Genetically marked hepatocytes from dipeptidyl peptidase (DPP) IV؉ Fischer 344 rats were transplanted into the liver of DPPIV ؊ mutant Fischer 344 rats after a combined treatment with retrorsine , a pyrrolizidine alkaloid that blocks the hepatocyte cell cycle, and two-thirds partial hepatectomy. In female rats , clusters of proliferated DPPIV ؉ hepatocytes containing 20 to 50 cells/cluster , mostly derived from single transplanted cells , were evident at 2 weeks , increasing in size to hundreds of cells per cluster at 1 month and 1000 to several thousand cells per cluster at 2 months, representing 40 to 60% of total hepatocyte mass. This level of hepatocyte replacement remained constant for up to 1 year , the duration of experiments conducted. In male rats , liver replacement occurred more rapidly and was more extensive , with transplanted hepatocytes representing 10 to 15% of hepatocyte mass at 2 weeks , 40 to 50% at 1 month , 90 to 95% at 2 months , 98% at 4 months , and 99% at 9 months.

A strategy for hepatocyte transplantation was recently developed whereby massive replacement of the recipient liver is achieved after a combined treatment with retrorsine, a pyrrolizidine alkaloid, and partial hepatectomy. We now investigated whether liver repopulation could occur in this animal model in the absence of any exogenous growth stimuli (eg, partial hepatectomy) for the transplanted cells. Dipeptidyl-peptidase type IV-deficient (DPPIV ؊ ) rats were used as recipients. Rats were given two injections of retrorsine (30 mg/kg each, 2 weeks apart), followed by transplantation of 2 ؋ 10 6 hepatocytes isolated from a normal, syngeneic, DPPIV ؉ donor. At 2 weeks after transplantation, clusters of DPPIV ؉ hepatocytes occupied 3.3 ؎ 0.9% of host liver, increasing to 38.2 ؎ 6.3% at 2 months, and to 65.9 ؎ 8.8% at 5 months. By 1 year, >95% of the original hepatocytes were replaced by donor-derived cells. Serum parameters related both to hepatocyte function and integrity (including glucose, bilirubin, total proteins, cholinesterase, alanine aminotransferase, and alkaline phosphatase) were in the normal range in retrorsine-treated and repopulated animals. These results provide further insights toward developing strategies for effective liver repopulation by transplanted hepatocytes with reduced toxicity for the host and potential clinical applicability. Transplantation of isolated hepatocytes is increasingly being considered as a possible alternative to whole organ replacement, which is currently the only available therapy to treat both inherited and acquired end-stage liver disease.

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We recently have shown that selective growth of transplanted normal hepatocytes can be achieved in a setting of cell cycle block of endogenous parenchymal cells. Thus, massive proliferation of donor-derived normal hepatocytes was observed in the liver of rats previously given retrorsine (RS), a naturally occurring alkaloid that blocks proliferation of resident liver cells. In the present study, the fate of nodular hepatocytes transplanted into RS-treated or normal syngeneic recipients was followed. The dipeptidyl peptidase type IV-deficient (DPPIV ؊ ) rat model for hepatocyte transplantation was used to distinguish donor-derived cells from recipient cells. Hepatocyte nodules were chemically induced in Fischer 344, DPPIV ؉ rats; livers were then perfused and larger (>5 mm) nodules were separated from surrounding tissue. Cells isolated from either tissue were then injected into normal or RS-treated DPPIV ؊ recipients. One month after transplantation, grossly visible nodules (2-3 mm) were seen in RS-treated recipients transplanted with nodular cells. They grew rapidly, occupying 80 -90% of the host liver at 2 months, and progressed to hepatocellular carcinoma within 4 months. By contrast, no liver nodules developed within 6 months when nodular hepatocytes were injected into the liver of recipients not exposed to RS, although small clusters of donor-derived cells were present in these animals. Taken together, these results directly point to a fundamental role played by the host environment in modulating the growth and the progression rate of altered cells during carcinogenesis. In particular, they indicate that conditions associated with growth constraint of the host tissue can drive tumor progression in vivo.