Synergistic role of sprouty2 inactivation and c‐Met up‐regulation in mouse and human hepatocarcinogenesis

Sprouty2 (Spry2), a negative feedback regulator of the Ras/mitogen‐activated protein kinase (MAPK) pathway, is frequently down‐regulated in human hepatocellular carcinoma (HCC). We tested the hypothesis that loss of Spry2 cooperates with unconstrained activation of the c‐Met protooncogene to induce hepatocarcinogenesis via in vitro and in vivo approaches. We found coordinated down‐regulation of Spry2 protein expression and activation of c‐Met as well as its downstream effectors extracellular signal‐regulated kinase (ERK) and v‐akt murine thymoma viral oncogene homolog (AKT) in a subset of human HCC samples with poor outcome. Mechanistic studies revealed that Spry2 function is disrupted in human HCC via multiple mechanisms at both transcriptional and post‐transcriptional level, including promoter hypermethylation, loss of heterozygosity, and proteosomal degradation by neural precursor cell expressed, developmentally down‐regulated 4 (NEDD4). In HCC cell lines, Spry2 overexpression inhibits c‐Met–induced cell proliferation as well as ERK and AKT activation, whereas loss of Spry2 potentiates c‐Met signaling. Most importantly, we show that blocking Spry2 activity via a dominant negative form of Spry2 cooperates with c‐Met to promote hepatocarcinogenesis in the mouse liver by sustaining proliferation and angiogenesis. The tumors exhibited high levels of activated ERK and AKT, recapitulating the subgroup of human HCC with a clinically aggressive phenotype. Conclusion : The occurrence of frequent genetic, epigenetic, and biochemical events leading to Spry2 inactivation provides solid evidence that Spry2 functions as a tumor suppressor gene in liver cancer. Coordinated deregulation of Spry2 and c‐Met signaling may be a pivotal oncogenic mechanism responsible for unrestrained activation of ERK and AKT pathways in human hepatocarcinogenesis. (H EPATOLOGY 2010)