Impact of salt and the osmoprotective transcription factor NFAT5 on macrophages during mechanical strain.

Myeloid cells regulate bone density responding to increased salt (NaCl) intake via osmoprotective transcription factor NFAT-5 (nuclear factor of activated T-cells-5). Since orthodontic tooth movement is a pseudo-inflammatory immunological process, we investigated the influence of NaCl and NFAT-5 on the expression pattern of macrophages in a model of simulated orthodontic tooth movement (OTM). RAW264.7 macrophages were exposed for 4 h with additional 0 mM or 40 mM NaCl to 2g/cm2 compressive or 16% tensile or no mechanical strain (control). We analysed expression of inflammatory genes and proteins (TNF, IL-6 and Ptgs-2/PG-E2) by RT-qPCR and ELISA. To investigate the role of NFAT-5 in these responses, NFAT-5 was both constitutively expressed and silenced. Salt and compressive strain, but not tensile strain increased expression of NFAT5 and most tested inflammatory factors in macrophages. NaCl induced the expression of Ptgs-2/PG-E2 and TNF, while IL-6 secretion was inhibited. Similarly, a constitutive expression of NFAT-5 reduced IL-6 expression, while increasing Ptgs-2/PG-E2 and TNF expression. Silencing of NFAT-5 upregulated IL-6 and reduced Ptgs-2/PG-E2 and TNF expression. Salt impacted on the expression profile of macrophages as a reaction to compressive and tensile strain as they occur during OTM. This was mediated via NFAT-5, which surprisingly also seems to play a regulatory role in mechanotransduction of compressive strain. Sodium accumulation in the periodontal ligament caused by dietary salt consumption might propagate local osteoclastogenesis via increased local inflammation and thus OTM velocity, but possibly also entail side effects like dental root resorptions or periodontal bone loss.