Methionine adenosyltransferase 2B (MAT2B) encodes for two variant proteins (V1 and V2) that promote cell growth. Using in-solution proteomics, GIT1 (G Protein Coupled Receptor Kinase Interacting ArfGAP 1) was identified as a potential interacting partner of MAT2B. Here, we examined the functional significance of this interplay. Coimmunoprecipitation experiments examined protein interactions. Tissue expression levels of proteins were examined using immunohistochemistry and western blotting. Expression levels of proteins were varied using transient knockdown or overexpression to observe the effect of alterations in each protein on the entire complex. Direct interaction among individual proteins was further verified using in vitro translated and recombinant proteins. We found both MAT2B variants interact with GIT1. Overexpression of V1, V2, or GIT1 activated mitogen-activated protein kinase kinase 1 (MEK1) and extracellular signal-regulated kinase (ERK), raised cyclin D1 protein level, and increased growth, whereas the opposite occurred when V1, V2, or GIT1 was knocked down. MAT2B and GIT1 require each other to activate MEK1/ERK and increase growth. MAT2B directly interacts with MEK1, GIT1, and ERK2. Expression level of V1, V2, or GIT1 directly influenced recruitment of GIT1 or MAT2B and ERK2 to MEK1, respectively. In pull-down assays, MAT2B directly promoted binding of GIT1 and ERK2 to MEK1. MAT2B and GIT1 interact and are overexpressed in most human liver and colon cancer specimens. Increased expression of V1, V2, or GIT1 promoted growth in an orthotopic liver cancer model, whereas increased expression of either V1 or V2 with GIT1 further enhanced growth and lung metastasis.
Conclusion: MAT2B and GIT1 form a scaffold, which recruits and activates MEK and ERK to promote growth and tumorigenesis. This novel MAT2B/GIT1 complex may provide a potential therapeutic gateway in human liver and colon cancer. (HEPATOLOGY 2012).
Copyright © 2013 American Association for the Study of Liver Diseases.