IP3 3-Kinase B Suppresses B Cell Lymphoma by Antagonizing PI3K/mTOR in B cells

Accounting for ~4% of all cancers in the US, Non-Hodgkin Lymphoma (NHL) is the most prevalent blood cancer. Diffuse large B-cell Lymphoma (DLBCL) is the most common and aggressive type of NHL. Here, we identify inositol-trisphosphate (IP3) 3-kinase B (Itpkb) as a novel tumor suppressor whose deficiency triggers DLBCL in mice. We found that aging Itpkb−/− mice die prematurely with anemia and splenomegaly. 15% of these mice showed multiorgan-infiltration with neoplastic germinal-center phenotype B cells reminiscent of DLBCL. Itpkb−/−B cell lymphomas are mono-or oligoclonal, transplantable, and constitutively hyperactivate the oncogenic phosphoinositide 3-kinase (PI3K) signaling pathway. Growth of Itpkb−/−B cell lymphoma xenografts in immunodeficient mice is sensitive to treatment with PI3K or mTOR inhibitors. In humans, subsets of DLBCL patients carry Itpkb missense mutations, deletions or copy number reductions. Additionally, Itpkb expression is altered in several other hematopoietic and non-hematopoietic human cancers. We propose that Itpkb suppresses tumors by producing inositol-tetrakisphosphate (IP4), a soluble analog of the PI3K product phosphatidylinositol-trisphosphate (PIP3). We and others have previously shown that IP4 antagonism with PIP3 for Akt effector kinase binding prevents excessive Akt/mTORC1 signaling in hematopoietic cells. Our new results suggest that this mechanism prevents oncogenic PI3K signaling in the B cell lineage and possibly in other cell types. Thus, Itpkb might be a novel biomarker for tumor aggressiveness or prognosis, and Itpkb activating drugs or IP4 might have therapeutic potential.