Abstract P5-07-04: Nuclear miR-133 expression is associated with breast cancer progression and survival
Authors: Hou M-F , Chen F-M , Yang S-F , Dai H-Y , Yeh Y-T 
Published: February 2018

Background:MiRNAs (microRNAs) can function as the tumor suppressor gene or oncogene to affect carcinogenesis, therapy response and progression in human cancer. The prevailing view is that miRNAs function to regulate mRNA stability and translation in the cytoplasm. However, multiple studies have detected miRNAs in the nuclear compartment. Nuclear miRNAs have been proven to regulate the functions of nuclear protein and synthesis of nuclear miRNA. We have found circulating muscle-enriched miR-133a can serve as prognostic biomarkers for breast cancer. Herein, we aim to explore the subcellular distribution, prognostic significance and treatment response of miR-133a in Taiwanese breast cancer.

Methods: Circulating miR-133a levels were conducted with the serum from 188 patients prior to surgery and from 82 healthy controls. The subcellular distribution patterns of miR-133a were analyzed by Chromogenic In Situ Hybridization (CISH) in paraffin embedded tissue sections from 224 cases. Cell viability and cell cycle were determined by XTT assay and flowcytometry. Protein and mRNA levels were examined by western blotting and RT-qPCR (TaqMan) assay.

Results: Our results showed that circulating miR-133a levels of breast cancer patient are significantly higher than those of health subjects, suggesting that increased miR-133a secretion from unknown tissues or cells may promote carcinogens in Taiwanese breast cancer. Using the receiver operating characteristic (ROC) curve to examine the diagnostic value of circulating miR-133a in breast cancer (AUC=0.834), we found that increased miR-133a levels could serve as a diagnostic biomarker in accessing Taiwanese breast cancer risk. Nevertheless, CISH analysis revealed that nuclear miR-133a frequency was inversely correlated with tumor grade, stage and lymph node metastasis (p=0.029, 0.012, and 0.0008), and cytoplasmic miR-133a intensity was positively correlated with recurrence (p=0.010). Furthermore, high frequency of nuclear miR-133a was correlated with a better overall survival rate (p=0.039) and improved the survival rate of patients who received hormone therapy (p=0.009). To further investigate the role of nuclear miR-133a, an additional miR-133a, denoted as Nu-miR-133a, ending in nuclear localization element was constructed and found to be highly enriched in the nucleus. Ectopic expression of wild-type and Nu-miR-133 inhibited proliferation of breast cancer cells. Both wild type miR-133a and Nu-miR-133a increased S phase arrest as well as the levels and foci of γ-H2AX. Unexpectedly, we found that wild type miR-133a decreased ATM expression but activated p38 MAPK. Notably, H2AX can be also phosphorylated by p38 MAPK kinase in addition to ATM and ATR. The underlying mechanism between nuclear miR-133a and potentially p38-mediated DNA damage response (DDR, γ-H2AX and S phase arrest) will be needed further elucidation.

Conclusions: In conclusion, we provide the first evidence that nuclear miR-133a may play a crucial role in the development and treatment response of breast cancer potentially through its mediated DDR. The interplay among nuclear miR-133a, p38 and DDR require further investigations.

Citation Format: Hou M-F, Chen F-M, Yang S-F, Dai H-Y, Yeh Y-T. Nuclear miR-133 expression is associated with breast cancer progression and survival [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P5-07-04.