S1. PI3K pathway and mutations in cancer. S2. PI3K inhibitor chemical structures. S3. Effect of INPP4B siRNA on Akt signaling. S4. Effect of INPP4B siRNA on PI3K inhibitor GI50s. S5. Effect of CRISPR-mediated knockout of INPP4B in HCC70 cells on Akt signaling in response to PI3K-αI. S6. Effect of CRISPR-mediated knockout of INPP4B in HCC70 cells on PI3K inhibitor GI50s. S7. Effect of CRISPR-mediated knockout of INPP4B in BT549 cells on Akt signaling. S8. Effect of CRISPR-mediated knockdown of INPP4B in BT549 cells on PI3K inhibitor GI50s. S9. Effect of the pan PI3Ki on in vitro PI3K activity.
ARTICLE ABSTRACTTriple-negative breast cancer [TNBC, lacks expression of estrogen receptor (ER), progesterone receptor (PR), and amplification of HER2/Neu] remains one of the most aggressive subtypes, affects the youngest patients, and still lacks an effective targeted therapy. Both phosphatidylinositol-3-kinase (PI3K)-α and -β contribute to oncogenesis of solid tumors, including the development of breast cancer. Inositol polyphosphate-4-phosphatase type II (INPP4B) catalyzes the removal of the 4′-phosphate of phosphatidylinositol-(3, 4)-bisphosphate (PI-3,4-P2), creating phosphatidylinositol-3-phosphate. There is debate concerning whether PI-3,4-P2 contributes to Akt and downstream effector activation with the known canonical signaling second messenger, phosphatidylinositol-(3, 4, 5)-trisphosphate (PIP3). If PI-3,4-P2 is a positive effector, INPP4B would be a negative regulator of PI3K signaling, and there is some evidence to support this. Utilizing phosphatase and tensin homolog deleted on chromosome ten (PTEN)-null triple-negative breast tumor cell lines, it was unexpectedly found that silencing INPP4B decreased basal phospho-Akt (pAkt) and cellular proliferation, and in most cases sensitized cells to PI3K-α and PI3K-β isoform-specific inhibitors. Conversely, overexpression of INPP4B desensitized cells to PI3K inhibitors in a phosphatase activity-dependent manner. In summary, the current investigation of INPP4B in PTEN-null TNBC suggests new mechanistic insight and the potential for targeted therapy for this aggressive subset of breast cancer.Implications: These data support a model where PI-3,4-P2 is inhibitory toward PI3K, revealing a novel feedback mechanism under conditions of excessive signaling, and potentially an indication for PI3K-β isoform-specific inhibitors in PTEN-null TNBC that have lost INPP4B expression. Mol Cancer Res; 15(6); 765–75. ©2017 AACR.