Repurposing antiretroviral drugs for treating triple-negative breast cancer via LINE-1 regulation

Abstract

The most common cancer in women is breast cancer with approximately 1 in 8 women developing this disease in their lifetime. Clinically, breast cancer can be divided into distinct subtypes based on the presence or absence of hormone receptors such as estrogen receptor (ER), progesterone receptor (PR) and expression of the HER2 gene. In this project, one of the most difficult to treat subclasses - triple negative (ER-/PR-/HER2-) breast cancer (TNBC) is studied. Until now, very limited drug treatment strategies are available for TNBCs because of a lack of hormone receptors as potential drug targets. Thus, it is a matter of urgency to seek specific treatment tailored to TNBC patients. This raises the prospect that antiretroviral drugs might be able to repurpose as anticancer drugs for TNBCs. In the mid-1990s, the incidence of the AIDS-related cancers was greatly reduced in the HIV patients due to the introduction of antiretroviral therapy to the HIV patients. It has been suggested that a direct inhibitory effect of antiretroviral drugs on the reverse transcriptase activity of long interspersed nuclear element 1 (LINE-1) in tumour cells could be a crucial factor. LINE-1 is the most important transposon with autonomous retro-transposition ability in humans. It can alter gene regulations and cause somatic mutations. Since LINE-1 has the potential to adversely affect individuals, it is silenced in differentiated tissues by diverse endogenous mechanisms. Nonetheless, LINE-1 is highly expressed in many cancers, especially in breast carcinomas. Therefore, it is worth examining whether antiretroviral drugs can be repurposed as anticancer drugs for treating TNBCs and to further understand the relationship between LINE-1 and TNBCs. Antiretroviral drug induced anticancer effects may be relevant to down-regulation of the fatty acid metabolism pathway. In this project, two antiretroviral drugs - Efavirenz and SPV122, had been shown to cause cell death and cell proliferation retardation, thus, effectively eliminating cancer cells in a range of TNBC cell lines. Additionally, LINE-1 suppression had been observed in the antiretroviral drugs-treated breast cancer cell lines implying a potential link between LINE-1 and TNBCs. Whole genome RNA sequencing data further highlighted the possible mechanisms involved in this anticancer process. It seemed the fatty acid metabolism pathway could be a key regulator in this anticancer process. Many key genes involved in fatty acid metabolism were down-regulated after drug treatments. However, the antiretroviral drugs-treated MCF10AT and MCF10CA1a cells were found to present some mesenchymal markers which are often characteristic signs of poor prognostic outcomes thereby highlighting the complexity of TNBCs. The RNA sequencing data also strongly implied that cancer stem cells (CSCs) could play a role in these confusing results. CSCs are a small group of cancer cells with stem cell-like abilities, and they are thought to be responsible for drug resistance, cancer metastasis, and cancer recurrence. Interestingly, in a series of experiments, different groups of CSCs showed various responses to a range of drugs. ALDHhigh epithelial-type CSCs were significantly reduced after antiretroviral drug treatment; whereas, CD44+/CD24- mesenchymal-type CSCs were increased after treatment. These results highlighted the importance of CSC heterogeneity and implied that mesenchymal-type CSCs have greater resistance to the drugs than other cancer cells. Finally, the functional CSC assay demonstrated that CSCs can be eliminated by the antiretroviral drugs indicating that Efavirenz and SPV122 might be able to target both non-CSCs and CSCs. To combine all the results together, Efavirenz and SPV122 could potentially be valid anticancer drugs for treating TNBCs by regulating cancer fatty acid metabolism. Follow-up experiments are necessary to further understand how antiretroviral drugs impact anticancer processes.