Contextual role of E2F1 in suppression of melanoma cell motility and invasiveness.

The general transcription factor E2F1 reportedly functions in a protumorigenic manner in several cancer models. We show that the genetic context of cancer cells influence E2F1's role to impede the protumorigenic role. Thirty to fifty percent of melanoma patients carry mutant BRAF with about 90% of mutant BRAF melanomas being V-600E mutation. Tissue microarrays from melanoma patients were used to establish an association between E2F1 and BRAF(V600E). We show for the first time that low E2F1 levels in BRAF(V600E) melanomas are associated with lymph node metastasis. Genetic manipulation of E2F1 in BRAF(V600E) and BRAF(wt) cells were used to determine its role in malignant melanoma progression by examining effects on migration and invasion. E2F1-mediated negative regulation of myosin light chain kinase (MYLK) increased migration and invasion in BRAF(V600E) cells by phosphorylating myosin light chain and increased stress fiber formation. We show that E2F1 inhibits extracellular signal-regulated kinase (ERK) activation in BRAF(V600E) cells and provide evidence for a negative feedback loop between E2F1 and ERK in these cells. This study shows for the first time that E2F1 has a cancer protective role in oncogenic BRAF-activated melanoma cells and that loss of E2F1 can allow disease progression through a novel mechanism of E2F1-mediated MYLK regulation. This study has implications for oncogenic BRAF-activated tumors and resistance to targeted oncogenic BRAF therapy.