A Novel Oncogenic Role of Inositol Phosphatase SHIP2 in ER-Negative Breast Cancer Stem Cells: Involvement of JNK/Vimentin Activation

Overexpression of SH2-containing-5'-inositol phosphatase-2 (SHIP2) correlates with poor survival in breast cancer. However, its role in breast cancer stem cells (BCSCs) remains unclear. Here, we showed that the percentage of SHIP2(+) cells was positively correlated with that of CD24(-)CD44(+) cells in 60 breast cancer specimens. Among 20 estrogen receptor (ER)-negative samples, 17 had greater SHIP2 expression in CD24(-)CD44(+) subpopulation than the remaining subpopulation. Data mining of microarray analysis of 295 breast tumors showed a significant correlation of higher SHIP2 expression with distant metastasis. Examination of patient-derived mouse xenografts revealed that SHIP2 protein and its tyrosine 1135 phosphorylation were significantly higher in BCSCs, identified as CD24(-)CD44(+) or aldehyde dehydrogenase (ALDH(+)), than non-BCSCs. SHIP2 silencing or inhibitor of SHIP2 phosphatase significantly decreased mammosphere-forming efficiency, ALDH(+) subpopulation in vitro and tumorigenicity of BCSCs in vivo. Overexpression of SHIP2 enhanced the expression of epithelial-mesenchymal transition markers including vimentin (VIM), which was mainly expressed in ER-negative breast cancer cells with higher level in mammospheres than monolayer culture. Ablation of c-Jun N-terminal kinase 1 (JNK1), JNK2, or VIM diminished the increased ALDH(+) population and tumorigenicity, induced by SHIP2 overexpression. BCSCs displayed greater expression of phospho-JNK than non-BCSCs and silencing of JNK suppressed SHIP2-mediated upregulation of VIM. Furthermore, SHIP2 overexpression enhanced Akt activation, but Akt inhibition failed to influence SHIP2-induced phospho-JNK/VIM upregulation. In conclusion, SHIP2 plays a key role in BCSCs of ER-negative breast cancers through activation of Akt and JNK with upregulation of VIM and may serve as a target for therapy directed at BCSCs.