A Targeted New Peptide Chemosensitizer Sensitizing Effects And Mechanisms Of Inhibition Of The Met Receptor Signaling

Chemotherapy has been shown to play an important role in the treatment of most solid tumors. Despite of their excellent anticancer activities, the clinical usages of the chemotherapeutics are often limited by their undesirable side effects. Moreover, malignant cells are either intrinsically resistant to chemotherapy or able to obtain resistance during the course of treatment. So there is an urgent need to develop new and improved methods to help promote the chemosensitivity and simultaneously to further illuminate the molecular mechanism involved in it. Currently, the receptor tyrosine kinases have become the important therapeutic targets in a variety of malignancies because their abnormal activation is associated with increased tumor growth, metastasis, resistance to chemotherapeutic agents and poor prognosis as well. In fact, design of specific tyrosine kinase receptor inhibitors is considered to be an emerging new paradigm in the development of cancer therapies. And combination of kinase inhibitors to traditional chemotherapy agents may give us a new strategy to improve the chemosensitivity during the tumor treatment.Here we tried to investigate the novel sensitizer based on inhibition of Met receptor. The tyrosine kinase of Met receptor is a high affinity receptor of hepatocyte growth factor. They play significant roles in cellular physiology such as mitogenesis, motogenesis and morphogenesis. Met receptor is always abnormally overexpressed and activated in a number of human solid tumors and the expression levels of Met are frequently correlated with tumor grade and tumor blood vessel density. Impairment of Met receptor activity signaling can be achieved via antagonism of ligand/receptor interaction, inhibition of TK catalytic activity, and blockade of intracellular receptor/effector interactions. The latter two strategies need to deliver the effector molecules intracellular.Previous study demonstrated the ability of a peptide derived from the carboxyl-terminal tail of Met receptor to inhibit kinase activity and invasive growth in normal and transformed epithelial cells in vitro. In our research, we coupled the carboxyl-terminal tail of the Met receptor to a cell-penetrating peptide TAT fragment (TAT-coupled Carboxyl-terminal tail peptide of Met receptor, TCTP), and successfully delivered the functional peptide intracellularly. We proved that the peptide derived from the Carboxyl-terminal tail of Met receptor coupled with the protein transduction domain TAT fragment could impair the HGF-induced Met activation and inhibit the Met-dependent downstream signaling of AKT and p44/42 MAPK. TCTP could markedly enhance the cytotoxicity of CDDP to glioma cell lines of T98G and U251. Then we tried to unravel the mysteries of sensitization mechanism to CDDP. Unexpectedly, Met-dependent downstream signaling of AKT and p44/42 MAPK impairment by TCTP was not the main cause for sensitization of CDDP. Either PI3K inhibitor LY294002 or p44/42 MAPK inhibitor PD98059 combination was nearly unaffected to the sensitization of CDDP in T98G cells. Further studies revealed that TCTP could activate the p38 MAPK phosphorylation. Activation of p38 MAPK by sorbitol pretreatment could resemble the sensitization effect of TCTP to chemosensitivity, while inhibition of p38 MAPK activation by its inhibitor SB202190 could counteract (retrieve) the sensitization to CDDP induced by TCTP in vitro. Therefore, p38 MAPK activation might be the major cause for sensitization induced by TCTP to CDDP.Taken together, our research has demonstrated a feasible therapeutic strategy to enhance the chemosensitivity by coupling the cell permeable peptide to another fragment of peptide functioned as tyrphostin. This system has great potentials to clinical applications in tumor chemotherapy.