Phorbol ester induction of rat hepatic metallothionein in vivo and in vitro

Effects of chronic cadmium poisoning on Zn, Cu, Fe, Ca, and metallothionein in liver and kidney of rats

An experiment was conducted to invest effects of chronic cadmium poisoning on Zn, Cu, Fe, Ca, and metallothionein gene expression and protein synthesis in liver and kidney in rats. Forty rats, 6 weeks old, were randomly allocated into two groups. A group was given CdCl(2) (1 mg/KgCd(2+)) by intraperitoneal injection once a day. The other group was treated with normal saline in the same way. Liver and kidney were collected for analysis at the end of the third week. Results showed that Cd exposure increased Cd (P<0.01) and Zn (P<0.01) content both in liver and kidney. Fe and Ca concentration had a considerable increase in kidney (P<0.01), while both had different degree reduction in liver. Discrepancies between MT mRNA and protein were observed in liver and kidney. In liver, both MT mRNA and protein had a significant increase (P<0.01), while in kidney, only MT gene increase was checked. Meanwhile, the expression levels of MT-1 mRNA and MT-2 mRNA were distinct between liver and kidney. The present study indicated that changes in tissue Cd and Zn levels tended to reflect MT mRNA expression, but bear no clear relationship with MT protein. There did not have a strict dose-dependent relationship among Cd content, MT gene expression, and MT protein synthesis. What is more, changes of Zn, Fe, Cu, and Ca had a certain interaction with both MT mRNA and protein.

Cloning and characterization of a tilapia (Oreochromis aureus) metallothionein gene promoter in Hepa-T1 cells following the administration of various heavy metal ions

Metallothioneins (MTs) are highly conserved intracellular metal-binding proteins that contribute to the homeostasis of essential metals and the detoxification of non-essential heavy metals. MT gene expression is induced by various heavy metal ions, and Zn(2+) is able to bind and activate a transcription factor associated with the MT gene that is known as the metal responsive element (MRE) binding transcription factor-1 (MTF-1). Heavy metals other than Zn(2+), such as Cd(2+) and Cu(2+), fail to activate the binding of MTF-1 to MREs despite their ability to induce the transcription of the MT gene. To study how different metal ions regulate MT gene expression, a tilapia (ti)-MT gene promoter was cloned and its responses to activation by various metal ions measured using a Hepa T1 cell culture model. The tiMT gene promoter contains six functional MREs within 2118bp 5' of the translational start site. A transient gene expression study showed the tiMT gene promoter fragment to be responsive to Cd(2+), Cu(2+), Hg(2+), Pb(2+), and Zn(2+). Deletions from the 5' end and the site-directed mutagenesis of individual MREs in the tiMT gene promoter confirmed that both proximal and distal clusters of MREs were required for the maximal metal induction of the tiMT gene. The distal cluster of MREs greatly enhanced the induction of tiMT gene expression by several of the heavy metal ions, and especially the non-Zn(2+) ions. Individual MREs showed a different responsiveness to metal ions, with MREe being the most potent, MREb being responsive to Zn(2+) but not to other metal ions, and MREa being mainly for the basal expression of the tiMT gene. Electrophoretic mobility shift assay (EMSA) identified a transcription factor that was able to bind most of the MREs, with the exception of MREd, but the binding was only activated by the in vivo administration of Zn(2+), not the administration of Cd(2+) or Cu(2+). In conclusion, the results of this study on a Hepa T1 cell model suggest that the mechanism of MT gene activation by non-Zn(2+) metal ions is different from that of activation by Zn(2+), and that different MREs may be involved in the activation of the tiMT gene by different metal ions without enhancing the binding of MTF-1 to MREs.

Efects of growth hormone and cadmium on the transcription regulation of two metallothionein isoforms

The effect of growth hormone (GH) and cadmium (Cd) on metallothionein (MT) expression was investigated in hepatoma cells. In fish the constitutive isoform MT-B and the metal-responsive MT-A are expressed. Real-time RT-PCR revealed that: Cd up-regulates mostly MT-A, GH slightly induces MT-B and the GH/Cd combination induces synergistically both MTs. Perturbations in Ca2+ levels suppressed or reduced the Cd-induction of MTs and abolished the GH/Cd synergy. Similar results were obtained by inhibition of tyrosine kinases. Also the signaling molecules recruited by the GH receptor responded differently to GH and Cd, with ERKs showing a synergistic activation upon GH/Cd. The following conclusions can be drawn: (1) cytosolic Ca2+ is mainly involved in MT-A regulation; (2) both Ca2+ and tyrosine phosphorylation are essential for Cd-induction and GH/Cd synergy on MTs. The synergy could depend on interactions in different signaling pathways, leading to a differential recruitment of MTF-1 and AP-1 transcription factors.

Activation of gene expression by metal-responsive signal transduction pathways

Metallothioneins are small, cysteine-rich, metal-binding proteins that play important roles in maintaining intracellular metal homeostasis and in transition metal detoxification. MTF-1 (metal transcription factor-1) plays a central role in regulating the metal-inducible, transcriptional activation of metallothionein. Here we report that the phosphorylation of MTF-1 plays a critical role in the activation of MTF-1/metal-responsive element-mediated transcription. Inhibitor studies indicate that signal transduction cascades, including those mediated by protein kinase C, tyrosine kinase, and casein kinase II, are essential for zinc- and cadmium-inducible transcription. In addition, calcium signaling is also involved in regulating transcription. In contrast, cAMP-dependent protein kinase may not be directly involved in the metal response. Contrary to what has been reported for other transcription factors, the inhibition of transcriptional activation does not impair the binding of MTF-1 to DNA, suggesting that phosphorylation is not regulating DNA binding. Elevated phosphorylation of MTF-1 is observed under conditions of protein kinase C inhibition, suggesting that dephosphorylation of this transcription factor mediates its activation.

Regulation of metallothionein transcription by the metal-responsive transcription factor MTF-1: identification of signal transduction cascades that control metal-inducible transcription

Every living organism must detoxify nonessential metals and carefully control the intracellular concentration of essential metals. Metallothioneins, which are small, cysteine-rich, metal-binding proteins, play an important role in these processes. In addition, the transcription of their cognate genes is activated in response to metal exposure. The zinc finger transcription factor MTF-1 plays a central role in the metal-inducible transcriptional activation of metallothionein and other genes involved in metal homeostasis and cellular stress response. Here we report that the phosphorylation of MTF-1 plays a critical role in its activation by zinc and cadmium. Inhibitor studies indicate that multiple kinases and signal transduction cascades, including those mediated by protein kinase C, tyrosine kinase, and casein kinase II, are essential for zinc- and cadmium-inducible transcriptional activation. In addition, calcium signaling is also involved in regulating metal-activated transcription. In contrast, cAMP-dependent protein kinase may not be directly involved in the metal response. Contrary to what has been reported for other transcription factors, inhibition of transcriptional activation does not impair the binding of MTF-1 to DNA, suggesting that phosphorylation is not regulating DNA binding. Elevated phosphorylation of MTF-1 is observed under condition of protein kinase C inhibition, suggesting that specific dephosphorylation of this transcription factor contributes to its activation.

Membrane phospholipid reorganization differentially regulates metallothionein and heme oxygenase by heme-hemopexin

Heme-hemopexin coordinately regulates genes encoding protective proteins including metallothionein-I (MT-I) and heme oxygenase 1 (HO-1). Hexamethylene-bisacetamide (HMBA), which induces differentiation and activates protein kinase C (PKC), synergistically augments the induction of both MT-I and MT-II mRNAs in response to heme-hemopexin, but attenuates the induction of HO-1. HMBA also augments the increase in MT mRNA in response to cobalt protoporphyrin-hemopexin, a hemopexin (HPX) receptor ligand that activates signaling cascades without tetrapyrrole uptake. Unlike the PKC-activating phorbol esters that induce MT-I and HO-1, HMBA has minimal effects on MT-I or HO-1. HMBA is an amphipathic molecule, and is shown here to interact physically with lipids in model membranes using differential scanning calorimetry (DSC). The data are consistent with a stabilization of the lipid bilayer and an HMBA-induced segregation of lipids into separate domains each relatively enriched in one of the lipids. HMBA also perturbs membrane-protein interactions, and causes a loss of PKC and G-protein subunits from plasma membranes in vitro. Taken together, these observations reveal an additional level of complexity in the regulation of protective proteins induced by HPX, and which may take place in vivo in response to natural compounds that reorganize membrane phospholipids. A model is proposed whereby a reorganization of lipids by HMBA alters signaling pathways and fusion events considered to be the etiology of the differential response of the MT-1 (and MT-II) and the HO-1 genes to HMBA and heme-HPX.

Links between cell-surface events involving redox-active copper and gene regulation in the hemopexin heme transport system

Heme is considered to play an instrumental role in the pathology of hemolysis, trauma, and reperfusion following ischemia. However, data are sparse and experimental models are required. The transport of heme by hemopexin to tissues is a specific, membrane receptor-mediated process. Hemopexin recycles after endocytosis like transferrin. Heme oxygenase-1 (HO-1), transferrin, the transferrin receptor, and ferritin are regulated by heme-hemopexin. Genes that encode proteins important for cellular defenses against oxidative stress, such as the cysteine-rich metallothioneins (MTs), are also activated by hemopexin, as are proteins that regulate cell cycle control including p21WAF1 and the tumor suppressor p53. The hemopexin system is being investigated to establish how intracellular events are affected by signal(s) from the plasma membrane due to hemopexin receptor occupancy and heme transport. A transient oxidative modification of proteins, shown by carbonyl production, takes place. Redox processes at the cell surface, which generate cuprous ions, are involved in the regulation of the MT-1 and HO-1 genes by heme-hemopexin before heme catabolism and intracellular release of iron. The "redox-sensitive" transcription factors activated by the hemopexin system include c- Jun, RelA/NFkappaB and MTF-1. The specific copper chelator bathocuproine disulfonate prevents carbonyl production, the nuclear translocation of MTF-1, and the induction of MT-1 revealing a novel, pivotal role for copper in the hemopexin system. In addition, surface redox-active copper is the first link shown for the concomitant regulation of HO-1 and MT-1 and is required for the activation of the amino-terminal c-Jun kinase (JNK) by heme-hemopexin.

Mechanism of metallothionein gene regulation by heme-hemopexin. Roles of protein kinase C, reactive oxygen species, and cis-acting elements

Heme-hemopexin or cobalt protoporphyrin (CoPP)-hemopexin (a model ligand for hemopexin receptor occupancy) is shown to increase transcription of the metallothionein-1 (MT-1) gene by activation of a signaling pathway. Promoter deletion analysis followed by transient transfection assays show that 110 base pairs (-153 to -43) of 5'-flanking region of the murine MT-1 promoter are sufficient for increasing transcription in response to heme-hemopexin or to CoPP-hemopexin in mouse hepatoma cells. The protein kinase C inhibitor, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H7), prevented the increase in MT-1 transcription by heme-hemopexin, CoPP-hemopexin, or phorbol 12-myristate 13-acetate, but the protein kinase A inhibitor, HA1004, was without effect. N-Acetylcysteine (NAC) and glutathione, as well as superoxide dismutase and catalase, inhibited both the increase in endogenous MT-1 mRNA and the activation of reporter gene activity by heme-hemopexin, CoPP-hemopexin, and phorbol 12-myristate 13-acetate. In sum, these data suggest that reactive oxygen intermediates are generated by heme-hemopexin via events associated with receptor binding, including protein kinase C activation. Induction of heme oxygenase-1 expression, in contrast to MT-1, is significantly less sensitive to NAC. Deletion and mutation analyses of the MT-1 proximal promoter revealed that the sequence 5'-GTGACTATGC-3' (from -98 to -89 base pairs) is, in part, responsible for the hemopexin-mediated regulation of MT-1 which is inhibited by H7. Regulation via this element is also induced by H2O2 showing that it is an antioxidant response element. Heme itself acts via more distal elements on the MT-1 promoter. In contrast to NAC and glutathione, diethyl dithiocarbamate and pyrrolidine dithiocarbamate, which inactivate reactive oxygen intermediates and chelate Zn(II), synergistically augment the induction of MT-1 mRNA levels and reporter gene activity in response to heme-hemopexin via the antioxidant response element by both metal-responsive element-dependent and -independent mechanisms.

Chemical modulation of metallothionein I and III mRNA in mouse brain

Metallothioneins (MTs) are sulfhydryl-rich proteins. MT-I and MT-II are found in all tissues of the body, while MT-III exists only in brain. Regulation of MT-I and MT-III mRNA was studied in brain and liver of control C57BL/6J mice and mice given chemicals known to increase MT-I, namely, lipopolysaccharide (LPS), zinc chloride (Zn), cadmium chloride (Cd), dexamethasone (Dex), ethanol, and kainic acid (KA). Northern blot analysis revealed that MT-I mRNA levels in liver were induced dramatically (12-27-fold over basal levels) by all of the chemicals, while in brain only LPS produced an increase in MT-I mRNA (2-fold). Interestingly, the MT-I inducers, Cd, Dex, ethanol, and KA, down-regulated brain MT-III mRNA levels by approx. 30%. Because brain is such a heterogenous tissue, in situ hybridization was used to localize MT-I and MT-III mRNA in control and treated mice. MT-I mRNA signal, which was most abundant in the glial cells of the Purkinje cell layer of the cerebellum in control mice, appeared to be enhanced in mice given the MT-I inducers (LPS, Zn, Cd, Dex, ethanol, and KA). MT-I mRNA hybridization signal was also enhanced in the olfactory bulbs from LPS- and Cd-treated mice, while this signal was present but weak in control brains. MT-III mRNA hybridization signals were localized in hippocampus and co-localized with MT-I message in the glial cells of the Purkinje cell layer of the cerebellum. In addition, diffuse MT-III mRNA signals were visible in areas of the cerebral cortex, and in the molecular layer of the cerebellum. Signals for MT-III in hippocampus appeared to be reduced by KA, Dex and LPS treatment, while in the cortical region, MT-III mRNA signals appeared to be enhanced by KA, Cd, and ethanol treatment. In conclusion, both MT-I and MT-III expression in brain appears to be modulated by exogenous treatment, however, the changes are small in relation to those observed in liver. Chemical-induced alterations of MT mRNA are non-uniform throughout the brain, and thus best studied in a region-specific manner.

Enhancement of sensitivity to platinum(II)-containing drugs by 12-O-tetradecanoyl-phorbol-13-acetate in a human ovarian carcinoma cell line

Sensitivity to platinum-containing drugs is believed to be a function of how much drug enters the cell, the extent of DNA adduct formation and the rate at which DNA is repaired. Activation of protein kinase C by 12-O-tetradecanoyl-phorbol-13-acetate (TPA) was found to enhance the sensitivity of human ovarian carcinoma 2008 cells to cisplatin (DDP), carboplatin (CBDCA) and (glycolato-O,O') diammineplatinum(II) (254-S). TPA was able to enhance the sensitivity of the DDP-resistant 2008/C13*5.25 subline to each of the three drugs to the same extent as for the 2008 cells. TPA produced no significant change in the uptake of [3H]cis-dichloro(ethylenediamine)-platinum(II). ([3H]DEP) or CBDCA. It did not alter glutathione content or glutathione-S-transferase activity, and induced rather than suppressed metallothionein IIA mRNA levels. TPA did increase the formation of intrastrand guanine-guanine cross-links by a factor of 1.5 +/- 0.3 (s.d.), and reduced the fraction of intrastrand adducts removed from DNA over the subsequent 24 h by a factor of 1.3 +/- 0.2 (s.d.) (n = 4; P < 0.05), however, these effects were too small to account for the degree of TPA-induced sensitisation. These results indicate that the mechanism of TPA-induced sensitisation is not specific to any one structural form of platinum-containing drug, and that it is not readily explicable on the basis of an effect on the four major parameters currently believed to regulate DDP sensitivity.

Involvement of metallothionein and copper in cell proliferation

Metallothionein is a low-molecular weight, cysteine-rich, metal-binding protein which has been implicated in the detoxification of toxic metals (cadmium, mercury), metabolism of zinc and copper, as well as in the scavenging of free radicals. Recent evidence suggests that the protein may also be involved in cell proliferation. Based on the experiments carried out so far, it is assumed that the fundamental role of metallothionein in cell proliferation may be to detoxify and/or transfer copper ions from the cytoplasm to the nucleus at the G1/S phase, which in turn participate in some way in nuclear DNA synthesis.

Inhibitors of Ca2+ channels, calmodulin and protein kinases prevent A23187 and other inductions of metallothionein mRNA in EC3 rat hepatoma cells

The role of calcium in the induction of MT mRNA has been studied in EC3 rat hepatoma cells, using various inducers (A23187, TPA, norepinephrine, and 2-chloroadenosine) and inhibitors (H7:PK-A and PK-C; W7:calmodulin; verapamil:calcium channel blocker; and TMB-8; cytosolic calcium chelator). The inhibitions of inductions observed in this study were consistent with calcium playing an important role in MT mRNA induction by itself and via crosstalk among the PK-A, PK-C, and calmodulin-dependent protein kinase pathways. Calcium has an important role in the complicated second messenger pathways which result in the positive interaction of transcription factors with the promoters of MT genes.