The Role of Carbamoyl Phosphate Synthetase 1 as a Prognostic Biomarker in Patients With Acetaminophen-induced Acute Liver Failure.

BACKGROUND & AIMS: Carbamoyl phosphate synthetase 1 (CPS1) is a highly abundant mitochondrial urea cycle enzyme that is expressed primarily in hepatocytes. CPS1 is constitutively and physiologically secreted into bile but is released into the bloodstream upon acute liver injury (ALI). Given its abundance and known short half-life, we tested the hypothesis that it may serve as a prognostic serum biomarker in the setting of acute liver failure (ALF). METHODS: CPS1 levels were determined using enzyme-linked immunosorbent assay and immunoblotting of sera collected by the ALF Study Group (ALFSG) from patients with ALI and ALF (103 patients with acetaminophen and 167 non-acetaminophen ALF etiologies). A total of 764 serum samples were examined. The inclusion of CPS1 was compared with the original ALFSG Prognostic Index by area under the receiver operating characteristic curve analysis. RESULTS: CPS1 values for acetaminophen-related patients were significantly higher than for non-acetaminophen patients (P < .0001). Acetaminophen-related patients who received a liver transplant or died within 21 days of hospitalization exhibited higher CPS1 levels than patients who spontaneously survived (P = .01). Logistic regression and area under the receiver operating characteristic analysis of CPS1 enzyme-linked immunosorbent assay values improved the accuracy of the ALFSG Prognostic Index, which performed better than the Model for End-Stage Liver Disease, in predicting 21-day transplant-free survival for acetaminophen- but not non-acetaminophen-related ALF. An increase of CPS1 but not alanine transaminase or aspartate transaminase, when comparing day 3 with day 1 levels was found in a higher percentage of acetaminophen transplanted/dead patients (P < .05). CONCLUSION: Serum CPS1 determination provides a new potential prognostic biomarker to assess patients with acetaminophen-induced ALF.

Mouse genetic background contributes to hepatocyte susceptibility to Fas-mediated apoptosis.

Liver disease progression is modulated by genetic modifiers in mouse strains and across human races and ethnicities. We hypothesized that hepatocyte culture duration and genetic background regulate hepatocyte susceptibility to apoptosis. Hepatocytes were isolated from FVB/N, C57BL/6, and C3H/He mice and cultured or treated with Fas ligand or acetaminophen after different culture times. Protein and mRNA expressions of Fas receptor, caspases-3/7/8, and Bak/Bax/Bid proteins were determined. FVB/N hepatocytes manifested rapid decreases of caspases-3/7 but not caspase-8 as culture time increased, which paralleled decreased susceptibility to apoptosis. Some changes were also found in Fas-receptor and Bak, Bax, and Bid proteins; caspase mRNA decreases were also noted. Caspase protein degradation was partially reversed by lysosomal protease but not proteasome or autophagy inhibitors. C57BL/6 and FVB/N hepatocytes behaved similarly in their limited susceptibility to apoptosis, whereas C3H/He hepatocytes show limited alterations in caspases, with consequent increased susceptibility to apoptosis. Similarly, C3H/He mice were more susceptible than C57BL/6 and FVB/N mice to Fas-mediated liver injury. Therefore there are significant mouse strain-dependent differences in susceptibility to apoptosis and selective loss of caspases upon short-term hepatocyte culture, with consequent decrease in susceptibility to apoptosis. These differences likely reflect genetic modifiers that provide resistance or predisposition to hepatocyte death.

Structural Requirements of Histone Deacetylase Inhibitors: SAHA Analogs Modified on the Hydroxamic Acid.

Histone deacetylase (HDAC) proteins have emerged as targets for anti-cancer therapeutics, with several inhibitors used in the clinic, including suberoylanilide hydroxamic acid (SAHA, vorinostat). Because SAHA and many other inhibitors target all or most of the 11 human HDAC proteins, the creation of selective inhibitors has been studied intensely. Recently, inhibitors selective for HDAC1 and HDAC2 were reported where selectivity was attributed to interactions between substituents on the metal binding moiety of the inhibitor and residues in the 14-Å internal cavity of the HDAC enzyme structure. Based on this earlier work, we synthesized and tested SAHA analogs with substituents on the hydroxamic acid metal binding moiety. The N-substituted SAHA analogs displayed reduced potency and solubility, but greater selectivity, compared to SAHA. Docking studies suggested that the N-substituent accesses the 14-Å internal cavity to impart preferential inhibition of HDAC1. These studies with N-substituted SAHA analogs are consistent with the strategy exploiting the 14-Å internal cavity of HDAC proteins to create HDAC1/2 selective inhibitors.

Carbamoyl phosphate synthetase-1 is a rapid turnover biomarker in mouse and human acute liver injury.

Several serum markers are used to assess hepatocyte damage, but they have limitations related to etiology specificity and prognostication. Identification of novel hepatocyte-specific biomarkers could provide important prognostic information and better pathogenesis classification. We tested the hypothesis that hepatocyte-selective biomarkers are released after subjecting isolated mouse hepatocytes to Fas-ligand-mediated apoptosis. Proteomic analysis of hepatocyte culture medium identified the mitochondrial matrix protein carbamoyl phosphate synthetase-1 (CPS1) among the most readily detected proteins that are released by apoptotic hepatocytes. CPS1 was also detected in mouse serum upon acute challenge with Fas-ligand or acetaminophen and in hepatocytes upon hypoosmotic stress, independent of hepatocyte caspase activation. Furthermore, CPS1 was observed in sera of mice chronically fed the hepatotoxin 3,5-diethoxycarbonyl-1,4-dihydrocollidine. Mouse CPS1 detectability was similar in serum and plasma, and its half-life was 126 ± 9 min. Immune staining showed that CPS1 localized to mouse hepatocytes but not ductal cells. Analysis of a few serum samples from patients with acute liver failure (ALF) due to acetaminophen, Wilson disease, or ischemia showed readily detectable CPS1 that was not observed in several patients with chronic viral hepatitis or in control donors. Notably, CPS1 rapidly decreased to undetectable levels in sera of patients with acetaminophen-related ALF who ultimately recovered, while alanine aminotransferase levels remained elevated. Therefore, CPS1 becomes readily detectable upon hepatocyte apoptotic and necrotic death in culture or in vivo. Its abundance and short serum half-life, compared with alanine aminotransferase, suggest that it may be a useful prognostic biomarker in human and mouse liver injury.

Mutation of caspase-digestion sites in keratin 18 interferes with filament reorganization, and predisposes to hepatocyte necrosis and loss of membrane integrity.

Keratin 18 (K18 or KRT18) undergoes caspase-mediated cleavage during apoptosis, the significance of which is poorly understood. Here, we mutated the two caspase-cleavage sites (D238E and D397E) in K18 (K18-DE), followed by transgenic overexpression of the resulting mutant. We found that K18-DE mice develop extensive Fas-mediated liver damage compared to wild-type mice overexpressing K18 (K18-WT). Fas-stimulation of K18-WT mice or isolated hepatocytes caused K18 degradation. By contrast, K18-DE livers or hepatocytes maintained intact keratins following Fas-stimulation, but showed hypo-phosphorylation at a major stress-kinase-related keratin 8 (K8) phosphorylation site. Although K18-WT and K18-DE hepatocytes showed similar Fas-mediated caspase activation, K18-DE hepatocytes were more 'leaky' after a mild hypoosmotic challenge and were more susceptible to necrosis after Fas-stimulation or severe hypoosmotic stress. K8 hypophosphorylation was not due to the inhibition of kinase binding to the keratin but was due to mutation-induced inaccessibility to the kinase that phosphorylates K8. A stress-modulated keratin phospho-mutant expressed in hepatocytes phenocopied the hepatocyte susceptibility to necrosis but was found to undergo keratin filament reorganization during apoptosis. Therefore, the caspase cleavage of keratins might promote keratin filament reorganization during apoptosis. Interference with keratin caspase cleavage shunts hepatocytes towards necrosis and increases liver injury through the inhibition of keratin phosphorylation. These findings might extend to other intermediate filament proteins that undergo proteolysis during apoptosis.

CD73 (ecto-5'-nucleotidase) hepatocyte levels differ across mouse strains and contribute to mallory-denk body formation.

UNLABELLED: Formation of hepatocyte Mallory-Denk bodies (MDBs), which are aggregates of keratins 8 and 18 (K8/K18), ubiquitin, and the ubiquitin-binding protein, p62, has a genetic predisposition component in humans and mice. We tested the hypothesis that metabolomic profiling of MDB-susceptible C57BL and MDB-resistant C3H mouse strains can illuminate MDB-associated pathways. Using both targeted and unbiased metabolomic analyses, we demonstrated significant differences in intermediates of purine metabolism. Further analysis revealed that C3H and C57BL livers differ significantly in messenger RNA (mRNA) level, protein expression, and enzymatic activity of the adenosine-generating enzyme, ecto-5'-nucleotidase (CD73), which was significantly lower in C57BL livers. CD73 mRNA levels were also dramatically decreased in human liver biopsies from hepatitis C and nonalcoholic fatty liver disease patients. Feeding mice with a diet containing the MDB-inducing agent, 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC), significantly decreased CD73 protein and activity in C57BL livers and resulted in loss of plasma membrane CD73 expression and activity in isolated mouse hepatocytes. To further examine the role of CD73 in MDB formation in vivo, we fed wild-type (WT) and CD73(-/-) mice a DDC-containing diet. Liver enlargement, p62 induction, and disappearance of the K8/K18 cytoskeleton were attenuated in CD73(-/-) , compared to WT livers. MDB formation, as assessed by biochemical and immunofluorescence detection of keratin and ubiquitin complexes, was nearly absent in CD73(-/-) mice. CONCLUSION: Purine metabolism and CD73 expression are linked to susceptibility to MDB formation in livers of different mouse strains. Expression of the adenosine-generating enzyme, CD73, contributes to experimental MDB induction and is highly regulated in MDB-associated liver injury in mice and in chronic human liver disease.

Energy determinants GAPDH and NDPK act as genetic modifiers for hepatocyte inclusion formation.

Genetic factors impact liver injury susceptibility and disease progression. Prominent histological features of some chronic human liver diseases are hepatocyte ballooning and Mallory-Denk bodies. In mice, these features are induced by 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) in a strain-dependent manner, with the C57BL and C3H strains showing high and low susceptibility, respectively. To identify modifiers of DDC-induced liver injury, we compared C57BL and C3H mice using proteomic, biochemical, and cell biological tools. DDC elevated reactive oxygen species (ROS) and oxidative stress enzymes preferentially in C57BL livers and isolated hepatocytes. C57BL livers and hepatocytes also manifested significant down-regulation, aggregation, and nuclear translocation of glyceraldehyde 3-phosphate dehydrogenase (GAPDH). GAPDH knockdown depleted bioenergetic and antioxidant enzymes and elevated hepatocyte ROS, whereas GAPDH overexpression decreased hepatocyte ROS. On the other hand, C3H livers had higher expression and activity of the energy-generating nucleoside-diphosphate kinase (NDPK), and knockdown of hepatocyte NDPK augmented DDC-induced ROS formation. Consistent with these findings, cirrhotic, but not normal, human livers contained GAPDH aggregates and NDPK complexes. We propose that GAPDH and NDPK are genetic modifiers of murine DDC-induced liver injury and potentially human liver disease.

The structural requirements of histone deacetylase inhibitors: Suberoylanilide hydroxamic acid analogs modified at the C3 position display isoform selectivity.

The FDA-approved drug suberoylanilide hydroxamic acid (SAHA, Vorinostat) was modified to improve its selectivity for a single histone deaetylase (HDAC) isoform. We show that attaching an ethyl group at the C3 position transforms SAHA from nonselective to an HDAC6-selective inhibitor. Theses results indicate that small structural changes in SAHA can significantly influence selectivity, which will lead future anti-cancer design efforts targeting HDAC proteins.

Fibrinogen-γ proteolysis and solubility dynamics during apoptotic mouse liver injury: heparin prevents and treats liver damage.

UNLABELLED: Fas ligand (FasL)-mediated hepatocyte apoptosis occurs in the context of acute liver injury that can be accompanied by intravascular coagulation (IC). We tested the hypothesis that analysis of selected protein fractions from livers undergoing apoptosis will shed light on mechanisms that are involved in liver injury that might be amenable to intervention. Proteomic analysis of the major insoluble liver proteins after FasL exposure for 4-5 hours identified fibrinogen-γ (FIB-γ) dimers and FIB-γ-containing high molecular mass complexes among the major insoluble proteins visible via Coomassie blue staining. Presence of the FIB-γ-containing products was confirmed using FIB-γ-specific antibodies. The FIB-γ-containing products partition selectively and quantitatively into the liver parenchyma after inducing apoptosis. Similar formation of FIB-γ products occurs after acetaminophen administration. The observed intrahepatic IC raised the possibility that heparin therapy may ameliorate FasL-mediated liver injury. Notably, heparin administration in mice 4 hours before or up to 2 hours after FasL injection resulted in a dramatic reduction of liver injury-including liver hemorrhage, serum alanine aminotransferase, caspase activation, and liver apoptosis-compared with heparin-untreated mice. Heparin did not directly interfere with FasL-induced apoptosis in isolated hepatocytes, and heparin-treated mice survived the FasL-induced liver injury longer compared with heparin-untreated animals. There was a sharp, near-simultaneous rise in FasL-induced intrahepatic apoptosis and coagulation, with IC remaining stable while apoptosis continued to increase. CONCLUSION: Formation of FIB-γ dimers and their high molecular mass products are readily detectable within the liver during mouse apoptotic liver injury. Heparin provides a potential therapeutic modality, because it not only prevents extensive FasL-related liver injury but also limits the extent of injury if given at early stages of injury exposure.

Keratin hypersumoylation alters filament dynamics and is a marker for human liver disease and keratin mutation.

Keratin polypeptide 8 (K8) associates noncovalently with its partners K18 and/or K19 to form the intermediate filament cytoskeleton of hepatocytes and other simple-type epithelial cells. Human K8, K18, and K19 variants predispose to liver disease, whereas site-specific keratin phosphorylation confers hepatoprotection. Because stress-induced protein phosphorylation regulates sumoylation, we hypothesized that keratins are sumoylated in an injury-dependent manner and that keratin sumoylation is an important regulatory modification. We demonstrate that K8/K18/K19, epidermal keratins, and vimentin are sumoylated in vitro. Upon transfection, K8, K18, and K19 are modified by poly-SUMO-2/3 chains on Lys-285/Lys-364 (K8), Lys-207/Lys-372 (K18), and Lys-208 (K19). Sumoylation affects filament organization and stimulus-induced keratin solubility and is partially inhibited upon mutation of one of three known K8 phosphorylation sites. Extensive sumoylation occurs in cells transfected with individual K8, K18, or K19 but is limited upon heterodimerization (K8/K18 or K8/K19) in the absence of stress. In contrast, keratin sumoylation is significantly augmented in cells and tissues during apoptosis, oxidative stress, and phosphatase inhibition. Poly-SUMO-2/3 conjugates are present in chronically injured but not normal, human, and mouse livers along with polyubiquitinated and large insoluble keratin-containing complexes. Notably, common human K8 liver disease-associated variants trigger keratin hypersumoylation with consequent diminished solubility. In contrast, modest sumoylation of wild type K8 promotes solubility. Hence, conformational changes induced by keratin natural mutations and extensive tissue injury result in K8/K18/K19 hypersumoylation, which retains keratins in an insoluble compartment, thereby limiting their cytoprotective function.

A histone deacetylase-dependent screen in yeast.

Histone deacetylase (HDAC) proteins are promising targets for cancer treatment, as shown by the recent FDA approval of the HDAC inhibitor suberoylanilide hydroxamic acid (SAHA, Vorinostat) for the treatment of cutaneous T-cell lymphoma. To identify additional potent inhibitors and characterize HDAC mutant proteins, there is interest to develop an inexpensive screening method dependent on native substrates. Here, we report the first yeast-based gene reporter screen dependent on the yeast Rpd3, which is a homolog of human class I HDAC proteins. The screen was sensitive to an inactive Rpd3 mutant and various inhibitors in qualitative, agar-based and quantitative, solution-phase formats. Interestingly, inclusion of the lytic enzyme zymolyase enhanced the inhibitor sensitivity of the screen. The gene reporter screen provides a tool to screen Rpd3 mutants and inhibitors of class I HDAC proteins.

Residues in the 11 A channel of histone deacetylase 1 promote catalytic activity: implications for designing isoform-selective histone deacetylase inhibitors.

Histone deacetylase 1 (HDAC1) has been linked to cell growth and cell cycle regulation, which makes it a widely recognized target for anticancer drugs. Whereas variations of the metal-binding and capping groups of HDAC inhibitors have been studied extensively, the role of the linker region is less well known, despite the potency of inhibitors with diverse linkers, such as MS-275. To facilitate a drug design that targets HDAC1, we assessed the influence of residues in the 11 A channel of the HDAC1 active site on activity by using an alanine scan. The mutation of eight channel residues to alanine resulted in a substantial reduction in deacetylase activity. Molecular dynamics simulations indicated that alanine mutation results in significant movement of the active-site channel, which suggests that channel residues promote HDAC1 activity by influencing substrate interactions. With little characterization of HDAC1 available, the combined experimental and computational results define the active-site residues of HDAC1 that are critical for substrate/inhibitor binding and provide important insight into drug design.

Synthesis and biological evaluation of histone deacetylase inhibitors that are based on FR235222: a cyclic tetrapeptide scaffold.

We outline the synthesis of six novel derivatives that are based on a recently discovered HDAC inhibitor FR235222. Our work is the first report utilizing a novel binding element, guanidine, as metal coordinators in HDAC inhibitors. Further, we demonstrate that these compounds show cytotoxicity that parallels their ability to inhibit deacetylase activity, and that the most potent compounds maintain an L-Phe at position 1, and a D-Pro at position 4. Both inhibition of HDAC activity and cytotoxicity against the pancreatic cancer cell line BxPC3 are exhibited by these compounds, establishing that a guanidine unit can be utilized successfully to inhibit HDAC activity.

Structural requirements of HDAC inhibitors: SAHA analogs functionalized adjacent to the hydroxamic acid.

Inhibitors of histone deacetylase (HDAC) proteins such as suberoylanilide hydroxamic acid (SAHA) have emerged as effective therapeutic anti-cancer agents. To better understand the structural requirements of HDAC inhibitors, a small molecule library with a variety of substituents attached adjacent to the metal binding hydroxamic acid of SAHA was synthesized. The presence of a substituent adjacent to the hydroxamic acid led to an 800- to 5000-fold decrease in inhibition compared to SAHA. The observed results have implications for drug design, suggesting that HDAC inhibitors with substituents near the metal binding moiety will have inhibitory activities in the micromolar rather than nanomolar range.