Akt‐FoxO1 axis controls liver regeneration
Authors: Leila Valanejad, Nikolai Timchenko
Journal: Hepatology (2016)

Potential conflict of interest: Nothing to report. See Article on Page 1660 Liver cancer is the fifth most common cancer and the third most common cause of cancer‐related death in the world.1 Despite significant progress in the investigation of mechanisms of liver cancer, therapeutic approaches for treatment of cancer at advanced stages are not developed and surgical resection is the main method to eliminate liver cancer. Because resections are mainly performed on unhealthy livers, liver regeneration is compromised by additional complications. Elucidation of the precise mechanisms of liver regeneration is highly significant for the development of approaches to support liver regeneration. The liver is a unique tissue which is able to regenerate after massive resection with the ability to restore its original mass and functions. The liver consists of several cell types, and regeneration after surgery includes communications between different cells and alterations of multiple pathways of signaling. During the last decade, significant progress has been made in the study of liver regeneration after surgery (partial hepatectomy [PH]). Usage of genetically modified animal models revealed that the process of liver regeneration is controlled by growth factors, hormones, and many transcription factors.2 It has also been shown that liver proliferation after surgery is significantly reduced in older organisms4 and that this reduction is associated with epigenetic silencing of cell cycle genes.5 The process of liver regeneration is also controlled by metabolic pathways such as glucose and fat metabolism.6 Despite the significant progress in investigating mechanisms of liver regeneration, the triggering events of liver proliferation are not well understood. In this issue of hepatology, Pauta and colleagues present evidence for the critical role of Akt‐FoxO1 signaling in liver regeneration after PH.7 The serine‐threonine kinase Akt/protein kinase B family consists of three members, Akt1, Akt2, and Akt3, which recognize the same phosphorylation motif and phosphorylate a number of substrates.8 While expression of Akt3 is mainly observed in the brain, Akt1 and Akt2 are expressed at high levels in the liver and are essential regulators of cell proliferation and glucose/fat metabolism. The authors bred Akt1 and Akt2 knockout (KO) mice, generated a double Akt1/Akt2 liver‐specific KO mouse line, and investigated liver regeneration after PH in wild‐type (WT), Akt1 KO, Akt2 KO, and Akt1/Akt2 double knockout (DLKO) mice. These studies demonstrated that the deficiency of one member of the family does not affect liver regeneration; however the liver‐specific deletion of both Akt1 and Akt2 leads to impaired liver regeneration and increased mortality after PH. Among multiple targets of Akt, transcription factor FoxO1 is one of the main proteins which regulate glucose metabolism. It has been shown that the liver‐specific deletion of FoxO1 leads to impaired glucose metabolism.9 To test if FoxO1 is involved in the inhibition of liver regeneration in Akt DLKO mice, Pauta et al. generated triple knockout mice (TLKO) where Akt1, Akt2, and FoxO1 are specifically deleted in the liver. Examination of liver regeneration of DLKO and TLKO mice revealed that the deletion of FoxO1 improves liver regeneration and increases the survival rate after PH. Searching for the mechanisms by which deletion of FoxO1 corrects liver proliferation and increases survival rate, the authors examined glucose homeostasis after PH in WT, DLKO, and TLKO mice. In WT mice, PH causes early hypoglycemia and subsequent normalization of glucose levels at 72 hours after surgery. DLKO mice also show early hypoglycemia; however, they develop severe hyperglycemia at 72 hours after PH, which presumably contributes to the high mortality of DLKO mice. Deletion of FoxO1 corrects glucose levels at 72 hours after PH. These observations clearly demonstrate that the proper coordination of Akt‐FoxO1 signaling is required for the regulation of glucose homeostasis after PH and that disruption of this signaling by deletion of Ak1/Akt2 results in a high rate of mortality. PH causes morphological alterations in the liver which are characterized by increases of the cellular area of hepatocytes. To further determine mechanisms by which FoxO1 corrects liver regeneration after PH, Pauta et al. performed histological examination of the livers of WT, DLKO, and TLKO mice before and after PH. The results indicate that Akt1/Akt2‐deficient mice have a significant reduction of cell size and that FoxO1 deficiency in TLKO mice partially corrects the poor hypertrophy before and after PH. Activation of the cell cycle proteins after PH is one of the hallmarks of successful liver regeneration.10 Pauta et al. found that liver‐specific deletion of Akt1/Akt2 inhibits activation of two critical cell cycle proteins, CCND1 and PCNA, and that the deficiency of FoxO1 in TLKO mice corrects expression of these cell cycle proteins. In agreement with these observations, examination of liver proliferation by ki67 staining showed that liver proliferation is dramatically inhibited in Akt1/Akt2 DLKO mice but significantly corrected by deletion of FoxO1 in TLKO mice. In addition to morphological changes, liver surgery causes lipid accumulation and alterations in the hepatic lipidome.11 To determine if hepatic deficiency of Ak1/Akt2 impairs liver regeneration through alterations in lipid metabolism, Pauta et al. examined these parameters in livers of WT, DLKO, and TLKO mice. Oil red O staining showed that fat droplets are accumulated at days 2‐4 after PH in livers of WT mice; however, no fat droplets were detected in livers of Akt1/Akt2 DLKO mice during 6 days after PH. Interestingly, deletion of FoxO1 in these mice completely corrected the profile and degree of fat droplet accumulation. In agreement with these observations, studies of the hepatic lipidome showed significant reduction of cholesterol ester, phosphatidylcholine, sphingomyelin, triglycerides, and fatty free acids after PH in livers of LKO mice. Deletion of FoxO1 in these mice normalized lipidome profiling. Although Akt signaling has been implicated in the regulation of liver regeneration, the precise mechanisms of this regulation have not been shown. The main discoveries of Pauta et al. identified FoxO1 as a main mediator of the Akt‐dependent regulation of liver proliferation after PH. Moreover, this work showed that the Akt‐FoxO1 axis regulates hepatocellular hypertrophy, hepatocyte proliferation, glucose homeostasis, and hepatic lipidome after PH (Fig. 1). The authors clearly demonstrated that genetic ablation of Akt activity causes impaired liver regeneration, which is associated with inhibition of all these signals. This work advances our understanding of the mechanisms which regulate liver regeneration after surgery.Figure 1: The main observations in Pauta et al. show that the Akt1/Akt2‐FoxO1 axis regulates liver regeneration after PH by controlling at least four signaling pathways.Although this work describes very important findings showing that deletion of both Ak1 and Akt2 affects liver regeneration, there are some questions that have not been addressed. Particularly, the role of individual Akt1 and Akt2 genes in liver regeneration is not clear. While deletion of a single gene, Akt1 or Akt2, did not affect the restoration of the liver mass, it would be interesting to analyze if Akt1 KO and Akt2 KO mice have alterations in primary events of liver regeneration after PH. Investigations of pathways which regulate G1 to S transition after PH in Akt1 KO and Ak2 KO mice would help in understanding this issue. The discovery of a key role of Akt‐FoxO1 in liver regeneration after surgery generated a background for future investigations of the Akt‐FoxO1 pathway in patients with chronic liver diseases. If liver surgery is required, examination of mutations in the Akt‐FoxO1 pathway in these patients might provide information for how the liver will regenerate after surgery and if patients might have unpredictable complications due to alterations within this pathway.