In summary, this report indicates that N-RAS(Q61K) and B-RAF(V600E) contribute to melanoma's resistance to apoptosis in part by downregulating Bim expression, suggesting that Bim is a possible treatment target for overriding melanoma's inherent defenses against cell death.
Here, we demonstrate that hyperactivation of the MAPK pathway following activation of an inducible form of oncogenic C-Raf induces a senescence-like proliferation arrest in B-Raf mutant melanoma cells.
Because of the very sensitive PAP technology, B-RAF mutations were found in cell lines and primary uveal melanomas, which suggests that they may occasionally play a role in the activation of the MAPK pathway in uveal melanoma and indicates a higher prevalence of B-RAF mutations in uveal melanoma than was reported earlier.
Inhibition of (V600E)B-Raf or Akt3 in advanced melanoma cells in which both pathways were active reduced anchorage-independent growth and tumor development in a cooperatively acting manner.
In conclusion, the T1799A BRAF mutation is present in a proportion of posterior uveal melanomas but within these tumours the distribution of the mutation is heterogeneous.
Critically, in melanoma cells MITF is required downstream of oncogenic BRAF because it regulates expression of key cell cycle regulatory proteins such as CDK2 and CDK4.
Our findings suggest that mitogen-activated protein kinase (MAPK) activity is subject to regulation even in BRAF/NRAS mutant melanoma cells and that high MAPK pathway signaling may be important only in distinct subsets of tumor cells.
These preliminary results suggest that BRAF and NRAS mutation status should be determined in prospective phase II studies of HSP90 inhibitors in melanoma.
Unexpectedly, ulceration was a significant adverse prognostic factor only in melanomas with BRAF mutations, whereas there was no effect of ulceration on overall survival in tumors with wild-type BRAF.
Using age < 55 y as a surrogate for BRAF mutation in an independent cohort of 4,785 patients of the Southern German Tumor Registry, we found a significant survival benefit (p < 0.0001) for patients who, based on their age, were predicted to have BRAF mutant melanomas in 69% of the cases.
Consistent with these observations, pharmacological inhibition of BRAF(V600E) or MEK1/2 in human melanoma cells (using PLX4720 and CI-1040 respectively) led to a striking elevation of BIM expression.
Mutant B-RAF is frequently expressed in melanoma and overrides growth factor and matrix adhesion control of cyclin D1 and p27(Kip1) levels in human melanocytes.
Our results suggest that although nevus propensity is important for the occurrence of both BRAF and NRAS-mutant melanomas, ambient UV irradiance influences risk differently based on the age of exposure.
As BRAF and its effectors could be good melanoma therapy targets, defining the repertoire of genes that are differentially regulated because of BRAF mutational activation is an important objective.
Taken together, these data support a model in which mutational activation of BRAF in human melanomas contributes to constitutive induction of NF-kappaB activity and to increased survival of melanoma cells.
The BRAF mutation is common in melanomas, but variation in rates across melanoma subtypes points to a complex interplay between BRAF activation and other factors (eg, sun exposure).Nevi also harbor the BRAF mutation.
Our observations suggest that Raf and MEK inhibitors may be effective for the treatment of B-RAF mutation-positive colorectal carcinomas as well as melanomas.