Low-dose UVA illumination of differentiating stromal cells reveals diverse effects on cell-mediated mineralisation in culture

In the present study the effect of low dose UVA illumination on cell-mediated mineralisation was tested. Rat marrow stromal cells were stimulated with dexamethasone (DEX) to induce mineralisation in the cultures. Different cultures were illuminated with low UVA dose, O.1 nJ/cm², using illumination protocols which differed either by day of exposure, for single-dose protocols, or by the number of exposure-days, for multiple-dose protocols. Different responses to UVA were induced by early (day 1) versus late (days 7-8) illumination. Single-dose illumination on days 1 or 8 decreased or increased day 21 mineralisation, respectively. The mineralisation differences were not equivalent to the differences in cell counts, and thus did not reflect selection of osteoprogenitor cells (OPC), in contrast to the effect of malonate found in previous studies. Late illumination, on day 7 resulted in a higher rhodamine 123 (Rho) retention on day 7 than the early (day 1) illumination effect on the day 7-Rho retention. The opposing effect of the early and late illumination protocols on mineralisation was also not accompanied by OPC selection. Thus, we have shown that low UVA dose modulates mineralisation together with modulation of mitochondrial Rho retention. These results suggest that low doses of UVA light may affect bone mineralisation.