Ploidy differences between hormone‐ and chemical carcinogen–induced rat mammary neoplasms: Comparison to invasive human ductal breast cancer*

Abstract

To ascertain differences between solely hormone- and chemical carcinogen-induced murine mammary gland tumors (MGTs), a direct comparison of their ploidy status was assessed. Nuclear image cytometry (NIC) was used to evaluate ploidy in ductal carcinoma in situ (DCIS) and MGTs induced solely by 17beta-estradiol (E(2)) in female A-strain Copenhagen Irish hooded gene rats (ACI) and E(2) plus testosterone propionate in male Noble rats. These results were compared to ploidy data from primary MGTs induced by two synthetic carcinogens, 7,12-dimethylbenz[a]antracene and nitrosomethylurea in female Brown Lewis Norway rats and an environmental carcinogen, 6-nitrochrysene, in female Sprague-Dawley rats. Both DCIS and primary MGTs induced solely by hormones were highly aneuploid (> 84%), whereas MGTs induced by either synthetic or environmental carcinogens were primarily diploid (> 85%). Examination of 76 metaphase plates obtained from eight individual E(2)-induced ACI female rat MGTs revealed the following consistent chromosome alterations: gains in chromosomes 7, 11, 12, 13, 19, and 20 and loss of chromosome 12. On Southern blot analysis, six of nine ACI female rat primary E(2)-induced MGTs (66%) exhibited amplified copy numbers (range: 3.4-6.9 copies) of the c-myc gene. Fluorescence in situ hybridization (FISH) analysis of these MGTs revealed specific fluorescent hybridization signals for c-myc (7q33) on all three homologs of a trisomy in chromosome 7. NIC analysis of 140 successive nonfamilial sporadic invasive human ductal breast cancers (BCs) showed an aneuploid frequency of 61%, while 31 DCISs revealed a 71% aneuploid frequency. These results clearly demonstrate that the female ACI rat E(2)-induced MGTs more closely resemble invasive human DCIS and ductal BC in two pertinent aspects: they are highly aneuploid compared with chemical carcinogen-induced MGTs and exhibit a high frequency of c-myc amplification.

Prevention of solely estrogen-induced mammary tumors in female aci rats by tamoxifen: evidence for estrogen receptor mediation

“…The female ACI rats were divided into five groups, each group contained 6-12 animals: group 1 remained untreated and served as control; groups 2 and 3 received a single 20 mg pellet of 17 -estradiol (E 2 ), containing either 2 or 3 mg E 2 , plus 18 or 17 mg cholesterol respectively; group 4 received a single 40 mg pellet of tamoxifen citrate (TAMc); and group 5 received a 40 mg pellet of TAMc, and 1 week later, a 20 mg pellet of E 2 containing 3 mg E 2 . Hormone pellets, prepared without binder by Hormone Pellet Press (Leawood, KS, USA), were implanted subpannicularly in the shoulder region as described previously (Li et al 2002). After 6 months of treatment, all the rats were killed by decapitation, subjected to macroscopic examination, and the presence of mammary gland tumors (MGTs), both number and location, was recorded.…”

“…However, crucial differences exist in MGT causation since both DCIS and primary MGTs induced by estrogens are highly aneuploid and exhibit a high frequency of c-myc amplification (Li SA et al 2000, Li et al 2002, whereas DCIS and MGTs induced by synthetic chemical carcinogens are largely diploid and exhibit high frequencies of H-ras activation (Zhang et al 1991, Li et al 2002. It is evident that the DCIS and MGTs induced by estrogen alone more closely resemble human DCIS and invasive ductal BCs, since the latter are also highly aneuploid (Alanen et al 1997, Arnerlov et al 2001, and show a high frequency of c-myc amplification (Escot et al 1986, Watson et al 1993.…”

“…Numerous early studies have shown that many rat strains are highly susceptible to estrogen-induced mammary tumors, including Noble,, and to a lesser extent, August, Wistar-Wag and Sprague-Dawley (36-42%) (Nelson 1944, Dunning et al 1953, Cutts & Noble 1964, Blankenstein et al 1977. Recently, we have shown that there are distinctive critical differences between synthetic chemical carcinogen-and environmental carcinogen-induced breast tumors, both ductal carcinoma in situ (DCIS) and primary tumors, and solely estrogen-induced mammary neoplasms (Li SA et al 2000, Li et al 2002. These differences include early genomic destabilization (aneuploidy) and c-myc amplification, both features commonly found in human BCs.…”

Protective roles of quinone reductase and tamoxifen against estrogen-induced mammary tumorigenesis

“…Estrogen-induced mammary cancer is also accompanied by genomic instability in the form of loss of heterozygosity (Harvell et al, 2000;Li et al, 2002a). TAM has been shown to prevent ACI rat mammary gland tumor formation, and this effect appears to be mediated by the ER (Li et al, 2002b).…”

Exposure to estrogen and ionizing radiation causes epigenetic dysregulation, activation of mitogen-activated protein kinase pathways, and genome instability in the mammary gland of ACI rats

“…[44][45][46][47][48] Considering that few other rat strains exhibit this propensity to develop mammary cancer in response to continuous E 2 treatment, it appears that the ACI rat possesses a unique genetic and epigenetic background that underlies its susceptibility to elevated estrogen levels. [49][50][51] Furthermore, the ACI rat has low levels of spontaneous and radiation-induced mammary tumors, thus it is an ideal model to analyze the combined effects of radiation and estrogen.…”