Abshire M K, Buzard G S, Shiraishi N, Waalkes M P
Inorganic Carcinogenesis Section, National Cancer Institute, Frederick Cancer Research and Development Center, Maryland 21702, USA.
J Toxicol Environ Health. 1996 Jul;48(4):359-77. doi: 10.1080/009841096161258.
Certain proto-oncogenes transfer growth regulatory signals from the cell surface to the nucleus. These genes often show activation soon after cells are exposed to mitogenic stimulation but can also be activated as a nonmitogenic stress response. Cadmium (Cd) is a carcinogenic metal in humans and rodents and, though its mechanism of action is unknown, it could involve activation of such proto-oncogenes. Metallothionein (MT), a metal-inducible protein that binds Cd, can protect against many aspects of Cd toxicity, including genotoxicity and possibly carcinogenesis. Thus, the effects of Cd on expression of c-myc and c-jun in rat L6 myoblasts, and the effect of preactivation of the MT gene by Zn treatment on such oncogene expression, were studied. MT protein levels were determined by the Cd-heme assay, and MT, c-myc, and c-jun mRNA levels were measured using oligonucleotide hybridization and standardized to beta-actin levels. Cd (5 microM CdCl2, 0-30 h) stimulated both c-myc and c-jun mRNA expression. An initial peak of activation of c-myc expression occurred 2 h after initiation of Cd exposure, and levels remained elevated throughout the assessment period. Zn pretreatment markedly reduced the activation of c-myc expression by Cd compared to cells not receiving Zn pretreatment. Cd treatment increased c-jun mRNA levels by up to 3.5-fold. Again, Zn pretreatment markedly reduced Cd-induced activation of c-jun expression as minimal increases occurred with Cd exposures of < or = 1 h, but otherwise the Zn pretreatment prevented activation of c-jun. The Zn pretreatment elevated MT protein levels > 5-fold over control at the point of Cd exposure, but Cd exposure did not further elevate these Zn-induced MT levels. Similarly, Zn pretreatment did not result in increased relative MT mRNA levels above Cd exposure alone at various time points after Cd exposure. Therefore, Zn pretreatment, possibly by providing elevated MT protein levels at the point of Cd exposure, inhibited the Cd-induced c-myc and c-jun proto-oncogene expression. The extent of Cd-induced proto-oncogene activation thus may be limited by the presence of cellular MT.
某些原癌基因可将生长调节信号从细胞表面传递至细胞核。这些基因在细胞受到促有丝分裂刺激后很快就会显示出激活状态，但也可作为非促有丝分裂应激反应而被激活。镉（Cd）是一种对人类和啮齿动物具有致癌性的金属，尽管其作用机制尚不清楚，但可能涉及此类原癌基因的激活。金属硫蛋白（MT）是一种可结合镉的金属诱导蛋白，能抵御镉毒性的诸多方面，包括遗传毒性以及可能的致癌作用。因此，研究了镉对大鼠L6成肌细胞中c-myc和c-jun表达的影响，以及锌处理对MT基因的预激活对此类癌基因表达的影响。通过镉-血红素测定法确定MT蛋白水平，并使用寡核苷酸杂交法测量MT、c-myc和c-jun mRNA水平，并将其标准化为β-肌动蛋白水平。镉（5 microM CdCl2，0 - 30小时）刺激了c-myc和c-jun mRNA的表达。镉暴露开始后2小时出现了c-myc表达激活的初始峰值，并且在整个评估期间水平一直保持升高。与未接受锌预处理的细胞相比，锌预处理显著降低了镉对c-myc表达的激活。镉处理使c-jun mRNA水平增加了高达3.5倍。同样，锌预处理显著降低了镉诱导的c-jun表达激活，因为在镉暴露≤1小时时增加幅度最小，但除此之外，锌预处理可防止c-jun的激活。在镉暴露时，锌预处理使MT蛋白水平比对照升高了5倍以上，但镉暴露并未进一步提高这些锌诱导的MT水平。同样，在镉暴露后的各个时间点，锌预处理并未导致相对MT mRNA水平高于单独镉暴露时的水平。因此，锌预处理可能通过在镉暴露时提供升高的MT蛋白水平，抑制了镉诱导的c-myc和c-jun原癌基因表达。因此，镉诱导的原癌基因激活程度可能受到细胞内MT存在的限制。
