Characterization of cancer stroma markers: in silico analysis of an mRNA expression database for fibroblast activation protein and endosialin

Abstract

Standardized, high-throughput RNA detection with microarray chips allows for the construction of genome-wide databases for tissue specimens suitable for in silico electronic Northern blot (eNorthern) analysis of marker genes. We used the BioExpress database, which contains transcriptional profiles of normal and cancer samples, to examine two putative markers of cancer stroma: fibroblast activation protein-alpha (FAP-alpha) and endosialin. Analyses for FAP-alpha showed that normal tissues generally lack RNA signals, with the exception of endometrium. Typing of tumors revealed prominent FAP-alpha signals in cancer types marked by desmoplasia, and localization of FAP-alpha in reactive cancer stroma was confirmed by immunohistochemistry. A subset of sarcomas displayed prominent FAP-alpha signals localizing to the malignant cells. For endosialin, eNorthern analyses showed low to moderate RNA signals in many normal organs, whereas immunohistochemistry revealed endosialin in only some tissues, such as endometrium. Endosialin was detected at the RNA and protein level in sarcomas, notably malignant fibrous histiocytomas. Low to moderate endosialin RNA signals were found in epithelial cancer types for which immunostaining identifies expression in subsets of tumor capillaries or fibroblasts. These findings extend the FAP-alpha and endosialin profiling in silico to an unbiased tumor database and place both molecules in a novel context of endometrial biology and sarcoma subtyping. Our findings suggest that BioExpress can be searched directly for tumor stroma markers but may need prior enrichment for markers with narrow cellular representation, such as endosialin. Constructing databases from microdissected cancer tissues may be an essential step for tumor stroma-targeted therapies.