c-Jun N-terminal kinase-interacting protein 1 inhibits gene expression in response to hypertrophic agonists in neonatal rat ventricular myocytes

G(q)-coupled receptor agonists, such as endothelin-1 (ET-1) and phenylephri ne (PE), initiate a hypertrophic response in cardiac myocytes that is chara cterized by increased expression of atrial natriuretic factor (ANF), beta - myosin heavy chain (beta -MHC), skeletal muscle alpha -actin (Sk alphaA) an d ventricular myosin light chain-2 (vMLC2). ET-1 and PE activate both the e xtracellular signal-regulated kinases and c-Jun N-terminal kinases (JNKs) i n cardiac myocytes, but the extent to which each contributes to the hypertr ophic response is uncertain. Here we have used the JNK-binding domain of JN K-interacting protein 1 (JIP-1), a cytosolic scaffold protein that binds to JNK and inhibits its signalling when overexpressed, to assess the contribu tion of JNK activation to the hypertrophic response. Expression of JIP-1 in hibited the increase in ANF, beta -MHC, Sk alphaA and vMLC2 reporter gene e xpression in response to ET-1 (by 45-86%) and PE (by 56-60%). However, acti vation of these reporter genes by PMA, which does not activate JNK signific antly in myocytes, was much less affected by overexpression of JIP-1. JIP-1 also failed to inhibit reporter gene activation in response to constitutiv ely active Ras or Raf, but attenuated reporter gene activation induced by a constitutively active mutant of mitogen-activated protein kinase kinase ki nase I (MEKK1), an upstream kinase that preferentially activates JNKs, by 5 0%. Overexpression of JIP-1 also significantly reduced the increase in cell area in response to PE from 63% to 56%, but had no effect on the increase in cell size in response to ET-1 (38%). These results suggest that activati on of the JNK pathway contributes to the transcriptional and morphological responses to G(q) receptor-coupled hypertrophic agonists.