Abstract P3191: Hur Mediates Calcium-driven Thermogenesis In Brown Adipose Tissue

Abstract

Since the discovery of brown adipose tissue (BAT) in adult humans, its high energy capacity has driven investigations targeting adipose tissue thermogenesis as a supplement to whole-body energy expenditure to mitigate the energy imbalances observed in obesity. Uncoupling protein 1 (UCP1) is the primary driver of thermogenesis, however several additional UCP1-independent pathways have been identified. We previously reported that adipocyte deletion of the RNA binding protein human antigen R (HuR) results in thermogenic dysfunction independent of UCP1. RNA sequencing revealed the downregulation of several genes involved in calcium ion transport upon HuR deletion. Although recently established as a thermogenic mechanism in beige adipocytes, the effects of calcium-mediated thermogenesis in brown fat and its contribution to whole-body metabolism remain unexplored. Therefore, we set out to investigate the impact and regulatory mechanism of HuR on SR calcium cycling and heat generation in BAT. We hypothesize that HuR regulates thermogenesis in brown adipose tissue through control of RyR-mediated SR calcium release and assessed this experimentally using murine models with a BAT-specific deletion of HuR. Brown adipocytes isolated from BAT-HuR -/- mice were imaged for cytosolic calcium and heat using Fluo-4 calcium indicator and ERthermAC dye respectively. Cohorts of mice were also subjected to acute cold challenge or placed on a 12-week high fat diet study to assess whole-body thermal and energy metabolism. We now show that loss of HuR decreases adrenergic-stimulated calcium release and heat generation in brown adipose tissue, and that this effect is rescued by pharmacological stabilization of RyR2. We also report that HuR directly binds RyR2 mRNA in vivo and loss of HuR reduces RyR2 mRNA transcript stability. In assessing whole-body metabolism, we observed impaired thermal tolerance of BAT-HuR -/- mice from controls, as well as sex-dependent effects on diet-induced weight gain between models. Our findings reveal, for the first time, that brown adipocytes participate in calcium-mediated thermogenesis and identify a regulatory role for HuR in coordinating heat production through maintenance of RyR2 calcium release activity.