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Mechanical stress activates protein kinase cascade of phosphorylation in neonatal rat cardiac myocytes
T Yamazaki1, I Komuro, S Kudoh
1Third Department of Medicine, University of Tokyo School of Medicine, Japan.
Abstract:
We have previously shown that stretching cardiac myocytes evokes activation of protein kinase C (PKC), mitogen-activated protein kinases (MAPKs), and 90-kD ribosomal S6 kinase (p90rsk). To clarify the signal transduction pathways from external mechanical stress to 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 kinase (MAPKK), MAPKs, and p90rsk in neonatal rat cardiac myocytes. Mechanical stretch transiently increased the activity of MAPKKKs. An increase in MAPKKKs activity was first detected at 1 min and maximal activation was observed at 2 min after stretch. The activity of MAPKK was increased by stretch from 1-2 min, with a peak at 5 min after stretch. In addition, MAPKs and p90rsk were maximally activated at 8 min and at 10 approximately 30 min after stretch, respectively. Raf-1 kinase (Raf-1) and (MAPK/extracellular signal-regulated kinase) kinase kinase (MEKK), both of which have MAPKKK activity, were also activated by stretching cardiac myocytes for 2 min. The angiotensin II receptor antagonist partially suppressed activation of Raf-1 and MAPKs by stretch. The stretch-induced hypertrophic responses such as activation of Raf-1 and MAPKs and an increase in amino acid uptake was partially dependent on PKC, while a PKC inhibitor completely abolished MAPK activation by angiotensin 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 p90rsk, and that angiotensin II, which may be secreted from stretched myocytes, may be partly involved in stretch-induced hypertrophic responses by activating PKC.
Insights
Mechanical stretch activates cardiac cell signaling pathways, including protein kinase C (PKC) and mitogen-activated protein kinases (MAPKs). Angiotensin II may play a role in this stretch-induced cardiac hypertrophy response.
Area of Science:
- Cardiovascular Biology
- Cell Signaling
- Molecular Cardiology
Background:
- Cardiac myocytes respond to mechanical stress through complex signaling pathways.
- Previous work identified activation of protein kinase C (PKC), mitogen-activated protein kinases (MAPKs), and 90-kD ribosomal S6 kinase (p90rsk) by stretching cardiac myocytes.
- Understanding the precise kinase cascade is crucial for elucidating mechanisms of cardiac hypertrophy.
Purpose of the Study:
- To elucidate the protein kinase cascade involved in signal transduction from mechanical stress to gene expression in cardiac hypertrophy.
- To examine the time course of activation of MAP kinase kinase kinases (MAPKKKs), MAP kinase kinase (MAPKK), MAPKs, and p90rsk in response to mechanical stretch.
- To investigate the role of angiotensin II and protein kinase C (PKC) in stretch-induced hypertrophic responses.
Main Methods:
- Neonatal rat cardiac myocytes were subjected to mechanical stretch.
- Time course analysis of MAPKKK, MAPKK, MAPK, and p90rsk activation was performed.
- Specific inhibitors and receptor antagonists (angiotensin II receptor antagonist, PKC inhibitor) were used to probe signaling pathways.
- Measurements included kinase activity and amino acid uptake.
Main Results:
- Mechanical stretch rapidly activated MAPKKKs (within 1-2 min), followed by MAPKK (peak at 5 min), MAPKs (peak at 8 min), and p90rsk (peak at 30 min).
- Raf-1 kinase (Raf-1) and MAPK/extracellular signal-regulated kinase kinase kinase (MEKK) were identified as activated MAPKKKs.
- Angiotensin II receptor antagonism partially inhibited stretch-induced activation of Raf-1 and MAPKs.
- Stretch-induced hypertrophic responses were partially dependent on PKC, with complete abolition of angiotensin II-induced MAPK activation by a PKC inhibitor.
Conclusions:
- Mechanical stretch activates a sequential protein kinase cascade in cardiac myocytes: Raf-1/MEKK → MAPKK → MAPKs → p90rsk.
- Angiotensin II, potentially secreted by stretched myocytes, contributes to hypertrophic responses partly through PKC activation.
- This cascade represents a key signaling pathway linking mechanical stress to cardiac hypertrophy.
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