title: Sodium-dependent phosphate cotransporters and phosphate-induced calcification of vascular smooth muscle cells: redundant roles for PiT-1 and PiT-2.
abstract: OBJECTIVE
  Elevated serum phosphate has emerged as a major risk factor for vascular calcification. The sodium-dependent phosphate cotransporter, PiT-1, was previously shown to be required for phosphate-induced osteogenic differentiation and calcification of cultured human vascular smooth muscle cells (VSMCs), but its importance in vascular calcification in vivo and the potential role of its homologue, PiT-2, have not been determined. We investigated the in vivo requirement for PiT-1 in vascular calcification using a mouse model of chronic kidney disease and the potential compensatory role of PiT-2 using in vitro knockdown and overexpression strategies.
  
  
  RESULTS
  Mice with targeted deletion of PiT-1 in VSMCs were generated (PiT-1(Δsm)). PiT-1 mRNA levels were undetectable, whereas PiT-2 mRNA levels were increased 2-fold in the vascular aortic media of PiT-1(Δsm) compared with PiT-1(flox/flox) control. When arterial medial calcification was induced in PiT-1(Δsm) and PiT-1(flox/flox) by chronic kidney disease followed by dietary phosphate loading, the degree of aortic calcification was not different between genotypes, suggesting compensation by PiT-2. Consistent with this possibility, VSMCs isolated from PiT-1(Δsm) mice had no PiT-1 mRNA expression, increased PiT-2 mRNA levels, and no difference in sodium-dependent phosphate uptake or phosphate-induced matrix calcification compared with PiT-1(flox/flox) VSMCs. Knockdown of PiT-2 decreased phosphate uptake and phosphate-induced calcification of PiT-1(Δsm) VSMCs. Furthermore, overexpression of PiT-2 restored these parameters in human PiT-1-deficient VSMCs.
  
  
  CONCLUSIONS
  PiT-2 can mediate phosphate uptake and calcification of VSMCs in the absence of PiT-1. Mechanistically, PiT-1 and PiT-2 seem to serve redundant roles in phosphate-induced calcification of VSMCs.
authors: From the Departments of Bioengineering (M.H.C., E.M.L., N.W.C., M.C.W., D.F.P., X.L., C.M.G.), Nephrology (W.L.L.), and Cardiology (Y.L., M.T.C.), University of Washington, Seattle; and Department of Medicine, University of Colorado, Denver (M.L.).
authors: Matthew H
authors: MH
authors: Crouthamel
authors: Wei Ling
authors: WL
authors: Lau
authors: Elizabeth M
authors: EM
authors: Leaf
authors: Nicholas W
authors: NW
authors: Chavkin
authors: Mary C
authors: MC
authors: Wallingford
authors: Danielle F
authors: DF
authors: Peterson
authors: Xianwu
authors: X
authors: Li
authors: Yonggang
authors: Y
authors: Liu
authors: Michael T
authors: MT
authors: Chin
authors: Moshe
authors: M
authors: Levi
authors: Cecilia M
authors: CM
authors: Giachelli
mesh_terms: D000818:Animals; D001011:Aorta / Q000166:cytology / Q000378:metabolism; D002478:Cells, Cultured; D004195:Disease Models, Animal; D006801:Humans; D051379:Mice; D018345:Mice, Knockout; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051436:Renal Insufficiency, Chronic / Q000235:genetics / Q000378:metabolism / Q000503:physiopathology*; D050611:Sodium-Phosphate Cotransporter Proteins, Type III / Q000235:genetics / Q000378:metabolism*; D014511:Uremia / Q000235:genetics / Q000378:metabolism / Q000503:physiopathology; D061205:Vascular Calcification / Q000235:genetics / Q000378:metabolism / Q000503:physiopathology*
chemical_list: D010710:Phosphates; D012333:RNA, Messenger; C493586:SLC20A1 protein, human; C493589:SLC20A2 protein, human; D050611:Sodium-Phosphate Cotransporter Proteins, Type III; C018279:sodium phosphate
keywords: PiT-1; PiT-2; phosphate; vascular calcification; vascular smooth muscle cell
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