Cyclin D1 is up-regulated in hepatocytes in vivo following cell-cycle block induced by retrorsine

A Pre-Clinical Large Animal Model of Sustained Liver Injury and Regeneration Stimulus

A feasible large animal model to evaluate regenerative medicine techniques is vital for developing clinical applications. One such appropriate model could be to use retrorsine (RS) together with partial hepatectomy (PH). Here, we have developed the first porcine model using RS and PH. RS or saline control was administered intraperitoneally to Göttingen miniature pigs twice, two weeks apart. Four weeks after the second dose, animals underwent PH. Initially, we tested different doses of RS and resection of different amounts of liver, and selected 50 mg/kg RS with 60% hepatectomy as our model for further testing. Treated animals were sacrificed 3, 10, 17 or 28 days after PH. Blood samples and resected liver were collected. Serum and liver RS content was determined by Liquid Chromatograph-tandem Mass Spectrometer. Blood analyses demonstrated liver dysfunction after PH. Liver regeneration was significantly inhibited 10 and 17 days after PH in RS-treated animals, to the extent of 20%. Histological examination indicated hepatic injury and regenerative responses after PH. Immunohistochemical staining demonstrated accumulation of Cyclin D1 and suppression of Ki-67 and PCNA in RS-treated animals. We report the development of the first large animal model of sustained liver injury with suppression of hepatic regeneration.

Polyploidization without mitosis improves in vivo liver transduction with lentiviral vectors

Lentiviral vectors are efficient gene delivery vehicles for therapeutic and research applications. In contrast to oncoretroviral vectors, they are able to infect most nonproliferating cells. In the liver, induction of cell proliferation dramatically improved hepatocyte transduction using all types of retroviral vectors. However, the precise relationship between hepatocyte division and transduction efficiency has not been determined yet. Here we compared gene transfer efficiency in the liver after in vivo injection of recombinant lentiviral or Moloney murine leukemia viral (MoMuLV) vectors in hepatectomized rats treated or not with retrorsine, an alkaloid that blocks hepatocyte division and induces megalocytosis. Partial hepatectomy alone resulted in a similar increase in hepatocyte transduction using either vector. In retrorsine-treated and partially hepatectomized rats, transduction with MoMuLV vectors dropped dramatically. In contrast, we observed that retrorsine treatment combined with partial hepatectomy increased lentiviral transduction to higher levels than hepatectomy alone. Analysis of nuclear ploidy in single cells showed that a high level of transduction was associated with polyploidization. In conclusion, endoreplication could be exploited to improve the efficiency of liver-directed lentiviral gene therapy.

Hepatocyte senescence in vivo following preconditioning for liver repopulation

In the retrorsine (RS)-based model of massive liver repopulation, preexposure to this naturally occurring alkaloid is sufficient to prime normal host parenchymal cells to be slowly replaced by transplanted normal hepatocytes. The basis for this striking effect is yet to be fully elucidated. In the present studies the possible involvement of cell senescence was investigated. Fischer 344 rats were treated according to the RS-based protocol for hepatocyte transplantation, i.e., two doses of RS, 2 weeks apart, and were killed at 4 or 8 weeks after treatment. Control groups were given saline. Expression of senescence-associated beta-galactosidase was greatly induced in hepatocytes exposed to RS. In addition, several other changes that have been related to cell senescence were observed: these included markers of persistent activation of a DNA damage response, an increased expression of mammalian target of rapamycin, and positive regulators of the cell cycle, together with the induction of p21 and p27 cyclin-dependent kinase inhibitors. Furthermore, RS treatment increased levels of interleukin-6 in the liver, consistent with the activation of a senescence-associated secretory phenotype.

CONCLUSION

These findings indicate that RS induces hepatocyte senescence in vivo. We propose that cell senescence and the associated secretory phenotype can contribute to the selective growth of transplanted hepatocytes in this system.

Activation of inactive hepatocytes through histone acetylation: a mechanism for functional compensation after massive loss of hepatocytes

The mechanisms by which hepatic function is maintained after extensive parenchymal loss are unclear. In this study, we propose a novel concept of "functional heterogeneity" of hepatocytes based on their different expression of acetylated histones, the markers of active gene transcription, to explain the powerful compensatory capability of the liver. In the healthy human liver, only a fraction of the hepatocytes were marked by acetylated histones (ac-H2AK5, ac-H2BK5, ac-H3K9, ac-H3K14, ac-H3K27, and ac-H3K9/14). With the progression of cirrhosis, the ratio of the positive cells was gradually elevated, accompanied by the gradual exhaustion of the negative cells. By examining the global transcriptome of the mouse hepatocytes, we observed that the primed genes in the positive cells were much more numerous than those in negative cells. In a 70% hepatectomized mouse, the remnant hepatocytes were extensively activated, and the liver function was well maintained even when regeneration was severely inhibited. The functional compensation was absolutely dependent on the elevated expression of acetyl-histones. Additionally, when liver regeneration was blocked, the metabolism-related genes seemed to be preferentially transcribed. In conclusion, we demonstrate that normally, part of the active hepatocytes are competent for routine physiological requirements. The inactive hepatocytes, delicately regulated by acetyl-histones, act as a functional reservoir for future activation to restore the liver function after massive parenchymal loss.

Repopulation by endogenous hepatocytes does not reconstitute liver mass in rats treated with retrorsine

The retrorsine (RS)-based model for massive liver repopulation was laid on the hypothesis that transplanted cells can proliferate in the recipient liver if the growth capacity of endogenous hepatocytes is persistently impaired. In order to directly test this hypothesis, we examined the long-term response to 2/3 partial hepatectomy (PH) in rats pretreated with RS, according to the protocol for liver repopulation. Rats were given RS or saline and 4 weeks later they underwent PH; they were killed up to 16 weeks thereafter. Liver weights, liver DNA, and protein content were significantly lower in the RS group throughout the experimental time considered (e.g., at 16 weeks post-PH relative liver weight was 1.99 +/- 0.30% in RS group vs. 3.06 +/- 0.5% in controls). Regenerative nodules were present in RS-treated livers; they occupied about 3% of the liver at 2 weeks post-PH and this value increased to nearly 50% at 8 weeks and to > 95% at 16 weeks. In conclusion, RS-treated rat liver is unable to recover its original mass for several months following PH, despite the development of regenerative nodules. This long-lasting effect is likely to contribute to the growth of transplanted hepatocytes, leading to massive liver repopulation.

STAT1 contributes to dsRNA inhibition of liver regeneration after partial hepatectomy in mice

Increasing evidence suggests that liver regeneration is suppressed in patients with chronic HCV infection; however, the underlying mechanisms remain unclear. Previously, we demonstrated that injection of the synthetic double-stranded RNA (dsRNA) poly I:C to mimic viral infection suppresses liver regeneration in the partial hepatectomy (PHx) model, whereby IFN-gamma contributes to the inhibition. In this study, we examined the role of the IFN-gamma-activated downstream signal (STAT1) and genes (IRF-1, p21(cip1), and SOCS1) in liver regeneration and hepatocyte proliferation. Results show that disruption of the STAT1 gene abolished poly I:C suppression of liver regeneration and the inhibitory effect of poly I:C on liver regeneration was diminished in IRF-1(-/-) and p21(cip1-/-)mice. Treatment with IFN-gamma in vitro inhibited cell proliferation of wild-type mouse hepatocytes, but not STAT1(-/-) hepatocytes. The inhibitory effect of IFN-gamma on cell proliferation was also diminished in IRF-1(-/-) and p21(cip1-/-) hepatocytes, but enhanced in SOCS1(-/-) hepatocytes. Hepatocyte proliferation was unaffected by treatment with poly I:C alone, but when hepatocytes were co-cultured with liver lymphocytes, proliferation was inhibited by IFN-gamma/STAT1-dependent mechanisms. Moreover, in HCV-infected livers with cirrhosis, activation of STAT1 was detected and correlated positively with liver injury (elevated serum levels of AST) but negatively with hepatocyte proliferation (hepatocyte PCNA and Ki-67 positive immunostaining). In conclusion, STAT1 is involved in dsRNA suppression of liver regeneration; not only does STAT1 activation contribute to liver injury, it may also block liver repair through inhibition of hepatocyte proliferation in HCV-infected patients, playing an important role in the pathogenesis of disease.