Expression and Regulation of Na-Dependent Pi Transport in Matrix Vesicles Produced by Osteoblast-like Cells

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Abstract
Extracellular matrix vesicles (MV) are the loci of initial mineralization in several calcifying tissues. We recently reported that MV isolated from chicken epiphyseal cartilage are equipped with a Na-dependent Pi transport (NaPiT) system. The activity of the NaPiT system appeared to be crucial for the development of MV-mediated calcification. In the present study we investigated the expression of NaPiT activity in MV produced by the osteoblast-like cells MC3T3-E1. The relationship between changes in NaPiT activity in the intact cells and in the released MV was also examined. NaPiT activity in MV harvested from cultured MC3T3-E1 cells was transiently expressed. It was markedly increased between Days 8 and 10 (5- to 6-fold), and then gradually decreased. NaPiT activity was enriched in MV as compared with the parent osteoblast-like cells, while the Na-dependent transport system for alanine (NaAIaT) was not. When NaPiT activity was enhanced in osteoblast-like cells by fetal calf serum (FCS) or Pi depletion, Pi transport stimulation was observed in the derived MV as well. Alkaline phosphatase (AP) was differentially expressed and regulated in MV from MC3T3-E1 cell cultures, as compared with NaPiT. In contrast to the transient expression of NaPiT, AP activity in MV increased continuously with time in culture. It was stimulated by FCS treatment of the parent cells, but decreased in MV obtained from Pi-depleted cultures. These results suggest that the presence in osteogenic cells of selective regulatory mechanisms for the insertion and enrichment of Pi transport activity in released MV. This process is probably of biological significance for the regulation of the cascade of events that leads to the mineralization of the bone matrix.
Funding Information
  • the Swiss National Science Foundation (32–032411)