Role of FoxO3a in Osteoblast Differentiation and Matrix Calcification

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Abstract

Forkhead Box O transcription factors play important roles in bone metabolism via defending against oxidative stress and apoptosis (Almeida, 2011; Ambrogini et al., 2010). FoxO3a is of special interest as it is the predominant isoform in bone cells (Ambrogini et al., 2010). In osteoblasts, the administration of 1,25 dihydroxyvitamin D₃ (1,25D₃) increases FoxO3a expression, and alters calcium handling. We therefore queried whether FoxO3a participates in vitamin D-mediated calcium regulation or osteoblast differentiation and matrix calcification, independent of reactive oxygen species (ROS) formation. This dissertation investigates the regulating role of Forkhead Box O3 transcription factors (FoxO3) in osteoblast differentiation and matrix mineralization. FoxO3a expression increased throughout differentiation. At day 3 and 7, FoxO3a mRNA and protein expression levels were significantly higher than predifferentiation. 1,25D₃ further enhanced both FoxO3a mRNA and protein expression levels in the 7-day differentiated osteoblast cells. Immunocytofluorescence localization of FoxO3a demonstrated the addition of 1,25D₃ led to nuclear localization. 7-day differentiated osteoblast cells incubated with H₂O₂ showed no obvious difference from control, however, increased FoxO3a expression was attenuated when H₂O₂ was added with 1,25D₃. In order to study the effect of 1,25D₃ on calcium regulation in osteoblasts, expression of calcium homeostasis mediators were assessed. Increased expression of the calcium channel Ca_v3.1 and plasma membrane Ca²⁺ ATPase (PMCA-1b) was observed throughout osteoblast differentiation. Addition of 1,25D₃ enhanced expression of Calbindin-D_9K and the sodium-calcium exchanger (NCX). 1,25D₃ increased calcium uptake and calcium deposition in differentiated osteoblasts as demonstrated by ratiometric live cell calcium imaging and alizarin red staining, respectively. FoxO3a overexpression led to a reduction in calcium uptake and calcium deposition, resulting in uncalcified matrix. When FoxO3a was over-expressed in MC3T3-E1 cells, pre-osteoblasts failed to differentiate into osteoblasts and unmineralized osteoid failed to calcify its matrix, as assessed by the biomarkers Runx2 and osteocalcin, respectively. Based on expression and functional data, the results suggest that high levels of FoxO3a inversely regulate osteoblast differentiation and matrix calcification, with a role in the repression of both processes.

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