The
glycoprotein A33 (GPA33) is a
colon cancer antigen. Phase I trials with 131I and 125I
monoclonal antibody A33 in colon
carcinoma patients showed excellent localization to
colorectal cancer and some evidence of
tumor response. Using
DNA microarrays, we have identified the GPA33 gene as a target of
PPARgamma in HT29-Cl.16E
colon cancer cells. Treatment of HT29-Cl.16E, Caco2, SW1116 and LS174T
colon cancer cells with the
PPARgamma agonist
GW7845 induced a 2- to 6-fold increase in GPA33
mRNA as determined by real-time PCR. This induction was also found in HT29-Cl.16E cells treated with
rosiglitazone and
ciglitazone and was prevented by cotreatment with the
PPARgamma antagonist
GW9662, indicating that this regulation was
PPARgamma dependent. No canonical
PPAR responsive
element was found in the GPA33 promoter. We therefore analyzed the expression of
transcription factors involved in GPA33 expression. CDXl, CDX2 and KLF5 expression was not modified by
PPARgamma activation. By contrast, a significant increase in KLF4 was seen, both at
mRNA and
protein levels. Furthermore,
chromatin immunoprecipitation studies demonstrated that an increased amount of
KLF4 protein was bound to the GPA33 promoter in cells treated with
rosiglitazone. Finally, downregulation of KLF4 expression by
siRNA reduced
rosiglitazone-induced GPA33 expression. This indicates that
PPARgamma activation induces KLF4 expression, which in turn increases GPA33 expression. We also demonstrate that
PPARgamma activation leads to increased (p21WAF1/Cip1 and
keratin 19) or decreased (
cyclin D1) expression of known KLF4 targets, suggesting that KLF4 is a nodal player in a network of
PPARgamma-regulated genes.