INPP4B Is a PtdIns(3,4,5)P3 Phosphatase That Can Act as a Tumor Suppressor

Inositol polyphosphate 4-phosphatase B (INPP4B) has been identified as a tumor suppressor mutated in human breast, ovary, and prostate cancers. The molecular mechanism underlying INPP4B's tumor-suppressive role is currently unknown. Here, we demonstrate that INPP4B restrains tumor development by dephosphorylating the PtdIns(3,4,5)P-3 that accumulates in situations of PTEN deficiency. In vitro, INPP4B directly dephosphorylates PtdIns(3,4,5)P-3. In vivo, neither inactivation of Inpp4b (Inpp4b boolean AND(/)boolean AND) nor heterozygous deletion of Pten (Pten(+/-)) in mice causes thyroid abnormalities, but a combination of these mutations induces malignant thyroid cancers with lung metastases. At the molecular level, simultaneous deletion of Inpp4b and Pten synergistically increases PtdIns(3,4,5)P-3 levels and activates AKT downstream signaling proteins in thyroid cells. We propose that the PtdIns(3,4,5)P-3 phosphatase activity of INPP4B can function as a "back-up" mechanism when PTEN is deficient, making INPP4B a potential novel therapeutic target for PTEN-deficient or PIK3CA-activated cancers. SIGNIFICANCE: Although INPP4B expression is reduced in several types of human cancers, our work on Inpp4B-deficient mice provides the first evidence that INPP4B is a bonafide tumor suppressor whose function is particularly important in situations of PTEN deficiency. Our biochemical data demonstrate that INPP4B directly dephosphorylates PtdIns(3,4,5)P-3. (C) 2015 AACR.