HnRNPM and CD44s expression affects tumor aggressiveness and predicts poor prognosis in breast cancer with axillary lymph node metastases.

HnRNPM is an essential splicing factor and its expression is closely correlated with invasion and metastasis of tumor cells. The CD44 cell adhesion molecule is aberrantly expressed in many breast tumors and CD44 splice variants have been implicated in specific oncogenic signaling pathways. To investigate the clinical significance and biological function of hnRNPM, immunohistochemistry, quantitative, and semiquantitative polymerase chain reaction, lentiviral transfection system and transwell invasion assays were performed. We found that hnRNPM expression was significantly upregulated in breast cancer tissues compared with benign breast lesions. Although there was no significant correlation between hnRNPM and total CD44 protein or mRNA level, there was a negative correlation between hnRNPM and CD44v6. HnRNPM and CD44s expression showed positive correlation and in particular, they were dually expressed in breast cancer tissues. Interestingly, cancer stem cells marker, ALDH1+ phenotype was positively associated with overexpression of CD44s or hnRNPM and negatively related to CD44v6. Patients with high hnRNPM tended to have higher levels of CD44s, shorter overall survival (OS) and higher rates of lymph node metastases (LNM). Remarkably, Kaplan-Meier and Cox regression analyses displayed that hnRNPM+ or CD44shigh was a poor prognostic factor for OS of patients with LNM. Upregulation of hnRNPM in MCF-7 cells caused a significant increase in cell invasion, and this effect may occur through the regulation of CD44s expression. In conclusion, overexpression of hnRNPM promotes breast cancer aggressiveness by regulating the level of CD44s, indicates a poor prognosis for patients with LNM, and has potential as therapeutic targets.