Abstract Body (Do not enter title and authors here): Introduction:
Obstructive sleep apnea (OSA) is highly prevalent and triples the risk for cardiovascular disease. Our previous studies showed that intermittent hypoxia (IH), the hallmark of OSA, triggers endothelial cell (EC) inflammation by reducing cellular protection against complement activity, which is cholesterol-dependent. We previously demonstrated that the cellular cholesterol efflux transporters ATP-binding cassette A1 (ABCA1) and G1 (ABCG1) are downregulated in EC exposed to IH. Nuclear factor erythroid 2-related factor 1 (Nrf1) is a recently identified transcription factor that suppresses the transcription of ABCA1 and ABCG1.
Aim:
To investigate cholesterol efflux in EC of OSA patients.
Hypothesis:
Cholesterol efflux in EC of OSA patients is reduced compared with OSA-free controls. The underlying mechanisms involve activation of Nrf1 triggered by IH in OSA.
Methods:
ECs were harvested with an angiocatheter inserted into forearm vein from OSA patients (n=17, age 44±13, 53% female, BMI 37.4±10.2 kg/m²) and OSA-free controls (n=13, age 38±13, 67% female, BMI 32.15±9.7 kg/m²). Cultured human umbilical vein ECs (HUVECs) exposed to IH (alternating 30 min 21% O₂ for normoxia/30 min 2% O₂ for hypoxia for 8 h), 2% O₂ for 8 h (continuous hypoxia, CH) or normoxia were used as the in vitro model.
Results:
The EC free cholesterol levels were greater in OSA patients than controls (OSA, n=10 vs. controls, n=10; 81% increase; p=0.005) and in HUVECs exposed to IH vs. normoxia or CH (IH vs. normoxia, 51% increase, p=0.002; IH vs. CH, 45% increase, p=0.001; n=4/each). Levels of ABCA1 and ABCG1 mRNA were reduced in OSA (ABCA1: OSA, n=17 vs. controls, n=13; 31% decrease; p=0.5) (ABCG1: OSA, n=16 vs. controls, n=12; 43% decrease; p=0.2). The ABCG1 protein level and consequently cholesterol efflux were lower in IH than normoxia or CH (ABCG1: IH vs. normoxia, 54% decrease, p=0.001; IH vs. CH, 55% decrease, p=0.02; n=3/each) (efflux: IH vs. normoxia, 24% decrease, p=0.003; IH vs. CH, 32% decrease, p=0.06; n=4/each). Nrf1 protein nuclear level was increased in ECs of OSA patients compared with controls (OSA, n=10 vs. controls, n=9; 66% increase; p<0.001)
Conclusions:
IH reduces endothelial cholesterol efflux, which contributes to the cellular accumulation of free cholesterol in EC in patients with OSA. Consequent impairment in endothelial protection against complement initiates endothelial inflammation in IH and increases cardiovascular risk in OSA.