Activation of protein synthesis in cardiomyocytes by the hypertrophic agent phenylephrine requires the activation of ERK and involves phosphorylation of tuberous sclerosis complex 2 (TSC2)

Abstract

The hypertrophic Gq-protein-coupled receptor agonist PE (phenylephrine) activates protein synthesis. We showed previously that activation of protein synthesis by PE requires MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase] and mTOR (mammalian target of rapamycin). However, it remained unclear whether ERK activation was required and which downstream components were involved in activating mTOR and protein synthesis. Using an adenovirus encoding the MKP3 (MAPK phosphatase 3) to inhibit ERK activity, we demonstrate that ERK is essential for the activation of protein synthesis by PE. Activation and phosphorylation of S6K1 (ribosomal protein S6 kinase 1) and phosphorylation of eIF4E (eukaryotic initiation factor 4E)-binding protein (both are mTOR targets) were also inhibited by MKP3, suggesting that ERK is also required for the activation of mTOR signalling. PE stimulation of cardiomyocytes induced the phosphorylation of TSC2 (tuberous sclerosis complex 2), a negative regulator of mTOR activity. TSC2 was phosphorylated only weakly at Thr1462, but phosphorylated at additional sites within the sequence RXRXX(S/T). This differs from the phosphorylation induced by insulin, indicating that MEK/ERK signalling targets distinct sites in TSC2. This phosphorylation may be mediated by p90RSK (90 kDa ribosomal protein S6K), which is activated by ERK, and appears to involve phosphorylation at Ser1798. Activation of protein synthesis by PE is partially insensitive to the mTOR inhibitor rapamycin. Inhibition of the MAPK-interacting kinases by CGP57380 decreases the phosphorylation of eIF4E and PE-induced protein synthesis. Moreover, CGP57380+rapamycin inhibited protein synthesis to the same extent as blocking ERK activation, suggesting that MAPK-interacting kinases and regulation of mTOR each contribute to the activation of protein synthesis by PE in cardiomyocytes.

Targeting survival cascades induced by activation of Ras/Raf/MEK/ERK, PI3K/PTEN/Akt/mTOR and Jak/STAT pathways for effective leukemia therapy

“…44,46 Recent evidence has highlighted that mTOR can also be activated by Raf/MEK/ERK. 59,60 This may well be another relevant cross talk between the Ras/Raf/MEK/ERK and the PI3K/ PTEN/Akt/mTOR pathways and might offer a further rationale for treatments combining drugs which inhibit both signaling networks. A diagram illustrating potential targets in these signal transduction pathways to target is presented in Figure 5.…”

Identification of cAMP-Dependent Kinase as a Third in Vivo Ribosomal Protein S6 Kinase in Pancreatic β-Cells

“…Given that these MEK inhibitors also activate AMPK, which clearly complicates the interpretation of these experiments, we used an alternative approach to inhibit the Erk signalling cascade and hence Rsk. MIN6 cells were infected with recombinant adenovirus encoding Mitogen-activated protein kinase phosphatase 3 (AdMPK3), a dualspecific phosphatase that dephosphorylates the activation loop of ERK1/2 with very high specificity 22 , and its effect on GLP1 stimulated rpS6 phosphorylation was assessed (figure 3b). The phorbol ester TPA, which potently activates Erk and Rsk, was used a positive control (figure 3b).…”

p90 Ribosomal S6 Kinase 1 (RSK1) and the Catalytic Subunit of Protein Kinase A (PKA) Compete for Binding the Pseudosubstrate Region of PKAR1α

“…RSK1 and RSK2 play a pivotal role in regulating a variety of biological functions including inhibition of apoptosis (14,15), activation of the mTOR pathway (16), and cell proliferation (17,18). RSK1 and RSK2 have also been shown to play an important role in the proliferation of breast and prostate tumor cells (17,18) and in augmenting cardiac hypertrophy (19). Previous studies from our laboratory have shown that the inactive form of RSK1 associates with PKARI␣, and upon its activation, RSK1 binds PKAc (20).…”