The liver regenerates by replication of differentiated hepatocytes after damage or removal of part of the liver. Although several growth factors and signaling pathways are activated during regeneration, it is unclear as to which of these are essential for hepatocyte replication. We show here that low-(1 mg/kg) and high-(10 mg/kg) dose hepatocyte growth factor (HGF) induced replication of 2.1% and 11.1% of hepatocytes in rats, respectively. Lipopolysaccharide (LPS), an inducer of the acute phase response, augmented hepatocyte replication in response to low-and high-dose HGF by 4-and 2-fold, respectively. HGF alone induced moderate levels of c-Jun-N-terminal kinase (JNK) and p44/ p42 mitogen-activated protein kinase (MAPK), resulting in moderate levels of AP-1-DNA binding activity. The combination of LPS Ψ HGF increased JNK and AP-1-DNA binding activity more than levels seen with LPS or HGF alone. The activation of Stat3 that was observed after administration of LPS Ψ HGF, but not HGF alone, could contribute to increased transcription of AP-1 components. Because phosphorylation of the c-Jun component of AP-1 by JNK increases its ability to activate transcription, the AP-1 in hepatocytes from animals treated with LPS Ψ HGF may be more active than in rats treated with LPS or HGF alone. LPS may contribute to hepatocyte replication by potentiating the effect of HGF on the activation of both AP-1-DNA binding and transcriptional activity. (HEPATOLOGY 1999;30: 1405-1416.)
The normal liver has a remarkable ability to replicate in response to injury or partial hepatectomy (PH). A better understanding of the molecular mechanisms responsible for this process might enable liver regeneration to be induced in liver insufficiency states such as cirrhosis. In addition, this information might facilitate gene therapy, as some vectors only transduce dividing cells, and hepatocytes are normally quiescent. 1 A number of factors can induce or augment hepatocyte replication in vitro or in vivo. [2][3][4][5] Hepatocyte growth factor (HGF), transforming growth factor β£, epidermal growth factor, keratinocyte growth factor, hepatic stimulatory substance, and heparin-binding-epidermal growth factor are complete liver mitogens, as they can stimulate hepatocyte replication in vitro in the absence of other factors. Factors that augment the effect of complete mitogens in vitro, but have no effect alone, are called incomplete or comitogens. These include insulin, glucagon, epinephrin, norepinephrin, augmenter of liver regeneration, thyroid hormone, parathormone, prolactin, estrogen, and tumor necrosis factor β£ (TNF-β£). These mitogens and comitogens function by activating various signal transduction pathways that result in cellular replication. [2][3][4][5] Factors that contribute to hepatocyte replication in vivo have been identified by documenting their elevation during liver regeneration, by testing their effect after administration to animals, and by analyzing the rate of liver regeneration in animals that do not express it. HGF is probably critical for liver regeneration, as it increases during liver regeneration, 6 can induce hepatocyte replication when administered to normal rodents, [7][8][9][10][11][12][13] and livers are small in mice that are homozygous deficient in HGF 14 or its receptor. 15 Although continuous infusion of 25 mg/kg of HGF induces replication of up to 20% of hepatocytes, 12 lower doses are less efficient. The effect of lower doses on hepatocyte replication can be augmented by performing an ancillary treatment such as 30% PH, 11 partial portal branch occlusion, 16 or administration of collagenase. 10 These treatments may act by inducing the acute phase response. This is a systemic reaction to injury, infection, or the administration of lipopolysaccharide (LPS) that involves the production of cytokines such as TNF-β£ and interleukin 6 (IL-6), and activation of the transcription factor CCAAT enhancer binding protein β€ (C/EBPβ€). First, liver regeneration after 70% PH is delayed in animals that do not produce LPS due to sterilization of the gastrointestinal tract. 17 Second, rats that are treated with anti-TNF-β£ antibodies, [18][19][20] or mice that are deficient in the TNF β£ receptor I, 21 IL-6, 22 or CCAAT enhancer binding protein β€ 23 show delayed liver regeneration after 70% PH.