MECHANICAL-STRESS ACTIVATES PROTEIN-KINASE CASCADE OF PHOSPHORYLATIONIN NEONATAL RAT CARDIAC MYOCYTES

We have previously shown that stretching cardiac myocytes evokes activ ation of protein kinase C (PKC), mitogen-activated protein kinases (MA PKs), and 90-kD ribosomal S6 kinase (p90(rsk)). To clarify the signal transduction pathways from external mechanical stress tb nuclear gene expression in stretch-induced cardiac hypertrophy, we have elucidated protein kinase cascade of phosphorylation by examining the time course of activation of MAP kinase kinase kinases (MAPKKKs), MAP kinase kina se (MAPKK), MAPKs, and p90(rsk) in neonatal rat cardiac myocytes. Mech anical stretch transiently increased the activity of MAPKKKs. An incre ase in MAPKKKs activity was first detected at 1 min and maximal activa tion was observed at 2 min after stretch, The activity of MAPKK was in creased by stretch from 1-2 min, with a peak at 5 min after stretch, I n addition, MAPKs and p90(rsk) were maximally activated at 8 min and a t 10 similar to 30 min after stretch, respectively, Raf-1 kinase (Raf- l) and (MAPK/extracellular signal-regulated kinase) kinase kinase (MEK K), both of which have MAPKKK activity, were also activated by stretch ing cardiac myocytes for 2 min, The angiotensin II receptor antagonist partially suppressed activation of Raf-1 and MAPKs by stretch, The st retch-induced hypertrophic responses such as activation of Raf-1 and M APKs and an increase in amino acid uptake was partially dependent on P KC, while a PKC inhibitor completely abolished MAPK activation by angi otensin II. These results suggest that mechanical stress activates the protein kinase cascade of phosphorylation in cardiac myocytes in the order of Raf-1 and MEKK, MAPKK, MAPKs and p90(rsk), and that angiotens in II, which may be secreted from stretched myocytes, may be partly in volved in stretch-induced hypertrophic responses by activating PKC.