Role of SUMO modification in hepatocyte differentiation

Abstract

Primary human hepatocytes are a scarce resource with variable function, which diminishes with time in culture. As a consequence their use in tissue modelling and therapy is restricted. Human embryonic stem cells (hESCs) could provide a stable source of human tissue due to their properties of self-renewal and their ability to give rise to all three germ layers. hESCs have the potential to provide an unlimited supply of hepatic endoderm (HE) which could offer efficient tools for drug discovery, disease modelling and therapeutic applications. In order to create a suitable environment to enhance HE formation, hESC culture needed to be standardised. As such, a media trail was carried out to define serum free media capable of maintaining hESC in a pluripotent undifferentiated state. We also ensured hESC cultured in the various media could be directly differentiated to HE in a reproducible and efficient manner. The project then focused on the effect of post-translational modifications (PTMs), specifically SUMOylation, in hepatocyte differentiation and its subsequent manipulation to enhance HE viability. SUMOylation is a PTM known to modify a large number of proteins that play a role in various cellular processes including: cell cycle regulation, gene transcription, differentiation and cellular localisation. We hypothesised that SUMO modification may not only regulate hESC self renewal, but also maybe required for efficient hESC differentiation. We therefore interrogated the role of SUMOylation in hESC differentiation to hepatic endoderm (HE). hESC were differentiated and the cellular lysates were analysed by Western blotting for key proteins which modulate the conjugation and de conjugation of SUMO. We demonstrate that peak levels of SUMOylation were detectable in hESC populations and during cellular differentiation to definitive endoderm (DE), day 5. Following commitment to DE we observed a decrease in the level of SUMO modified proteins during cellular specialisation to a hepatic fate, corresponding with an increase in SENP 1, a SUMO deconjugation enzyme. We also detected reduced levels of hepatocyte nuclear factor 4 α (HNF4α), a critical regulator of hepatic status and metabolic function, as SUMOylation decreased. As a result, we investigated if HNF4α was SUMOylated and if this process was involved in modulating HNF4α’s critical role in HE. HNF4α is an important transcription factor involved in liver organogenesis during development and is a key regulator for efficient adult liver metabolic functions. We observed a decreasing pattern of HNF4α expression at day 17 of our differentiation protocol in conjunction with a decrease in SUMO modified proteins. In order to further investigate and validate a role of SUMOylation on HNF4α stability Immunoprecipitation (IP) was employed. HNF4α protein was pulled down and probed for SUMO 2. Results show an increase in the levels of SUMO2 modification as the levels of HNF4α decrease. Through deletion and mutation analysis we demonstrated that SUMO modification of HNF4α was restricted to the C-terminus on lysine 365. Protein degradation via the proteasome was responsible for the decrease in HNF4α, demonstrated by the use of a proteasome 26S inhibitor MG132. Additionally, a group at the University of Dundee has shown that polySUMOylation of promyelocytic leukaemia protein (PML) leads to its subsequent ubiquitination via RNF4, an ubiquitin E3 ligase, driving its degradation. Using an in vitro ubiquitination assay, we show that polySUMOylated HNF4α is preferentially ubiquitinated in the presence of RNF4. Overall polySUMOylation of HNF4α may reduce its stability by driving its degradation, hence regulating protein activity. In conclusion, polySUMOylation of HNF4α is associated with its stability. HNF4α is subsequently important for HE differentiation both driving the formation of the hepatocytes and in maintaining a mature phenotype, in agreement with a number of different laboratories. Creating the ideal environment for sustaining mature functional hepatocytes, primary and those derived from hESCs and iPSCs, is essential for further use in applications such as drug screening, disease modelling and extracorporeal devices.