Zinc increases EGF-stimulated DNA synthesis in primary mouse hepatocytes. Studies in tumor promoter-treated cell cultures

Zinc promotes proliferation and activation of myogenic cells via the PI3K/Akt and ERK signaling cascade

Skeletal muscle stem cells named muscle satellite cells are normally quiescent but are activated in response to various stimuli, such as injury and overload. Activated satellite cells enter the cell cycle and proliferate to produce a large number of myogenic progenitor cells, and these cells then differentiate and fuse to form myofibers. Zinc is one of the essential elements in the human body, and has multiple roles, including cell growth and DNA synthesis. However, the role of zinc in myogenic cells is not well understood, and is the focus of this study. We first examined the effects of zinc on differentiation of murine C2C12 myoblasts and found that zinc promoted proliferation, with an increased number of cells incorporating EdU, but inhibited differentiation with reduced myogenin expression and myotube formation. Furthermore, we used the C2C12 reserve cell model of myogenic quiescence to investigate the role of zinc on activation of myogenic cells. The number of reserve cells incorporating BrdU was increased by zinc in a dose dependent manner, with the number dramatically further increased using a combination of zinc and insulin. Akt and extracellular signal-regulated kinase (ERK) are downstream of insulin signaling, and both were phosphorylated after zinc treatment. The zinc/insulin combination-induced activation involved the phosphoinositide 3-kinase (PI3K)/Akt and ERK cascade. We conclude that zinc promotes activation and proliferation of myogenic cells, and this activation requires phosphorylation of PI3K/Akt and ERK as part of the signaling cascade.

Efficacy of zinc supplementation in preventing acute hepatitis in Long-Evans Cinnamon rats

OBJECTIVES

Long-Evans Cinnamon (LEC) rats are characterized by an abnormal hepatic deposition of copper (Cu) due to a lack of the Cu-transporter P-type adenosine triphosphatase: accordingly, the strain is a good animal model of Wilson's disease. The effect of oral zinc (Zn) acetate treatment on the development of acute hepatitis and the biochemical parameters of Cu-induced liver damage was studied in 5-week-old LEC rats (n=52).

METHODS

Rats receiving 50 or 80 mg/ml/day Zn acetate by gavage and control rats receiving a daily dose of glucose solution 0.02 g/ml by gastric intubation were killed at 1, 2 or 8 weeks after the start of treatment.

RESULTS

Treatment with Zn acetate resulted in the prevention of acute hepatitis: 10 of the 13 untreated rats developed signs and symptoms compatible with acute hepatitis between the 6th and 7th week of treatment. Tissue metallothionein (MT) significantly increased in the treated rats and positively correlated with Zn concentrations within the liver. Control rats had a significantly higher iron concentration in the liver and kidneys compared with supplemented rats, after both short- and long-term experiments. 8-hydroxy-2'-deoxyguanosine amounts were significantly lower in untreated rats.

CONCLUSIONS

Zn acetate prevents acute hepatitis, by increasing tissue MT concentrations, reducing Cu absorption and interfering with Fe metabolism.

Gap junction-mediated intercellular communication in a long-term primary mouse hepatocyte culture system

Gap junction-mediated intercellular communication (GJIC) is critical for maintaining integral cellular processes including differentiation and growth control. The disruption of GJIC has been correlated with aberrant function in many cell types, including hepatocytes in vivo; therefore it is imperative that cellular model systems support intercellular communication to simulate normal cellular functions. Functional GJIC has been shown in long-term primary rat hepatocyte cultures, which have been implemented widely to study various aspects of hepatocellular function; however, the onset of transgenic technology in murine species has necessitated the development of a primary mouse hepatocyte system. In this report, we analyze GJIC in a dimethylsulfoxide (DMSO)-containing long-term primary mouse hepatocyte culture system. The cells retain morphologic and biochemical characteristics of differentiated hepatocytes through day 30 post plating, including liver-specific gene expression. We further show that connexin32 and connexin26 expression and gap junction plaque formation increase over time in culture concomitant with an increase in GJIC between adjoining primary mouse hepatocytes. In conclusion, the findings described in this study make it possible to maintain differentiated primary mouse hepatocytes that also show GJIC in long-term culture for 30 days. In addition, this system has the potential to be extended to study primary mouse hepatocytes isolated from genetically engineered mice.

Inhibition of insulin-like growth factor-I mitogenic action by zinc chelation is associated with a decreased mitogen-activated protein kinase activation in RAT-1 fibroblasts

The mechanisms responsible for the resistance to the anabolic actions of IGF-I induced by zinc deficiency are not understood. We showed that zinc chelation by DTPA (diethylenetriaminepenta-acetic acid) inhibits [3H]thymidine incorporation stimulated by IGF-I in Rat-1 fibroblasts. This inhibition was specific of zinc chelation since it was prevented by the addition of zinc to DTPA. The stimulation of MAPK, which is crucial for the [3H]thymidine incorporation induced by IGF-I in Rat-1 cells, was partially blunted by DTPA. Therefore, the inhibition of the mitogenic action of IGF-I in Rat-1 fibroblasts by DTPA is potentially caused by decreased MAPK activation by IGF-I.

Flow cytometric evaluation of the DNA profile and cell cycle of zinc supplemented human Chang liver cells

Zinc, an essential trace element, is important for normal cell growth. Growing children, especially at puberty, require increased zinc (2.8 mg/day for males and 2.65 mg/day for females). The DNA profile and cell cycle of human Chang liver cells grown in 0-900 mumol/L zinc chloride supplemented serum-free media for 24 h were analyzed using a Coulter flow cytometer. There was no significant difference in the G1, S and G2/M phases between zinc treated cells and control cultures except at 90 and 900 mumol/L zinc chloride. At these two higher dosages, fragmentation of genomic DNA into sub-2N DNA (sub-G1 DNA), generally considered a hallmark of programmed cell death (PCD), was noted. Results of the present study seem to suggest that growth regulation by zinc during growth spurts such as at puberty, could also be influenced by other factors besides its direct effect on DNA synthesis. In addition, high dosages of zinc could be cytotoxic.

Metabolic alterations of zinc and prostaglandins in both human and animal colonic tumor cells

OBJECTIVE

To understand the relationship between zinc and prostaglandin (PG) metabolisms in inducing colon cancer incidence in human and animals.

METHODS

Human colonic tumor and normal cells were obtained from Departments of Surgery and Pathology at the Kaiser Permanente Medical Center, Los Angeles, CA and US VA Medical Center, North Hills, CA. Rat colonic tumor and normal cells were isolated from the rats that received two injections of 50 mg/kg of Azoxymethan (AOM) in 2 weeks and then kept 30 weeks in the animal facility. Then, the effects of zinc on the PGE2 synthesis and PGE2 on zinc metabolism in tumor and normal cells were determined.

RESULTS

PGE2 concentrations in both human and AOM-induced rat colonic tumor cells increased compared to those in adjacent normal colonocytes, whereas PGF2 alpha concentrations did not change. Gene expression of inducible form of prostaglandin synthase (PGS-2) is stimulated in rat colonocytes by epidermal growth factor and by tetradecanoyl 13-phorbol acetate (a tumor promoter and mitogen) only in the presence of zinc. PGE2 binding activity of rat enterocytes was maximum at 15 microM of zinc (normal plasma zinc concentration), but PGE2 synthesis activity increased for the first 15 minutes when extracellular zinc concentrations were either higher or lower than the normal extracellular zinc concentration. However, variations in extracellular zinc concentrations did not change the rate of PGF2 alpha synthesis in the normal rat enterocytes. PGE2 significantly increased zinc uptake rates of colonic tumor cells but PGF2 alpha showed only moderate effect.

CONCLUSIONS

These results suggest that zinc is required for PGS-2 gene expression, that maintaining an optimal zinc nutriture is important for normal PG synthesis of intestinal cells, and that only PGE2 synthesis mechanisms rather than PGS-2 gene expression are altered in colonic tumor cells resulting in the abnormal zinc nutriture of these cells.

Topical zinc oxide treatment increases endogenous gene expression of insulin-like growth factor-1 in granulation tissue from porcine wounds

Application of zinc oxide has been shown to accelerate the healing of both chronic and acute wounds, but the mechanisms are unknown. We quantified the gene expression (mRNA) for one important growth factor, insulin-like growth factor-1 (IGF-1) in 12 full-thickness wounds in each of three domestic pigs treated with or without topical zinc oxide. We used a RNAase protection/solution hybridisation technique to measure IGF-1 mRNA concentrations, which were 50% higher in the granulation tissue in wounds treated with zinc oxide compared with control wounds on days 3-4 (p < 0.05), but not thereafter (up to postoperative day 11). Topical zinc oxide increased the healing rate of wounds compared to the control group (p < 0.01). The cell composition of the granulation tissue was similar in the two groups. The increased gene expression of IGF-1 may be one mechanism by which topical zinc oxide enhances wound healing.

Measurement of zinc in hepatocytes by using a fluorescent probe, zinquin: relationship to metallothionein and intracellular zinc

Zinquin [ethyl (2-methyl-8-p-toluenesulphonamido-6-quinolyloxy)acetate], a new intracellular zinc fluorophore, was used to reveal and to measure Zn in cultured rat hepatocytes before and after metallothionein (MT) induction. Hepatocytes labelled with an intense extranuclear fluorescence. Culture with combinations of Zn, dexamethasone and interleukin-6, increased intracellular MT by 24-fold, Zn 3-fold, and Zinquin fluorescence by approx. 2-fold above control values. Zinquin fluorescence correlated in descending order with the total cellular Zn (r = 0.747), exchangeable Zn (r = 0.735), soluble cytosolic Zn (r = 0.669) and MT (r = 0.666). When Zinquin was incubated with a cytosolic fraction of liver proteins before Sephadex G-75 column chromatography, it fluoresced with free, MT-incorporated and protein-bound Zn. Although only a slight attenuation of fluorescence was seen with high-molecular-mass protein-bound Zn, MT was degraded by 60% in the presence of Zinquin. The undegraded Zn-MT fluoresced at about 20% of the expected intensity. Although Zinquin fluoresces with all cytosolic Zn, caution is required when comparisons are made between samples with different concentrations of MT. This limitation was demonstrated by staining liver slices from adjuvant-treated rats where MT was increased 24-fold, intracellular Zn by 77%, but Zinquin fluorescence by only 19% above controls. Nevertheless, Zinquin should prove to be a useful tool for studying the distribution of Zn in living cells.

Role of Ca2+ in stimulation of DNA synthesis by epidermal growth factor and tumor promoters in cultured rat hepatocytes

This study examines the effects of extracellular Ca2+ concentrations, [Ca2+]o, and of treatments known to modulate intracellular Ca2+ levels on the extent and timing of DNA synthesis in primary cultures of adult rat hepatocytes. In cultures exposed to insulin and EGF, the extent of DNA synthesis between 40 h and 70 h in culture was independent of [Ca2+]o in the range 25-1,800 microM, although the peak of DNA synthesis occurred 5-10 h earlier with 1.2 mM Ca2+ than with 25 microM Ca2+. Complete removal of extracellular Ca2+ using EGTA blocked DNA synthesis if Ca2+ was removed on the second day after EGF addition but not if Ca2+ was absent only on day 1. Treatment of cultures in 1.2 mM Ca(2+)-containing media with Ca(2+)-ionophore A23187 or with thapsigargin, agents expected to raise cytosolic [Ca2+], failed to augment the stimulation of DNA synthesis by EGF. These observations suggest that hepatocytes may have a permissive requirement for [Ca2+]o > 0 at least late in the sequence of events leading from growth factor stimulation to DNA synthesis. However, sustained elevation of cytosolic [Ca2+] does not appear to be important as an early signalling event either in mediating or augmenting EGF action in hepatocytes. The ability of liver tumor promoters alpha-hexachlorocyclohexane or DDT to stimulate DNA synthesis in combination with EGF was independent of [Ca2+]o. By contrast, the skin tumor-promoting phorbol ester, TPA, or liver tumor promoter, phenobarbital, were without effect or inhibitory at low [Ca2+]o but in combination with EGF, stimulated DNA synthesis at [Ca2+]o > 0.4 mM, suggesting that Ca2+ may have some role in mediating or modulating the stimulatory effects of these agents.

Growth stimulation of primary rat hepatocytes by 2,3,7,8-tetrachlorodibenzo-p-dioxin

The modulation of liver growth control by the tumor promoter, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), was investigated in primary hepatocytes of adult rats. Under defined conditions in serum-free cultures, the interaction of TCDD with growth-related hormones was studied. TCDD-treatment of the cultured hepatocytes for two days caused a transient stimulation of both DNA synthesis and mitotic activity. This effect was maximal at the very low nontoxic concentration of 10(-12) M TCDD, i.e., two orders of magnitude below the optimal concentrations for induction of drug metabolizing enzymes. Growth stimulation by TCDD was dependent on the presence of growth-related hormones; in primary rat hepatocytes, TCDD acted synergistically with insulin and epidermal growth factor (EGF) and antagonized the growth inhibition by dexamethasone. Under culture conditions allowing high rates of DNA synthesis, e.g., at low concentrations of dexamethasone, in the presence of EGF plus alpha 1-adrenergic agonists or rat serum, no significant effect of TCDD on cellular growth was observed. Furthermore, TCDD failed to stimulate DNA synthesis in a rat hepatoma cell line, H4IIE, which is less sensitive to growth controlling factors than normal hepatocytes. Therefore, the results suggest that the growth modulation of primary rat hepatocytes by TCDD is the most sensitive parameter of the agent thus far observed. This effect may involve both a release from the growth inhibition caused, for instance, by glucocorticoids, as well as a direct growth-stimulating effect, synergistic to the one induced by insulin.