Copper is a catalytic and structural cofactor for many proteins, and it has been implicated in growth factor signaling and cell proliferation. Turski et al. report that copper promotes mitogen-activated protein kinase (MAPK) signaling downstream of Ras activation by growth factors. Reducing the abundance of the high-affinity copper transporter Ctr1 with double-stranded RNA (dsRNA) in the fruit fly prothoracic gland, which controls body size, resulted in pupae and adults that were smaller than wild-type animals, similarly to expression of a dominant-negative form of Ras. dsRNA-mediated knockdown of Ctr1 rescued phenotypes caused by expression of constitutively active Ras, and experiments in cultured cells revealed that copper and the copper transporting activity of Ctr1 were required for maximal phosphorylation of ERK (a MAPK) in response to insulin, which activates the MAPK cascade through Ras. In Ctr1-deficient cultured mouse embryo fibroblasts (MEFs), phosphorylation of ERK1 and ERK2 (ERK1/2) in response to insulin stimulation was greatly reduced but was partially restored by incubating the cells with copper before insulin stimulation. Copper was also required for maximal ERK1/2 phosphorylation in MEFs downstream of Ras activation by a recombinant human fibroblast growth factor (FGF). In vitro experiments with purified proteins indicated that MEK1 (a MAPKK) bound to copper with high affinity and that copper enhanced MEK1-mediated phosphorylation of ERK1/2. Coimmunoprecipitation of endogenous MEK1 and ERK1/2 from MEFs was reduced in Ctr1-deficient MEFs, suggesting that copper promoted MEK1 binding to ERK1/2. Protein extracts from the hearts of mice lacking cardiac Ctr1 exhibited copper deficiency, decreased ERK1/2 phosphorylation, and increased MEK1 phosphorylation compared to the hearts of wild-type mice. Thus, copper binding by MEK1 is important for optimal phosphorylation of ERK in response to growth factor signaling through Ras, perhaps by inducing or stabilizing a conformation that promotes the association with or catalytic activity toward ERK.
M. L. Turski, D. C. Brady, H. J. Kim, B.-E. Kim, Y. Nose, C. M. Counter, D. R. Winge, D. J. Thiele, A novel role for copper in Ras/mitogen-activated protein kinase signaling. Mol. Cell Biol. 32, 1284–1295 (2012). [Abstract] [Full Text]