Adhesion‐dependent redistribution of MAP kinase and MEK promotes muscarinic receptor‐mediated signaling to the nucleus

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Abstract
The mitogen‐activated protein kinases (MAPKs) are activated by extracellular signals, and translocate to the nucleus where they modulate transcription. Integrin‐mediated cell adhesion to extracellular matrix (ECM) proteins is required for efficient transmission of MAPK‐based signals initiated by growth factors. However, the modulation of G protein‐coupled receptor (GPCR) signaling by adhesion is less well understood. In the present study, we assessed the impact of cell adhesion on MAPK activation by muscarinic M3 receptors. The muscarinic agonist carbachol more efficiently promoted stress fiber formation and tyrosine phosphorylation of focal adhesion‐associated proteins in M3 receptor‐expressing cells adherent to fibronectin or collagen type I, as compared to polylysine. Overall MAPK activation was robust in cells adherent to all three substrata. However, total levels of MAPK and mitogen‐activated protein kinase kinase (MEK) in the nucleus were significantly greater in cells adherent to ECM proteins for 2.5 h, and levels of activated MAPK and MEK in the nuclei of these cells were higher following carbachol stimulation, relative to levels in cells adherent to polylysine. MEK inhibitors did not prevent adhesion‐dependent translocation of MAPK and MEK to the nucleus, and increased nuclear phospho‐MEK levels in carbachol‐stimulated cells. The results suggest that adhesion of cells to ECM triggers the redistribution of MAPK and MEK to the nucleus, possibly as a result of the cytoskeletal rearrangements that accompany cell spreading. This may represent a mechanism for priming the nucleus with MEK and MAPK, leading to more rapid and pronounced increases in intranuclear phospho‐MAPK upon GPCR stimulation.

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