Expression of a newly identified phosphate transporter retrovirus receptor in human SaOS-8 osteoblast-like cells and its regulation by insulin-like growth factor I

The cell surface receptor for gibbon ape leukemia virus (Glvr-1) was recently demonstrated to serve normal cellular functions as a sodium-dependent phosphate (NaPi) transporter. This protein belongs to a newly identified phosphate transporter/retrovirus receptor gene family distinct from renal type I and II NaPi transporters. Although inorganic phosphate (Pi) transport is an important function of osteoblasts and of the matrix vesicles produced by these cells in the context of bone matrix calcification, the molecular identity of the NaPi transport system(s) present in this cell type is still unknown. In contrast to Pi uptake mediated by renal NaPi transporters, the activities of both the osteoblastic transport system and Glvr-1 are decreased at alkaline pH, and this observation led us to investigate expression of this transporter in human SaOS-2 osteosarcoma cells. Northern blotting analysis revealed the presence of a 4-kilobase Glvr-1 transcript. The expression of Glvr-1 messenger RNA (mRNA) was increased in response to insulin-like growth factor I (IGF-I). Associated with this effect, a selective, dose-and time-dependent stimulation of NaPi transport was observed. Actinomycin D and cycloheximide abolished the increase in NaPi transport, which thus appeared to be dependent on RNA and protein synthesis. The increase in Glvr-1 mRNA induced by IGF-I was dose dependent and transient, peaking after 4 h (similar to 4-fold increase in response to 10(-7) M IGF-I). It preceded the maximal expression of NaPi transport stimulation (173-235% of control), which was observed after 18-24 h. Induction of Glvr-1 mRNA expression by IGF-I was inhibited by actinomycin D, suggesting that this effect was related to an increase in gene transcription. The stability of Glvr-1 mRNA was not altered by IGF-I, and Glvr-1 mRNA induction did not require the synthesis of new proteins. These data demonstrate for the first time regulated expression of mRNA encoding the type III NaPi transporter Glvr-1 in osteoblast-like cells. They also suggest that this new transporter family may be involved in Pi handling in osteogenic cells and in its regulation by osteotropic factors.