of the insulin signaling pathway was down-regulated in myocytes isolated from the femoral muscle of Znt7 knock-out (KO) mice. High fat diet consumption (45% kcal) induced weight gain in male Znt7 KO mice but not female Znt7 KO mice. Male Znt7 KO mice fed the high fat diet at 5 weeks of age for 10 weeks exhibited hyperglycemia in the non-fasting state. Oral glucose tolerance tests revealed that male Znt7 KO mice fed the high fat diet had severe glucose intolerance. Insulin tolerance tests showed that male Znt7 KO mice were insulin-resistant. Diet-induced insulin resistance in male Znt7 KO mice was paralleled by a reduction in mRNA expression of Insr, Irs2, and Akt1 in the primary skeletal myotubes isolated from the KO mice. Overexpression of ZnT7 in a rat skeletal muscle cell line (L6) increased Irs2 mRNA expression, Irs2 and Akt phosphorylation, and glucose uptake. We conclude that a combination of decreased insulin secretion and increased insulin resistance accounts for the glucose intolerance observed in Znt7 KO mice.

Over-expression of ZnT7 increases insulin synthesis and secretion in pancreatic beta-cells by promoting insulin gene transcription

The mechanism by which zinc regulates insulin synthesis and secretion in pancreatic beta-cells is still unclear. Cellular zinc homeostasis is largely maintained by zinc transporters and intracellular zinc binding proteins. In this study, we demonstrated that zinc transporter 7 (ZnT7, Slc30a7) was co-expressed with insulin in the islet of Langerhans in the mouse pancreas. In RIN5mF cells (rat insulinoma cells), ZnT7 was found mainly residing in the perinuclear region of the cell, which is consistent with its Golgi apparatus localization. Over-expression of ZnT7 in RIN5mF cells increased the total cellular insulin content leading to a high basal insulin secretion. Furthermore, glucose-induced insulin secretion was not altered in RIN5mF cells over-expressing ZnT7. Quantitative RT-PCR and (35)S metabolic labeling analysis demonstrated that over-expression of ZnT7 in RIN5mF cells led to an increase of insulin mRNA expression and subsequent insulin protein synthesis in the cell. Metal-responsive elements (MREs) were identified in the promoter regions of the Ins1 and Ins2 genes. Mtf1, a metal-responsive transcription factor, was shown to specifically bind to the MRE in the Ins genes and activated the insulin gene transcription. Together, the data strongly suggest that ZnT7 plays an important role in regulating insulin expression by modulating Mtf1 transcriptional activity.

Expression of the ZNT (SLC30) family members in the epithelium of the mouse prostate during sexual maturation

A prostate contains approximately 10-fold higher zinc than other soft organs. The function of the prostate is to produce a zinc-enriched seminal fluid. To establish a protein expression profile for zinc transporters involved in zinc efflux and intracellular sequestration/storage in the mouse prostate during sexual maturation, ZNT expression were investigated by immunohistochemistry. Our study demonstrated that ZNT proteins were differentially expressed in the prostate during sexual maturation. ZNT1 was mainly detected on the lateral membrane of the epithelium. Other ZNTs examined resided intracellularly. Among differences were a staining of ZNT2/ZNT5 in the ER-rich area of the epithelium in the anterior lobe, a staining of ZNT2 along the lateral and apical membrane, a luminal border staining of ZNT4, a staining of ZNT5 in the Golgi area of the epithelium in the ventral lobe, a uniform expression of ZNT6 across the lobes and ages, and a staining of ZNT7 in all lobes across ages.

A di-leucine sorting signal in ZIP1 (SLC39A1) mediates endocytosis of the protein

It has been demonstrated that the plasma membrane expression of ZIP1 is regulated by endocytic mechanisms. In the zinc-replete condition, the level of surface expressed ZIP1 is low due to the rapid internalization of ZIP1. The present study aimed to identify a sorting signal(s) in ZIP1 that mediated endocytosis of ZIP1. Four potential sorting signals (three di-leucine-and one tyrosine-based) were found by searching the eukaryotic linear motif resource for functional sites in proteins (http://elm.eu.org). Site-directed mutagenesis and immunofluorescence microscopic analyses demonstrated that the di-leucine sorting signal, ETRALL144-149, located in the variable loop region of ZIP1, was required for the ZIP1 internalization and lysosomal degradation. Substitutions of alanines for the di-leucine residues (LL148,149/AA) severely impaired the internalization of ZIP1 and subsequent protein degradation, leading to an accumulation of the mutant ZIP1 on the cell surface, as well as inside the cell. Using chimeric proteins composed of an alpha-chain of interleukin-2 receptor fused to the peptides derived from the variable loop region of ZIP1, we found that the di-leucine sorting signal of ZIP1 was required and sufficient for endocytosis of the chimeric proteins.

Immunohistochemical Analysis of ZnT1, 4, 5, 6, and 7 in the Mouse Gastrointestinal Tract

Expression of five zinc transporters (ZnT1, 4, 5, 6, and 7) of the Slc30 family in the mouse gastrointestinal tract was studied by immunohistochemical analysis. Results demonstrated unique expression patterns, levels, and cellular localization among ZnT proteins in the mouse gastrointestinal tract with some overlapping. ZnT1 was abundantly expressed in the epithelium of the esophagus, duodenum of the small intestine, and cecum of the large intestine. ZnT4 was predominantly detected in the large intestine. ZnT5 was mainly expressed in the parietal cell of the stomach and in the absorptive epithelium of the duodenum and jejunum. ZnT6 was predominantly detected in the chief cell of the stomach, columnar epithelial cells of the jejunum, cecum, colon, and rectum. Lastly, ZnT7 was observed in all epithelia of the mouse gastrointestinal tract with the highest expression in the small intestine. Expression of ZnT proteins in the absorptive epithelial cell of the gastrointestinal tract suggests that ZnT proteins may play important roles in zinc absorption and endogenous zinc secretion.

Decreased intracellular zinc in human tumorigenic prostate epithelial cells: a possible role in prostate cancer progression

Background

Zinc plays important roles in maintaining normal function of the prostate and in development of prostate malignancy. It has been demonstrated that prostate malignant epithelial cells contain much less cellular zinc than the surrounding normal epithelial cells. However, the pathway(s) which leads to lower zinc accumulation in malignant prostate epithelial cells is poorly understood. In this study, the zinc homeostatic features of two human prostate epithelial cell lines (non-tumorigenic, RWPE1, and tumorigenic, RWPE2) were investigated. Effects of over-expression of ZIP1 in RWPE2 on cell proliferation and apoptosis were also studied.

Results

RWPE2 accumulated less intracellular zinc than RWPE1 due to the decreased zinc uptake activity. The mRNA expression of ZIP1 and ZIP3 in RWPE1 and RWPE2 was comparable. However, the protein expression of ZIP1 in RWPE2 was lower than that in RWPE1. ZIP3 was detected in a lysosomal compartment of RWPE2 while no ZIP3 was detected in the same compartment of RWPE1. Over-expression of ZIP1 in RWPE2 resulted in an elevation of intracellular zinc concentration and suppression of cell growth of RWPE2 due to the increased apoptosis.

Conclusion

These findings suggest that tumorigenic prostate epithelial cells accumulated less intracellular zinc than non-tumorigenic prostate epithelial cells. The reduction in capacity for accumulation of intracellular zinc in tumorigenic prostate epithelial cells may be caused by the decrease in the ZIP1 protein expression and the intracellular redistribution of ZIP3 in RWPE2. RWPE1 and RWPE2 are excellent cellular models to study the association of intracellular zinc levels with prostate cancer progression.

Investigation of lymphocyte gene expression for use as biomarkers for zinc status in humans

A bioassay for zinc status in humans has been sought due to the importance of zinc, an essential trace metal, for many divergent functions in the human body; however, a sensitive bioassay for zinc status in humans is lacking. To address this issue, we established gene expression profiles of human lymphoblastoid cells treated with 0 or 30 micro mol/L ZnSO(4) using microarray technology. A limited number of genes were responsive to 30 micro mol/L zinc based on the analysis of Affymetrix human genome U133A GeneChips. We also examined the gene expression patterns of zinc transporters in human lymphoblastoid cells using quantitative RT-PCR analysis. ZNT1 was upregulated in lymphoblastoid cells, whereas ZIP1 was downregulated in response to the increased zinc concentrations in the culture media. To evaluate the potential applications of using both zinc transporter genes as biomarkers of zinc status, we measured the expression levels of ZIP1 and ZNT1 in the peripheral leukocytes collected from 2 different age groups of Korean women. After administration of a zinc supplement (22 mg zinc gluconate/d for 27 d), ZIP1 expression decreased by 17% (P < 0.01) and 21% (P < 0.05) in the peripheral leukocytes collected from 15 young (20-25 y) and 10 elderly (64-75 y) subjects, respectively. ZNT1 expression was not affected by taking the zinc supplement. These data suggest a potential application of ZIP1 as a biomarker of zinc status in humans.

ZnT7, a novel mammalian zinc transporter, accumulates zinc in the Golgi apparatus

ZnT7, a novel member of the zinc transporter (ZnT) family, was identified by searching the expressed sequence tag (EST) databases with the amino acid sequence of ZnT1. Like the other ZnT proteins, the protein (387 amino acids) predicted from this gene contains six transmembrane domains and a histidine-rich loop between transmembrane domains IV and V. We show that Znt7 is widely transcribed in mouse tissues with abundant expression in the liver and small intestine and moderate expression in the kidney, spleen, brain, and lung. An affinity-purified antibody raised against the amino acids 299-315 of mouse ZnT7 specifically reacted with the proteins with apparent molecular masses of 85, 43, and 65 kDa in small intestine and lung tissues by Western blot analysis. Immunofluorescence microscope analysis reveals that ZnT7 is localized in the Golgi apparatus and cytoplasmic vesicles. Exposure of the ZnT7-expressing Chinese hamster ovary (CHO) cells to zinc causes an accumulation of zinc in the Golgi apparatus, suggesting that ZnT7 facilitates zinc transport from the cytoplasm into the Golgi apparatus.

Functional characterization of a novel mammalian zinc transporter, ZnT6

We describe ZnT6, a new member of the CDF (cation diffusion facilitator) family of heavy metal transporters. The human ZNT6 gene was mapped at 2p21-22, while the mouse Znt6 was localized to chromosome 17. Overexpression of ZnT6 in both wild-type yeast and mutants that are deficient in cytoplasmic zinc causes growth inhibition, but this inhibition is abolished in mutant cells with high cytoplasmic zinc. ZnT6 may function in transporting the cytoplasmic zinc into the Golgi apparatus as well as the vesicular compartment, as evidenced by its overlapping intracellular localization with TGN38 and transferrin receptor in the normal rat kidney cells. We also demonstrate that the intracellular distributions of ZnT6 as well as ZnT4 are regulated by zinc in the normal rat kidney cells. The results from this report, combined with those from other studies, suggest that the intracellular zinc homeostasis is mediated by many ZnT proteins, which act in tissue-, cell-, and organelle-specific manners.