Expression of growth factor receptors, the focal adhesion kinase, and other tyrosine kinases in human soft tissue tumors

Background: The tyrosine kinases are a family of genes that includes many growth factor receptors and protooncogenes. They appear to have a role in many cancers, but have not been systematically studied in the pathogenesis and progression of human sarcomas. Methods: To characterize the protein tyrosine kinases that are expressed in human sarcomas, we used a polymerase chain reaction (PCR)-based method to construct kinase-specific cDNA libraries from low-grade and high-grade primary tumors. Thereafter, individual tyrosine kinase gene expression was assessed in a panel of sarcoma cell lines and primary tumors using Northern blotting and PCR. Results: We identified 19 species of tyrosine kinase genes, including many growth factor receptors, the human homolog of the focal adhesion kinase (FAK) gene, and a novel trk-related kinase designated HGK2. Messenger RNA expression analyses showed relative overexpression of the two forms of the platelet-derived growth factor receptors (PDGFRs) with expression of the α form restricted to a subgroup of high-grade and metastatic sarcomas. We were unable to demonstrate coexpression of the PDGF isoforms in primary tumors that overexpressed the receptors, suggesting that a PDGF/PDGFR autocrine pathway may not be a central mechanism in the malignant transformation of sarcomas in vivo. FAK expression was observed in a variety of sarcomas, with increased levels in several high-grade and metastatic leiomyosarcomas. Conclusions: When grouped together by histologic cell type and grade, the expression data of the 19 kinases in primary tumors described a greater degree of heterogeneity than is generally appreciated by clinicopathologic classification schemes. This diversity suggests that sarcomas, even those that appear to be clinically similar, arise through a variety of molecular pathways involving tyrosine kinases.