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

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author Li Xingyao, Gu Chunfang, Hostachy Sarah, Sahu Soumyadip, Wittwer Christopher, Jessen Henning J., Fiedler Dorothea, Wang Huanchen, Shears Stephen B.,
Project Discovery and mechanistic dissection of novel signaling pathways controlling phosphate homeostasis in eukaryotes
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Original article (peer-reviewed)

Journal Proceedings of the National Academy of Sciences
Volume (Issue) 117(7)
Page(s) 3568 - 3574
Title of proceedings Proceedings of the National Academy of Sciences
DOI 10.1073/pnas.1908830117

Open Access

URL https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7035621/
Type of Open Access Repository (Green Open Access)

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 (InsP 8 ). XPR1-mediated Pi efflux was inhibited by reducing cellular InsP 8 synthesis, either genetically (knockout [KO] of diphosphoinositol pentakisphosphate kinases [PPIP5Ks] that synthesize InsP 8 ) 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 InsP 8 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 InsP 8 ; PCP analogs of other PP-InsP signaling molecules were ineffective. High-affinity binding of InsP 8 to the XPR1 N-terminus ( Kd = 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 PPIP5K mutations might extend an epistatic repertoire for XPR1 dysregulation, with pathological consequences for bone maintenance and ectopic calcification.
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