Role of GPS2 in epigenome alterations linked to metaflammation

Abstract: Current evidence suggests that transcriptional and epigenomic reprogramming events, triggered by transcription factors and chromatin-modifying co-regulators, are of central importance for disease development. G-protein pathway suppressor 2 (GPS2) is a key component of the HDAC3 co-repressor complex that has been earlier implicated in cholesterol homeostasis and anti-inflammatory crosstalk. More recent work revealed that the expression and function of GPS2 is altered in obese humans and correlated to the inflammation status and the risk for developing type 2 diabetes. Although this potentially suggests the involvement of GPS2 in metaflammation, i.e. closely linked metabolic and inflammatory disease pathways, the underlying mechanisms and the precise role of GPS2 remained unknown. The aim of this thesis was to characterize the functions of GPS2 at the molecular and physiological level with an emphasis on obesity-associated inflammation, insulin resistance, and fatty liver disease. In Paper I, we identified GPS2 as a key regulator of ABCA1-dependent cholesterol efflux in inflammatory macrophages. This study potentially implicates the GPS2-ABCA1 axis in linking obesity and type 2 diabetes to cardiovascular diseases. In Paper II, we identified GPS2-repressed pro-inflammatory enhancers and deeply characterized enhancer structure and function at the Ccl2 gene locus in macrophages. The study revealed that GPS2-repressed enhancers are non-redundant and that inhibiting enhancer-transcribed eRNAs reduced gene expression, thus suggesting eRNA function. In Paper III, we characterized macrophage-specific Gps2 knockout mice along with in vitro models and expression analysis in humans to identify a potent anti-inflammatory role of GPS2 and the underlying genomic actions. Upon diet-induced obesity, Gps2 knockout mice display hallmarks of metaflammation typical for obese humans, i.e. elevated inflammation and insulin resistance. In Paper IV, we describe hitherto unknown liver functions of GPS2 in the development of the non-alcoholic fatty liver disease. Through integrated genomic and phenotypic characterization of hepatocyte-specific Gps2 knockout mice, we found that GPS2 specifically antagonizes the fatty acid receptor PPARa. Thus, the selective modulation of GPS2-PPARa interactions could be of therapeutic interest for future interventions. In conclusion, this thesis revealed novel insights into the multifaceted regulatory roles of GPS2 in altering epigenomes and transcription linked to metabolic and inflammatory processes. These insights should help to better understand the development of obesity, type 2 diabetes, atherosclerosis, and fatty liver disease, and they may help to define novel therapeutic strategies.