Unique phosphorylation mechanism of Gab1 using PI 3-kinase as an adaptor protein

Kirsten Ras* oncogene: significance of its discovery in human cancer research

The KRAS/ K-RAS oncogene is crucially involved in human cancer. The term "oncogene" -- i.e., a gene able to transform a normal cell into a tumor cell - was introduced in 1969, but the word was not used in the human carcinogenesis literature until much later. Transforming Kras and Hras oncogenes from the Kirsten and Harvey sarcoma viruses were not identified until the early 1980s due to the complicated structures of the viral genomes. Orthologs of these viral oncogenes were then found in transforming DNA fragments in human cancers in the form of mutated versions of the HRAS and KRAS proto-oncogenes. Thus, RAS genes were the first human oncogenes to be identified. Subsequent studies showed that mutated KRAS acted as an in vivo oncogenic driver, as indicated by studies of anti-EGFR therapy for metastatic colorectal cancers. This review addresses the historical background and experimental studies that led to the discovery of Kirsten Ras as an oncogene, the role of mutated KRAS in human carcinogenesis, and recent therapeutic studies of cancer cells with KRAS mutations.

Neurotrophins in the regulation of cellular survival and death

The neurotrophins play crucial roles regulating survival and apoptosis in the developing and injured nervous system. The four neurotrophins exert profound and crucial survival effects on developing peripheral neurons, and their expression and action is intimately tied to successful innervation of peripheral targets. In the central nervous system, they are dispensable for neuronal survival during development but support neuronal survival after lesion or other forms of injury. Neurotrophins also regulate apoptosis of both peripheral and central neurons, and we now recognize that there are regulatory advantages to having the same molecules regulate life and death decisions. This chapter examines the biological contexts in which these events take place and highlights the specific ligands, receptors, and signaling mechanisms that allow them to occur.

The involvement of the docking protein Gab1 in mitogenic signalling induced by EGF and HGF in rat hepatocytes

Grb2-associated binder (Gab) family proteins are docking molecules that can interact with receptor tyrosine kinases (RTKs) and cytokine receptors and bind several downstream signalling proteins. Studies in several cell types have shown that Gab1 may have a role in signalling mediated by the two RTKs epidermal growth factor (EGF) receptor (EGFR) and Met, the receptor for hepatocyte growth factor (HGF), but the involvement of Gab1 in EGFR and Met signalling has not been directly compared in the same cell. We have studied mechanisms of activation and role in mitogenic signalling of Gab1 in response to EGF and HGF in cultured rat hepatocytes. Gab1, but not Gab2, was expressed in the hepatocytes and was phosphorylated upon stimulation with EGF or HGF. Depletion of Gab1, using siRNA, decreased the ERK and Akt activation, cyclin D1 expression, and DNA synthesis in response to both EGF and HGF. Studies of mechanisms of recruitment to the receptors showed that HGF induced co-precipitation of Gab1 and Met while EGF induced binding of Gab1 to Grb2 but not to EGFR. Gab1 activation in response to both EGF and HGF was dependent on PI3K. While EGF activated Gab1 and Shc equally, within the same concentration range, HGF very potently and almost exclusively activated Gab1, having only a minimal effect on Shc. Collectively, our results strongly suggest that although Gab1 interacts differently with EGFR and Met, it is involved in mitogenic signalling mediated by both these growth factor receptors in hepatocytes.

Hepatocyte growth factor protects small airway epithelial cells from apoptosis induced by tumor necrosis factor-alpha or oxidative stress

Involvement of hepatocyte growth factor (HGF) in lung morphogenesis and regeneration has been established by in vitro and in vivo experiments in animals. In the present study, the protective activity of HGF against tumor necrosis factor (TNF)-alpha or hydrogen peroxide (H2O2)-induced damage of pulmonary epithelial cells was examined using the human small airway epithelial cell line (SAEC). Western blot analysis revealed that the receptor for HGF (c-Met) was highly expressed on the surface of SAEC and its downstream signal transduction pathway was functional. The SAEC was induced into apoptosis by the treatment with TNF-alpha or H2O2 in a dose-dependant manner, but was significantly rescued from apoptosis in the presence of HGF. The HGF effect was evident when added not only at the same time but also within several hours after treatment. This protective activity of HGF against the TNF-alpha- or H2O2-induced apoptosis was mediated, at least in part, by up-regulating the nuclear factor kappaB activity and an increase in the ratio of apoptosis-suppressing to apoptosis-inducing proteins. These results suggest that administration of HGF might exhibit a potent function in vivo for protection and improvement of acute and chronic lung injuries induced by inflammation and/or oxidative stress.

Regulation of TRPC6 channel activity by tyrosine phosphorylation

Various hormonal stimuli and growth factors activate the mammalian canonical transient receptor potential (TRPC) channel through phospholipase C (PLC) activation. However, the precise mechanism of the regulation of TRPC channel activity remains unknown. Here, we provide the first evidence that direct tyrosine phosphorylation by Src family protein-tyrosine kinases (PTKs) is a novel mechanism for modulating TRPC6 channel activity. We found that TRPC6 is tyrosine-phosphorylated in COS-7 cells when coexpressed with Fyn, a member of the Src family PTKs. We also found that Fyn interacts with TRPC6 and that the interaction is mediated by the SH2 domain of Fyn and the N-terminal region of TRPC6 in a phosphorylation-independent manner. In addition, we demonstrated the physical association of TRPC6 with Fyn in the mammalian brain. Moreover, we showed that stimulation of the epidermal growth factor receptor induced rapid tyrosine phosphorylation of TRPC6 in COS-7 cells. This epidermal growth factor-induced tyrosine phosphorylation of TRPC6 was significantly blocked by PP2, a specific inhibitor of Src family PTKs, and by a dominant negative form of Fyn, suggesting that the direct phosphorylation of TRPC6 by Src family PTKs could be caused by physiological stimulation. Furthermore, using single channel recording, we showed that Fyn modulates TRPC6 channel activity via tyrosine phosphorylation. Thus, our findings demonstrated that tyrosine phosphorylation by Src family PTKs is a novel regulatory mechanism of TRPC6 channel activity.