Trimming the Fat From Liver Regeneration * #

The liver is remarkable in its ability to regenerate itself after acute injury.1-3 This highly regulated response is the mechanistic basis for the precision with which the liver-to-body mass ratio is controlled and is also critically important for a host's ability to recover from functional deficits induced by acute hepatic injuries. However, clinical experience suggests that derangement of the hepatic regenerative response in the setting of chronic liver diseases may lead to the complications of diseases such as cirrhosis and hepatocellular carcinoma. Thus, elucidating the mechanisms that regulate the hepatic regenerative response has motivated generations of investigators with hopes of characterizing fundamental mechanisms of growth regulation, identifying candidate targets for novel clinical therapeutic strategies, and even discovering paradigms for analyses of tissue regeneration in other organ systems. NFκB, nuclear factor κB; MCD, methionine-choline deficient. The rodent partial hepatectomy model, characterized by Higgins and Anderson over 70 years ago,4 continues to be the most extensively used tool for studying the hepatic regenerative response. Experimental analyses using this system show that liver regeneration is precisely regulated in its initiation and duration, and that after hepatic mass, function, and structure are restored, the regenerative response is terminated. These analyses also show that, under most circumstances, regeneration does not depend on a stem cell population, but, instead, all of the normally quiescent mature cellular populations of the liver can re-enter the cell cycle and proliferate. The nature of the signal that initiates hepatic regeneration was suggested by the elegant studies of Bucher and others,5, 6 who showed that, after partial hepatectomy, a factor, whose source was unknown, appeared in the rodent's blood that could promote hepatocellular proliferation in a second non-hepatectomized animal. Over the last several decades, the partial hepatectomy model has been used to further characterize the mechanisms that regulate liver regeneration by analyses of the regenerative response in pharmacologically and genetically manipulated mice and characterization of signals that are activated during and necessary for normal liver regeneration. For example, these studies have shown that cytokine-7-9 and growth factor–dependent10 activation of specific transcription factors11-13 direct an immediate-early gene expression program,14, 15 which promotes hepatocellular re-entry into and progression through the cell cycle. Proliferation continues until hepatocellular mass is restored, after which growth factor–dependent termination of hepatocellular proliferation occurs.16 Despite the increased understanding gained from these studies, the precise nature of the most proximal molecular events that initiate regeneration and the distal signals that terminate this process remain elusive. The report by DeAngelis, Lambris, and colleagues in this issue of HEPATOLOGY17 offers new insight into the mechanisms that regulate hepatic regeneration and also has important implications with respect to the pathogenesis, natural history, and management of fatty liver disease and other human liver diseases. DeAngelis et al. evaluated the effects of chronic exposure to a high-fat diet on liver regeneration in mice subjected to partial hepatectomy. Their results showed that, in addition to obesity and steatosis, animals exposed to a high-fat diet (40.8% of calories from fat) exhibited significantly impaired liver regeneration compared with those fed a standard diet (17.3% of calories from fat). This disruption of regeneration was associated with derangements in many of the specific signaling events known to be regulated during liver regeneration, including increased expression of inhibitor of nuclear factor κB alpha, decreased activation of nuclear factor κB (NFκB), and inhibition of cyclin D1 and Bcl-xL expression, and was also associated with increased hepatocellular apoptotic cell death. The investigators concluded that exposure to a high-fat diet and the subsequent development of steatosis impairs liver regeneration and predisposes to increased hepatic injury. These observations are homologous to the results of previous analyses showing that liver regeneration is markedly impaired in genetically based models of steatosis resulting from leptin deficiency (ob/ob mice18-20) or leptin-resistance (db/db mice21). In contrast, liver regeneration has been reported to be normal or only mildly delayed in other models of diet-induced hepatic steatosis, including methionine-choline–deficient (MCD)22, 23 and orotic-acid–supplemented24 diets. DeAngelis et al.17 speculated on explanations for these differences, suggesting that choline deficiency in the MCD diet might augment the regenerative response based on the known inhibitory effect of choline supplementation and stimulatory effect of choline deficiency on liver regeneration25, 26 and that the relatively mild degree of steatosis in the orotic acid-fed model was not as great, and therefore perhaps not as potent, as that seen in other models. The clinical implications of these data are substantial. The greatest risk factors for the development of hepatic steatosis are obesity and type 2 diabetes. Indeed, obesity has become an epidemic in modern Western society, affecting approximately 30% of adults and approximately 15% of teenage children in the United States.27 Until recently, simple steatosis has generally been considered a benign condition without significant clinical consequences and relatively unlikely to progress to steatohepatitis, fibrosis, or cirrhosis. However, the impaired regenerative response seen in association with hepatic steatosis that results from exposure to a high-fat diet, as reported here,17 or from genetically disrupted leptin signaling, as published previously,18-20 suggests that in at least some cases hepatic steatosis places obese individuals at increased risk for complications of liver disease secondary to an inhibitory effect on the hepatic regenerative response. Indeed, accumulating clinical experience indirectly supports this conclusion, demonstrating that obese patients with fatty liver disease have poorer outcomes with partial hepatic resection, liver transplantation, chronic hepatitis C infection, and drug- and alcohol-induced liver injury than do their lean counterparts28 and that fatty liver grafts have an increased risk of primary graft nonfunction compared with non-fatty grafts.29 Together, these observations predict that as the prevalence of obesity and fatty liver disease continues to increase, the morbidity and mortality associated with a host of other liver diseases also may increase. Perhaps even more profound than the clinical implications of DeAngelis' data is the potential insight they offer with respect to elucidating the mechanisms that regulate liver regeneration. That similar mechanisms might account for the impaired regenerative response seen in the high-fat diet and ob/ob models of hepatic steatosis, as suggested above, is supported by the overlap in derangements in signaling during liver regeneration in each of these models. For example, in both models, NFκB activation during liver regeneration is impaired.17, 18 However, differences between the 2 models are also apparent. For example, in contrast to the results reported by DeAngelis et al., hepatocellular apoptotic cell death is not increased but STAT3 activation is augmented during liver regeneration in ob/ob mice.18 Furthermore, in contrast to the results of Farrell and colleagues,19 who showed that leptin supplementation rescues impaired hepatic regeneration in ob/ob mice, DeAngelis et al. noted that the presence of increased circulating leptin levels in their dietary model of steatosis suggests that the impaired regenerative response in these animals is more likely to be the direct result of hepatic steatosis and not secondary to disrupted leptin signaling. However, other possibilities bear some consideration. For example, elevated circulating leptin levels in DeAngelis et al.'s model may result from the development of leptin resistance, as has been reported in other high-fat diet mouse models30; thus, impaired regeneration in DeAngelis et al.'s model may indeed be linked to deranged leptin signaling. Alternatively, the observation that leptin treatment impairs liver regeneration in wild-type mice31 raises the possibility that elevated circulating leptin levels directly or indirectly impair liver regeneration in still-undefined ways. Taken together, these studies show that hepatocellular fat accumulation alone is not sufficient to inhibit liver regeneration. Indeed, the earliest studies using the partial hepatectomy model recognized that transient hepatic steatosis develops during the early stages of liver regeneration,32 and more recent analyses demonstrate that elements of this steatotic response are precisely regulated during and likely to be essential for normal liver regeneration.31 Recent clinical experiences also add support to this idea, showing that some steatotic hepatic donor grafts can function comparably to non-fatty grafts after liver transplantation,33 suggesting that additional genetic or environmental factors impact the effects of steatosis on hepatic regeneration. Perhaps the impaired hepatic regenerative response observed in some, but not all, forms of hepatic steatosis offer us a novel clue about the specific nature of Bucher's liver regeneration-promoting signal.