Temporal Activation of p42/44 Mitogen-Activated Protein Kinase and c-Jun N-Terminal Kinase by Acetaldehyde in Rat Hepatocytes and Its Loss after Chronic Ethanol Exposure

Abstract

Several cell-damaging effects of ethanol are due to its major metabolite acetaldehyde but its mechanisms are not known. We have studied the effect of acetaldehyde on p42/44 mitogenactivated protein kinase (MAPK) and p46/p54 c-Jun N-terminal kinase (JNK 1/2) in rat hepatocytes. Acetaldehyde caused peak activation of p42/44 MAPK at 10 min followed by JNK activation at 1 h. These responses were acetaldehyde dose-dependent (0.2-5 mM). There was a consistently higher activation of p46 JNK than p54 JNK. Ethanol also activated both p42/44 MAPK and p46/p54 JNK. The activation of JNK by ethanol, however, was not significantly affected by treatment of hepatocytes with 4-methylpyrazole, an alcohol dehydrogenase inhibitor. Cells treated with 200 mM ethanol for 1 h accumulated 0.35 Ϯ 0.02 mM acetaldehyde, but the magnitude of JNK activation was greater than that expected with 0.35 mM acetaldehyde. Thus, ethanol-activated JNK may be both acetaldehyde-dependent and -independent. The activation of JNK by ethanol or acetaldehyde was insensitive to the treatment of hepatocytes with genistein (tyrosine kinase inhibitor) and 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)maleimide (GF109203X) (protein kinase C inhibitor). Remarkably, in contrast to the above-mentioned effects on normal hepatocytes, acetaldehyde was unable to increase JNK activity in hepatocytes isolated from rats chronically fed ethanol for 6 weeks and indicated a loss of this acetaldehyde response. Thus, temporal activation of the p42/44 MAPK and p46/p54 JNK, the greater activation of p46 JNK than p54 JNK, and loss of JNK activation after chronic ethanol exposure indicate that these kinases are differentially affected by ethanol metabolite acetaldehyde.

Effects of ethanol on insulin-like growth factor-I system in primary cultured rat hepatocytes: Implications of JNK1/2 and alcoholdehydrogenase

“…Hepatocytes express two JNK genes (JNK1 and JNK2) and bile acids cause activation of both JNK1 and JNK2, but JNK1 activation causes apoptosis whereas JNK2 activation protects against apoptosis [21] . It has been reported that ethanol causes more pronounced activation of JNK 1 compared to JNK 2, suggesting a role for this preferential activation of JNK 1 in ethanol-induced apoptosis of hepatocytes [15] .…”

“…It has been reported that ethanol activates MAPKs [1,2] , and acute exposure of primary cultured rat hepatocytes to ethanol for 60 min increases the activities of p42/44 MAPK and p-JNK1/2, with both activities gradually decreasing thereafter [15] . Exposure to ethanol has been shown to cause prolonged activation of p-JNK1/2; however, this response was attenuated in hepatocytes obtained from rats chronically exposed to ethanol for 6 wk [16] .…”

“…Ethanol rapidly activates p-JNK1/2, which is associated with the response of the endoplasmic reticulum to stress, which in turn causes inhibition of ADH [23] . Acute exposure to 200 mmol/L ethanol may also activate p-JNK1/2 via acetaldehyde-dependent [15] and acetaldehyde-independent [24] pathways in rat hepatocytes. It was reported that acetaldehyde produced by ethanol oxidation activates p42/44 MAPK and p-JNK1/2 in rat hepatocytes [1,2,15] .…”

“…Acute exposure to 200 mmol/L ethanol may also activate p-JNK1/2 via acetaldehyde-dependent [15] and acetaldehyde-independent [24] pathways in rat hepatocytes. It was reported that acetaldehyde produced by ethanol oxidation activates p42/44 MAPK and p-JNK1/2 in rat hepatocytes [1,2,15] . We previously reported that ethanolinduced changes of the IGF-Ⅰ system are related to ADH activity [11] .…”

Some Findings on Apoptosis in Hepatocytes

“…MAPKs regulate various biological processes including cell growth, proliferation, movement, inflammation, fatty degeneration, necrosis and apoptosis [70]. In a primary culture of rat hepatocytes, ethanol showed modest activation of ERK1/2 and notable activation of JNK [71]. In rat cultured hepatocytes, when ERK1/2 phosphorylation was inhibited by U-0126 (a MEK1/2 inhibitor), phosphorylation of JNK by ethanol was increased [72].…”