The role of DNA damage associated microRNAs in enhancing the chemotherapeutic responses in triple negative breast cancer (TNBC)

Abstract

Triple negative breast cancer (TNBC) is a heterogeneous aggressive type of mammary tumour which lacks a definite tailored therapeutic approach, consequently limiting treatment options to chemotherapeutic agents. Doxorubicin has demonstrated significant efficacy in the treatment of TNBC although many patients experience recurrent malignancies as a result of chemoresistance. In addition, an enhanced immune response of the tumour microenvironment (TME) is proposed to be associated with better clinical outcomes in different types of cancer including breast cancer. However, the parameters which define a positive anti-tumour inflammatory response are still unclear and limits the application of immune target therapies in the clinic. This thesis hypothesised that by understanding the biological responses to a minimum DNA damage in response to a low dose radiation in TNBC cells, new opportunities in reversing the chemoresistant status of this aggressive type of cancer can be achieved. Hence, the main aim of the presented thesis was to investigate the effects of low-dose radiation on tumour immunity and the modulation of Doxorubicin responses with microRNAs to elucidate novel strategies to kill TNBC cells.Characterizing the response of TNBC cells to a minimum DNA damage following an exposure to low-dose radiation demonstrated that a sublethal radiation dose has the potential to propagate an anti-tumour pro-inflammatory activity. In cells exposed to low dose 0.5 Gy radiation the expression of NFκB-P65 was correlated with proinflammatory endpoints and revealed that an increase in the nuclear expression of P65 was associated with concurrent increases in IL-6 and CXCL10 levels (p < 0.01 and p< 0.001 respectively). Meanwhile, these low dose radiation effects were associatedwith a reduction in miR-21 levels (p < 0.05). To verify this association, the miR-21 expression was inhibited using an anti-miR-21 transient transfection assay. Data showed a significant upregulation of the anti-tumour mediators (CXCL10, p < 0.01). MicroRNA-21 inhibition with anti-miR-21 confirmed the pro-survival protumorigenic activity of this miRNA in TNBC cells in which miR-21 inhibition slowed tumour cell migration and inhibited tumour cell proliferation. The anti-miR-21 effect on Doxorubicin cytotoxicity was further studied and was shown to be significantly increased in cells pretreated with anti-miR-21, therefore demonstrating that these agents could be delivered as a sequential therapy regimen. In conclusion, the in vitro assays show the potential to treat TNBC cells with a sequential combination of anti-miR-21 and Doxorubicin. Moreover, downregulating the expression of this DNA damage-associated miRNA indicated that it may also have effects on TME cellular content and could influence an anti-tumour immunity response. Therefore, elucidating anti-miR-21 as a promising treatment modality for TNBC.

Date of Award	Jul 2020
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Fiona Furlong (Supervisor), Jonathan Coulter (Supervisor) & Niamh Buckley (Supervisor)