Increased FoxO3a expression prevents osteoblast differentiation and matrix calcification

Forkhead Box O transcription factors play important roles in bone metabolism by defending against oxidative stress and apoptosis. FoxO3a is of special interest as it is the predominant isoform expressed in bone. In osteoblasts, the administration of 1,25 dihydroxyvitamin D(3) (1,25D(3)) increases FoxO3a expression, and alters calcium handling. We therefore queried whether FoxO3a participates in vitamin D-mediated regulation of calcium transport pathways or matrix calcification, independent of reactive oxygen species (ROS) formation. To examine this possibility, we differentiated MC3T3-E1 cells into mature osteoblast-like cells over 7 days. This coincided with an increased ability to mineralize extracellular matrix. FoxO3a expression increased throughout differentiation. 1,25D(3) enhanced both FoxO3a mRNA and protein expression. Immunofluorescence microscopy found increased FoxO3a nuclear localization with differentiation and after treatment with 1,25D(3). Live cell ratiometric imaging with Fura-2AM identified significant L-type calcium channel mediated calcium uptake that was enhanced by 1,25D(3). We observed expression of both Ca(v)1.2 and Ca(v)1.3, although expression decreased throughout differentiation and was not altered by 1,25D(3) treatment. FoxO3a overexpression reduced calcium uptake and calcium deposition. FoxO3a overexpression also prevented alterations in calcium channel expression and the cell differentiation associated decrease in expression of Runx2 and increased expression of osteocalcin, findings consistent with a failure for the cells to differentiate. Based on both our expression and functional data, we suggest that high levels of FoxO3a prevent osteoblast differentiation and matrix calcification.