FOXO1 transcription factor regulates chondrogenic differentiation through transforming growth factor β1 signaling.

The forkhead box O (FOXO) proteins are transcription factors involved in the differentiation of many cell types. () Cre--knockout and -Cre- triple-knockout mice exhibit growth plate malformation. Moreover, recent studies have reported that in some cells, the expressions and activities of FOXOs are promoted by transforming growth factor β1 (TGFβ1), a growth factor playing a key role in chondrogenic differentiation. Here, using a murine chondrogenic cell line (ATDC5), mouse embryos, and human mesenchymal stem cells, we report the mechanisms by which FOXOs affect chondrogenic differentiation. FOXO1 expression increased along with chondrogenic differentiation, and FOXO1 inhibition suppressed chondrogenic differentiation. TGFβ1/SMAD signaling promoted expression and activity of FOXO1. In ATDC5, FOXO1 knockdown suppressed expression of (), a master regulator of chondrogenic differentiation, resulting in decreased α () and () expression after TGFβ1 treatment. On the other hand, chemical FOXO1 inhibition suppressed and expression without suppressing To investigate the effects of FOXO1 on chondrogenic differentiation independently of SOX9, we examined FOXO1's effects on the cell cycle. FOXO1 inhibition suppressed expression of p21 and cell-cycle arrest in G/G phase. Conversely, FOXO1 overexpression promoted expression of p21 and cell-cycle arrest. FOXO1 inhibition suppressed expression of nascent RNA by TGFβ1, and FOXO1 bound the promoter. p21 inhibition suppressed expression of and during chondrogenic differentiation. These results suggest that FOXO1 is necessary for not only SOX9 expression, but also cell-cycle arrest during chondrogenic differentiation via TGFβ1 signaling.