Cytokine-dependent activation of small hepatocyte-like progenitor cells in retrorsine-induced rat liver injury

Complete liver regeneration after partial hepatectomy (PH) in rats exposed to the pyrrolizidine alkaloid retrorsine is accomplished through the activation, expansion, and differentiation of a population of small hepatocyte-like progenitor cells (SHPCs). The mechanism(s) governing activation of SHPCs after PH in retrorsine-injured rats has not been investigated. We examined the possibility that SHPCs require cytokine priming prior to becoming growth factor responsive in this model of liver injury and regeneration. Male Fischer 344 rats were treated with retrorsine (30 mg/kg ip) at 6 and 8 weeks of age. Retrorsine-exposed and age-matched control rats were randomized into dexamethasone-treated and no DEX groups. DEX-treated animals were either given a single dose of DEX (2 mg/kg ip) at the time of PH or multiple DEX treatments (2 mg/kg ip each) at 24 and 1 h before PH and 1, 2, and 3 days post-PH. A subset of rats received 10 mu g of recombinant IL6 protein, administered intravenously 30 min after PH. Liver tissues were harvested at 7, 14, 21, and 30 days post-PH. Treatment of retrorsine-exposed rats with the cytokine inhibitor dexamethasone (DEX) effectively blocked the emergence of SHPCs resulting in an inhibition of liver regeneration and producing significant short-term mortality. The livers of DEX-treated retrorsine-exposed rats displayed decreased numbers and smaller SHPC clusters compared to retrorsine-exposed rats in the absence of DEX treatment. Administration of recombinant IL6 to DEX-treated retrorsine-exposed rats restored the emergence of SHPCs and SHPC-mediated regenerative response. The livers of DEX-treated retrorsine-exposed rats that received IL6 displayed numbers of expanding SHPC clusters comparable to that of retrorsine-exposed rats in the absence of DEX treatment. These results combine to suggest that SHPC activation after PH in retrorsine-exposed rats is cytokine dependent and may specifically require IL6. (C) 2009 Elsevier Inc. All rights reserved.