Augmentation of urea-synthetic capacity by inhibition of nitric oxide synthesis in butyrate-induced differentiated human hepatocytes

Augmentation of Butyrate-induced Differentiation of Human Hepatocyte by Cyclin E Over-expression

In mammalian cells, cellular differentiation into specific cell types is usually preceded by growth arrest. On the other hand, the induced differentiation may also be preceded by an enhanced G1–S transition of the cell cycle prior to the growth arrest. This suggests that an early increase in proliferation is in some way a prerequisite for subsequent differentiation. We therefore attempted to assess whether we could produce human hepatocytes with further differentiated functions by promoting G1-S transition in a butyrate-treated human hepatocyte cell line. A cyclin E-over-expressing cell line was established by transfecting human cyclin E cDNA. Upon butyrate treatment, the cyclin E-over-expressing cells exhibited a significantly increased albumin-secreting and ammonia-detoxifying capacity when compared to the control cells. In particular, the ornithine transcarbamylase activity was increased in these cells. Collectively, these results implicate that the cyclin E over-expression may augment the hepatocyte-specific functions during the butyrate-induced differentiation process of human hepatocytes by enhancing G1-S cell cycle transition.

Differentiation Effects by the Combination of Spheroid Formation and Sodium Butyrate Treatment in Human Hepatoblastoma Cell Line (Hep G2): A Possible Cell Source for Hybrid Artificial Liver

The aim of this study was to investigate the feasibility of human hepatoblastoma cell line (Hep G2), which differentiates by spheroid formation, and treatment with sodium butyrate (SB) as a cell source for hybrid artificial liver (HAL). Hep G2 spontaneously formed spheroids in polyurethane foam (PUF) within 3 days of culture and restored weak ammonia removal activity. Treatment with SB, which is a histone deacetylase inhibitor, further increased the ammonia removal activity of Hep G2 spheroids in a concentration-dependent manner. The activation of ornithine transcarbamylase—a urea cycle enzyme—was significantly related to the upregulation of ammonia removal by spheroid formation, but scarcely contributed to the further upregulation following SB treatment. In contrast with ammonia removal, treatment with SB reduced the albumin secretion of Hep G2 spheroids in a concentration-dependent manner. In the PUF-HAL module in a circulation culture, the ammonia removal rate and albumin secretion rate (per unit volume of the module) of Hep G2 spheroids treated with 5 mM SB were almost the same as those of primary porcine hepatocyte spheroids. These results suggest that simultaneous use of spheroid formation and SB treatment in Hep G2 is beneficial in enhancing the functions of human hepatocytes with potential applications in regenerative medicine and drug screening.

Dynamics of HBV cccDNA expression and transcription in different cell growth phase

Background

The covalently closed-circular DNA (cccDNA) of hepatitis B virus (HBV) is associated with viral persistence in HBV-infected hepatocytes. However, the regulation of cccDNA and its transcription in the host cells at different growth stages is not well understood.

Methods

We took advantages of a stably HBV-producing cell line, 1.3ES2, and examine the dynamic changes of HBV cccDNA, viral transcripts, and viral replication intermediates in different cellular growth stages.

Results

In this study, we showed that cccDNA increased suddenly in the initial proliferation phase of cell growth, probably attributable to its nuclear replenishment by intracellular nucleocapsids. The amount of cccDNA then decreased dramatically in the cells during their exponential proliferation similar to the loss of extrachromosomal plasmid DNA during cell division, after which it accumulated gradually while the host cells grew to confluency. We found that cccDNA was reduced in dividing cells and could be removed when proliferating cells were subjected to long term of lamivudine (3TC) treatment. The amounts of viral replicative intermediates were rapidly reduced in these proliferating cells and were significantly increased after cells reaching confluency. The expression levels of viral transcripts were increased in parallel with the elevated expression of hepatic transcription factors (HNF4α, CEBPα, PPARα, etc.) during cell growth confluency. The HBV transcripts were transcribed from both integrated viral genome and cccDNA, however the transcriptional abilities of cccDNA was less efficient then that from integrated viral genome in all cell growth stages. We also noted increases in the accumulation of intracellular viral particles and the secretion of mature virions as the cells reached confluency and ceased to grow.

Conclusions

Based on the dynamics of HBV replication, we propose that HBV replication is modulated differently in the different stages of cell growth, and can be divided into three phases (initial proliferation phase, exponential proliferation phase and growth confluency phase) according to the cell growth curve. The regulation of cccDNA in different cell growth phase and its importance regarding HBV replication are discussed.

Enrichment of hepatocyte-like cells with upregulated metabolic and differentiated function derived from embryonic stem cells using S-NitrosoAcetylPenicillamine

The generation of a large number of fully functional hepatocytes from a renewable cell source can provide an unlimited resource for bioartificial liver devices and cell replacement therapies. We have established a directed differentiation system using sodium butyrate treatment to generate an enriched population of hepatocyte-like cells from embryonic stem cells. A metabolic analysis of the hepatocyte populations revealed glycolytic and mitochondrial phenotypes similar to mouse hepatoma cells, implying that these cells represent an immature hepatocyte phenotype. To mediate further differentiation, S-NitrosoAcetylPenicillamine (SNAP), a nitric oxide donor, was utilized to induce mitochondrial development in the precursor populations. A comparative analysis of the different treated populations showed that 500microM SNAP treatment resulted in the generation of an enriched population of metabolically mature hepatocyte-like cells with increased differentiated function. Specifically, 500microM SNAP treatment increased glucose consumption, lactate production rates, mitochondrial mass, and potential as compared to untreated populations. In addition, functional analysis revealed that intracellular albumin content, urea secretion rates, and cytochrome P450 7a1 promoter activity were increased in the treated population. The methodology described here to generate an enriched population of metabolically and functionally mature hepatocyte-like cells may have potential implications in drug discovery and regenerative medicine.

Argininosuccinate Synthetase is Reversibly Inactivated by S-Nitrosylation in Vitro and in Vivo

Prior studies have demonstrated that the substrate for NO synthesis, l-arginine, can be regenerated from the NOS co-product l-citrulline. This requires the sequential action of two enzymes, argininosuccinate synthetase (AS) and argininosuccinate lyase (AL). AS activity has been shown to be rate-limiting for high output NO synthesis by immunostimulant-activated cells and represents a potential site for metabolic control of NO synthesis. We now demonstrate that NO mediates reversible S-nitrosylation and inactivation of AS in vitro and in lipopolysaccharide-treated cells and mice. Using a novel mass spectrometry-based method, we show that Cys-132 in human AS is the sole target for S-nitrosylation among five Cys residues. Mutagenesis studies confirm that S-nitrosylation of Cys-132 is both necessary and sufficient for the inhibition of AS by NO donors. S-nitroso-AS content is regulated by cellular glutathione levels and selectively influences NO production when citrulline is provided to cells as a protosubstrate of NOS but not when l-arginine is provided. A phylogenetic comparison of AS sequences suggests that Cys-132 evolved as a site for post-translational regulation of activity in the AS in NOS-expressing species, endowing NO with the capacity to limit its own synthesis by restricting arginine availability.

Hepatocyte nuclear factor-4alpha mediates redox sensitivity of inducible nitric-oxide synthase gene transcription

The underlying redox-sensitive mechanisms that regulate hepatocyte expression of inducible nitric-oxide synthase (iNOS) and its antioxidant functions are largely unknown. We have demonstrated previously that oxidative stress induced by benzenetriol-mediated superoxide production increases interleukin-1beta-induced iNOS protein synthesis, steady state iNOS mRNA expression, NO production, iNOS gene transcription, and trans-activation of the iNOS promoter in primary cultures of rat hepatocytes. In this study, we extend these studies by establishing the sequence specificity and binding of nuclear protein to the previously described 15-base cis-regulatory element of the rat hepatocyte iNOS promoter, isolating and identifying the cis-regulatory element transcription factor as hepatocyte nuclear factor-4alpha (HNF-4alpha), and confirming the functional role of HNF-4alpha in mediating redox-sensitive iNOS promoter trans-activation. In addition, we demonstrate that binding of HNF-4alpha to the transcriptional coactivator, PC4, in the presence of oxidative stress and interleukin-1beta stimulation is essential for increased iNOS promoter activity in this setting. Our results indicate that HNF-4alpha is the transcription factor that mediates redox regulation of hepatocyte iNOS gene transcription.