Control of XPR1-dependent cellular phosphate efflux by InsP ₈ is an exemplar for functionally-exclusive inositol pyrophosphate signaling

Abstract

Homeostasis of cellular fluxes of inorganic phosphate (Pi) supervises its structural roles in bones and teeth, its pervasive regulation of cellular metabolism, and its functionalization of numerous organic compounds. Cellular Pi efflux is heavily reliant on Xenotropic and Polytropic Retrovirus Receptor 1 (XPR1), regulation of which is largely unknown. We demonstrate specificity of XPR1 regulation by a comparatively uncharacterized member of the inositol pyrophosphate (PP-InsP) signaling family: 1,5-bis-diphosphoinositol 2,3,4,6-tetrakisphosphate (InsP₈). XPR1-mediated Pi efflux was inhibited by reducing cellular InsP₈ synthesis, either genetically (knockout [KO] of diphosphoinositol pentakisphosphate kinases [PPIP5Ks] that synthesize InsP₈) or pharmacologically [cell treatment with 2.5 µM dietary flavonoid or 10 µM N2-(m-trifluorobenzyl), N6-(p-nitrobenzyl) purine], to inhibit inositol hexakisphosphate kinases upstream of PPIP5Ks. Attenuated Pi efflux from PPIP5K KO cells was quantitatively phenocopied by KO of XPR1 itself. Moreover, Pi efflux from PPIP5K KO cells was rescued by restoration of InsP₈ levels through transfection of wild-type PPIP5K1; transfection of kinase-dead PPIP5K1 was ineffective. Pi efflux was also rescued in a dose-dependent manner by liposomal delivery of a metabolically resistant methylene bisphosphonate (PCP) analog of InsP₈; PCP analogs of other PP-InsP signaling molecules were ineffective. High-affinity binding of InsP₈ to the XPR1 N-terminus (<i>K</i>_d = 180 nM) was demonstrated by isothermal titration calorimetry. To derive a cellular biology perspective, we studied biomineralization in the Soas-2 osteosarcoma cell line. KO of PPIP5Ks or XPR1 strongly reduced Pi efflux and accelerated differentiation to the mineralization end point. We propose that catalytically compromising <i>PPIP5K</i> mutations might extend an epistatic repertoire for <i>XPR1</i> dysregulation, with pathological consequences for bone maintenance and ectopic calcification.