Hepatic metallothionein synthesis in neonatal Mottled-Brindled mutant mice

Cadmium-induced inhibition of proliferation and differentiation of embryonal carcinoma cells and mechanistic aspects of protection by zinc

Murine embryonal carcinoma cells have been used in in vitro models to study the effects of cadmium chloride on proliferation and differentiation of early embryonic cells. This approach allows the various cell types within the early embryo as well as several developmental mechanisms to be dissected and studied in isolation using larger numbers of cells than would be readily available from the embryo itself. The present study shows that both embryonal carcinoma cell proliferation and differentiation into parietal endoderm are inhibited by cadmium chloride. The effects are counteracted by the additional presence of zinc chloride. The uptake of cadmium into the cells is inhibited in the presence of zinc chloride, suggesting that competition between these metals for passage into the cells contributes to the mechanism underlying the protective effect of zinc. In addition, metallothionein gene expression is enhanced more rapidly after simultaneous incubation with zinc chloride, indicating that the attenuating effect of zinc on cadmium toxicity is also partly attributable to detoxification by metallothioneins.

Metallothionein mRNA expression in mice homozygous for chromosomal deletions around the albino locus

Deletions in chromosome 7 of the mouse affect the expression of the metallothionein gene Mt-1, which maps on chromosome 8, and steady-state levels of Mt-1 mRNA are reduced to 15-40% of normal in livers of newborn mice homozygous for either the c3H or c14CoS deletion. Glucocorticoids fail to induce hepatic Mt-1 mRNA levels in deletion homozygotes in contrast to normal littermates. However, zinc chloride is effective in inducing Mt-1 mRNA levels in livers of deletion homozygotes as well as of their normal littermates. Other tissues (e.g., kidney and intestine) of deletion homozygotes express basal levels of Mt-1 mRNA higher than those of normal littermates. In the intestine these are furthermore inducible by both hormonal and metal agents. Thus, loss of inducibility of the Mt-1 gene in deletion homozygotes concerns glucocorticoids only and is furthermore restricted to specific cell types (i.e., hepatocytes). The trans-acting factor(s) normally encoded in the deleted region of chromosome 7 appears to be instrumental in conferring on the metallothionein gene in hepatocytes the essential competence to respond to hormonal inducing signals.

Age-dependent variation for inducibility of metallothionein genes in mouse liver by cadmium

Cadmium is a toxic metal that induces the expression of metallothionein genes in many tissues and that binds avidly to metallothionein, a soluble transition metal binding protein. The present study examined the temporal pattern and magnitude of accumulation of metallothionein mRNA in liver of C57BL/6J mice of various ages treated with cadmium. In adult female mice, accumulation was dependent on the dosage level of cadmium and related to the concentration of this metal in liver. The accumulation of metallothionein mRNA in liver depended on age at exposure to cadmium. Intraperitoneal administration of 2 mg of cadmium per kg provoked small increases (two- to threefold) in levels of metallothionein mRNA in livers of 7- and 14-day-old mice. In contrast, cadmium treatment of 28- and 56-day-old mice resulted in 12- to 19-fold increases in levels of metallothionein mRNA in liver with maximum increases occurring 3 to 4 hr after treatment. Because similar patterns for the accumulation of cadmium of liver were found in 7-, 28-, and 56-day-old mice, observed age-dependent differences in induction of metallothionein mRNA in liver were probably not due to differences in the accumulation of cadmium in this organ. Taken together, these data suggest that tissue-specific factors controlling the expression of metallothionein genes may account for developmental variation in the inducibility of these genes by cadmium. Ontogenic variation in accumulation of metallothionein mRNA after cadmium treatment may be a factor in developmental variation in the acute lethality of cadmium in C57BL/6J mice.

The ontogeny of expression of murine metallothionein: comparison with the alpha-fetoprotein gene

The ontogeny of expression of mouse metallothionein was studied by RNA dot and Northern blot hybridization using a cloned cDNA probe. In some instances the synthesis of metallothionein was analyzed by cell-free translation of RNA as well as pulse-labeling of proteins in short-term organ cultures followed by polyacrylamide gel electrophoresis. Interesting parallels between metallothionein and alpha-fetoprotein gene expression during development were noted. Like alpha-fetoprotein mRNA ( Dziadek and Andrews, 1983), metallothionein mRNA was found to be abundant in developing liver as well as in visceral yolk sac endoderm. In addition, metallothionein mRNA was abundant in parietal yolk sac. During liver development metallothionein and alpha-fetoprotein mRNAs were abundant by Day 12 of gestation, increasing to maximal levels on Day 16 and decreasing during late fetal and neonatal life to basal levels in adult. Metallothionein mRNA increased in maternal liver and was also abundant in certain hepatomas. Synthesis of metallothionein and levels of metallothionein mRNA in visceral yolk sac increased from Day 9 of gestation to maximal levels on Days 11-12 and then decreased abruptly after Day 15. RNA from differentiated teratocarcinoma cells with primitive, parietal or visceral endoderm characteristics each contained high levels of metallothionein mRNA, whereas, levels of this mRNA varied widely among embryonal carcinoma stem cell lines. alpha-Fetoprotein mRNA was not detected in embryonal carcinoma cells but was expressed in visceral endoderm-like differentiated cells. These results indicate that parietal and visceral endoderm cells actively express the metallothionein gene and further suggest that expression may be initiated at the earlier stage of primitive endoderm.