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A2670 - Mechanisms of Microlith Clearance Induced by Dietary Phosphate Restriction in Pulmonary Alveolar Microlithiasis
Author Block: Y. Uehara1, N. M. Nikolaidis2, Y. Hasegawa3, A. Saito4, L. B. Pitstick1, H. Wu3, K. LaSance3, J. C. Woods5, J. Guo6, F. X. McCormack3; 1Pulmonary, Critical Care and Sleep Medicine, University of Cincinnati, Cincinnati, OH, United States, 2Pulm and Crit Care, Univ of Cincinnati, Cincinnati, OH, United States, 3University of Cincinnati, Cincinnati, OH, United States, 4Dept. of Respiratory Medicine and Allergology, Sapporo Medical University, Sapporo, Japan, 5Pulmonary Medicine, Cincinnati Children's Hospital Medical Center, Cincinnati, OH, United States, 6Cincinnati Children's Hospital Medical Center, Cincinnati, OH, United States.
Rationale: Pulmonary alveolar microlithiasis (PAM) is an autosomal recessive disorder caused by deficiency of Npt2b, a sodium phosphate cotransporter on type II alveolar epithelial cells (AECII), which is required for the export of phosphate from the alveolar lining fluid. Progressive calcium phosphate microlith deposition and macrophage rich inflammation and fibrosis often results in respiratory failure and death in patients with PAM. We have previously reported that the burden of microliths in the Npt2b-/- murine model of PAM is reduced by dietary phosphate restriction. Here we investigated the upregulation of alternative phosphate transporters, osteoclastogenesis and osteoclast activation as potential mechanisms of dietary modulation of the microlith burden in PAM. Methods: Npt2b-/- and Npt2b+/+ mice were fed diets with typical (0.7% Pi), low (0.02%, 0.1% and 0.4% Pi) or high (2% Pi) phosphate content, denoted RPD, LPD and HPD, respectively. Disease progression was monitored with chest radiographs, lung weights and stone volume calculated from serial quantitative microCT imaging. Serum levels of key mediators of phosphate metabolism were measured including fibroblast growth factor 23 (FGF-23), parathyroid hormone (PTH) and 1,25-dihydroxy vitamin D (1,25(OH)Vit D). Phosphate and calcium levels in serum and bronchoalveolar lavage fluid (BALF) were also measured. rtPCR was used to analyze the expression of phosphate and calcium transporters in AECII and osteoclast related genes in whole lung homogenates. Results: In Npt2b-/- mice with advanced stone burdens, treatment with LPD or HPD for 8 weeks decreased and increased microCT stone burden and lung weights relative to RPD, respectively. LPD reduced serum levels of FGF-23 and PTH, increased serum 1,25(OH)Vit D levels and reduced BAL levels of phosphate and calcium. Alternative sodium-phosphate cotransporters, Pit1 and Pit2, were upregulated in isolated AECII of Npt2b-/- mice following LPD. Osteoclast related genes were upregulated in whole lung homogenates of Npt2b-/- mice after LPD. The osteoclastogenesis inhibitory factor, osteoprotegerin, and calcium and phosphate levels were increased in the BALF of Npt2b-/- mice on HPD. Conclusion: LPD prevents progressive microlith accumulation in young Npt2b-/- mice and reverses stone burden in Npt2b-/- mice with well-established PAM by a mechanism that includes upregulation of alternative AECII sodium phosphate co-transporters Pit1 and Pit2, reduction of alveolar calcium and phosphate levels, and activation of osteoclastogenesis and osteoclastic function. HPD increase alveolar calcium, phosphate and osteoprotegerin levels. We conclude that dietary phosphate restriction may be a promising approach for the treatment of PAM. Funding: RLDC(U54HL127672), RO1(R01HL127455)