The PDGFRα-laminin B1-keratin 19 cascade drives tumor progression at the invasive front of human hepatocellular carcinoma

Human hepatocellular carcinomas (HCCs) expressing the biliary/hepatic progenitor cell marker keratin 19 (K19) have been linked with a poor prognosis and exhibit an increase in platelet-derived growth factor receptor a (PDGFR alpha) and laminin beta 1 (LAMB1) expression. PDGFR alpha has been reported to induce de novo synthesis of LAMB1 protein in a Sjogren syndrome antigen B (La/SSB)-dependent manner in a murine metastasis model. However, the role of this cascade in human HCC remains unclear. This study focused on the functional role of the PDGFR alpha-La/SSB-LAMB1 pathway and its molecular link to K19 expression in human HCC. In surgical HCC specimens from a cohort of 136 patients, PDGFR alpha expression correlated with K19 expression, microvascular invasion and metastatic spread. In addition, PDGFR alpha expression in pre-operative needle biopsy specimens predicted poor overall survival during a 5-year follow-up period. Consecutive histological staining demonstrated that the signaling components of the PDGFR alpha-La/SSB-LAMB1 pathway were strongly expressed at the invasive front. K19-positive HCC cells displayed high levels of alpha 2 beta 1 integrin (ITG) receptor, both in vitro and in vivo. In vitro activation of PDGFR alpha signaling triggered the translocation of nuclear La/SSB into the cytoplasm, enhanced the protein synthesis of LAMB1 by activating its internal ribosome entry site, which in turn led to increased secretion of laminin-111. This effect was abrogated by the PDGFR alpha-specific inhibitor crenolanib. Importantly LAMB1 stimulated ITG-dependent focal adhesion kinase/Src proto-oncogene non-receptor tyrosine kinase signaling. It also promoted the ITG-specific downstream target Rho-associated coiled-coil containing protein kinase 2, induced K19 expression in an autocrine manner, invadopodia formation and cell invasion. Finally, we showed that the knockdown of LAMB1 or K19 in subcutaneous xenograft mouse models resulted in significant loss of cells invading the surrounding stromal tissue and reduced HepG2 colonization into lung and liver after tail vein injection. The PDGFR alpha-LAMB1 pathway supports tumor progression at the invasive front of human HCC through K19 expression.