Tamoxifen suppresses histologic progression to atypia and DCIS in MCFIOAT xenografts, a model of early human breast cancer.

We evaluated the effects of tamoxifen on the growth and progression of MCFIOAT xenografts, an estrogen responsive model of human breast tumor progression, in which cells are injected orthotopically into the mammary fat pad of female nude mice. At 10 weeks following implantation, histologic sections of each graft were evaluated microscopically for histologic lesions analogous to human breast tumor progression, graded as simple hyperplasia, complex hyperplasia, atypical hyperplasia, ductal carcinoma in situ and invasive carcinoma. Three out of five xenografts in (endocrine intact) control animals progressed to atypical hyperplasia, one progressed to ductal carcinoma in situ and one to invasive carcinoma. The latter two control grafts also contained foci of putative precursor lesions (i.e. atypical hyperplasia and in situ carcinoma, respectively). Tamoxifen supplemented xenografts (N= 17) were uniformly smaller than controls, but contained invasive carcinoma in a similar proportion (4/17, 24%). However, none of these grafts exhibited ductal carcinoma in situ and only one contained atypical hyperplasia. Most grafts in tamoxifen supplemented animals (10/17, including all four with carcinomas) showed complex hyperplasia, which typically dominated the graft. We conclude that tamoxifen selectively inhibits the appearance or growth of preinvasive index lesions. Development of malignancy in the absence of such precursors, though, implies selection for alternative histogenetic pathways as a result of endocrine manipulation.

Estrogen inducibility of c-Ha-ras transcription in breast cancer cells. Identification of functional estrogen-responsive transcriptional regulatory elements in exon 1/intron 1 of the c-Ha-ras gene.

Although mutation of ras gene is rare in human breast cancer, overexpression of normal c-Ha-ras gene is frequently observed. Using a mouse mammary metastasis model consisting of genetically related mammary tumor sublines with variant metastatic potential, we have previously (i) demonstrated a direct correlation between c-Ha-ras mRNA and protein levels and metastatic potential and (ii) identified a novel hormone-responsive transcriptional regulatory element in intron 1 of the mouse c-Ha-ras gene that contains the consensus half-site of a glucocorticoid response element and flanking consensus half-sites for estrogen response element. Here, we have examined the functionality of intron 1 sequence in context of upstream sequences by using transient transfection assays with plasmids expressing chloramphenicol acetyltransferase. Intron 1 sequence and sequences similar to intron 1 element located in exon 1 function as transcriptional regulatory elements that confer hormonal inducibility to chloramphenicol acetyltransferase gene expression both independently and in context of 5'-flanking sequences. Measurement of c-Ha-ras transcription rates and protein expression by nuclear run-on and metabolic labeling assays showed a 5-12-fold enhancement, respectively, following treatment with 17beta-estradiol that was blunted by ICI 182,780 in the nonmetastatic variant. In contrast, constitutive overexpression of c-Ha-ras transcripts and protein in the metastatic subline was unaffected by estrogen and ICI 182,780. Gel shift assays demonstrated specific interaction of c-Ha-ras exon 1 sequence with nuclear proteins of human breast cancer MCF-7 cells with formation of two complexes, one of which contains estrogen receptor. Our data demonstrate a direct (i) interaction of c-Ha-ras sequence with estrogen receptor and (ii) stimulatory effect of estrogen on c-Ha-ras gene transcription and suggest that alteration in transcriptional regulation of c-Ha-ras gene by estrogen may play an important role in progression of breast cancer.

p53 and protein kinase C independent induction of growth arrest and apoptosis by bryostatin 1 in a highly metastatic mammary epithelial cell line: In vitro versus in vivo activity.

We have evaluated the effects of bryostatin 1 on growth of a highly malignant p53-null mouse mammary tumor line, 4T1, and the mechanism by which bryostatin 1 inhibits in vitro growth and in vivo development of tumor and metastases from the orthotopic site. Bryostatin 1 at 20-400 nM concentrations inhibits growth of 4T1 cells by approximately 60% in two-day cultures. Inhibition of growth is associated with an increase in the number of cells undergoing apoptosis with concomitant elevation in the steady state levels of bax protein and drop in bcl-2 levels. The cytotoxic effect of bryostatin 1 on 4T1 cells occurs independently of p53, since there was no evidence of p53-mediated transcriptional activity in 4T1 cells following treatment with bryostatin 1.4T1 cells respond in vivo to bryostatin 1 therapy (75 microg/kg body weight). Intraperitoneal administration of bryostatin 1 inhibits both primary and secondary tumor growth by approximately 50%. However, although bryostatin 1 has a remarkable capacity to slow tumor growth and progression, it is unable to completely eradicate tumor growth and progression due to in vivo development of tumor resistance to bryostatin 1. Levels and cellular distribution of PKCalpha and delta do not correlate with the growth inhibitory effects of bryostatin 1 on 4T1 cells; however, reduction in cytosolic PKCalpha and delta without associated increase in membrane compartment appear to correlate with bryostatin-resistance. Our results suggest that the therapeutic effects of bryostatin 1 in our system do not involve alterations in levels and distribution of PKC but rather a direct upregulation of bax/ bcl-2 ratios that is independent of p53.

Transcriptional activation of functional endogenous estrogen receptor gene expression in MCF10AT cells: a model for early breast cancer.

Utilizing the MCF10AT xenograft model for progression of human proliferative breast disease, we detected expression of the endogenous estrogen receptor (ER) gene only in MCF10AneoT and cells of the MCF10AT system, all of which stably express a transfected mutated T24 Ha-ras gene. ER transcripts were undetectable in the parental MCF10A cells and in MCF10A cells transfected with normal c-Ha-ras or vector. ER transcripts expressed in MCF10AT cells contain a normal full-length ER coding region and direct synthesis of a normally sized ER protein. The protein is functional based on its ability to mediate estradiol (E2)-induced increases of transcription from both endogenous and exogenous E2-regulated genes. Transcriptional activation of the endogenous ER gene does not appear to be related to a change in methylation status of the gene since a diagnostic CpG site in exon 1 that is methylated in ER-negative breast tumors and completely unmethylated in ER-positive breast tumors is hypomethylated to the same extent in ER-negative MCF10A cells and ER-positive MCF10AT cells. E2 increased both the number and size of soft-agar colonies formed by MCF10AT3c cells, a line from a third generation MCF10AT xenograft lesion. This suggests that xenograft passage has selected for growth regulatory pathways that are E2-responsive and that identification of these pathways and their role in progression will aid in determining how E2 acts to increase risk of breast cancer.

Environmental estrogen stimulation of growth and estrogen receptor function in preneoplastic and cancerous human breast cell lines.

BACKGROUND: DDT and polychlorinated biphenyls (PCBs), which are widespread in the ecosystem, can mimic estrogen-mediated cell activities. Thus, they can potentially interfere with many physiologic processes. We compared the effects of organochlorines belonging to the DDT and PCB families, alone and in combination, for their ability to influence the estrogen receptor-mediated activities in preneoplastic breast epithelial cells and breast cancer cells. METHODS: Multiple assay systems requiring functional estrogen receptor were employed to test estrogen-like activity of organochlorine ligands. Two-sided statistical tests were used to compare the data. RESULTS: p,p'-DDT, the predominant form of DDT in the environment, is a more potent estrogen than o,p'-DDT (P<.001), although it is less effective than o,p'-DDT in inhibiting the binding of estradiol (natural estrogen) to estrogen receptor. Among the PCBs, Heptachlor is estrogenic (in transient reporter assays; P< or =.001), whereas Aroclor 1221 and Aroclor 1254, both individually and in combination, are only weakly estrogenic. CONCLUSION: p,p'-DDT is the most effective organochlorine in regulating estrogen receptor-mediated cellular responses. In estrogen receptor-positive breast cancer cells, p,p'-DDT evokes responses by itself and enhances the responses in collaboration with estradiol or o,p'-DDT.

Heterogeneity in p53 mutations in mouse mammary tumor subpopulations with different metastatic potential from the orthotopic site.

We have found that p53 expression is altered with progression to the metastatic phenotype of a series of neoplastic subpopulations of a single mouse mammary tumor. Single strand conformation polymorphism (SSCP) analysis of p53 transcripts indicate that the expressed p53 genes in each of the subpopulations contain one or more differences from wild type p53. Although the mutations in the coding sequence of p53 are different in each of the sublines, suggesting that independent mutational events have occurred, the mutations are clustered in exons 2-4 and in exon 6. In the metastatic subpopulations, there is either a complete loss of p53 transcriptional activity or accumulation of transcripts with an additional alteration in exons 4-5.

Correlation of differences in modulation of ras expression with metastatic competence of mouse mammary tumor subpopulations.

The ras family of cellular oncogenes is one of the most frequently detected families of transformation-inducing genes in human solid tumors. The capacity of breast cancers to grow and metastasize have been related to enhanced expression of normal p21ras rather than the mutant form. Transformation in tumours that lack the mutant p21ras has been suggested to result from transcriptional deregulation of ras. cis-Acting sequence elements that participate in the regulation of gene expression in normal tissues and that could serve as potential targets for the deregulation of expression in tumors have been localized in several genes including c-myc and N-ras. Using a mouse mammary metastasis model system of closely related tumor subpopulations that vary in metastatic potential and with defined deficiencies, we show that c-Ha-ras plays a prominent role as a metastasis-modulating gene in this system. We have identified a highly conserved cis-acting sequence element in the first intron of the mouse and rat, and in the first exon of Ha- and Ki-ras genes of human, mouse and rat. This regulatory sequence confers strong transcription enhancer activity that is differentially modulated by steroid hormones in metastatic and nonmetastatic subpopulations. Our results indicate that perturbations in the regulatory activities of cis-acting sequences such as the one we have identified may play an important role in governing oncogenic potency of Ha-ras through transcriptional control mechanisms.

Molecular events in metastatic progression.

A number of genes have been implicated in the metastatic process but relatively little is known regarding the specific event(s) in dissemination of malignant cells which are altered by specific genetic changes. In most cases, only a correlation between gene expression and a final outcome, e.g. patient survival or enumeration of metastatic nodules at necropsy for experimental animal models, exists. In other cases, gene expression has been correlated with one part of the metastatic cascade but possible alterations in additional steps were not determined. The ultimate effect of expression of any gene on specific steps in the metastatic cascade varies with the cell type. To minimize differences due to unspecified differences in background genetic mutations and expression, a model of closely related tumor subpopulations, which vary in metastatic potential and whose specific deficiencies are known, is described. This model may prove to be valuable in assessing the steps in the metastatic cascade affected by specific metastasis modulating genes.