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 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

Retroviral vectors can result in therapeutic andstable levels of expression of proteins from the liver. However, m ost retroviral vectors transduce only dividing cells, and hepatocytes are norm ally quiescent. The goal of this study was to determ ine if an adenoviral vector could transiently express hepatocyte growth factor (HG F) in order to induce hepatocyte replication and facilitate retroviral vector transduction of the liver. Intramuscular injection of an adenoviral vector that expressed human HGF from the cytomegalovirus promoter (Ad.C M V.HGF) resulted in m oderate levels of H GF in blood and liver, and replication of 3 to 12% of hepatocytes. No cytopathic effect was observed in the liver, and a control adenoviral vector indu ced no or lower levels of replication. W hen a retroviral vector expressing b -galactosidase cD NA was injected into a peripheral vein during the peak period of hepatocyte replication induced by intram uscularly adm inistered Ad.C M V.HG F, 8% of hepatocytes were transduced. W e conclude that intram uscular injection of Ad.C M V.HG F is a safe and effective way to induce transient systemic expression of HG F and hepatocyte replication, and to facilitate transduction of hepatocytes with a retroviral vector. 911 OVERVIEW SU M M A RY Retroviral vectors can resu lt in stable and therapeutic levels of expression of proteins from the liver. H owever, m ost retroviral vectors transduce the liver in vivo only when the norm ally quiescen t hepatocytes are induced to replicate. Although 70% partial hepatectom y, 70% portal branch occlusion, or adm inistration of a liver toxin have been used to induce hepatocyte rep lication for gene therapy, their potential m orbidity m akes them less attractive for use in hum ans. G rowth factors that induce hepatocyte rep lication are a prom ising approach for facilitating retro viral vector transduction. H owever, they are expensive, and optim al results req uire continuous infusion or m ultiple injections. W e dem onstrate here that intram uscular injection of Ad.C M V.H G F resu lted in the transient appearance of H G F in the seru m and liver, hepatocyte rep lication, and retro viral vector transduction without apparent toxicity. This approach will be a useful tool for perform ing gene therapy experim ents in large num bers of animals with a specific genetic disorder.

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