Role of epigenetics in liver-specific gene transcription, hepatocyte differentiation and stem cell reprogrammation

HAT/HDAC-mediated transcriptional control. (A), Histone-related pathway: HATs/HDACs acetylate/deacetylate histones resulting in reduced/augmented chromatin compaction and alternations of the histone code, respectively. (B) Non-histone related pathway: HDACs directly interfere with non-histone protein targets, including transcription factors, nuclear hormone receptors, nuclear import factors, structural proteins and adhesion proteins. Deacetylation of latter non-histone proteins might affect diverse aspects of their protein physiology, resulting in either decreased or increased activity of the target protein. Both pathways interconnect with each other. The transcriptional outcome thus relies on the sum of all – transcription-stimulating/inhibiting – actions.

Nucleoside analogue DNMTi.

Epigenetic control of gene transcription. Inhibition of gene transcription typically corresponds to hypermethylated CpG islands in gene promoter regions and deacetylated histone tails at local chromatin domains. The indirect mechanism of gene silencing may involve binding of methyl-binding proteins (MeCp) to methylated cytosine and subsequent recruitment of HDAC-corepressor (CoR) complexes, resulting in a non-permissive heterochromatin status that blocks binding of transcription factors (TF) and polymerase II RNA complexes (PolII) to target promoter sequences. The direct mechanism may involve the direct interference of TF with HDAC or methylated CpG sites within the promoter. HDAC inhibitors (HDACi) and DNMT inhibitors (DNMTi) modulate the chromatin structure. They create an open, transcriptionally active euchromatin configuration at gene coding and regulatory regions, accessible for TF, thereby facilitating gene transcription. Abbreviations: 5-AzaC, decitabine; M, 5-methyl cytosine at CpGs; SB, sodium butyrate; TSA, trichostatin A; VPA, valproic acid.

Effects of TSA on phase I CYP-dependent biotransformation activity. Hepatocytes were cultured and remained either unexposed [C] or were exposed to 0.083% (v/v) ethanol as solvent control [SC] or 25μM TSA [T] for 7 days. (A) After 2 [D2], 4 [D4] and 7 days [D7], CYP1A1, CYP2B1 and CYP3A2 protein expression were analysed by means of immunoblotting. In order to control for equal loading of proteins, expression levels of HNF-1α were determined, as latter LETF, in contrast to C/EBPα and HNF-4, is not affected by culture time or exposure to TSA. Representative images for three independent experiments are shown. (B) After 2, 4 and 7 days of culture PROD (CYP2B1)-dependent activities were measured. Data are expressed as percentage of the values found for freshly isolated primary rat hepatocytes, i.e. 7.7±3.1pmol/min/mg microsomal protein. Values represent mean±SD.

Effects of 4-Me₂ N-BAVAH on phase I CYP-dependent biotransformation activity. Cultured hepatocytes were either exposed to 0.05% (v/v) ethanol as a solvent control [C] or to 50μM 4-Me₂N-BAVAH [B] for 7 days. (A) After 4 [D4] and 7 days [D7], CYP1A1, CYP2B1 and CYP3A2 protein expression were analysed by means of immunoblotting. Representative images for three independent experiments are shown. (B) After 4 and 7 days of culture PROD (CYP2B1)-dependent activities were measured. Data are expressed as percentage of the values found for freshly isolated primary rat hepatocytes, i.e. 7.7±3.1pmol/min/mg microsomal protein. Values represent mean±SD (n⩾3). (^∗p<0.05 compared to control values, paired Student’s t-test).