Essential Roles of c-JUN and c-JUN N-Terminal Kinase (JNK) in Neuregulin-Increased Expression of the Acetylcholine Receptor ε-Subunit

Neuregulin is a neural factor implicated in upregulation of acetylcholine receptor (AChR) synthesis at the neuromuscular junction. Previous studies have demonstrated that the extracellular signal–regulated kinase (ERK) subgroup of MAP kinases is required for neuregulin-induced AChR gene expression. We report here that the neuregulin-mediated increase in AChR ε-subunit mRNA was a delayed response in C2C12 muscle cells. Neuregulin induced expression of immediate early genesc-junandc-fos, which followed and depended on the ERK activation. Treatment of muscle cells with cycloheximide to inhibit c-JUN synthesis at the protein level and suppression of c-JUN function by a dominant-negative mutant blocked neuregulin-induced expression of the ε-subunit gene, indicating an essential role of c-JUN in neuregulin signaling. Furthermore, neuregulin activated c-JUN N-terminal kinase (JNK) in C2C12 muscle cells. Blockade of JNK activation by overexpressing dominant-negative MKK4 inhibited ε-promoter activation. Moreover, overexpression of the JNK dominant-negative mutant inhibited neuregulin-mediated expression of the εtransgene and endogenous ε-mRNA. Taken together, our results demonstrate important roles of c-JUN and JNK in neuregulin-mediated expression of the AChR ε-subunit gene and suggest that neuregulin activates multiple signaling cascades that converge to regulate AChR ε-subunit gene expression.