c‐Fos is required for TGFβ1 production and the associated paracrine migratory effects of human colon carcinoma cells

In tumor cells that have lost responsiveness to the growth inhibitory effects of transforming growth factor β (TGFβ), increased TGFβ production by the tumor cells often contributes to cancer progression, primarily through paracrine mechanisms. Here we investigated the major components of the activator protein‐1 (AP‐1) complex in the TGFβ1 promoter of human colon carcinoma cells (HCCCs). In contrast to untransformed epithelial cells (UECs), HCCCs displayed constitutive activation of AP‐1 at the proximal AP‐1 site in the human TGFβ1 promoter. Further, in contrast to the JunD and Fra‐2 components present in the AP‐1 complex at this AP‐1 site in UECs, c‐Fos was the major detectable AP‐1 component in HCCCs. Thus, transcriptional factor switching had occurred in HCCCs relative to the UECs, with regard to the proximal AP‐1 site of the human TGFβ1 promoter. Small interfering RNAs (siRNAs) against c‐Fos significantly suppressed AP‐1 activity at the relevant AP‐1 site, and led to a decrease in TGFβ1 secretion by the HCCCs. Our results indicate for the first time that c‐Fos binding at the TGFβ1 promoter proximal AP‐1 site in HCCCs is required for TGFβ1 production by the tumor cells. Further, we demonstrated that blockade of TGFβ1 secretion by c‐Fos siRNA led to a suppression of the cellular migration and mitogenesis of NIH 3T3 fibroblasts in a paracrine fashion. Thus, c‐Fos may have utility as a target for blocking tumor cell‐secreted TGFβ1, thereby suppressing the migratory behavior associated with the malignant phenotype of HCCCs. © 2006 Wiley‐Liss, Inc.