MET and epidermal growth factor signaling: The pillars of liver regeneration?
Authors: Jeffrey H. Albrecht
Journal: Hepatology (2016)

When caring for patients with acute liver injury, a bedrock assumption is that normal hepatic function will usually be restored once the injurious stimuli have resolved. The main factor underlying the restoration of liver function is the remarkable ability of differentiated hepatocytes to undergo compensatory hyperplasia ("regeneration"), which is observed after partial liver resection, transplantation, or insults causing widespread cell death, such as viruses, alcohol, drugs, toxins, ischemia, and autoimmune hepatitis. The capacity of the liver to regenerate is conserved among species and suggests an evolutionary imperative to restore appropriate functional hepatocyte mass. Indeed, extensive studies in models of liver regeneration have shown that there is considerable redundancy in the cellular pathways and signaling molecules controlling this process, such that deletion or inhibition of key mediators delays, but does not abolish, the hyperplastic response. Furthermore, in models where hepatocyte proliferation is blocked by chemical or genetic means, proliferation of hepatic progenitor cells can reconstitute hepatocyte populations. Decades of research have shed considerable light on the myriad factors regulating liver regeneration, but the "essential" signals that trigger this response have not been clearly identified.1, 2 In this issue of Hepatology, Paranjpe et al. provide new insight into the key growth-factor-mediated events that control liver regeneration.3 They focus on the two known "complete" growth factor signaling pathways in hepatocytes: the MET receptor (activated by hepatocyte growth factor [HGF]) and the epidermal growth factor (EGF) receptor (EGFR), which is activated by EGF and related ligands. Both HGF and EGF are sufficient to trigger proliferation of hepatocytes in culture and in vivo, and both are present in physiological concentrations in the liver. HGF is synthesized by several cell types in the liver, and during regeneration, it is liberated from stores within the hepatic extracellular matrix. EGF is primarily delivered to the liver by the portal circulation, arising from several extrahepatic sites including the Brunner's glands of the duodenum. Additional EGFR ligands are produced in the liver, including transforming growth factor α and amphiregulin. Previous studies have shown that inhibition of either MET or EGFR signaling can impair liver regeneration in rodents, but neither is absolutely necessary for this process—in part, because each system can compensate for the loss of the other.4-6 The article by Paranjpe et al. addressed whether combined loss of both MET and EGFR signaling is sufficient to prevent liver regeneration in the most common mouse model, two-thirds partial hepatectomy (PHx). PHx stimulates robust hepatocyte proliferation within 1-2 days, and liver mass is restored within weeks.1, 2 The investigators used an inducible Cre/Lox system to ablate MET function in the adult animal and eliminated EGFR signaling by feeding Canertinib, a potent irreversible inhibitor of this receptor (this compound also affects other EGFR family members with variable efficacy). As expected, inhibition of either MET or EGFR led to distinct alterations in downstream signaling pathways and some diminution in hepatocyte proliferation, but substantial restoration of liver mass had occurred by 14 days. In striking contrast, elimination of both MET and EGFR signaling essentially prevented hepatocyte proliferation or any restoration of liver mass by day 14 post-PHx. At this time point, the investigators terminated the study because the mice were ill and had evidence of liver failure (ascites and elevated ammonia levels). Morphological studies revealed that hepatocytes and the liver lobules were substantially smaller in size than in control animals. Interestingly, evidence of cell death was not apparent, suggesting that the liver failure was attributed to deficient hepatocyte growth, proliferation, and/or differentiation. Furthermore, hepatic progenitor cell proliferation was not observed, indicating a complete "shutdown" of the redundant systems to promote liver regeneration in this model. The gradual development of liver failure post-PHx in mice with combined MET and EGFR inhibition, in the absence of apparent cell death, raises very interesting questions about the complex relationship between liver regeneration and the essential metabolic functions of the liver. In the PHx model, metabolic insufficiency caused by acute loss of functional hepatocytes helps to trigger the regenerative response.7 Conversely, robust cell proliferation and tissue growth requires significant alterations in metabolic pathways to provide for the synthesis of new cellular components,8 yet the liver must continue to perform metabolic functions necessary to sustain homeostasis. The molecular mechanisms by which hepatocytes balance the metabolic needs of proliferation/growth and systemic homeostasis have not been well characterized. To examine the metabolic state of the liver post-PHx in the setting of MET and EGFR inhibition, the investigators performed gene array analysis at 14 days. In the combined MET-EGFR-inhibited mice, there was marked down-regulation of a large number of genes associated with differentiated hepatocyte functions, including de novo lipogenesis, mitochondrial function, drug metabolism, urea cycle, et al. These results suggest that in the setting of PHx, the absence of proper growth factor signaling leads to a critical impairment in the essential function of differentiated hepatocytes—maintenance of metabolic homeostasis. This does not likely simply reflect a diminished number of hepatocytes, given that the effect worsened over time, but rather a gradual atrophy of metabolic function that paralleled the reduction in hepatocyte cell size. This novel finding may be analogous to the major metabolic function of muscle tissue, glucose uptake, which is enhanced by muscle hypertrophy and impaired in atrophic conditions.9 The study by Paranjpe et al. does not elucidate the mechanisms by which combined MET and EGFR inhibition lead to the loss of hepatocyte metabolic capacity. The investigators point out that in normal liver, cells are constantly exposed to HGF and EGF, and suggest that these factors may support hepatocyte function even in the absence of proliferation. Thus, in addition to providing potent mitogenic signals, these growth factors may promote essential metabolic processes in the liver. Future studies to examine signal transduction pathways and transcription factors that regulate key metabolic functions in the liver may provide insight into the critical effects of MET and EGFR in hepatic metabolism. In summary, this article provides the first report of signals that are required to allow liver regeneration post-PHx. The MET and EGFR signaling pathways provide redundancy for each other, but at least one of these is required to sustain liver function. This study may provide a new paradigm to examine forms of human liver failure that are not associated with marked hepatocyte cell death. Jeffrey H. Albrecht, M.D. Minneapolis VA Health Care System and University of Minnesota Minneapolis, MN