Selective induction of chemotherapy resistance of mammary tumors in a conditional mouse model for hereditary breast cancer

Abstract

We have studied in vivo responses of "spontaneous" Brca1- and p53-deficient mammary tumors arising in conditional mouse mutants to treatment with doxorubicin, docetaxel, or cisplatin. Like human tumors, the response of individual mouse tumors varies, but eventually they all become resistant to the maximum tolerable dose of doxorubicin or docetaxel. The tumors also respond well to cisplatin but do not become resistant, even after multiple treatments in which tumors appear to regrow from a small fraction of surviving cells. Classical biochemical resistance mechanisms, such as up-regulated drug transporters, appear to be responsible for doxorubicin resistance, rather than alterations in drug-damage effector pathways. Our results underline the promise of these mouse tumors for the study of tumor-initiating cells and of drug therapy of human cancer.

Fig. 1.

Chemotherapy treatment of mammary tumor-bearing Kcre;Brca1^F/F;p53^F/F mice results in resistance to doxorubicin and docetaxel but not to cisplatin. (A) Schematic overview of therapeutic intervention studies in female mice with spontaneous Brca1^−/−;p53^−/− mammary tumors. (B and C) Tumor-bearing animals (T1–T18) were either left untreated (B) or treated with doxorubicin (5 mg/kg i.v.), docetaxel (25 mg/kg i.v.), or cisplatin (6 mg/kg i.v.), as indicated by the arrows (C). Curves are represented as relative tumor volume (0.5 × length × width², y axis) over time (days, x axis) and show examples of various responses to drug treatment observed.

Fig. 2.

Gene expression fingerprints of individual Brca1^−/−;p53^−/− mammary tumors are retained during chemotherapy treatment. Unsupervised hierarchical cluster analysis of gene expression data showing coclustering of samples derived from the same parental tumor. Amplified RNA (aRNA) from indicated tumor samples and common reference aRNA were fluorescently labeled and cohybridized to mouse 31,769 oligonucleotide microarrays. Samples derived from a single tumor are marked with the same color and black squares indicate whether the tumor was grafted or of spontaneous origin.

Fig. 3.

Expression of Mdr1a, Mdr1b, and other selected genes in doxorubicin-sensitive and -resistant tumors. Ratios of gene expression in doxorubicin-resistant tumors and samples from the corresponding drug-sensitive tumors before treatment. Shown are RT-MLPA analyses of 13 doxorubicin-resistant tumors. The sum of the values for Actinβ and MRP1 was used as internal reference for Mdr1a, Mdr1b, Bcrp1, Top2α (two different sequences targeted), and Top2β. MRP1 was compared with Actinβ and vice versa. The presented values represent the mean ratio of three independent reactions. Error bars indicate standard deviation. For the complete data set, see SI Table 3.

Fig. 4.

Functional imaging of doxorubicin-resistant tumors shows increased drug transporter activity in vivo. Nuclear imaging was used to measure ^99mTc-sestamibi uptake and washout in a doxorubicin-sensitive tumor (T*23-con) (Left) and a doxorubicin-resistant tumor (T*23-dox2-res), with (Right) or without (Center) pretreatment of the Mdr1a/Mdr1b inhibitor cyclosporin A.