The significance of Pyk2 in hepatocellular carcinoma invasiveness

Abstract

Background and Objective: Our previous study showed that proline rich tyrosine kinase (Pyk2) is over-expressed in tumor tissues as compared to their adjacent non-tumor tissue. Statistical analysis suggests that this over-expression is significantly correlated with tumor growth and venous invasion. Over-expression of Pyk2 is also significantly correlated with shorter overall and disease-free survival. However, the precise mechanism of Pyk2 on tumor invasiveness is still unclear due to the limited reports. In the current study, we aim to investigate the role of Pyk2 on the invasiveness of hepatocellular carcinoma cells by both in vitro functional study and in vivo animal models. Materials and Methods: In the in vitro study, plasmids containing the full length or dominant negative form of Pyk2 was transfected into HCC cell line (PLC). After selection with antibiotics (G418), stable clones with the expression of full length or dominant negative form of Pyk2 was isolated. The invasiveness was compared according to their ability to adhere to the extracellular matrix, colony formation assay and wound healing assay. The mechanism of the Pyk2 signaling was investigated by western blotting and immunoprecipipation assay. In vivo tumor models were done in athymic nude mice. Tumors produced from the different transfectants were implantanted into the liver of the mice. After 48 days the mice were sacrificed and tissue samples were collected. Tumor growth pattern including invasiveness was examined by H&E staining. Tumor cell proliferation (Ki67) and apoptosis (TUNEL) were compared among the groups of mice with liver tumor from different transfectants. Results: The full length Pyk2 transfectant possessed the highest cell motility as compared to the vector control and C-terminal transfectants by wound healing assay. Pyk2 full length transfectant also presented significantly stronger adhesiveness towards collagen I, fibronectin and laminin by adhesion assay. It promoted the anchorage-independent growth as well as the anchorage dependent growth by the soft agar assay and colony formation assay. Western blotting and co-precipipation assays indicated that Pyk2 forms a signaling complex with c-Src. Phosphorylation of c-Src, MEK and ERK 1/2 were up-regulated in full length Pyk2 transfectants as compared to the vector control and C-terminal transfectants. The tumors from the full length Pyk2 transfectants got the highest incidence of Ki67 positive staining tumor cells and least apoptotic cells as compared to empty vector control and C-terminal transfectants. Conclusion: Over-expression of Pyk2 may contribute to an invasive phenotype of HCC.