Activation of mitogen-activated protein kinase in freshly isolated rat hepatocytes by both a calcium- and a protein kinase C—dependent pathway

doi:10.1016/S0026-0495(97)90193-1

Metabolism

Volume 46, Issue 5, May 1997, Pages 548-555

https://doi.org/10.1016/S0026-0495(97)90193-1 Get rights and content

Abstract

In the present study, we investigated the role of calcium and protein kinase C (PKC) in the activation of mitogen-activated protein kinase (MAPK) in isolated rat hepatocytes. We found that the glycogenolytic hormone norepinephrine (NE), acting through the α₁-adrenergic receptor and the G protein Gq, was able to induce a dose- and time-dependent activation of MAPK in hepatocytes. Vasopressin, which acts through a different receptor but also through stimulation of the Gq-dependent pathway, also caused a twofold activation of MAPK. Activation of MAPK by both agonists required the presence of free extracellular calcium and was blocked by the specific PKC inhibitor, Ro 31-8220. MAPK activation was also induced by phorbol myristate acetate (PMA), confirming that a PKC-dependent pathway exists for MAPK activation in liver. Furthermore, calcium-mobilizing agents such as thapsigargin and ionomycin were able to induce an activation of MAPK by a PKC-independent pathway that was totally abolished by preincubation of cells with EGTA. A second pathway for MAPK activation that relies solely on calcium may therefore exist. Ro 31-8220 did not affect phosphorylase activation by NE, vasopressin, thapsigargin, and ionomycin, indicating that PKC inhibition did not interfere with the signaling pathway leading to inositol-1,4,5-triphosphate (IP₃)-induced calcium mobilization or with changes in calcium fluxes. The role of MAPK activation by NE and vasopressin in the regulation of hepatic carbohydrate metabolism is discussed.

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Adipose tissue plays an important role providing energy to other tissues and functioning as an energy reserve organ. The energy supply is produced by triglycerides stored in a large vacuole representing approximately 95% of adipocyte volume. In the fasting period, triglyceride hydrolysis produces glycerol and free fatty acids which are important oxidative fuels for other tissues such as liver, skeletal muscle, kidney and myocardium. Hormone-sensitive lipase (HSL) is the enzyme that hydrolyzes intracellular triacylglycerol and diacylglycerol, and is one of the key molecules controlling lipolysis. Hormones and physiological factors such as dieting, physical exercise and ageing regulate intensively the release of glycerol and free fatty acids from adipocytes. One of the best known mechanisms that activate lipolysis in the adipocyte is the cAMP dependent pathway. cAMP production is modulated by hormone receptors coupled to Gs/Gi family of GTP binding proteins, such as β-adrenergic receptors, whereas cAMP degradation is controlled by modulation of phosphodiesterase activity, increased by insulin receptor signalling. cAMP activates PKA which activates HSL by promoting its phosphorylation. Hormonal control of lipolysis can also be achieved by receptors coupled G proteins of the Gq family, through molecular mechanisms that involve PKC and MAPK, which are currently under investigation. cGMP and PKG have also been found to activate lipolysis in adipocytes. In this review we have compiled data from literature reporting both the classical and the alternative mechanisms of lipolysis.
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Citation Excerpt :
This hypothesis also offers an explanation for the synergistic effect of insulin on ATP-induced MAPK stimulation observed in the present study. Indeed, for both Gq-coupled receptor agonists and insulin, MAPK activation may implicate PKC-dependent pathways in hepatocytes,6 as in other cells.40 It is therefore possible that convergent stimulation of MAPK, through different PKC isoforms induced by insulin and ATP, explains the synergy of the 2 hormones at that level in HTC cells.
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^☆: Supported by Grant No. MT-10804 from the Medical Research Council of Canada.

¹: Present address: A.R., Department of Cell Biology, Harvard Medical School, 240 Longwood Ave, Boston, MA 02115.

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Activation of mitogen-activated protein kinase in freshly isolated rat hepatocytes by both a calcium- and a protein kinase C—dependent pathway☆

Abstract

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

Cell

Cell

Hepatology

J Biol Chem

J Biol Chem

Hepatology

J Biol Chem

J Biol Chem

Hepatology

Toxicology

FEBS Lett

J Biol Chem

J Biol Chem

J Biol Chem

Arch Biochem Biophys

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

Biochim Biophys Acta

An insulin-stimulated protein kinase similar to yeast kinases involved in cell cycle control

Science