Colorectal cancer (CRC) is the third most common cancer worldwide with invasion and metastatic spread as primary causes of death from this disease. Fos related antigen-1 (Fra-1) is an AP-1 transcription factor frequently over-expressed in a variety of cancers and plays a critical role in driving CRC cell migration and invasion. The aims of this study were to unravel genetic programs orchestrated by Fra-1 in invasive CRC cells.
Stable silencing of Fra-1 in BE CRC cells resulted in dramatic morphological changes, with cells switching from a mesenchymal phenotype to an epithelial-like appearance and complete inhibition of cell migration and invasion. To determine how Fra-1 silencing invokes this striking phenotypic switch, we searched for its transcriptional targets in BE cells using a combination of microarrays to identify Fra-1-regulated genes, and ChIP-Seq to identify genomic binding sites of Fra-1.We found that Fra-1 is bound in close proximity to many genes identified in the microarrays and that the largest ontological class of genes over-represented in both datasets encoded proteins involved in a process describes as epithelial-mesenchymal transition (EMT) (termed Fra-1EMT genes). Amongst these, mesenchymal genes were suppressed upon Fra-1 silencing, whereas epithelial genes were up-regulated. Thus, stable silencing of Fra-1 leads to a reversal of EMT in BE cells. To determine whether changes in the expression of Fra-1EMT genes occur in primary CRCs, we performed unsupervised clustering on existing microarray data from stage B and C CRCs.This analysis demonstrated that the Fra-1EMT signature is associated with poor prognosis in CRC patients. Furthermore, histochemical staining of Fra-1 in CRC specimens revealed that Fra-1 expression is significantly enriched in budding tumour cells, which represent the invasive front of CRCs.
In conclusion, we demonstrate that Fra-1 is a key regulator of transcriptional programs required to maintain EMT in CRC cells.