Different effects of point mutations within the B-Raf glycine-rich loop in colorectal tumors on mitogen-activated protein/extracellular signal-regulated kinase kinase/extracellular signal-regulated kinase and nuclear factor kappaB pathway and cellular transformation.
Recently, mutations in the B-Raf gene have been identified in a variety of human cancers, such as melanoma and colorectal carcinoma, and more than 80% of the B-Raf mutations have been V599E. Although other mutations have been reported, their functional consequences are poorly understood. In our earlier study, we demonstrated that colon tumor-associated B-Raf mutations within the kinase activation segment are not necessarily associated with an increase in mitogen-activated protein/extracellular signal-regulated kinase kinase/extracellular signal-regulated kinase (MEK/Erk) or nuclear factor kappaB (NFkappaB) signaling activity or in NIH3T3-transforming ability. In this study, we examined the effect of colon tumor-associated mutations within the B-Raf glycine-rich loop (G loop) on MEK/Erk and NFkappaB signaling and on the transformation of NIH3T3 fibroblasts or IEC-6 intestinal epithelial cells. Of the six G loop mutations examined, only the B-Raf G468A significantly increased MEK/Erk and NFkappaB signaling and NIH3T3 transformation. Only this mutation induced transformed phenotypes of IEC-6 cells. In contrast, the B-Raf G468E mutation significantly decreased MEK/Erk signaling and NIH3T3 transformation and had no effect on NFkappaB signaling. The B-Raf F467C mutation moderately elevated MEK/Erk signaling and NIH3T3 transformation. The other three B-Raf mutations, R461I, I462S, and G463E, did not increase MEK/Erk or NFkappaB signaling or NIH3T3 transformation. Except for F467C, none of the tumors with B-Raf mutations examined in this study had K-Ras mutations. These results suggest that some of the B-Raf G loop mutations reported in colorectal tumors do not increase kinase or transforming activities but might contribute to carcinogenesis via other mechanisms or be irrelevant to carcinogenesis.
Ikenoue
Tsuneo
T
Division of Gastroenterology, The Institute for Adult Diseases, Asahi Life Foundation, Tokyo, Japan.
Hikiba
Yohko
Y
Kanai
Fumihiko
F
Aragaki
Jun
J
Tanaka
Yasuo
Y
Imamura
Jun
J
Imamura
Takaaki
T
Ohta
Miki
M
Ijichi
Hideaki
H
Tateishi
Keisuke
K
Kawakami
Takayuki
T
Matsumura
Masayuki
M
Kawabe
Takao
T
Omata
Masao
M
D000595:Amino Acid Sequence; D000818:Animals; D019556:COS Cells; D002448:Cell Adhesion / Q000235:genetics; D002455:Cell Division / Q000235:genetics; D002471:Cell Transformation, Neoplastic / Q000235:genetics* / Q000378:metabolism; D002522:Chlorocebus aethiops; D015179:Colorectal Neoplasms / Q000201:enzymology / Q000235:genetics*; D004789:Enzyme Activation; D006801:Humans; D051550:I-kappa B Kinase; D020935:MAP Kinase Signaling System / Q000235:genetics*; D051379:Mice; D020928:Mitogen-Activated Protein Kinases / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D016328:NF-kappa B / Q000378:metabolism*; D041681:NIH 3T3 Cells; D017354:Point Mutation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D048493:Proto-Oncogene Proteins B-raf; D019908:Proto-Oncogene Proteins c-raf / Q000235:genetics* / Q000378:metabolism; D016415:Sequence Alignment; D015398:Signal Transduction; D014158:Transcription, Genetic
D016328:NF-kappa B; C482120:Braf protein, mouse; D017346:Protein Serine-Threonine Kinases; D048493:Proto-Oncogene Proteins B-raf; D019908:Proto-Oncogene Proteins c-raf; C496559:CHUK protein, human; C496564:Chuk protein, mouse; D051550:I-kappa B Kinase; C496560:IKBKB protein, human; C496565:IKBKE protein, human; C496562:Ikbkb protein, mouse; C496566:Ikbke protein, mouse; D020928:Mitogen-Activated Protein Kinases