Cancers arise, evolve and develop progressively due to the accumulations of mutations and/or modifications in the genomic DNA. Loss-of-function mutations in "tumor suppressor genes" induce cell immortalization, while gain-of-function mutations in "proto-oncogenes" induce cell transformation. Cell immortalization prevents the induction of apoptosis and/or senescence, which is a defense mechanism against cancer development. Cell transformation involves the gain of autonomous cell growth, the loss of cell communication, morphological changes, and the elevated production of matrix proteases and growth factors that participate in invasion, metastasis and angiogenesis. These cellular events thus lead to the malignant conversion of cancer cells. The primary focus of this department is to understand the molecular basis of the cell transformation that is induced by the gain-of-function mutations in proto-oncogenes. As a representative proto-oncogene, we have focused on the c-Src proto-oncogene, which encodes a non-receptor tyrosine kinase. To date, we have analyzed its physiological roles in development and the mechanisms by which its specific regulators, such as Csk and Cbp, regulate it. To obtain a full picture of the cell signaling pathways that lead to c-Src-mediated cell transformation and to search for new therapeutic targets that will block c-Src-mediated cancer progression, the following projects are currently in progress:
I. Molecular mechanisms that suppress c-Src-mediated cell transformation.
In normal cells, c-Src is present in an inactive form that is phosphorylated by its negative regulatory kinase Csk. Extracellular stimuli transiently activate it (Fig. 1), after which it in turn activates downstream pathways such as the MAPK pathway, thereby inducing the gene expression that is required for cell growth and the phenotypic changes that are involved in cell transformation (Fig. 2). While the c-Src gene is rarely mutated in human cancers, its protein is frequently hyperactivated and overexpressed. This aberrant activation of c-Src is suggested to contribute to cancer malignancy (Fig. 2).
Recently, we found a new system by which the aberrant activation of c-Src could be suppressed. We previously showed that the inactivation of c-Src is facilitated when Csk is recruited to lipid rafts by the specific adaptor Cbp. Further analysis then revealed that Cbp can recruit activated c-Src to lipid rafts directly, and that this is sufficient for suppressing cell transformation. Furthermore, we found that the expression of Cbp is substantially downregulated in various human cancers, which suggests that the Cbp gene serves as a tumor suppressor gene (ref. 3, Fig. 3). Currently, we are analyzing the molecular mechanisms behind the downregulation of the Cbp gene in cancer cells.
Fig.1. Functions andregulation of c-Src. |
Fig.2. c-Src and human cancer. |
Fig.3. Tumor suppressing role of Cbp.
II. Cell signaling pathway of c-Src-mediated cell transformation.
To elucidate the main pathway by which c-Src induces malignant transformation, we searched for new targets of c-Src. Recently, one such potential c-Src target was found to be the novel adaptor protein p18, which is exclusively localized at the lipid rafts of late endosomes. p18 can recruit a branch of MAPK pathway to late endosomes by directly binding the p14/MP1 complex, which is known to be a specific scaffold of MEK1. Analyses of p18 KO mice (embryonic lethal), p18 KO cells, and epidermis-specific p18 KO mice revealed that p18 plays a pivotal role in endosome dynamics by regulating membrane fusion between endosomal vesicles. More recently, we also found that the p18-dependent MAPK pathway is essential for the cell transformation induced by Src, K-Ras and Pak1 (ref. 2, Fig. 4). We are currently analyzing the details of this pathway. We have also started a project that seeks to identify anti-cancer agents that target the p18-dependent pathway.
Fig.4. Roles played by the p18-MAPK pathway in endosome dynamics and cancer growth.