Elucidating the metabolic regulation of liver regeneration. Am J Pathol. 2014;184:309-321. [PMID: 24139945 DOI: 10.1016/j.ajpath.2013.04.034]

Regulation of hepatocyte identity and quiescence

The liver is a highly differentiated organ with a central role in metabolism, detoxification and systemic homeostasis. To perform its multiple tasks, liver parenchymal cells, the hepatocytes, express a large complement of enabling genes defining their complex phenotype. This phenotype is progressively acquired during fetal development and needs to be maintained in adulthood to guarantee the individual's survival. Upon injury or loss of functional mass, the liver displays an extraordinary regenerative response, mainly based on the proliferation of hepatocytes which otherwise are long-lived quiescent cells. Increasing observations suggest that loss of hepatocellular differentiation and quiescence underlie liver malfunction in chronic liver disease and pave the way for hepatocellular carcinoma development. Here, we briefly review the essential mechanisms leading to the acquisition of liver maturity. We also identify the key molecular factors involved in the preservation of hepatocellular homeostasis and finally discuss potential strategies to preserve liver identity and function.

Fibroblast Growth Factor Signaling Controls Liver Size in Mice With Humanized Livers

BACKGROUND & AIMS

The ratio of liver size to body weight (hepatostat) is tightly controlled, but little is known about how the physiologic functions of the liver help determine its size. Livers of mice repopulated with human hepatocytes (humanized livers) grow to larger than normal; the human hepatocytes do not recognize the fibroblast growth factor (FGF)-15 produced by mouse intestine. This results in up-regulation of bile acid synthesis in the human hepatocytes and enlargement of the bile acid pool. We investigated whether abnormal bile acid signaling affects the hepatostat in mice.

METHODS

We crossed Fah(-/-), Rag2(-/-), Il2r(-/-) mice with nonobese diabetic mice to create FRGN mice, whose livers can be fully repopulated with human hepatocytes. We inserted the gene for human FGF19 (ortholog to mouse Fgf15), including regulatory sequences, into the FRGN mice to create FRGN19(+) mice. Livers of FRGN19(+) mice and their FRGN littermates were fully repopulated with human hepatocytes. Liver tissues were collected and bile acid pool sizes and RNA sequences were analyzed and compared with those of mice without humanized livers (controls).

RESULTS

Livers were larger in FRGN mice with humanized livers (13% of body weight), compared with control FRGN mice; they also had much larger bile acid pools and aberrant bile acid signaling. Livers from FRGN19(+) normalized to 7.8% of body weight, and their bile acid pool and signaling more closely resembled that of control FRGN19(+) mice. RNA sequence analysis showed activation of the Hippo pathway, and immunohistochemical and transcription analyses revealed increased hepatocyte proliferation, but not apoptosis, in the enlarged humanized livers of FRGN mice. Cell sorting experiments showed that although healthy human liver does not produce FGF19, nonparenchymal cells from cholestatic livers produce FGF19.

CONCLUSIONS

In mice with humanized livers, expression of an FGF19 transgene corrects bile acid signaling defects, resulting in normalization of bile acid synthesis, the bile acid pool, and liver size. These findings indicate that liver size is, in part, regulated by the size of the bile acid pool that the liver must circulate.

Identification of an epigenetic signature of early mouse liver regeneration that is disrupted by Zn-HDAC inhibition

Liver regeneration has been well studied with hope of discovering strategies to improve liver disease outcomes. Nevertheless, the signals that initiate such regeneration remain incompletely defined, and translation of mechanism-based pro-regenerative interventions into new treatments for hepatic diseases has not yet been achieved. We previously reported the isoform-specific regulation and essential function of zinc-dependent histone deacetylases (Zn-HDACs) during mouse liver regeneration. Those data suggest that epigenetically regulated anti-proliferative genes are deacetylated and transcriptionally suppressed by Zn-HDAC activity or that pro-regenerative factors are acetylated and induced by such activity in response to partial hepatectomy (PH). To investigate these possibilities, we conducted genome-wide interrogation of the liver histone acetylome during early PH-induced liver regeneration in mice using acetyL-histone chromatin immunoprecipitation and next generation DNA sequencing. We also compared the findings of that study to those seen during the impaired regenerative response that occurs with Zn-HDAC inhibition. The results reveal an epigenetic signature of early liver regeneration that includes both hyperacetylation of pro-regenerative factors and deacetylation of anti-proliferative and pro-apoptotic genes. Our data also show that administration of an anti-regenerative regimen of the Zn-HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) not only disrupts gene-specific pro-regenerative changes in liver histone deacetylation but also reverses PH-induced effects on histone hyperacetylation. Taken together, these studies offer new insight into and suggest novel hypotheses about the epigenetic mechanisms that regulate liver regeneration.

Cell-autonomous decrease in proliferative competitiveness of the aged hepatocyte

BACKGROUND & AIMS

The regenerative potential of the liver declines with age, this might be dependent on a decrease in the intensity of the stimulus and/or an increased refractoriness of the target. In the present study, we compared the in vivo growth capacity of young and old hepatocytes transplanted into the same host.

METHODS

We utilized the retrorsine (RS)-based model for liver repopulation, which provides a specific and effective stimulus for transplanted hepatocytes. Rats of the dipeptidyl-peptidase type IV (DPP-IV)-deficient strain were given RS and were injected with a mix of hepatocytes isolated from either a 2-month old or an 18-month old donor. To follow the fate of transplanted cells, they were each identified through a specific tag: young hepatocytes expressed the green fluorescent protein (GFP(+)), while those from old donors were DPP-IV-positive.

RESULTS

At 1 month post-transplantation, DPP-IV-positive clusters (derived from old donor) were consistently smaller than those GFP(+) (young donor); the cross sectional area of clusters was decreased by 50%, while the mean volume was reduced to 1/3. Furthermore, when 2/3 partial hepatectomy (PH) was performed, the S-phase response of old hepatocyte-derived clusters was only 30-40% compared to that observed in cluster originating from young hepatocytes. No markers of cell senescence were expressed in clusters of transplanted hepatocytes.

CONCLUSIONS

This is the first direct evidence in vivo that hepatocytes in the aged liver express a cell-autonomous decline in their replicative capacity and in their regenerative response to PH compared to those from a young animal.

Delayed ethanol elimination and enhanced susceptibility to ethanol-induced hepatosteatosis after liver resection

AIM: To investigate ethanol-induced hepatic steatosis after liver resection and the mechanisms behind it.

METHODS: First, the preliminary examination was performed on 6 sham-operated (Sham) and 30 partial hepatectomy (PH) male Wistar rats (8-wk-old) to evaluate the recovery of the liver weight and liver function after liver resection. PH rats were sacrificed at the indicated time points (4, 8, and 12 h; 1, 3, and 7 d) after PH. Second, the time point for the beginning of the chronic ethanol exposure (1 wk after sham- or PH-operation) was determined based on the results of the preliminary examination. Finally, pair-feeding was performed with a controlled diet or with a 5-g/dL ethanol liquid diet for 28 d in another 35 age-matched male Wistar rats with a one-week recovery after undergoing a sham- (n = 15) or PH-operation (n = 20) to evaluate the ethanol-induced liver injury after liver resection. Hepatic steatosis, liver function, fatty acid synthase (Fas) gene expression level, the expression of lipid metabolism-associated enzyme regulator genes [sterol regulatory element binding protein (Srebp)-1 and peroxisome proliferator-activated receptor (Ppar)-α], the mediators that alter lipid metabolism [plasminogen activator (Pai)-1 gene expression level and tumor necrosis factor (Tnf)-α production], and hepatic class-1 alcohol dehydrogenase (Adh1)-associated ethanol elimination were investigated in the 4 groups based on histological, immunohistochemical, biochemical, Western blotting, reverse transcriptase chain reaction, and blood ethanol concentration analyses. The relevant gene expression levels, liver weight, and liver function were assessed before and 1 wk after surgery to determine the subject’s recovery from the liver resection using the rats that had been subjected to the preliminary examination.

RESULTS: In the PH rats, ethanol induced marked hepatic steatosis with impaired liver functioning, as evidenced by the accumulation of fatty droplets within the hepatocytes, the higher increases in their hepatic triglyceride and blood alanine aminotransferase and blood aspartate aminotransferase levels after the 28-d pair-feeding period. The Sham-ethanol rats, not the PH-ethanol rats, demonstrated the up-regulation of Srebp-1 and the down-regulation of Ppar-α mRNA expression levels after the 28-d pair-feeding period. The 28-d ethanol administration induced the up-regulation of Pai-1 gene expression level and an overproduction of TNF-α in the Sham and the PH rats; however, the effect was more significant in the PH rats. The PH-ethanol rats (n = 4) showed higher residual blood ethanol concentrations than did the Sham-ethanol rats (n = 6) after a 5-h fast (0.66 ± 0.4 mg/mL vs 0.2 ± 0.1 mg/mL, P < 0.05); these effects manifested without up-regulation of Adh1 gene expression, which was present in the Sham-ethanol group after the 28-d pair-feeding period. One week after the liver resection, the liver weight, function, the gene expression levels of Fas, Srebp-1, Ppar-α, Pai-1 and Tnf-α recovered; however, the Adh1 gene expression did not recover in rats.

CONCLUSION: Desensitization to post-hepatectomy ethanol treatment and slow recovery from PH in Adh1 gene expression enhanced the susceptibility to ethanol-induced hepatic steatosis after PH in rats.

Loss of α-catenin elicits a cholestatic response and impairs liver regeneration

The liver is unique in its capacity to regenerate after injury, during which hepatocytes actively divide and establish cell-cell contacts through cell adhesion complexes. Here, we demonstrate that the loss of α-catenin, a well-established adhesion component, dramatically disrupts liver regeneration. Using a partial hepatectomy model, we show that regenerated livers from α-catenin knockdown mice are grossly larger than control regenerated livers, with an increase in cell size and proliferation. This increased proliferation correlated with increased YAP activation, implicating α-catenin in the Hippo/YAP pathway. Additionally, α-catenin knockdown mice exhibited a phenotype reminiscent of clinical cholestasis, with drastically altered bile canaliculi, elevated levels of bile components and signs of jaundice and inflammation. The disrupted regenerative capacity is a result of actin cytoskeletal disorganisation, leading to a loss of apical microvilli, dilated lumens in the bile canaliculi, and leaky tight junctions. This study illuminates a novel, essential role for α-catenin in liver regeneration.

Hepatic regenerative medicine: exploiting the liver's will to live

This Guest Editorial introduces this month's special Liver Pathobiology Theme Issue, a series of reviews that encompass the discipline of hepatic regenerative medicine.

Bile acid flux is necessary for normal liver regeneration

BACKGROUND & AIMS

Many signals governing liver regeneration (LR) following 2/3 partial hepatectomy (PH) are recognized, but the primary signal(s) remains unknown. The aim of the study was to confirm that the remnant liver after PH lacks capacity to secrete the BA pool returning via the enterohepatic ciruculation (EHC), which may in turn stimulate LR.

METHODS

After standard PH, BA flux was documented and BA signaling (Fgf15) and synthesis (Cyp7a) determined by qPCR. Rat biliary fistula (BF) and Asbt knockout mouse models interrupted the EHC prior to PH, and standard assays for LR employed along with complete RNA sequencing. CCl4 intoxication after BF tested the hypothesis in an alternate injury model.

RESULTS

BA rise in systemic blood immediately following PH, confirming that the remnant liver cannot handle the BA returning via portal circulation. When the BA pool is drained prior to PH in the rat BF model, LR is markedly attenuated, a phenomenon reversed with duodenal BA replacement. Hepatocyte proliferation is similarly attenuated after PH in the Asbt knockout mouse as well as after CCl44 intoxication in rats with BF. Complete RNA sequencing in the rat PH model shows that early c-jun and AP-1 gene expression pathways are down regulated in the absence of BA, coincident with attenuated LR.

CONCLUSIONS

Absent BA return to the liver after PH or CCl4 injury markedly attenuates LR, though hepatocyte proliferation still occurs, inferring that BA flux and signaling are not the sole signals governing LR. Transcriptional networks involving c-jun and AP-1 are involved in the BA-specific effects on hepatocyte proliferation.