The transcriptional co-regulator LDB1 is required for brown adipose function

Abstract

Objective Brown adipose tissue (BAT) is critical for thermogenesis and glucose/lipid homeostasis. Exploiting the energy uncoupling capacity of BAT may reveal targets for obesity therapies. This exploitation requires a greater understanding of the transcriptional mechanisms underlying BAT function. One potential regulator of BAT is the transcriptional co-regulator LIM domain-binding protein 1 (LDB1), which acts as a dimerized scaffold, allowing for the assembly of transcriptional complexes. Utilizing a global LDB1 heterozygous mouse model, we recently reported that LDB1 might have novel roles in regulating BAT function. However, direct evidence for the LDB1 regulation of BAT thermogenesis and substrate utilization has not been elucidated. We hypothesize that brown adipocyte-expressed LDB1 is required for BAT function. Methods LDB1-deficient primary cells and brown adipocyte cell lines were assessed via qRT-PCR and western blotting for altered mRNA and protein levels to define the brown adipose-specific roles. We conducted chromatin immunoprecipitation with primary BAT tissue and immortalized cell lines. Potential transcriptional partners of LDB1 were revealed by conducting LIM factor surveys via qRT-PCR in mouse and human brown adipocytes. We developed a Ucp1 -Cre-driven LDB1-deficiency mouse model, termed Ldb1 ΔBAT , to test LDB1 function in vivo . Glucose tolerance and uptake were assessed at thermoneutrality via intraperitoneal glucose challenge and glucose tracer studies. Insulin tolerance was measured at thermoneutrality and after stimulation with cold or the administration of the β3-adrenergic receptor (β3-AR) agonist CL316,243. Additionally, we analyzed plasma insulin via ELISA and insulin signaling via western blotting. Lipid metabolism was evaluated via BAT weight, histology, lipid droplet morphometry, and the examination of lipid-associated mRNA. Finally, energy expenditure and cold tolerance were evaluated via indirect calorimetry and cold challenges. Results Reducing Ldb1 in vitro and in vivo resulted in altered BAT-selective mRNA, including Ucp1 , Elovl3 , and Dio2 . In addition, there was reduced Ucp1 induction in vitro . Impacts on gene expression may be due, in part, to LDB1 occupying Ucp1 upstream regulatory domains. We also identified BAT-expressed LIM-domain factors Lmo2 , Lmo4 , and Lhx8 , which may partner with LDB1 to mediate activity in brown adipocytes. Additionally, we observed LDB1 enrichment in human brown adipose. In vivo analysis revealed LDB1 i...

Adipocyte Mitochondria: Deciphering Energetic Functions across Fat Depots in Obesity and Type 2 Diabetes

“…Gaining further insights into the molecular processes that regulate the browning of adipocytes could pave the way for the development of pharmacological interventions to prevent or reduce obesity and offset its associated metabolic issues. As such, recent studies have reported strategies to facilitate the development of pharmaceutical approaches to raise energy expenditure as a means of addressing obesity and metabolic disorders [ 131 , 132 , 133 ].…”

“…Although BeAT is highly abundant in the postnatal period, when maternally supplied nutrients are oxidized, in part, to preserve body heat [ 140 , 141 ], recent research indicates that WAT contains both beige and white Adipocyte stem-like Cells (ASCs), which are established in neonates and determined in committed ASC subtypes [ 5 , 118 , 142 ]. In mice and humans, selectively increasing and/or prolonging BeAT increased energy expenditure and improved insulin sensitivity and glucose control when challenged with an obesogenic diet [ 14 , 33 , 120 , 131 , 132 , 133 , 143 , 144 ]. Thus, mature beige adipocytes initially arise de novo from ASCs and remain primed to burn nutrients and protect against diet-induced obesity [ 118 ].…”