The role of NR1D1 in obesity-related fibrosis, inflammation and insulin resistance

Background. Obesity- and age-related metabolic dysfunction are major public health concerns in the UK and globally. In

health, white adipose tissue (WAT) is an essential metabolic buffer, safely storing and supplying energy according to intake and metabolic demand. As obesity develops, expansion of the WAT beds accommodates excess lipid storage. However, profound adipocyte hypertrophy and WAT expansion ultimately drive tissue hypoxia, adipocyte dysfunction, immune cell

infiltration, and tissue fibrosis. This is especially evident in visceral WAT depots, and the degree of visceral adiposity is associated with poor metabolic outcomes (e.g. insulin resistance) in humans and animal models. Despite many decades of

research, the events which trigger a transition from safe and efficient fat storage to tissue dysfunction remain a subject of debate. It is well recognised that aberrant WAT function during obesity is a prominent feature of compromised metabolic

health. Here, attenuated metabolic activity, endocrine function and adipogenic capacity of WAT reduces healthy metabolic

profile, and progression to WAT inflammation and fibrosis elevates risk for pathologies such as diabetes and cancer. We have

identified the nuclear hormone receptor, NR1D1, as a major regulator of adipose tissue, whereby selective deletion of this factor in adipocytes prevents development of WAT inflammation and fibrosis even amid profound obesity1 . In work leading

up to this PhD proposal, components extracellular matrix (ECM) have emerged as clear regulatory target of NR1D1 activity in

adipocytes, and we propose that altered collagen dynamics underlie the 'healthy' obese phenotype driven by Nr1d1 deletion.

Adipocytes are embedded in an ECM that must allow tissue expansion and regression, but also provide structural support for

relatively fragile lipid laden adipocytes. Critical here is the tissue and metabolic-state specific expression ECM proteins such

as collagens and their modifiers. The impact of ECM on WAT function and remodelling during obesity has gained increasing

attention, and the role of WAT ECM rigidity and fibrosis in obesity-related pathology is clear5,6. Nevertheless, there remain profound gaps in our understanding of how production and modification of ECM components is coordinated in WAT, and

which key mechanisms facilitate adaptation to tissue hypertrophy, but eventually transition to drive tissue fibrosis. We

discovered an intriguing paradox in which genetic deletion of NR1D1 in the entire animal or selectively in adipocytes enhances

diet-induced obesity, yet without the development of WAT inflammation, fibrosis and concomitant loss of insulin sensitivity. Under normal conditions adipocyte-specific deletion of Nr1d1, mice do not exhibit increased adipocyte and show few phenotypic or transcriptional differences in WAT1 . The almost exclusive targets of NR1D1 are a subset of collagen and

collagen-modifying genes1,7 . We hypothesis that it is this regulation of influential 'minor' collagens (e.g. Col5a3, Col6a1-3, Col6a6) and collagen modifying enzymes (e.g. Loxl4, P4ha2) that so dramatically alters gWAT response to obesity-related

expansion. It is thought that rapid expansion of WAT under obesogenic conditions leads to hypoxia, in turn driving cellular

dysfunction, inflammation, and pro-fibrotic processes13. HIF1a, as a major regulator of cellular response to hypoxia, is induced

in WAT in obesity. Unlike its actions elsewhere, HIF1a does not drive a proangiogenic response in WAT, rather it engages a potent pro-fibrotic transcriptional program13 . Our data suggest that NR1D1 acts downstream of HIF1a, and that Nr1d1 deletion modulates adipocyte response to hypoxia/HIF1a activation. This PhD project will characterise WAT ECM dynamics

in responses to obesity and Nr1d1 manipulation and define mechanisms that underlie adipose collagen dynamics and response to hypoxic stress.