RAF
proteins are well known
oncoproteins. The B-RAF has been shown to be activated by mutations in a multitude of human
cancers. Alterations of C-RAF expression are discussed to play a role in
lung cancer. Only for A-RAF no link to
tumorigenesis has been published so far.
Malignant gliomas are the most prevalent
primary brain tumors of adults. They are highly invasive and very difficult to treat, despite of surgery, gamma-irradiation and
chemotherapy. Although a role of the mitogenic Ras-RAF-
MEK-ERK signalling cascade in
brain tumor development is well established, there are only few reports available addressing alterations in RAF sequence or
protein expression and function in human
gliomas. We analysed the mutational status of A-RAF and B-RAF in human
glioblastomas (GBM) by sequencing. Then we checked for RAF gene amplification by dot blot hybridization and examined RAF
mRNA and
protein expression patterns in human
astrocytic gliomas of WHO grade II (LGA) and IV (GBM) by semiquantitative RT-PCR and Western blotting, respectively. The results were correlated with patients prognosis. Finally, we performed functional assays to address a putative function of A-RAF in
glioma cell proliferation and migration. We showed that RAF mutations are a rare event in
glioblastoma multiforme. A-raf gene amplification was more often detected and overexpression of all three RAF
proteins on
mRNA and
protein level was regularly found in human
malignant gliomas. Whereas A-RAF and C-RAF expression was negatively correlated with the patients prognosis, B-RAF expression had a positive effect. Since neither A-RAF, nor C-RAF expression had any influence on proliferation and migration of GBM cells, putative functions of C-RAF in angiogenesis and of A-RAF in regulation of metabolism are discussed. Our data indicate that RAF
proteins might be valuable targets for small molecule
therapies. However, initially specific functions of RAF during
tumorigenesis have to be elucidated.