Amino acid response (AAR) pathway is activated when cells are deprived of
amino acids. In the present study, using the human
colon cancer cell line SW480, we observed that DKK1, an antagonist of the Wnt pathway, was significantly induced at the
mRNA level after the removal of
amino acids from the medium. Addition of the amino alcohol
histidinol, which prevents the formation of histidinyl-
tRNA(His), also increased DKK1
mRNA to a level similar to that observed when cells were deprived of all
amino acids. Transcriptional activity and stability of DKK1
mRNA were both increased in the
amino acid-deprived condition. The induction of DKK1 gene expression was confirmed by the increased immunofluorescent staining of the DKK1
protein in the
amino acid deprived condition. Although
chromatin immunoprecipitation assays showed increased
RNA Polymerase II binding at the DKK1 promoter in
amino acid-limited conditions, ATF4 binding to the promoter is absent.
Luciferase reporter assays did not detect any functional AARE within the DKK1 gene structure. Knockdown of ATF4 by
siRNA did not affect the increase of DKK1
mRNA during
amino acid limitation. Inhibition of ERK phosphorylation abolished the induction of DKK1. Our study revealed that DKK1 is a novel target gene in the response to
amino acid deficiency and that the expression of DKK1 is up-regulated through an ATF4-independent and an ERK-dependent pathway.