By E3 ubiquitin ligases can be recognized and degraded by the 26S proteasome, a multi-subunit, multi-catalytic protease machine. Proteasome inhibitors have shown great promise as cancer therapeutics because they MCE Company 133407-82-6 impact a variety of mechanisms affecting tumor cell proliferation and survival; proteasome inhibition interferes with cell cycle progression, upregulates tumor suppressors such as p53, and diminishes activation of pro-proliferation pathways such as those controlled by NFkB and extracellular signal-regulated kinases. The mitogen-activated protein kinases ERK2/ MAPK1 and ERK1/MAPK3 are activated by phosphorylation in a canonical Raf R MEK R ERK kinase cascade in response to most growth factors and cytokines, and ERK1/2 phosphorylate more than 150 cytosolic and nuclear substrates. Thus, they are master controllers of cell proliferation, differentiation, and migration. ERK signaling is inappropriately activated in a wide array of human cancers, which can be caused by an activating mutation in one of the upstream signaling proteins or through overexpression of growth factors or growth factor receptors. The dual specificity phosphatases have been linked to dephosphorylation of ERK1/2 and other MAPKs, and in many contexts, DUSP expression levels are known to be regulated through the ubiquitin-proteasome degradation pathway. Accordingly, cells treated with MG132 or other proteasome inhibitors exhibit higher expression of MKP3/DUSP6, an ERK1/ 2-specific DUSP, accompanied by lower levels of ERK phosphorylation stimulated by growth factors. Considering that a host of intracellular proteins are 220551-92-8 affected by proteasome inhibition, coupled with evidence that knockdown of MKP3 expression enhances growth factor-stimulated ERK phosphorylation in some contexts but not in others, led us to question whether or not the diminution of ERK signaling in MG132-treated cells could be attributed solely to upregulation of MKP3 and other DUSPs. In this short paper, we confirm that MG132 treatment reduces phosphorylation of ERK in fibroblasts stimulated with platelet-derived growth factor or basic fibroblast growth factor and show that this is caused by two parallel effects. For a given level of MEK activation, ERK phosphorylation is reduced, consistent with the proposed upre
