Regulation of cell proliferation and cell density by the inorganic phosphate transporter PiT1

We found that knock-down of PiT1 in MC3T3-E1 cells led to impaired proliferation supporting that at least a certain level of PiT1 is important for wildtype level of proliferation. We, however, also observed that MC3T3-E1 and NIH3T3 cells themselves regulate their endogenous PiT1 mRNA levels with lower levels in general correlating with decreased proliferation/increased cell density. Moreover, over-expression of human PiT1 led to increased proliferation of both MC3T3-E1 and NIH3T3 cultures and resulted in higher cell densities in cultures of these two strictly density-inhibited cell lines. In addition, when we transformed NIH3T3 cells by cultivation in fetal bovine serum, cells over-expressing human PiT1 formed more colonies in soft agar than control cells.We conclude that not only is a certain level of PiT1 necessary for normal cell division as suggested by previously published studies, rather the cellular PiT1 level is involved in regulating cell proliferation and cell density and an increased PiT1 expression can indeed make NIH3T3 cells more sensitive to transformation. We have thus provided the first evidence for that expression of the type III Pi transporter, PiT1, above the endogenous level can drive cell proliferation and overrule cell density constraints, and the results bridge previous observations showing that a certain PiT1 level is important for regulating normal embryonic growth/development and for tumorigenicity of HeLa cells.The mammalian type III sodium-dependent inorganic phosphate (NaPi) symporters, PiT1 (SLC20A1; formerly GLVR1) and PiT2 (SLC20A2; formerly GLVR2 and Ram1) [1-6] belong to the Pi transport (PiT) family (SLC20; TC #2.A.20 [7]), which members are present in all kingdoms of life [8]. The mammalian PiT proteins were originally identified as receptors for gammaretroviruses [2,3,5]. Thus, e.g., human PiT1 (hPiT1) is receptor for gibbon ape leukemia virus (GALV) [3] and feline leukemia virus subgroup B (FeLV-B) [9] and human PiT2 (hPiT2) an