Activation of Extracellular-Signal Regulated Kinase by Epidermal Growth Factor Is Potentiated by cAMP-Elevating Agents in Primary Cultures of Adult Rat Hepatocytes

Abstract

Epidermal growth factor (EGF) is well recognized as a potent mitogen, and appears to trigger liver regeneration after partial hepatectomy or after acute liver cell necrosis caused by chemicals in vivo. [1][2][3][4] The response of adult rat hepatocytes to EGF has also been extensively investigated with respect to DNA synthesis and proliferation in vitro. EGF is now known to be a potent hepatocyte mitogen and induces multiple biological activities in a wide variety of cell types. [5][6][7][8] More recently, we reported that EGF rapidly stimulates hepatocyte DNA synthesis and proliferation during short-term (e.g., 4 h) culture.9) In addition, the hepatocyte DNA synthesis and proliferation induced by EGF was inhibited depending on the initial plating density. Furthermore, we found that EGF-induced hepatocyte DNA synthesis and proliferation were potentiated by b 2 -adrenergic agonists. 9)The signal transduction pathways activated in response to EGF in hepatocytes and other cell types are now more clearly understood. [10][11][12] Using specific inhibitors of signal transducers, we pharmacologically demonstrated that EGF-receptor tyrosine kinase and ribosomal p70 S6 kinase activities, but not phosphoinositide 3-kinase (PI3K), are essential for EGFinduced DNA synthesis and proliferation in primary cultures of adult rat hepatocytes. 9) PI3K is considered to be involved in hepatocyte growth factor (HGF) signal transduction. 13)In addition, extracellular-signal regulated kinase (ERK)1/2, also known as mitogen-activated protein kinase (p42/44 MAPK), is now known to be activated in response to a large number of mitogenic stimuli, and this enzyme is a key participant in the response to various growth factors and cytokines.14,15) In order to better understand the EGF-mediated signaling pathway, we investigated whether activation of the ERK isoforms, ERK1 and ERK2, is involved in EGFinduced DNA synthesis and proliferation in primary cultures of adult rat hepatocytes.Catecholamines (e.g., norepinephrine and its analogs) have been shown to be involved in the regulation of liver function (e.g., lipid metabolism, carbohydrate metabolism and cell growth). There are several types of catecholamine receptors, b 1 -and b 2 -receptors, that stimulate adenylate cyclase (AC), while a 2 -receptor inhibits its phosphorylation. 5,16) a 1 -Receptor is involved in phospholipase C activation and subsequent increases in inositolphosphate turnover and diacylglycerol production.17,18) However, some investigators have reported that a 1 -and b-adrenergic responses are involved in adrenergic regulation of carbohydrate metabolism in the liver of normal adult rats and in cultured hepatocytes. 19,20) There are few studies regarding the adrenergic regulation of ERK1/2 phosphorylation induced by growth factors in liver cells. Therefore, in the present study, we examined whether a 1 -, a 2 -and b 2 -adrenergic agonists can modulate EGF-induced ERK1/2 isoform activities. The physiological significance of crosstalk between the EGF pathway and a 1 -, a 2 -and...

Udział szlaku sygnałowego cAMP/PKA w hamującym wpływie melatoniny na wydzielanie oksytocyny i wazopresyny z układu podwzgórze–część nerwowa przysadki szczura

“…Our findings show, for the first time, that both PKA inhibitors are capable of negating the stimulatory effect of cAMP analogue on OT and VP secretion from the rat H-NH system. This observation is in agreement with those of previous stu dies demonstrating the ability of H-89 and/or cAMPS-Rp to antagonise various effects of cAMP/PKA stimulators under a range of experimental conditions [25,26,28,29]. For example, H-89 antagonised the effect of 8-Br-cAMP on epidermal growth factor (EGF)-induced ERK2 phosphorylation in primary cultures of rat hepatocytes [29], while in another study, 8-Br-cAMP and PACAP treatment significantly increased nitric oxide synthase type 1 (NOS1) level in rat pituitary gonadotrophs in vitro, and H-89 was able to inhibit such stimulation [25].…”

“…One such inhibitor is H-89, which has been found to act as a competitive antagonist of ATP at its binding site on the PKA catalytic subunits [20], and to offer great value in discriminating between the effects of PKA and other cAMP-regulated proteins such as EPACs. As a potent PKA inhibitor, H-89 has been found to abolish various effects of cAMP/PKA stimulation by forskolin, PACAP, or 8-Br-cAMP in vitro [21,24,25,28,29]. However, H-89 has also been found to have a range of properties not asso ciated with PKA, and to restrain the activity of several kinases other than PKA, including protein kinase C (PKC), Rho-associated kinase (ROCK-II), extracellular signalregulated kinase 1 and 2 (ERK 1/2), mitogen-activated protein kinase (MAPK), mitogen-and stress-activated protein kinase 1 (MSK1), and ribosomal protein s6 kinase (S6K1) [20,39].…”

“…Two of them, i.e. cAMPS-Rp (which acts as a competitive cAMP antagonist which inhibits PKA activation) [19,20] and H-89 (a potent inhibitor of PKA and some other kinases) [20][21][22], have been shown to antagonise the effects of cAMP/PKA stimulators, such as forskolin [21,23,24], pituitary adenylate cyclaseactivating polypeptide (PACAP) [25][26][27], or cAMP analogue(s) [25][26][27][28][29]. These two PKA inhibitors have also been used frequently when studying the role of the cAMP/PKA-dependent signal transduction cascade in CNS function and behaviour [5,22,[30][31][32][33].…”

Adenosine 3′,5′-Cyclic Monophosphate Involvement in Hepatic Triacylglyceride Lipase Release from Prazosin-Stimulated Primary Cultured Rat Hepatocytes

“…HTGL release was suppressed by the PKA inhibitors H-89 and KT5720. 24,25) Thus, we investigated whether the release of HTGL involved PKA activity in hepatocytes with or without prazosin treatment. It was observed that intracellular PKA activity increased after treatment with 100 µM prazosin in a time-dependent manner for up to 60 min (Fig.…”

Effect of Selective Serotonin (5-HT)_2B Receptor Agonist BW723C86 on Epidermal Growth Factor/Transforming Growth Factor-α Receptor Tyrosine Kinase and Ribosomal p70 S6 Kinase Activities in Primary Cultures of Adult Rat Hepatocytes

“…On the other hand, p70S6K phosphorylation induced by 5-HT or BW723C86 was not affected by an inactive structural analogue of U-73122 U-73343 (10 −6 M), a protein kinase A inhibitor H-89 (10 −6 M), 33) a protein kinase C inhibitor GF109203X (10 −6 M), 34) or a direct adenylate cyclase inhibitor 2,4-dideoxyadenosine (10 −6 M). 35) The proposed mechanism for the induction of hepatocyte DNA synthesis and proliferation via the 5-HT 2B receptor is shown in a schematic in Fig. 6.…”