Abstract
Forkhead members of the 'O' class (FoxO) are transcription factors crucial for the regulation of metabolism, cell cycle, cell death and cell survival. FoxO factors are regulated by insulin-mediated activation of PI3K (phosphoinositide 3-kinase)-PKB (protein kinase B) signalling. Activation of PI3K-PKB signalling results in the phosphorylation of FoxO factors on three conserved phosphorylation motifs, which are essential for the translocation of FoxO factors from the nucleus to the cytosol. FoxO6, however, remains mostly nuclear due to the fact that its shuttling ability is dramatically impaired. FoxO1, FoxO3 and FoxO4 all contain an N- and C-terminal PKB motif and a motif located in the forkhead domain. FoxO6 lacks the conserved C-terminal PKB motif, which is the cause of the shuttling impairment. Since FoxO6 can be considered constitutively nuclear, we investigated whether it is also a constitutively active transcription factor. Our results show that FoxO6 transcriptional activity is inhibited by growth factors, independent of shuttling, indicating that it is not constitutively active. The PKB site in the forkhead domain (Ser184) regulated the DNA binding characteristics and the N-terminal PKB site acted as a growth factor sensor. In summary, FoxO6 is not a constitutively active transcription factor and can be regulated by growth factors in a Thr26- and Ser184-dependent manner, independent of shuttling to the cytosol.