Phospho-ERK Staining Is a Poor Indicator of the Mutational Status of BRAF and NRAS in Human Melanoma

Abstract

Mutated BRAF and NRAS are suspected to contribute to melanomagenesis by activation of extracellular signal-regulated kinase (ERK). To test this notion, we analyzed the presence of phosphorylated ERK1/2 in 170 melanomas with established NRAS/BRAF mutational status and well-documented clinical follow-up by immunohistochemistry. Several notable observations were obtained: (i) phospho-ERK staining was very heterogeneous within the tumor; (ii) in most cases, ERK was phosphorylated in only a minority of tumor cells; (iii) the percentage of phospho-ERK-positive cells was not correlated with the mutational status of NRAS and/or BRAF; (iv) the Raf kinase inhibitor protein (RKIP) was expressed homogeneously in virtually all melanoma samples not reflecting the inhomogeneity of phospho-ERK; and, finally, (v) neither the portion of phospho-ERK-positive tumor cells nor the RKIP staining intensity showed any correlation to the clinical course of the patients. Furthermore, the ability of BRAF mutant melanoma cells to downregulate mitogen-activated protein kinase activation was shown in melanoma cell lines cultured at high densities or under nonadherent conditions. 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.

Frequent loss of RAF kinase inhibitor protein expression in acute myeloid leukemia

“…9 Interestingly, analysis of AML cell lines and patient samples did not reveal any significant correlation between RKIP loss and phosphorylation of ERK, an observation that has been reported in breast cancer and malignant melanoma previously. 14,41 This finding may suggest the involvement of additional RKIP effector pathways in primary leukemic cells.…”

BRAF, a target in melanoma

“…34,35 The correlation between BRAF mutation and ERK phosphorylation in clinical nevi or melanoma specimens was identified as quite weak. 36,37 Furthermore, it was reported that the introduction of V600E BRAF into melanocytes induced p16-mediated senescence, a putatively irreversible state of cell cycle arrest. 38 Although this latter observation is consistent with the effect of other, previously described ''strong'' oncogenes, such as RAS and the v-myc myelocytomastosis viral oncogene homolog MYC, it appeared that BRAF mutations were a very early event in melanocytic proliferation and that compensatory mechanisms could reverse its activation of the MAP kinase pathway and its effect on cell proliferation.…”

Bcr-Abl-mediated Raf kinase inhibitor protein suppression promotes chronic myeloid leukemia progenitor cells proliferation

“…These data suggest RKIP might play as a tumor and/or metastasis suppressor for solid tumors. However, it has been reported that RKIP expression has no relationship with ERK phosphorylation or the clinical course of disease in patients with melanoma or breast cancer [24,27]. These different observations suggest that RKIP contributes to the regulation of not only MAPK signaling including phosphorylated ERK but also other signal pathways.…”