Jin Z, Houle B, Mikheev A M, Cha R S, Zarbl H
Division of Toxicology and Center for Environmental Health Sciences, Massachusetts Institute of Technology, Cambridge 02139, USA.
Cancer Res. 1996 Nov 1;56(21):4927-35.
We previously demonstrated that H-ras1 oncogene mutations detected in N-nitroso-N-methylurea (NMU)-induced mammary tumors arose as background mutations within rat mammary cells (RMCs) and that NMU promoted the outgrowth of these preexisting mutants. We have now detected a putative DNA structure in the H-ras1 promoter of RMCs in vivo that was absent in NMU-induced mammary tumor cells. Analysis of the promoter in RMCs as a function of time after exposure to carcinogens indicated that NMU, but not 7,12-dimethylbenz(a)anthracene, initiated the loss of this structure with a half-life of 7 days. Although loss of the structure was irreversible in cells that gave rise to tumors, it was restored in normal RMCs by 120 days after exposure and was present in normal RMCs of animals bearing tumors, even 1 year after NMU exposure. The structure was also abrogated in RMCs during pregnancy and restored after lactation was terminated, suggesting that reversible regulation of the structure by hormones contributed to normal RMC growth. Thus, NMU may promote abnormal RMC growth by mimicking the effects of hormones on DNA conformation. We hypothesize that the NMU-induced alterations in promoter conformation irreversibly deregulates H-ras1 expression in initiated cells, thereby increasing the phenotypic penetrance of the conditional H-ras1 mutations.
我们之前证明，在N-亚硝基-N-甲基脲（NMU）诱导的乳腺肿瘤中检测到的H-ras1癌基因突变是作为大鼠乳腺细胞（RMC）内的背景突变出现的，并且NMU促进了这些预先存在的突变体的生长。我们现在在体内RMC的H-ras1启动子中检测到一种推定的DNA结构，而在NMU诱导的乳腺肿瘤细胞中不存在这种结构。分析RMC中启动子在接触致癌物后的时间函数表明，NMU而非7,12-二甲基苯并(a)蒽引发了这种结构的丧失，半衰期为7天。虽然这种结构的丧失在产生肿瘤的细胞中是不可逆的，但在接触后120天，正常RMC中的这种结构得以恢复，并且在NMU暴露后1年的荷瘤动物的正常RMC中也存在。在怀孕期间，RMC中的这种结构也被消除，在哺乳期结束后恢复，这表明激素对该结构的可逆调节有助于正常RMC的生长。因此，NMU可能通过模拟激素对DNA构象的影响来促进RMC的异常生长。我们假设，NMU诱导的启动子构象改变不可逆地解除了起始细胞中H-ras1表达的调控，从而增加了条件性H-ras1突变的表型外显率。
