Gene regulation in human metabolic homeostasis: PPARg perturbation and Cold exposure as models of intrinsic and extrinsic metabolic regulation

Abstract

Metabolism refers to the integrated network of all biochemical reactions that sustains organismal function. In humans and other mammals, systemic metabolic homeostasis arises from the interplay of multiple organs that exchange circulating metabolites. Two major human metabolic organs are adipose tissue (AT) and the skeletal muscle. While systemic homeostasis depends on the integrated function of multiple organs, the ability of each organ to adapt to the dynamic energy demands is ultimately determined by metabolism at the cellular level. The metabolic state of a cell is tightly connected to the regulation of gene expression: the genome encodes the proteins that define metabolic capacity, while intracellular metabolites can regulate gene expression by influencing transcription factor function and chromatin structure. Thus, gene regulation and metabolism are reciprocally linked. This thesis addresses transcriptional regulation from two perspectives: (1) intrinsic disruption of homeostasis, exemplified by Peroxisome proliferator-activated receptor gamma (PPARγ) mutations causing lipodystrophy (Chapters 2 and 3), and (2) extrinsic modulation of homeostasis, illustrated by skeletal muscle adaptation to cold exposure (Chapters 4-6). Chapter 2 introduces the essential role of the nuclear receptor (NR) PPARγ in the AT as a direct coupling mechanism between metabolism and gene expression. Loss-of-function mutations in PPARγ cause familial partial lipodystrophy type 3 (FPLD3), a rare syndrome characterised by severe metabolic consequences such as insulin resistance, type 2 diabetes mellitus (T2DM) and dyslipidemia. This chapter establishes an accessible and robust set of complementary experimental assays to investigate how disease-associated missense variants disrupt the function of PPARγ. Chapter 3 focuses on a novel patient-derived PPARγ-K184N mutation, exploring its structural implications. PPARγ, like all NRs, consists of multiple domains including a DNA-binding domain (DBD) and a ligand-binding domain (LBD) that integrate multiple roles into a specific transcriptional output. This chapter explores the involvement of K184N in DBD-LBD interdomain interaction and exposes sequence-dependent sensitivity. Due to the dominant role of skeletal muscle in thermogenesis and metabolic regulation, Chapter 4 investigates the effect of shivering cold acclimation on metabolic health in adults with overweight or obesity. Repeated cold exposure with shivering improved oral glucose tolerance, fasting glucose, triglycerides, non- esterified fatty acid concentrations and blood pressure, highlighting it as a promising strategy to improve metabolic health. In Chapter 5 we used a multilayered bioinformatics approach to characterise the shivering skeletal muscle transcriptome response and included a pipeline to identify putatively secreted proteins with potential endocrine functions during cold exposure. Chapter 6 examines how the intensity of repeated cold exposure, and thereby the degree of shivering, translates into the metabolic benefits of cold acclimation. We demonstrate that the intensity of shivering determines the effectiveness of cold acclimation on metabolic improvement. Additional in-depth assessment of the effect of shivering showed a beneficial shift in plasma lipid profiles in metabolically compromised adults.