Objective: To explore the expression and function of insulin-like growth factor II (IGFII) mRNA binding protein (IMP3) in the Triple Negative Breast Cancer (TNBC).

Materials and methods: According to previously reported gene expression array, we found that IMP3 had significantly higher expression in the CD44+CD24-ESA+ cell cluster, tumor initiating cell or cancer stem cell (CSCs), compared to other tumor cells. Based on the GEO database (GEO accession No. GSE6883), we detected the mRNA levels of IMP 1,2 and 3 by quantitative polymerase chain reaction (q-PCR) in CD44+CD24-ESA+ cell cluster and other breast tumor cell clusters. Besides, we measured IMP3 expression in microsphere of breast cancer, which exerted more significant tumor stem cell properties. The effects of IMP3 on breast cancer cell stem cell properties were studied by RNA interference and overexpression approaches in vitro. Furthermore, we predicted and identified microRNA, which could target and regulate IMP3 from bioinformatics analysis, and verified the interaction by luciferase assays and rescue experiments.

Results: Previously reported data showed that IMP3 expression was significantly upregulated in CD44+CD24-ESA+ cell cluster from breast cancer tissues. Besides, we found IMP3 had higher expression in mesenchymal cells rather than epithelial cells, which was also significantly elevated in SUM159 and T49D cell lines cultured as microsphere rather than adherent cells or differentiated cells. CD44+CD24-ESA+ cell cluster proportion was significantly decreased after silencing IMP3 in SUM1315, and its ability to develop into microsphere was significantly inhibited. By re-expressing IMP3 in SUM315, we restored the self-renewal capacity and tumorigenesis potential of SUM315. Through relative predicting website, we found several miRNAs which could regulate IMP3. miR-34a with highest score was chosen for further analysis. Mimicking miR-34a significantly downregulated IMP3 expression and inhibited its ability to develop into microsphere, while overexpressing IMP3 could rescue this process.

Conclusions: IMP3 plays a vital role in maintaining stem cell properties of breast cancer cells, which could be regulated by mir-34a.