Differential Activation of Protein Kinase Cascade in Cardiac Volume Overload Following Aorto-Caval Shunt in Rats • 141

Abstract

Volume overload (VO) results in increased expression of specific subset of cardiac genes. Since VO hypertrophy is associated with distinct molecular phenotypes than that seen with pressure hypertrophy, we tested the hypothesis that this distinction is due to differential activation of protein kinase cascade. Mechanical stress associated with pressure hypertrophy activates protein kinase cascade of MAP kinase family, including c-Jun NH2-terminal kinase (JNK) and extracellular signal regulated kinase(ERK) but not p38 kinase. These kinases are regulated by different MAP kinase kinase, and regulate growth by their ability to phosphorylate and activate specific nuclear transcription factors. Cardiac VO was induced in adult Sprague-Dawley rats (300±20 gm) by creating aorto-caval shunt. Sham-operated animals served as controls. Significant increase in heart/body weight ratio occurred as compared to shams one week post-surgery(20±3.2% p≤.05). Phenotype of these hearts was characterized for mRNA levels of α-and β-myosin heavy chain (MHC). Time-course of ERK, JNK and p38 kinase activities was analyzed in heart cell lysates after 2,5,10,15,30,60,120 and 240 min VO. These kinases were immunoprecipated with antibodies specific to respective kinases and immobilized on protein A-sepharose beads. In vitro kinase assay involved incubating beads withγP32 ATP and respective substrates (GST c-jun for JNK, myelin basic protein for ERK and for p38 kinase). Phosphorylated products were resolved by SDS-PAG; after drying the gels and autoradiography, signal intensity was quantitated by densitometer scanning. Northern blot analysis on total RNA revealed 4-fold increase in α-MHC mRNA but no change inβ-MHC mRNA in VO, an entirely different phenotype than that seen in pressure hypertrophy (i.e., ↑β-MHC and ↓α-MHC mRNA). Time course on MAP kinase cascade showed rapid induction of p38 kinase activity in VO within 5 min (50±15%), reaching peak at 30 min (180±20%), and returning to basal level in 2 hrs. Activity of other two kinases remained unchanged. These data implicate for the first time the role of p38 kinase in VO hypertrophy and show that α-MHC mRNA is increased in this form of hypertrophy.