p27^Kip1 Inactivation Provides a Proliferative Advantage to Transplanted Hepatocytes in DPPIV/Rag2 Double Knockout Mice after Repeated Host Liver Injury

Studies were conducted to develop a new DPPIV^-/-Rag2 ^-/- mouse model for hepatocyte transplantation by allogeneic and xenogeneic cells and to compare the proliferative capacity of p27 null hepatocytes versus normal hepatocytes in this system. Dipeptidyl peptidase IV (DPPIV) gene knockout mice, in which wild-type (wt) DPPIV⁺ donor hepatocytes can be readily identified by enzyme histochemistry, were bred with Rag2 null mice to prepare immunotolerant DPPIV^-/-/Rag2^-/- double knockout mice. DPPIV^-/-/Rag^-/- mice were transplanted with wt hepatocytes or p27 null mouse hepatocytes, which show enhanced cell cycle activity due to disruption of the Kip1 cyclin kinase inhibitor gene, and liver repopulation was assessed under nonproliferative versus proliferative experimental conditions. After their initial engraftment, transplanted wt hepatocytes did not proliferate in untreated livers or increase significantly in response to an acute liver regenerative stimulus. p27 null hepatocytes engrafted with the same efficiency as wt hepatocytes, but showed a noticeable, although not statistically significant, increase in proliferation in response to partial hepatectomy or acute CCl₄ administration. Repeated treatments with CCl₄ substantially increased proliferation and liver repopulation by p27 null hepatocytes but not by wt hepatocytes. These results suggest that p27 gene inactivation does not overcome proliferative restrictions imposed on hepatocytes by the normal liver, but that after repeated episodes of toxic liver injury, the augmented proliferative capacity of p27 null hepatocytes leads to significant liver repopulation compared with wt hepatocytes. These properties of p27-deficient hepatocytes could prove useful as a target for liver repopulation in patients with intermittent or a low level of chronic liver injury.