Brown Adipose Tissue Activation Is Involved in Atherosclerosis of ApoE−/− Mice Induced by Chronic Intermittent Hypoxia

Background: Obstructive sleep apnea is an atherogenesis factor of which chronic intermittent hypoxia is a prominent feature. Chronic intermittent hypoxia (CIH) exposure can sufficiently activate the sympathetic system, which acts on the β3 adrenergic receptors of brown adipose tissue (BAT). However, the activity of BAT and its function in CIH-induced atherosclerosis have not been fully elucidated. Methods: This study involved ApoE-/- mice which were fed with a high-fat diet for 12 weeks and grouped into control and CIH group. During the last 8 weeks, mice in the CIH group were housed in cages to deliver CIH (12 h per day, cyclic inspiratory oxygen fraction 5-20.9%, 180 s cycle). Atherosclerotic plaques were evaluated by Oil Red O, hematoxylin and eosin, Masson staining, and immunohistochemistry. Afterward, we conducted immunohistochemistry, western blotting, and qRT-PCR of uncoupling protein 1 (UCP1) to investigate the activation of BAT. The level of serum total cholesterol (TC), triglyceride, low-density lipoprotein cholesterol (LDL-c), high-density lipoprotein cholesterol (HDL-c), and free fatty acid (FFA) were measured. Finally, RNA-Sequencing was deployed to explore the differentially expressed genes (DEGs) and their enriched pathways between control and CIH groups. Results: Chronic intermittent hypoxia exposure promoted atherosclerotic plaque area with increasing CD68, α-SMA, and collagen in plaques. BAT activation was presented during CIH exposure with UCP1 up-regulated. Serum TC, triglyceride, LDL-c, and FFA were increased accompanied by BAT activation. HDL-c was decreased. Mechanistically, 43 lipolysis and lipid metabolism-associated mRNA showed different expression profiling between the groups. Calcium, MAPK, and adrenergic signaling pathway included the most gene number among the significantly enriched pathways. Conclusion: This study first demonstrated that BAT activation is involved in the progression of CIH-induced atherosclerosis, possibly by stimulating lipolysis.