Pattern recognition receptors (PRR), Toll-like receptors (TLR), and nucleotide-oligomerization domain-containing proteins (NOD) play critical roles in mediating inflammation and modulating functions in white adipocytes in obesity. However, the role of PRR activation in brown adipocytes, which are recently found to be present in adult humans, has not been studied. Here we report that mRNA of TLR4, TLR2, NOD1, and NOD2 is upregulated, paralleled with upregulated mRNA of inflammatory cytokines and chemokines in the brown adipose tissue (BAT) of the obese mice. During brown adipocyte differentiation, mRNA and protein expression of NOD1 and TLR4, but not TLR2 and NOD2, is also increased. Activation of TLR4, TLR2, or NOD1 in brown adipocytes induces activation of NF-κB and MAPK signaling pathways, leading to inflammatory cytokine/chemokine mRNA expression and/or protein secretion. Moreover, activation of TLR4, TLR2, or NOD1 attenuates both basal and isoproterenol-induced uncoupling protein 1 (UCP-1) expression without affecting mitochondrial biogenesis and lipid accumulation in brown adipocytes. Cellular bioenergetics measurements confirm that attenuation of UCP-1 expression by PRR activation is accompanied by suppression of both basal and isoproterenol-stimulated oxygen consumption rates and isoproterenol-induced uncoupled respiration from proton leak; however, maximal respiration and ATP-coupled respiration are not changed. Further, the attenuation of UCP-1 by PRR activation appears to be mediated through downregulation of the UCP-1 promoter activities. Taken together, our results demonstrate the role of selected PRR activation in inducing inflammation and downregulation of UCP-1 expression and mitochondrial respiration in brown adipocytes. Our results uncover novel targets in BAT for obesity treatment and prevention.

Our results demonstrate the suppressive effects of 1,25(OH)2D3/VDR signaling on brown adipocyte differentiation and mitochondrial respiration. The role of 1,25(OH)2D3/VDR system in regulating BAT development and function in obesity warrant further investigation.

Brown adipose tissue, now known to be present in adult humans, is a specialized tissue responsible for heat production through non‐shivering thermogenesis. Understanding of the regulation of the differentiation from the precursors to functional brown adipocytes holds the promise for novel strategies to combat obesity. The purpose of the study was to determine the effects of 1,25‐dihydroxyvitamin D/Vit D receptor on the differentiation and function of brown adipocytes from the immortalized precursor cells derived from interscapular brown adipose tissue of newborn C57BL/6 mice. Here we report that Vitamin D dose‐dependently (1‐100nM) suppressed brown adipocyte differentiation, as measured by both mRNA and protein expression of brown adipocyte markers and oil red O staining of lipid accumulation. Consistently, over‐expression of Vit D receptor also significantly impaired differentiation of the brown adipocytes. Further, results from the cellular bioenergetics measurements showed that suppression of differentiation by Vit D was accompanied by significant impairment of isoproterenol‐induced oxygen consumption rates, maximal respiration and the uncoupled respiration from proton leaks. Together, our results conclude that Vit D signaling suppresses brown adipocyte differentiation and function. Grant Funding Source: Supported by the University of Tennessee faculty start‐up funds to L.Z.

Brown adipose tissue (BAT) plays a critical role in nonshivering thermogenesis and energy disposal in contrast to white adipose tissue (WAT), which is responsible for energy storage. Recent evidence shows that two families of pattern recognition receptors (PRR), Toll‐like receptors (TLR) and Nucleotide‐oligomerization domain containing proteins (NOD), play critical roles in inflammation in white adipocytes and WAT. However, the role of TLR and NOD in brown adipocytes and BAT remains to be determined. Here, we report that mRNA of TLR4, TLR2, NOD1 and NOD2 is increased in BAT of diet‐induced obese mice, genetically obese ob/ob mice, and differentiated brown adipocytes compared to undifferentiated cells. Moreover, stimulation of TLR4, TLR2 and NOD1 with their respective synthetic ligand induce mRNA expression of proinflammatory chemokine/cytokine MCP‐1, RANTES, IL‐6 and TNF‐α and suppress mRNA expression of uncoupling protein‐1 through modulation of MAPK‐dependent and/or NF‐κB‐responsive pathways in brown adipocytes. Together, our results suggest that PRRs may play important roles in inducing inflammation and dysfunction in BAT in obesity. The work is supported by The University of Tennessee faculty start‐up funds to L.Z.