Characterization of roles for inositol polyphosphate kinases in the control of myeloid cell functions and signaling actions

Abstract

Inositol polyphosphate multikinase (IPMK) and inositol hexakisphosphate 1 (IP6K1) are pleiotropic enzymes essential for the biosynthesis of inositol polyphosphate. However, little is known about the precise control of IPMK expression in macrophages. I observed that both mRNA and protein levels of IPMK were downregulated in TLR4-activated macrophages stimulated with lipopolysaccharide (LPS). miR-181c which is induced by LPS treatment was predicted to bind to the 3'UTR of mouse IPMK mRNA. I performed genomic deletion of miR-181c-binding sequence within IPMK 3'UTR. Stable IPMK expression in macrophages showed both TLR4-signaling events and proinflammatory cytokine induction were markedly attenuated at 2-6 hours of LPS treatment in mutant IPMK macrophages. IPMK was further shown to directly inhibit K63-linked ubiquitination of TRAF6 via direct interaction. Therefore, these findings reveal that this miRNA-181c-mediated suppression of IPMK as a nodal point of regulation for the resolution of TLR4 signaling and inflammation. Also, regarding tumor formation and progression, the role of host IP6K1 in this pathology remains poorly understood. Here I investigated whether host IP6K1 knockout (KO) could control tumor growth using a syngeneic MC38 model which is a syngeneic mouse colon adenocarcinoma tumor model on C57BL/6 background. Using the global IP6K1 KO mice, I showed that the absence of the host IP6K1 gives rise to significantly shortened survival than normal mice due to more rapidly growing tumors. By flow cytometric analysis, tumors in IP6K1 KO mice contained high levels of CD11b+Gr1+IL10+ myeloid cells and lower infiltration of M1-polarized tumor-associated macrophage (TAM), dendritic cell, and CD8 killer T cell into murine colon carcinoma tumors. These data indicate that host IP6K1 acts as a tumor suppressor to regulate anti-tumor immune activities and control the recruitment and activation of cytotoxic T cells to the primary tumor through reprogramming tumor-immune control, and loss of this pathway increases tumor progression.