Apolipoprotein A-I gene expression is regulated by cellular zinc status in hep G2 cells

A systematic review and meta-analysis on the impact of oral vitamin E supplementation on apolipoproteins A1 and B100

BACKGROUND AND AIM

Cardiovascular diseases (CVDs) are the number one cause of mortality worldwide. Apolipoprotein B (ApoB), apolipoprotein A1 (ApoA1), and ApoB/ApoA1 ratio are considered as predictors of CVD alongside with lipid profile. Evidence suggest that nutrients with antioxidant properties, especially vitamin E, are essential for a healthy cardiovascular system. The aim of present meta-analysis was to determine the effect alpha-tocopherol on ApoA1 and ApoB levels.

METHODS

PubMed-Medline and SCOPUS databases and Google Scholar were searched up to July 2021. Random-effects model was employed to perform meta-analysis. In order to find heterogeneity sources, subgroup analysis was performed. Trim and fill analysis was performed in case of presence of publication bias. Quality assessment was performed using Cochrane Collaboration's tool.

RESULTS

Seven eligible studies, involving 1284 individuals were included. Mean age of participants ranged between 25.4 and 59 years. There was no significant effect of vitamin E supplementation on Apo A1 (SMD = 0.22 IU/d; 95% CI: -0.38, 0.28; P = 0.481) and Apo B levels (SMD = -0.62 IU/d; 95% CI: -1.94, 0.70; P = 0.360).

CONCLUSION

No remarkable effect of vitamin E supplementation was observed on ApoA1 and ApoB levels in adults. Additional studies investigating the influence of vitamin E on apolipoproteins as primary outcome with larger sample size are suggested.

A moderate increase in dietary zinc reduces DNA strand breaks in leukocytes and alters plasma proteins without changing plasma zinc concentrations

BACKGROUND

Food fortification has been recommended to improve a population's micronutrient status. Biofortification techniques modestly elevate the zinc content of cereals, but few studies have reported a positive impact on functional indicators of zinc status.

OBJECTIVE

We determined the impact of a modest increase in dietary zinc that was similar to that provided by biofortification programs on whole-body and cellular indicators of zinc status.

DESIGN

Eighteen men participated in a 6-wk controlled consumption study of a low-zinc, rice-based diet. The diet contained 6 mg Zn/d for 2 wk and was followed by 10 mg Zn/d for 4 wk. To reduce zinc absorption, phytate was added to the diet during the initial period. Indicators of zinc homeostasis, including total absorbed zinc (TAZ), the exchangeable zinc pool (EZP), plasma and cellular zinc concentrations, zinc transporter gene expression, and other metabolic indicators (i.e., DNA damage, inflammation, and oxidative stress), were measured before and after each dietary-zinc period.

RESULTS

TAZ increased with increased dietary zinc, but plasma zinc concentrations and EZP size were unchanged. Erythrocyte and leukocyte zinc concentrations and zinc transporter expressions were not altered. However, leukocyte DNA strand breaks decreased with increased dietary zinc, and the level of proteins involved in DNA repair and antioxidant and immune functions were restored after the dietary-zinc increase.

CONCLUSIONS

A moderate 4-mg/d increase in dietary zinc, similar to that which would be expected from zinc-biofortified crops, improves zinc absorption but does not alter plasma zinc. The repair of DNA strand breaks improves, as do serum protein concentrations that are associated with the DNA repair process. This trial was registered at clinicaltrials.gov as NCT02861352.

Low zinc environment induces stress signaling, senescence and mixed cell death modalities in colon cancer cells

Currently it is not clear what type of the final cellular response (i.e. cell death modality or senescence) is induced upon chronic intracellular zinc depletion in colon cancer cells. To address this question, isogenic colon cancer lines SW480 and SW620 exposed to low zinc environment were studied over the period of 6 weeks. Low zinc environment reduced total as well as free intracellular zinc content in both cell lines. Decreased intracellular zinc content resulted in changes in cellular proliferation, cell cycle distribution and activation of stress signaling. In addition, colonocytes with low zinc content displayed increased levels of oxidative stress, changes in mitochondrial activity but in the absence of significant DNA damage. Towards the end of treatment (4th-6th week), exposed cells started to change morphologically, and typical markers of senescence as well as cell death appeared. Of two examined colon cancer cell lines, SW480 cells proved to activate predominantly senescent phenotype, with frequent form of demise being necrosis and mixed cell death modality but not apoptosis. Conversely, SW620 cells activated mostly cell death, with relatively equal distribution of apoptosis and mixed types, while senescent phenotypes and necrosis were present only in a small fraction of cell populations. Addition of zinc at the beginning of 4th week of treatment significantly suppressed cell death phenotypes in both cell lines but had no significant effect on senescence. In conclusion, presented results demonstrate variability of responses to chronic zinc depletion in colon cancer as modeled in vitro.

The effect of nutritional supplements on serum high-density lipoprotein cholesterol and apolipoprotein A-I

One of the factors contributing to the increased risk of developing premature atherosclerosis is low plasma concentrations of high-density lipoprotein (HDL) cholesterol. Multiple potential mechanisms account for the cardioprotective effects of HDL and its main protein apolipoprotein A-I (apo A-I). Diet has an important role in modulating HDL cholesterol level. The widespread use of nutritional supplements may also alter the biology of HDL. In this review, we discuss the effect of select nutritional supplements on serum HDL cholesterol and apo A-I levels. Some nutritional supplements, such as phytosterols, soy proteins, and black seed extracts, may increase HDL cholesterol levels, while others such as cholic acid and high doses of commonly used antioxidant vitamins may downregulate HDL cholesterol levels and reduce its cardioprotection. Multiple mechanisms are involved in the regulation of HDL levels, so changes in production and clearance of HDL may have different clinical implications. The clinical relevance of the changes in HDL and apo A-I caused by nutrient supplementation needs to be tested in controlled clinical trials.

Feeding low or pharmacological concentrations of zinc oxide changes the hepatic proteome profiles in weaned piglets

Pharmacological levels of zinc oxide can promote growth and health of weaning piglets, but the underlying molecular mechanisms are yet not fully understood. The aim of this study was to determine changes in the global hepatic protein expression in response to dietary zinc oxide in weaned piglets. Nine half-sib piglets were allocated to three dietary zinc treatment groups (50, 150, 2500 mg/kg dry matter). After 14 d, pigs were euthanized and liver samples taken. The increase in hepatic zinc concentration following dietary supplementation of zinc was accompanied by up-regulation of metallothionein mRNA and protein expression. Global hepatic protein profiles were obtained by two-dimensional difference gel electrophoresis following matrix-assisted laser desorption ionization/time-of-flight mass spectrometry. A total of 15 proteins were differentially (P<0.05) expressed between groups receiving control (150 mg/kg) or pharmacological levels of zinc (2500 mg/kg) with 7 down- (e.g. arginase1, thiosulfate sulfurtransferase, HSP70) and 8 up-regulated (e.g. apolipoprotein AI, transferrin, C1-tetrahydrofolate synthase) proteins. Additionally, three proteins were differentially expressed with low zinc supply (50 mg/kg Zn) in comparison to the control diet. The identified proteins were mainly associated with functions related to cellular stress, transport, metabolism, and signal transduction. The differential regulation was evaluated at the mRNA level and a subset of three proteins of different functional groups was selected for confirmation by western blotting. The results of this proteomic study suggest that zinc affects important liver functions such as blood protein secretion, protein metabolism, detoxification and redox homeostasis, thus supporting the hypothesis of intermediary effects of pharmacological levels of zinc oxide fed to pigs.

Influence of insulin-like growth factor I and nutrition during phases of postnatal growth in very preterm infants

Pronounced growth restriction (GR) occurs after very preterm birth. The interaction between IGF-I, nutritional intake, and growth was evaluated prospectively in 64 infants with a mean (SD) GA of 25.7 (1.9) wk. Blood sampling of IGF-I and measurements of weight, length, and head circumference were performed weekly until discharge. Daily calculation of nutritional intake was performed. Standard deviation scores (SDSs) for growth parameters defined two growth phases: GR phase (birth until lowest SDS) and catch-up (CU) phase (lowest SDS until 35 gestational weeks). IGF-I concentrations during the first postnatal weeks were low and increased at 30 wk GA, irrespective of GA at birth, coinciding with initiation of CU growth. Concentrations of IGF-I were positively associated with change in weight SDS during the GR phase, p = 0.001 and CU phase, p = 0.004-0.027. Protein and energy intake were not associated with change in SDS weight during the GR phase as opposed to the CU phase (p < 0.001, respectively). Nutritional intake did not correlate to concentrations of IGF-I before 30 wk GA. IGF-I is associated with growth at an earlier postnatal age than nutrient intake and the effect of nutrition on levels of IGF-I may be restricted to the period of established CU growth.

Responses of human gingival and periodontal fibroblasts to a low-zinc environment

Morphology, motility, proliferation rate and markers of oxidative stress in primary human gingival fibroblasts (GF) and periodontal ligamental fibroblasts (PDL-F) grown in zinc-deficient cultivation medium (ZDM), were studied over a 5-week culture period. A low-zinc environment effectively reduced the total, as well as the free, intracellular zinc content in both cell types, over the course of the experiment. Decreased intracellular zinc content resulted in altered cellular morphology, reduced motility, and rearrangement of actin and tubulin in the cytoskeleton. In addition, fibroblasts with low zinc content exhibited decreased proliferation, accompanied by changes in cell cycle distribution, expression of specific biochemical markers, increased oxidative stress and the activation of caspase-3. Supplementation of ZDM with exogenous zinc prevented the loss of intracellular zinc, while also restoring the morphology, cell proliferation and mitogenic signalling of the cultured cells. Moreover, such supplemented cells were protected against oxidative stress and cell death. Of the two primary cell cultures examined, GF were more sensitive to decreased intracellular zinc content, when compared to PDL-F. The results obtained suggest that the human primary cell cultures can be useful for the longer-term evaluation of the effects of nutritional factors originating from the environment.

Influence of zinc deficiency on Akt-Mdm2-p53 and Akt-p21 signaling axes in normal and malignant human prostate cells

Phosphorylated Akt (p-Akt), a phosphoinositide-3-OH-kinase-activated protein kinase, is highly expressed in prostate tumors. p-Akt can indirectly hinder p53-dependent growth suppression and apoptosis by phosphorylating Mdm2. Alternatively, p-Akt can directly phosphorylate p21 and restrict it to the cytoplasm for degradation. Because the prostate is the highest zinc-accumulating tissue before the onset of cancer, the effects of physiological levels of zinc on Akt-Mdm2-p53 and Akt-p21 signaling axes in human normal prostate epithelial cells (PrEC) and malignant prostate LNCaP cells were examined in the present study. Cells were cultured for 6 days in low-zinc growth medium supplemented with 0 [zinc-deficient (ZD)], 4 [zinc-normal (ZN)], 16 [zinc-adequate (ZA)], or 32 [zinc-supplemented (ZS)] microM zinc. Zinc status of both cell types was altered in a dose-dependent manner, with LNCaP cells reaching a plateau at >16 microM zinc. For both cell types, p-Akt was higher in the ZD than in the ZN cells and was normalized to that of the ZN cells by treatment with a PI3K inhibitor, LY-294002. PTEN, an endogenous phosphatase targeting Akt dephosphorylation, was hyperphosphorylated (p-PTEN, inactive form) in ZD PrEC. Nuclear p-Mdm2 was raised, whereas nuclear p53 was depressed, by zinc deficiency in PrEC. Nuclear p21 and p53 were lowered by zinc deficiency in LNCaP cells. Higher percentages of ZD, ZA, and ZS than ZN LNCaP cells were found at the G(0)/G(1) phase of the cell cycle, with proportionally lower precentages at the S and G(2)/M phases. Hence, the increased p-PTEN in ZD PrEC would result in hyperphosphorylation of p-Akt and p-Mdm2, as well as reduction of nuclear p53 accumulation. For ZD LNCaP cells, Akt hyperphosphorylation was probably mediated through p21 phosphorylation and degradation, thus restricting p21 nuclear entry to induce cell cycle arrest. Thus zinc deficiency differentially modulated the Akt-Mdm2-p53 signaling axis in normal prostate cells vs. the Akt-p21 signaling axis in malignant prostate cells.

Transient catabolic state with reduced IGF-I after antenatal glucocorticoids

Glucocorticoid (GC) administration before preterm birth reduces neonatal morbidity but may restrain growth. Here we explored the effect of antenatal GC on nutrient substrates [glucose, FFA, amino acids (AA)], and on IGF-I and IGF-binding protein-1 (IGFBP-1). We analyzed umbilical vein (UV) plasma obtained at birth from 91 preterm newborns that received one course of GC (last exposure 1-1358 h before birth) and 49 newborns that did not. We found that recent GC exposure (-48 h) raised glucose, FFA, and AA concentrations, and the homeostasis model assessment of insulin resistance (HOMA-IR) index, but lowered IGF-I concentrations. The AA surge was greater in newborns with a birth weight z score <0 than in those with a z score >0. Although all AA were transiently increased, the increment was most robust for glutamine and alanine. Shorter duration since GC administration and lower IGF-I concentrations independently predicted AA levels. In conclusion, an antenatal course of GC elicited a transient catabolic state encompassing all nutrient substrates, and a temporary drop in IGF-I concentrations. These changes may explain the growth-inhibitory effects of repeated antenatal GC administration. Future research should clarify the role of IGF-I in the protein-catabolic response to GC.

Nuclear accumulations of p53 and Mdm2 are accompanied by reductions in c-Abl and p300 in zinc-depleted human hepatoblastoma cells

The influence of zinc status on the expression of proteins known to be involved in the stability of p53, the human tumor suppressor gene product, was examined in hepatoblastoma (HepG2) cells. Cells were cultured in zinc-deficient (ZD0.2, ZD0.4), zinc normal (ZN), zinc adequate (ZA), or zinc-supplemented (ZS) medium, which contained 0.2, 0.4, 4, 16, or 32 microM zinc, respectively. Nuclear p53 levels were almost 100% and 40% higher in ZD0.2 and ZD0.4 cells, respectively, than in ZN cells. Mdm2 protein, which mediates p53 degradation, was 174% and 148% higher in the nucleus of ZD0.2 and ZD0.4 cells, respectively, than in ZN cells. In addition, the observed reductions of nuclear c-Abl in ZD0.2 and ZD0.4 cells to 50% and 60% of ZN cells, respectively, may be a cellular response attempting to normalize nuclear p53 accumulation because nuclear c-Abl is known to down-regulate ubiquitination and nuclear export of p53. Moreover, no changes in total cellular p53, Mdm2, and c-Abl or nuclear Mdmx were observed among the treatment groups. Furthermore, in ZD0.2 and ZD0.4 cells, the reduction in total and nuclear p300, which is known to complex with CREB-binding protein and Mdm2 in the nucleus for the generation of degradable polyubiquitinated form of p53, may have depressed the degradation pathway for p53 and Mdm2, and contributed to the nuclear accumulation of these proteins in ZD cells.

The effect of select nutrients on serum high-density lipoprotein cholesterol and apolipoprotein A-I levels

One of the factors contributing to the increased risk of developing premature atherosclerosis is low plasma concentrations of high-density lipoprotein (HDL) cholesterol (HDLc). Multiple potential mechanisms account for the cardioprotective effects of HDL and its main protein apolipoprotein A-I (apo A-I). The low plasma concentrations of HDL could be the result of increased fractional clearance and reduced expression of apo A-I. To this end, nutrients play an important role in modulating the fractional clearance rate, as well as the rate of apo A-I gene expression. Because medical nutrition therapy constitutes the cornerstone of management of dyslipidemias, it is essential to understand the mechanisms underlying the changes in HDL level in response to alterations in dietary intake. In this review, we will discuss the effect of select nutrients on serum HDLc and apo A-I levels. Specifically, we will review the literature on the effect of carbohydrates, fatty acids, and ketones, as well as some of the nutrient-related metabolites, such as glucosamine and the prostanoids, on apo A-I gene expression. Because there are multiple mechanisms involved in the regulation of serum HDLc levels, changes in gene transcription do not necessarily correlate with clinical observations on serum levels of HDLc.

Intrauterine growth restriction ameliorates the effects of gradual hemorrhagic hypotension on regional cerebral blood flow and brain oxygen uptake in newborn piglets

Data are scant regarding the development of cerebrovascular autoregulation in intrauterine growth-restricted (IUGR) newborns. We tested the hypothesis that IUGR improves the ability of neonates to withstand critical periods of gradual hemorrhagic hypotension by optimizing cerebrovascular autoregulation. Studies were conducted on 1-d-old anesthetized piglets divided into groups of normal weight (NW, n = 14, body weight = 1518 +/- 122 g) and IUGR (n = 14, body weight = 829 +/- 50 g) animals. Physiologic parameters, including regional cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO(2)), were similar in NW and IUGR piglets under baseline conditions. Controlled arterial blood loss [hemorrhagic hypotension (HH)] induced a stepwise reduction of the mean arterial blood pressure of 49 +/- 3 mm Hg (mild HH), 39 +/- 3 mm Hg (moderate HH), and 30 +/- 3 mm Hg (severe HH) in seven NW and seven IUGR piglets (p < 0.05). In NW piglets, cortical CBF and CMRO(2) was reduced already at moderate HH (p < 0.05). A similar CMRO(2) reduction occurred during severe HH in NW and IUGR piglets (p < 0.05). In addition, during mild and moderate HH, primarily in IUGR piglets, an increase in regional CBF of brainstem, cerebellum, and thalamus was shown compared with baseline values (p < 0.05). Furthermore, under these conditions, cerebral cortex blood flow was maintained in newborn IUGR animals. In contrast, NW piglets exhibited a significant reduction in CBF (p < 0.05) during moderate HH. Thus, IUGR resulted in an improved ability to withstand critical periods of gradual oxygen deficit as shown by improved cerebrovascular autoregulation during hemorrhagic hypotension.

Effect of glucosamine on apolipoprotein AI mRNA stabilization and expression in HepG2 cells

Previously published studies suggest that an alteration in hexosamine flux induces a state of insulin resistance in muscle, liver, and other cell types. Glucosamine also alters the expression of several genes through an effect on transcription factors such as Sp1. Since the anti-atherogenic protein apolipoprotein AI (apoAI) is positively regulated by insulin, at least partly through its effect on Sp1, we investigated the effect of glucosamine on apoAI gene expression in the hepatocyte cell line, HepG2. By 24 hours of treatment with 0.1, 1, or 3 mmol/L glucosamine, the amount of apoAI protein secreted into the culture media increased 1.8-fold, 5.5-fold, and 2.3-fold, respectively. The decline in apoAI secretion at the highest glucosamine levels may be due to toxicity since the percentage of cells able to exclude trypan blue was lower in this group than in control cells (98.5% +/- 1.5% in control cells v 89.2% +/- 2.1% in cells treated with 3 mmol/L glucosamine, P <.01). ApoAI mRNA levels increased 2.4-fold in hepatocytes treated with 1 mmol/L glucosamine for 24 hours (1,158.1 +/- 78.8 v 482.2 +/- 24.3 arbitrary integrator units [AIU], P <.02), suggesting that the increase in apoAI protein secretion was due, at least partly, to an increase in apoAI mRNA levels. However, glucosamine had no effect on apoAI gene transcription rate as measured by nuclear runoff analysis (3,155 +/- 46.0 in control cells v 3,181 +/- 30.0 AIU in glucosamine-treated cells). Similarly, apoAI promoter activity measured in HepG2 cell transfected with an apoAI reporter plasmid containing the full-length apoAI promoter including an insulin-responsive Sp1 binding site did not change with glucosamine addition. In this assay, the chloramphenicol acetyltransferase (CAT) activity was 12.4% +/- 3.1%, 10.1% +/- 2.4%, 9.8% +/- 2.0%, 9.7% +/- 2.2%, and 11.9% +/- 2.9% in cells treated with 0, 0.03, 0.1, 0.3, and 1 mmol/L glucosamine, respectively. The apoAI mRNA turnover studies showed that 1 mmol/L glucosamine treatment of HepG2 cells was associated with increased apoAI mRNA half-life, from 7.6 to 16.6 hours. These findings suggest that increases in apoAI gene expression by glucosamine occur primarily through stabilizing apoAI mRNA.

Transcriptional control of apolipoprotein A-I gene expression in diabetes

Cardiovascular disease continues to be the leading cause of mortality in diabetes. One of the factors contributing to the increased risk is the high prevalence rate of low plasma concentrations of HDL cholesterol. Multiple potential mechanisms account for the cardioprotective effects of HDL and its main protein apolipoprotein (apo) A-I. The reduced plasma concentrations of HDL could be the result of increased fractional clearance of HDL and reduced expression of apo A-I. In animal models of diabetes and in cell cultures treated with high concentrations of glucose, apo A-I expression is reduced. In this review we will discuss the alterations in transcriptional control of apo A-I in diabetes. The role of select nutritional and hormonal alterations commonly found in diabetes will be reviewed. Specifically, we will review the literature on the effect of hyperglycemia, hypoinsulinemia, and ketoacidosis, as well as the role of various mediators of insulin resistance, such as fatty acids, cytokines, and prostanoids, on apo A-I promoter activity. Identifying the mechanisms that modulate apo A-I gene expression will aid in the new development of therapeutic agents that increase plasma apo A-I and HDL concentrations.

Effect of chromium on apolipoprotein A-I expression in HepG2 cells

OBJECTIVE

Chromium is a key micronutrient required for lipid and carbohydrate metabolism. Some but not all clinical trials have associated use of chromium supplements with improved insulin sensitivity and lipid profile including increased high-density lipoprotein cholesterol levels.

METHODS

Because apolipoprotein A-I (apoA-I) is the principal protein of high-density lipoprotein, the molecular pathways underlying chromium-related changes in apoA-I expression were studied in a human hepatoma cell line (HepG2) transfected with full-length apoA-I promoter attached to the reporter chloramphenicol acetyl transferase gene.

RESULTS

Exposure of these cells to different concentrations of chromium chloride (0, 0.5, 1.0, and 3.0 mM) resulted in a dose-dependent decrease in apoA-I promoter activity (chloramphenicol acetyl transferase activity expressed as a percentage of an internal control was 99.4 +/- 7.2% in control cells versus 87.6 +/- 5.0%, 73.4 +/- 2.3%, and 36.6 +/- 3.9%, respectively, P < 0.01). Chromium chloride at 10 mM concentration was toxic and caused death in a large number of cells. Treating HepG2 cells with other minerals known to have insulin-sensitizing effects such as magnesium (1 mM), zinc (0.2 mM), and vanadyl sulfate (0.1 mM) significantly reduced apoA-I promoter activity in the presence and absence of 100 microU/mL of insulin. Northern blot analyses showed that the apoA-I mRNA content of cells treated with 0.2 mM of chromium chloride relative to G3PDH mRNA was not significantly increased compared with controls (0.652 +/- 0.122 versus 0.745 +/- 0.143, the ratio of apoA-I to glyceraldehyde 3-phosphate dehydrogenase (G3PDH) mRNA in control and chromium-treated cells, respectively). Western blot analyses of proteins secreted in culture media indicated that neither chromium treatment of the HepG2 cells (858.0 +/- 151.4 arbitrary units) nor treatment with magnesium (1323.3 +/- 175.7) or vanadium (1102 +/- 78.7) significantly altered apoA-I concentrations compared with controls (1061.7 +/- 114.7). However treatment of HepG2 cells with 0.2 mM of zinc significantly reduced apoA-I concentrations (291.0 +/- 29.2 versus 1061.7 +/- 114.7; P < 0.001).

CONCLUSIONS

Supraphysiologic concentrations of chromium and other minerals with known insulin-sensitizing activity may reduce apoA-I promoter activity in cultured cells. Whether similar changes may occur in vivo remains to be shown. However, these observations do not support the use of pharmacologic amounts of chromium supplementation to enhance the cardioprotective lipid profile.

Zinc depletion reduced Egr-1 and HNF-3beta expression and apolipoprotein A-I promoter activity in Hep G2 cells

We examined the influence of zinc status on expression of certain transcription factors involved in regulation of apolipoprotein A-I (apoAI) expression in human hepatoblastoma Hep G2 cells. A low zinc basal medium (zinc deficient, ZD) consisting of DMEM and 10% Chelex100-treated fetal bovine serum was used to deplete cellular zinc over one passage. Cells were also cultured for one passage in medium supplemented with 0.4 (ZD0.4), 4.0 (zinc normal, ZN), 16.0 (zinc adequate, ZA), or 32.0 microM zinc (zinc supplemented, ZS). Compared with ZN cells, cellular zinc levels were 43 and 31% lower in ZD and ZD0.4 cells but 70 and 146% higher in ZA and ZS cells, respectively. Supplementation of 0.4 microM zinc significantly increased DNA contents per plate, from 65% in ZD cells to 83% in ZD0.4 cells compared with ZN cells. Addition of >4 microM zinc in medium did not further increase DNA contents. The proportion of cells in G(1)/S and S phase was about fourfold higher and threefold lower, respectively, in ZD cells compared with ZN and other groups. Nuclear Egr-1 protein was markedly decreased in ZD and ZD0.4 cells. Moreover, hepatocyte nuclear factor (HNF)-3beta was severely degraded in ZD and ZD0.4 cells. In contrast, HNF-4alpha remained stable in all groups and was not significantly lower in ZD and ZD0.4 cells. Furthermore, downregulation of trans-acting factor Egr-1 and cleavage of HNF-3beta were associated with reduction of apoAI promoter activity in zinc-deficient Hep G2 cells. Thus zinc is critical in transcriptional regulation of apoAI gene expression in hepatocytes.

p53 protein and p21 mRNA levels and caspase-3 activity are altered by zinc status in aortic endothelial cells

The influence of zinc status on the levels of p53, as well as downstream targets of p53 in cell repair and survival, was examined in human aortic endothelial cells (HAECs). A serum-reduced low-zinc medium (ZD) was used to deplete zinc over one passage. Other treatments included zinc-normal control (ZN), zinc-adequate (ZA), and zinc-supplemented (ZS) treatment with 3.0, 16.0, and 32.0 microM zinc, respectively. Cellular zinc levels in the ZD cells were 64% of ZN controls; levels in the ZA cells were not different, but levels in ZS cells were significantly higher (40%) than in ZN cells. No difference in p53 mRNA abundance was detected among all treatments; however, p53 nuclear protein levels were >100% higher in the ZD and ZS cells and almost 200% higher in the ZA cells than in ZN controls. In addition, p21 mRNA abundance, a downstream target of p53 protein, was increased in the ZS cells compared with both the ZN control and ZD cells. In the ZS cells, bax and mcl-1 were also approximately 50% higher compared with ZN controls, whereas bcl-2 mRNA was increased compared with ZA cells. Moreover, caspase-3 activity of ZD cells was not different from that of ZN controls but was reduced to 83 and 69% of ZN controls in ZA and ZS cells, respectively. Thus p53 protein and p53 downstream target genes appeared to be modulated by intracellular zinc status in HAECs.

Epidermal growth factor induction of apolipoprotein A-I is mediated by the Ras-MAP kinase cascade and Sp1

Insulin induces apolipoprotein A-I, apoA-I gene transcription via a membrane receptor with intrinsic tyrosine kinase activity. This finding prompted us to ask whether the gene is stimulated by epidermal growth factor (EGF), EGF a peptide hormone that binds to another member of the receptor superfamily with tyrosine kinase activity. Our data showed that like insulin, EGF increased abundance of apoA-I protein and transcription of the gene in human hepatoma, Hep G2 cells. The effects of both hormones appeared direct because their induction of apoA-I gene transcription was not affected by the protein synthesis inhibitor, cycloheximide. Although both insulin and EGF stimulate apoA-I expression, each hormone binds to a distinct membrane receptor thus suggesting differential intracellular signaling. Therefore, we used a panel of inhibitors to define the pathway(s) that mediate the actions of these hormones. Whereas, the actions of EGF required only the Ras-mitogen-activated protein, MAP kinase, those of insulin were mediated by equal participation of both the Ras-MAP kinase and protein kinase C, PKC cascades. Despite differences in signaling pathways triggered by each hormone receptor, the activation of apoA-I transcription required the participation of a single transcription factor, Sp1. Furthermore, EGF induction of transcription was attenuated by mutating the MAP kinase site at amino acid, Thr(266) rendering Sp1 phosphorylation deficient. In summary, EGF stimulation of apoA-I expression is mediated solely by the Ras-MAP kinase cascade and enhanced activity of this pathway requires Sp1 with an intact phosphorylation site at Thr(266). However, insulin induction of this gene is different and requires both Ras-MAP kinase and PKC pathways but their actions are also mediated by Sp1.

Retinol binding protein expression is induced in HepG2 cells by zinc deficiency

Zinc (Zn) deficiency is often associated with low plasma vitamin A (retinol) concentrations. It has been suggested that the reduction in plasma retinol is secondary to reduced liver retinol binding protein (RBP) synthesis. In the present study, RBP expression was determined in HepG2 cells cultured in either Zn adequate media or chelated media containing varying concentrations of Zn. Levels of RBP mRNA increased in a time- and Zn concentration-dependent manner such that 0.5 microM Zn-treated cells exhibited a >7.5-fold increase while cells treated with 15 microM Zn were increased 2.9-fold at 72 h compared to controls. RBP protein also progressively increased by 72 h to levels >8-fold and 3-fold higher than controls, in 0.5 microM and 15 microM Zn-treated cells, respectively. The increase in RBP occurred without any change in DNA concentration between groups through 72 h. The Zn deficiency-induced elevations in RBP transcript levels could be reversed within 24-48 h of repletion in Zn adequate media. Thus, the reductions in plasma retinol observed in Zn deficiency are in part a direct consequence of the deficiency.

Activation of apolipoprotein AI gene expression by protein kinase A and kinase C through transcription factor, Sp1

Our previous finding that insulin induces apolipoprotein AI (apoAI) transcription points to the participation of intracellular signaling. This finding prompted us to ask whether two classical G-protein-coupled signaling pathways requiring activated protein kinase A (PKA) or kinase C (PKC) may also regulate apoAI. Therefore, human hepatoma, Hep G2 cells stably transfected with pAI.474-CAT, a reporter construct spanning -474 to -7 of apoAI DNA fused to chloramphenicol acetyltransferase (CAT) were treated with 10 microm forskolin (FSK) or 50 nm phorbol dibutyrate (PDBu) to activate PKA and PKC, respectively. Results showed that the apoAI promoter activity increased 4-5-fold following 24 h of treatment with either FSK or PDBu. Induction by either agent was blocked with actinomycin D but not the protein synthesis inhibitor, cycloheximide. The PKA inhibitor, PKI 14-22 amide, abrogated induction by FSK, 100 microm 8-bromo-cAMP, or 100 ng/ml cholera toxin, but it had no effect on activation via PKC. Similarly, PDBu induction was attenuated by 2 microm of the PKC inhibitor, GF109203X, but it did not affect FSK activity. Next we used deletional constructs to show that the actions of FSK and PDBu required the insulin-responsive core element (IRCE). This motif matched the consensus binding site for the transcription factor, Sp1. The binding of Sp1 to the IRCE was confirmed by gel-retardation and supershift analysis. Site-directed mutagenesis of the IRCE eliminated Sp1 action and induction by FSK or PDBu. Whereas overexpression of Sp1 enhanced basal and FSK or PDBu induced promoter activity, transfection of an antisense oligomer against Sp1 mRNA attenuated both parameters. In summary, activation of PKA or PKC increases apoAI promoter activity. The activity of both signaling pathways is mediated by the IRCE, a motif that binds the transcription factor, Sp1.

Expression of the p53 tumor suppressor gene is up-regulated by depletion of intracellular zinc in HepG2 cells

Expression and activation of the p53 tumor suppressor protein are modulated by various cellular stimuli. The objective of this work was to examine the influence of zinc depletion on the expression of p53 in HepG2 cells. Two different low Zn (ZD) media, Zn-free Opti-MEM and a ZD medium containing Chelex-100 treated serum, were used to deplete cellular zinc over one passage. Cellular zinc levels of ZD cells were significantly lower than in their controls in both the Opti-MEM and Chelex studies. p53 mRNA abundance was 187% higher in ZD Opti-MEM cells and >100% higher in ZD Chelex cells compared with their respective controls. To examine whether the effects were specific to zinc depletion, a third, zinc-replenished group (ZDA) was included in the Opti-MEM study in which cells were cultured in ZD media for nearly one passage before a change was made to zinc-adequate (ZA) medium for the last 24 h. Zinc levels in the ZDA cells were significantly higher than in ZD cells, and p53 mRNA abundance was normalized to control levels. Nuclear p53 protein levels were >100% higher in the ZD Opti-MEM cells than in ZA cells. Interestingly, the ZDA Opti-MEM cells had significantly lower levels of nuclear p53 protein than both the ZA and ZD cells. These data suggest that expression of p53, a critical component in the maintenance of genomic stability, may be affected by reductions in cellular zinc.