Overexpression of focal adhesion kinase in primary colorectal carcinomas and colorectal liver metastases: immunohistochemistry and real-time PCR analyses

Abstract

Purpose: Focal adhesion kinase (FAK), a protein tyrosine kinase that functions in signaling events between cells and their extracellular matrix, is overexpressed in a variety of human solid tumors. To determine whether FAK expression is up-regulated in colorectal cancer, we analyzed FAK mRNA and protein levels in primary colorectal tumors and colorectal liver metastases.

Experimental design: p125(FAK) expression in formalin-fixed paraffin-embedded (FFPE) tissue was studied using immunohistochemical assays on 24 matched primary colorectal carcinomas and colorectal liver metastases as well as 18 different colorectal liver metastases using monoclonal anti-FAK 4.47. FAK mRNA expression was quantitated by real-time PCR on 39 matched normal colorectal mucosa and primary colorectal carcinomas as well as on 17 separate liver metastases.

Results: Elevated levels of p125(FAK) expression were demonstrated in both primary colorectal tumors and colorectal liver metastases compared with normal colorectal mucosa. Immunohistochemistry experiments demonstrated equivalent FAK expression in matched samples of colorectal primary tumors and liver metastases. Using real-time PCR in 39 matched samples, FAK mRNA copy number was significantly higher in primary colorectal tumors compared with normal colorectal mucosa. FAK expression was analyzed by both real-time PCR and immunohistochemistry in a separate set of colorectal liver metastases. Immunohistochemistry demonstrated high levels of FAK expression in 89% of samples. Furthermore, FAK mRNA copies in these unmatched liver metastases were significantly higher than the primary tumor FAK mRNA copies.

Conclusion: These experiments have shown that both primary colorectal cancers and colorectal liver metastases express high levels of FAK mRNA and p125(FAK) protein. Furthermore, the majority of colorectal liver metastases demonstrated robust FAK expression equivalent to or greater than that in the primary colorectal tumor.