18P - Low levels of miR-18a is associated with immunosuppression and increased chemo-unresponsiveness in ER-negative tumors

Background

Chemotherapy is the main mode of treatment for the ER negative (ER-ve) subtype of breast cancer. However, despite an initial good response there is a high risk of recurrence in this subtype. microRNAs play an important role in mediating this drug resistance by regulation of the tumor-stromal interactions. Here we have explored the role played by miR-18a in immune modulation and chemo-responsiveness in ER-ve breast cancer.

Methods

Chemosensitivity to Paclitaxel (20-200 μM) was evaluated using MTT assay in an ER-ve cell line; MDA-MB-468 after inhibition of miR-18a using synthetic inhibitors. Expression of integrin β3 was evaluated by immunohistochemistry and that of miR-18a by q-PCR in surgically excised breast tumors with residual disease from partial and non-responders to neo-adjuvant chemotherapy (NACT) (n=57). The expression of miR-18a was further analysed in ER-ve breast tumors from the TCGA (n=116) and the METABRIC cohort (n=107). Differentially expressed genes (DEGs) and deregulated pathways were analysed between miR-18a high and low groups using G:profiler. Immune cell identification was done using CIBERSORT analysis.

Results

miR-18a inhibition in MDA-MB-468 led to increased chemoresistance at 200 μM paclitaxel (p=0.005). In the residual tumors of non-responders to NACT, 78% of the ER-positive and 60% of the ER-ve tumors had a lower expression of miR-18a. miR-18a low tumors also expressed high levels of integrin β3, a marker for chemo-unresponsiveness (p=0.01); the same association was observed in residual tumors of ER-ve subtype (p=0.1). Functional enrichment of DEGs in miR-18a low tumors of the TCGA and METABRIC cohorts demonstrated activation of pathways involving integrin cell surface interactions and drug transporter genes like ABCC11, ABCG1, & ABCG2. (p<0.05). Immune cell stratification using CIBERSORT algorithm revealed a higher proportion of M2 macrophages (p<0.008) and a lower proportion of M1 macrophages in the low miR-18a expressing tumors (p=0.01).

Conclusions

Our results support the existence of a miR-18a mediated chemoresistance within ER-ve tumors. Immunomodulation brought about by the less expression of miR-18a may contribute to immune suppression and chemoresistance in these tumors.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

St. John's Research Institute.

Funding

Department of Health Research and ICMR-India Welcome Trust-DBT.

Disclosure

All authors have declared no conflicts of interest.