Transcription profiling of primary and reccurent spontaneous mammary gland tumors in female MMTV-HER2/neu mice in response to varying 17-DMAG/Stavudine treatment regiments

Activation of endogenous retrotransposons frequently occurs in cancer cells and contributes to tumor genomic instability. To test whether inhibition of retrotranspositions has an anticancer effect, we used treatment with the nucleoside reverse transcriptase inhibitor (NRTI) stavudine in mouse cancer models, MMTV-HER2/Neu and Th-MYCN, that spontaneously develop breast cancer and neuroblastoma, respectively. In both cases, stavudine in drinking water did not affect tumor incidence nor demonstrate any direct antitumor effects. However, stavudine dramatically extended progression-free survival in both models following an initial complete response to chemotherapy. To approach the mechanism underlying this phenomenon, we analyzed the effect of NRTI on the selection of treatment-resistant variants in tumor cells in culture. Cultivation of mouse breast carcinoma 4T1 in the presence of stavudine dramatically reduced the frequency of cells capable of surviving treatment with anticancer drugs. Global transcriptome analysis demonstrated that the acquisition of drug resistance by 4T1 cells was accompanied by an increase in the constitutive activity of interferon type I and NF-?B pathways and an elevated expression of LINE-1 elements, which are known to induce inflammatory responses via their products of reverse transcription. Treatment with NRTI reduced NF-?B activity and reverted drug resistance. Furthermore, the inducible expression of LINE-1 stimulated inflammatory response and increased the frequency of drug resistant variants in a tumor cell population. These results indicate a new mechanism by which retrotransposon desilencing can stimulate tumor cell survival during treatment and suggest reverse transcriptase inhibition as a potential new therapeutic approach for targeting the development of drug resistant cancers. Overall design: Total RNA was isolated from individual spontaneous mammary gland tumors ("tumor") representing several experimental groups and from cancer-free mammary glands ("mammary gland"; "untreated") of female MMTV-HER2/neu mice . Samples marked as "untreated" represent RNAs from tumor that developed in control untreated mice. Samples marked as "STV" represent tumors that developed in mice that received reverse transcriptase inhibitor stavudine in drinking water. Samples marked as "DMAG" represent RNAs from recurrent tumors that regrew following initial complete response following treatment with HSP90 inhibitor 17-DMAG. Samples marked as "DMAG+STV" represent RNAs from recurrent tumors that regrew following initial complete response following treatment with HSP90 inhibitor 17-DMAG under constitutive treatment with stavudine in drinking water.