Phosphate uptake-independent signaling functions of the type III sodium-dependent phosphate transporter, PiT-1, in vascular smooth muscle cells

Abstract

Vascular calcification (VC) is prevalent in chronic kidney disease and elevated serum inorganic phosphate (Pi) is a recognized risk factor. The type III sodium-dependent phosphate transporter, PiT-1, is required for elevated Pi-induced osteochondrogenic differentiation and matrix mineralization in vascular smooth muscle cells (VSMCs). However, the molecular mechanism(s) by which PiT-1 promotes these processes is unclear. In the present study, we confirmed that the Pi concentration required to induce osteochondrogenic differentiation and matrix mineralization of mouse VSMCs was well above that required for maximal Pi uptake, suggesting a signaling function of PiT-1 that was independent of Pi transport. Elevated Pi-induced signaling via ERK1/2 phosphorylation was abrogated in PiT-1 deficient VSMCs, but could be rescued by wild-type (WT) and a Pi transport-deficient PiT-1 mutant. Furthermore, both WT and transport-deficient PiT-1 mutants promoted osteochondrogenic differentiation as measured by decreased SM22α and increased osteopontin mRNA expression. Finally, compared to vector alone, expression of transport-deficient PiT-1 mutants promoted VSMC matrix mineralization, but not to the extent observed with PiT-1 WT. These data suggest that both Pi uptake-dependent and -independent functions of PiT-1 are important for VSMC processes mediating vascular calcification.

Phosphate (Pi)-regulated heterodimerization of the high-affinity sodium-dependent Pi transporters PiT1/Slc20a1 and PiT2/Slc20a2 underlies extracellular Pi sensing independently of Pi uptake

“…Our present study brings evidence for a role of PiT1 and PiT2 in transmitting the Pi signal to the cell by showing that the Pi-dependent upregulation of Mgp and Opn, and ERK1/2 phosphorylation were blunted in PiT1-or PiT2-depleted cells. A role for PiT1 in mediating ERK1/2 signaling has been reported earlier (48,55), however a similar role for PiT2 has never been illustrated before.…”

“…Since PiT1 and PiT2 have very close Pi transport characteristics (51), they may also share Pi-sensing properties and thus have interconnected roles in Pi sensing. Moreover, since Pi-independent functions have been highlighted recently for PiT1 (52)(53)(54)(55)(56), the involvement of Pi transport in the Pi sensing by PiT1 or PiT2 remains to be investigated.…”

Slc20a2 deficiency results in fetal growth restriction and placental calcification associated with thickened basement membranes and novel CD13 and lamininα1 expressing cells

“…This result supports that Slc20a2 is required in both the maternal and fetal placental compartments. Furthermore, these data suggest that placental phosphate transport may involve a two-step transport mechanism, for example (1) transport out of the maternal circulation and (2) transport into the fetal circulation, or there may be unidentified phosphate transport-independent roles of Slc20a2, as is the case for Slc20a1 [36]. …”

A new in vitro model to delay high phosphate-induced vascular calcification progression

“…VSMC osteoblastic differentiation is one of the main features of the process of actively mediated calcification of blood vessels induced by high Pi [18]. The main characteristics of this transformation are the loss of smooth muscle lineage markers and the upregulation or induction of proteins typical of osteoblastic cells [19].…”