Purpose: Many proteins are proteolytically released from the cell surface by a process known as ectodomain shedding. Shedding occurs under normal physiologic conditions and can be increased in certain pathologies. Among the many receptors for which ectodomain shedding has been shown is c-Met, the hepatocyte growth factor (HGF) receptor tyrosine kinase. HGF stimulates mitogenesis, motogenesis, and morphogenesis in a variety of cellular targets during development, homeostasis, and tissue regeneration. Inappropriate HGF signaling resulting in unregulated cell proliferation, motility, and invasion occurs in several human malignancies. This can occur through paracrine signaling, autocrine loop formation, receptor mutation, gene amplification, or gene rearrangement, accompanied frequently with overexpression of ligand and/or receptor proteins. We hypothesized that c-Met overexpression in cancer might result in increased ectodomain shedding, and that its measure could be a useful biomarker of tumor progression.
Experimental design: We developed a sensitive electrochemiluminescent immunoassay to quantitate c-Met protein in cell lysates, culture supernatants, and biological samples.
Results: A survey of cultured cell models of oncogenic transformation revealed significant direct correlations (P < 0.001, t test or ANOVA) between malignant potential and the rate of c-Met ectodomain shedding that was independent of steady-state receptor expression level. Moreover, weekly plasma and urine samples from mice harboring s.c. human tumor xenografts (n = 4 per group) displayed soluble human c-Met levels that were measurable before tumors became palpable and that correlated directly with tumor volume (R2 > 0.92, linear regression).
Conclusions: For a variety of human cancers, c-Met ectodomain shedding may provide a reliable and practical indicator of malignant potential and overall tumor burden.