Short-term Modulation of Cell Proliferation and Apoptosis and Preventive/Therapeutic Efficacy of Various Agents in a Mammary Cancer Model

Abstract

Purpose: The methylnitrosourea (MNU)-induced mammary cancer model in rats is similar to estrogen receptor^positive breast cancer in women. In prevention studies using this model, tumor incidence and multiplicity were typically primary end points. The ability of various agents administered for a short period to modulate cell proliferation [proliferation index (PI)] and apoptosis [apoptotic index (AI)] in mammary cancers was compared with their efficacy in long-term prevention and therapy studies. Experimental Design: Rats were injected with MNU to induce mammary cancers. For the prevention studies, agents were administered by gavage or in the diet beginning 5 days after MNU. For proliferation (PI) and apoptosis (AI) experiments, animals with a palpable mammary cancer were treated with the agents for only 4 to 7 days. PI was determined following 5-bromodeoxyuridine labeling whereas AI was determined using the terminal deoxyribonucleotidyl transferase^mediated dUTP nick end labeling assay. Therapeutic efficacy was evaluated by measuring cancer size over a 6-week period. Results: Treatments with differing chemopreventive efficacy and mechanism(s) of action were examined: (a) hormonal treatments [tamoxifen, vorozole (an aromatase inhibitor), and ovariectomy]; (b) retinoid X receptor agonists (targretin, 9-cis retinoic acid, and UAB30); (c) inducers of drug-metabolizing enzymes (indole-3-carbinol, 5,6 benzoflavone, and diindoylmethane); (d) agents that alter signal transduction (R115777, a farnesyltransferase inhibitor); Iressa (an epidermal growth factor receptor inhibitor); sulindac and celecoxib (cyclooxygenase 1/2 and cyclooxygenase 2 inhibitors); and (e) diverse agents including meclizine, vitamin C, and sodium phenylbutyrate. Correlations between inhibition of PI, increase of AI, and chemopreventive efficacy were observed. Although most agents with moderate or low preventive efficacy suppressed PI, they minimally affected AI. Conclusions: The data confirmed that the short-term effects of various agents on cell proliferation and apoptosis in small mammary cancers can predict their preventive/therapeutic efficacy. Thus, these biomarkers can be used to help determine the efficacy of compounds in phase II clinical prevention trials.The methylnitrosourea (MNU) model of mammary carcinogenesis in rats was developed f30 years ago by Gulliano et al.(1). The resulting mammary tumors are primarily carcinomas (1, 2) and show many of the histopathologic and biological characteristics of estrogen receptor -positive breast cancer in women (3). In this model, almost all animals develop multiple mammary tumors after a relatively short latency (3-4 months). In addition, the morphology, origin (terminal end buds or terminal ductal lobular units), and preinvasive stages (hyperplasia, ductal carcinoma in situ) of these tumors seem to be similar to human breast cancer (2, 3). More recently, studies on gene expression have shown that MNU-induced mammary cancers are similar in gene expression to that of welldifferentiated duct...

Modulation of Gene Expression and Cell-Cycle Signaling Pathways by the EGFR Inhibitor Gefitinib (Iressa) in Rat Urinary Bladder Cancer

“…Results in humans have shown clearly the 2 known positive "preventive" agents selective estrogen receptor modulators (SERM) examined in large prevention trials and aromatase inhibitors in prevention trials, but clear data from inhibition of tumors of the contralateral breast in an adjuvant setting yield major changes in biomarkers in a presurgical setting with early stage cancer (28). We have used such an approach to examine a wide variety of effective and ineffective agents in a breast cancer model and found a close correlation between alterations of biomarkers in this presurgical setting and efficacy in long-term prevention studies particularly when initiated further along in tumor progression (29). This is not an argument that there are agents which might be effective early in the process of tumor progression but are ineffective at later times and will not give a strong signal in this later intervention.…”

Effects of gefitinib (Iressa) on mammary cancers: preventive studies with varied dosages, combinations with vorozole or targretin, and biomarker changes

“…After sacrifice of the animals by CO 2 asphyxiation, cancers were removed and fixed overnight in 10% formalin for assessment of histopathology, BrdUrd labeling, and apoptosis. Differences in proliferation index and apoptotic index were evaluated using Wilcoxon rank sum analysis (17,18).…”

“…We have recently examined a series of short-term end points that might reflect the long-term efficacy of specific treatments (18). One group of end points is the effects of short-term exposure to agents on proliferation and apoptosis in small palpable mammary cancers.…”

“…One group of end points is the effects of short-term exposure to agents on proliferation and apoptosis in small palpable mammary cancers. These end points have been used in examining a wide variety of agents, including aromatase inhibitors, selective ER modulators, and farnesyltransferase inhibitors (18), as well as in screening a variety of RXR agonists for chemopreventive activity (19). An excellent correlation was found between decreases in proliferation and increases in apoptosis and long-term chemopreventive and therapeutic efficacies.…”

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