Interaction with MEK Causes Nuclear Export and Downregulation of Peroxisome Proliferator-Activated Receptor γ

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Abstract
The mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) cascade plays a central role in intracellular signaling by many extracellular stimuli. One target of the ERK cascade is peroxisome proliferator-activated receptor γ (PPARγ), a nuclear receptor that promotes differentiation and apoptosis. It was previously demonstrated that PPARγ activity is attenuated upon mitogenic stimulation due to phosphorylation of its Ser84 by ERKs. Here we show that stimulation by tetradecanoyl phorbol acetate (TPA) attenuates PPARγ9s activity in a MEK-dependent manner, even when Ser84 is mutated to Ala. To elucidate the mechanism of attenuation, we found that PPARγ directly interacts with MEKs, which are the activators of ERKs, but not with ERKs themselves, both in vivo and in vitro. This interaction is facilitated by MEKs9 phosphorylation and is mediated by the basic D domain of MEK1 and the AF2 domain of PPARγ. Immunofluorescence microscopy and subcellular fractionation revealed that MEK1 exports PPARγ from the nucleus, and this finding was supported by small interfering RNA knockdown of MEK1 and use of a cell-permeable interaction-blocking peptide, which prevented TPA-induced export of PPARγ from the nucleus. Thus, we show here a novel mode of downregulation of PPARγ by its MEK-dependent redistribution from the nucleus to the cytosol. This unanticipated role for the stimulation-induced nuclear shuttling of MEKs shows that MEKs can regulate additional signaling components besides the ERK cascade.