The MEKK-JNK Pathway Is Stimulated by α₁-Adrenergic Receptor and Ras Activation and Is Associated with in Vitro and in Vivo Cardiac Hypertrophy*

In neonatal rat ventricular myocytes, stimulation of the α₁-adrenergic receptor (α₁-AdrR) activates a program of genetic and morphological changes characterized by transcriptional activation of the atrial natriuretic factor (ANF) gene and enlargement (hypertrophy) of the cells. The low molecular weight GTPase Ras has been established as an important regulator of hypertrophy bothin vitro and in vivo. Ras activates a kinase cascade involving Raf, the mitogen-activated protein kinase kinase (MEK), and the extracellular signal-regulated protein kinase (ERK). However, the extent of involvement of this pathway in regulating hypertrophic responses is controversial. We demonstrate here that both α₁-AdrR stimulation and Ras can also activate the c-Jun NH₂-terminal kinase (JNK) in cardiomyocytes. The α₁-AdrR effect on JNK occurs through a pathway requiring Ras and MEK kinase (MEKK). A constitutively activated mutant of MEKK that preferentially activates JNK, stimulates ANF reporter gene expression, while a dominant negative MEKK mutant inhibits ANF expression induced by PE. Furthermore, JNK activity is increased in the ventricles of mice overexpressing oncogenic Ras, whereas ERK activity is not. These results suggest that the α₁-AdrR mediates ANF gene expression through a Ras-MEKK-JNK pathway and that activation of this pathway is associated with in vitro and in vivo hypertrophy.