Transforming Growth Factor-β Gene Expression Signature in Mouse Hepatocytes Predicts Clinical Outcome in Human Cancer

Hepatocellular carcinoma (HCC) is the fifth most common cancer world-wide, accounting for at least 600,000 deaths annually.1 Although most frequent in southeast Asia and sub-Sahara Africa, the incidence and mortality rate of HCC have doubled in the United States and Europe in the past 4 decades and are expected to double again over the next 10 to 20 years.2 As the incidence of HCC has increased, the age-distribution of HCC has shifted toward relatively younger ages.2 These observations make it clear that liver cancer is becoming a major heath problem in the United States and Europe. Although recent advances in genomics analysis of HCC provide an increasingly comprehensive portrayal of hepatocarcinogenesis,3 the molecular pathogenesis of HCC remains poorly understood. Patients have a highly variable clinical course, indicating that HCC comprises several biologically distinctive subgroups reflecting the molecular heterogeneity of the tumors.4,5 Previously, we have demonstrated that gene expression signatures estab-lished in well-controlled experimental conditions can successfully identify new subgroups of tumors and thus improve the molecular classification of human HCC.6–9 For example, a gene signature specific for hepatocyte growth factor/c-Met signaling pathway allowed us to define a clinically relevant subset of human HCC characterized by a poor prognosis and an aggressive phenotype.10 Therefore, this approach provides a practical experimental framework for a systematic analysis of the contributions of growth factor signaling pathways or oncogenic signaling pathways to the tumor phenotype, as well as identifying homogeneous subgroups of heterogeneous cancers such as HCC. Transforming growth factor beta (TGF-β) is a pleiotropic cytokine that controls many fundamental aspects of cell biology, including proliferation, migration, adhesion, differentiation, and modification of the cellular microenvironment.11,12 TGF-β exerts its effects through the TGFBR1 and TGFBR2 serine-threonine kinases receptors and the small mothers against decapentaplegic (SMAD) family of transcriptional regulators. TGF-β binding to TGFBR2 induces phosphorylation and the activation of TGFBR1. The activated TGFBR1 then phosphorylates SMAD2 and SMAD3, which in turn bind to SMAD4 and move into the nucleus to form complexes that regulate transcription.12,13 Genetic alterations in the TGF-β signaling pathway have been reported in cancer, but the implication of TGF-β signaling in carcinogenesis is complex, because TGF-β may exhibit both tumor suppressive and oncogenic properties.14–17 TGF-β acts as a tumor suppressor at the early stages of tumor development by inhibiting proliferation and inducing apoptosis, but TGF-β also possesses oncogenic potential, which contributes to tumor progression later in carcinogenesis. Notably, as a potent stimulator of epithelial-mesenchymal transitions (EMT), TGF-β promotes tumor cell invasiveness and metastasis.18 Based on these considerations, we hypothesized that a TGF-β gene expression signature established under well-controlled experimental conditions in vitro may contain gene sets characteristic of both tumor suppressive and oncogenic properties and thus be relevant for the molecular classification of tumors. Here we report that the genes embedded in the TGF-β expression signature could identify novel homogeneous and clinically distinct subgroups of human HCC.