Combination of MEK and SRC inhibition suppresses melanoma cell growth and invasion

The RAS-RAF-MEK-ERK pathway is deregulated in over 90% of malignant melanomas, and targeting MEK as a central kinase of this pathway is currently tested in clinical trials. However, dose-limiting side effects are observed, and MEK inhibitors that sufficiently reduce ERK activation in patients show a low clinical response. Apart from dose limitations, a reason for the low response to MEK targeting drugs is thought to be the upregulation of counteracting signalling cascades as a direct response to MEK inhibition. Therefore, understanding the biology of melanoma cells and the effects of MEK inhibition on these cells will help to identify new combinatorial approaches that are more potent and allow for lower concentrations of the drug being used. We have discovered that in melanoma cells MEK inhibition by selumetinib (AZD6244, ARRY-142886) or PD184352, while efficiently suppressing proliferation, stimulates increased invasiveness. Inhibition of MEK suppresses actin-cortex contraction and increases integrin-mediated adhesion. Most importantly, and surprisingly, MEK inhibition results in a significant increase in matrix metalloproteases (MMP)-2 and membrane-type 1-MMP expression. All together, MEK inhibition in melanoma cells induces a 'mesenchymal' phenotype that is characterised by protease-driven invasion. This mode of invasion is dependent on integrin-mediated adhesion, and because SRC kinases are the main regulators of this process, the SRC kinase inhibitor, saracatinib (AZD0530), completely abolished the MEK inhibitor-induced invasion. Moreover, the combination of saracatinib and selumetinib effectively suppressed the growth and invasion of melanoma cells in a 3D environment, suggesting that combined inhibition of MEK and SRC is a promising approach to improve the efficacy of targeting the ERK/MAP kinase pathway in melanoma.