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To study the mechanisms by which catecholamines regulate hepatocyte proliferation after partial hepatectomy (PHX), hepatocytes were isolated from adult male rats 24 h after sham operation or two-thirds PHX and treated with catecholamines and other agonists. In freshly isolated sham cells, p42 mitogen-activated protein (MAP) kinase activity was stimulated by the ␣ 1 -adrenergic agonist phenylephrine (PHE). Activation of p42 MAP kinase by growth factors was blunted by pretreatment of sham hepatocytes with glucagon but not by that with the ␤ 2 -adrenergic agonist isoproterenol (ISO). In PHX cells, the ability of PHE to activate p42 MAP kinase was dramatically reduced, whereas ISO became competent to inhibit p42 MAP kinase activation. PHE treatment of sham but not PHX and ISO treatment of PHX but not sham hepatocytes also activated the stress-activated protein (SAP) kinases p46/54 SAP kinase and p38 SAP kinase. These data demonstrate that an Partial hepatectomy (PHX) or acute dissociation and primary culturing of hepatocytes trigger their entry into the cell cycle, which is accompanied by alterations in the levels of expression of various liver-specific proteins (14, 34). Epidermal growth factor (EGF), hepatocyte growth factor, and insulin have been shown to induce DNA synthesis in cultured hepatocytes or in quiescent liver (14,29,34). Catecholamines have been shown to increase DNA synthesis in quiescent liver via stimulation of ␣ 1 -adrenergic receptors (ARs) (6,14,18,24,27,34,44,51). The role(s) of ␤ 2 -ARs in the control of hepatic DNA synthesis is less clear, as both stimulatory and inhibitory effects have been described elsewhere (13,15,35). Previous studies have demonstrated that PHX or primary culture of rat hepatocytes significantly increases ␤ 2 -AR, and decreases ␣ 1 -AR, expression and function (24,35,38). However, the capacity of this receptor switch to modulate many of the recently discovered signal transduction pathways, such as the Raf/mitogen-activated protein (MAP) kinase cascade, and its role in the regulation of hepatocyte regeneration are unclear.Raf-1 is a member of a family of serine/threonine protein kinases termed Raf-1, B-Raf, and A-Raf (41). Proto-oncogenes of the Raf family have been implicated in hepatic carcinogenesis (2,11,17,34,40,41). Raf family members function in a signaling cascade that extends from the plasma membrane, via tyrosine kinase (16) and serpentine (22, 36) receptors, to the low-molecular-weight GTP-binding protein Ras. GTP-Ras binds and translocates Raf-1 to the plasma membrane, which leads to the activation of Raf-1 by multiple mechanisms (12,16,30,36). Raf family members in turn activate MEK1/2, which in turn activates the p42/44 MAP kinases, whose activation leads to modulation of downstream transcription factor activities (30, 46). The MAP kinase pathway can be negatively regulated by agonists that elevate cyclic AMP and activate protein kinase A (PK-A), such as glucagon and ␤ 2 -AR agonists (8,28,32,45). Activation of PK-A has been shown to inhibit or delay the act...

Elevation of serum interleukin-6 (IL-6) levels is always associated with alcoholic liver disease (ALD), but the significance of such elevation is not clear. Here we show that chronic ethanol consumption induces significant apoptosis in the liver of IL-6 (-/-) mice but not IL-6 (+/+) mice. IL-6 (-/-) hepatocytes are more susceptible to ethanol- and tumor necrosis factor alpha- (TNFalpha-) induced apoptotic killing, which can be corrected by IL-6. Expression of both anti-apoptotic (such as Bcl-2 and Bcl-x(L)) and proapoptotic (such as Bax) proteins is markedly elevated in the liver of human ALD and chronically ethanol-fed IL-6 (+/+) mice. On the contrary, induction of Bcl-2 and Bcl-x(L) is not observed in the liver of chronically ethanol-fed IL-6 (-/-) mice, whereas expression of Bax protein remains elevated. Injection of IL-6 markedly induces expression of Bcl-2 and Bcl-x(L) but not Bax in the liver. Finally, high concentrations of ethanol inhibit IL-6-activated anti-apoptotic signal, but increasing the concentrations of IL-6 is able to overcome such inhibitory effect. These findings suggest that elevated serum IL-6 levels in ALD may overcome the inhibitory effect of ethanol on IL-6-mediated anti-apoptotic signals and prevent alcohol-induced hepatic apoptosis by induction of Bcl-2 and Bcl-x(L).

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To understand the mechanisms by which ethanol inhibits hepatocyte proliferation, we studied the effects of ethanol on p42/44 mitogen-activated protein kinase (MAPK), p38 mitogen-activated protein kinase (p38 MAPK) and c-Jun N-terminal kinase (JNK) in normal and regenerating rat liver. Treatment of rat hepatocytes with 100 mM ethanol in vitro for 16 h prolonged the activation of p42/44 MAPK and p38 MAPK induced by various agonists. Such treatment also increased basal JNK activity, but did not potentiate or prolong agonist-induced JNK activation. Ethanol potentiation of the activation of p42/44 MAPK was abolished by pertussis toxin. In contrast, chronic ethanol consumption in vivo inhibited the activation of p42/44 MAPK, p38 MAPK and JNK induced either by partial hepatectomy or by various agonists. However, both acute and chronic ethanol inhibited hepatocyte proliferation induced by insulin and epidermal growth factor. A selective inhibitor of p42/44 MAPK partially prevented the inhibition of hepatocyte proliferation caused by acute, but not by chronic, ethanol exposure, whereas a selective inhibitor of p38 MAPK further inhibited hepatocyte proliferation under both conditions. These data suggest that acute and chronic ethanol inhibit hepatocyte proliferation by different mechanisms. The effect of acute ethanol may be related to the prolongation of p42/44 MAPK activation, whereas inhibition of hepatocyte proliferation by chronic ethanol may be due to inhibition of p38 MAPK activation.