Increased plasma ANGPTL7 levels with increased obstructive sleep apnea severity

Abstract

Background: Weight-loss surgery is one of the recommended methods for treating obstructive sleep apnea (OSA) in obese patients. While weight reduction is critical to relieve symptoms of OSA, the biochemical factors involved in post-surgery improvement are still unknown. We aimed to explore the link between ANGPTL7 and OSA in patients with different OSA severity. Furthermore, we examined the effect of treating OSA with bariatric surgery on ANGPTL7 level.

Methods: We quantified levels of circulating ANGPTL7 in fasting plasma and adipose tissue samples of 88 participants before and after bariatric surgery. Confocal microscopy analyses were also performed to assess the ANGPTL7 expression in subcutaneous white adipose tissue biopsies obtained from people with moderate-to-severe OSA compared to those with none or mild OSA. The study involved 57 individuals with none or mild OSA and 31 patients with moderate-to-severe OSA.

Results: Levels of circulating ANGPTL7 were significantly higher in people with moderate-to-severe OSA (1440 ± 1310 pg/ml) compared to the none or mild OSA group (734 ± 904 pg/ml, p = 0.01). The increase in ANGPTL7 correlated significantly and positively with the apnea-hypopnea index (AHI, r = .226, p = .037), and AHI-supine (r = .266, p = .019) in participants with moderate-to-severe OSA. Multivariate logistic regression analysis demonstrated an association between ANGPTL7 and OSA severity (log₂ ANGPTL7; OR =1.24, p = 0.024). ANGPTL7 levels exhibited significant positive correlations with the levels of TG and oxLDL (p-value = 0.002 and 0.01 respectively). Bariatric surgery reduced the levels of both ANGPTL7 and AHI significantly.

Conclusion: Here we report significantly increased levels of ANGPTL7 both in the circulation and in adipose tissue of patients with OSA, which concurred with increased inflammation and OSA severity. Levels of ANGPTL7 decreased significantly as OSA showed a significant improvement post-surgery supporting a potential role for ANGPTL7 in either OSA progression or a role in an OSA-related mechanism.

Keywords: ANGPTL7; apnea hypopnea index; bariatric surgery; hypoxia; obstructive sleep apnea; polysomnography.

Figure 1

Flowchart of subjects included in the study.

Figure 2

OSA diagnosis by polysomnography and OSA indices. (A) The apnea hypopnea index (AHI) demonstrating a significant difference in OSA severity between patients with none or mild OSA (5.27 ± 0.57 events/hour, white bar) and those with moderate-to-severe OSA (28.7 ± 3.02 events/hour, p-value <0.001, black bar). (B) Apnea hypopnea-supine index (AHI-supine) reflecting a significant increase in OSA severity in patients with moderate-to-severe OSA (33.5 ± 4 events/hour, p-value <0.001, black bar) compared to the none or mild OSA group (8.41 ± 1.63 events/hour, white bar). (C) The number of shallow breathing events reflected by the hypopnea index (HI) showing a significant increase in patients with moderate-to-severe OSA (17.4 ± 9.29 events/hour, p-value <0.001, black bar) in comparison to those with none or mild OSA (3.76 ± 3.55 events/hour, white bar). (D) The apnea index (AI) showing that the number of complete paused breathing events in patients with moderate-to-severe OSA (9.88 ± 12 events/hour, p-value <0.001, black bar) is significantly higher compared to patients with none or mild OSA (1.08 ± 1.74 events/hour, white bar). **: p-value <0.001.

Figure 3

Baseline levels of ANGPTL7 in patients with and without OSA. (A) Circulating ANGPTL7 is significantly higher in patients with moderate-to-severe OSA (1440 ±243.46 pg/ml, p-value = 0.01) compared to those with none or mild OSA (734.904 ± 121.85 pg/ml). (B) ANGPTL7 gene expression showing 3.2-fold increase in patients with moderate-to-severe OSA compared to patients with none or mild OSA. (C) Increased ANGPTL7 protein expression in adipose tissue determined by confocal microscopy. Representative images taken at 10x showing expression level in patients with AHI < 15 events/hour and moderate-to-severe OSA with AHI ≥ 15 events/hour, quantification of ANGPTL7 protein expression plotted as bar chart showing a significant difference in ANGPTL7 protein expression. (D) Images of ANGPTL7 protein expression at 40x with a quantification of expression level by a bar chart plot. **: p-value <0.001.

Figure 4

Spearman correlation analysis between circulating ANGPTL7 and; (A) apnea-hypopnea index (AHI) that demonstrates a significant positive correlation (ρ= 0.227; p-value= 0.038), (B) TG displaying a significant positive correlation (ρ=0.331; p-value=0.002), (C) oxLDL reflecting a significant positive correlation (ρ=0.291; p-value=0.010). ANGPTL7 showed a significant negative correlation with (D) HDL (ρ=-0.239; p-value=0.028), (E) IL-10 (ρ=-0.304; p-value=0.005).

Figure 5

Post-surgery improvement in OSA and a decline in circulating ANGPTL7. Apnea-hypopnea index (AHI) demonstrates a significant decline after six months of bariatric surgery in (A) people with none or mild OSA (2.82 ± 2.6 events/hour, p < 0.001) and those with (B) moderate-to-severe OSA (9.10 ± 9.69 events/hour, p < 0.001), reflecting a substantial improvement in OSA. Levels of circulating ANGPTL7 dropped in patients with none or mild OSA (114 ± 807 pg/ml, p = 0.014, (C), and it showed a significant reduction in levels of circulating ANGPTL7 in patients with moderate-to-severe OSA (495 ± pg/ml, p = 0.0113) (D) after six months of surgery.

Figure 6

Hypoxic conditions increase ANGPTL7 expression. Effect of CoCl₂ (150 µM), an eminent hypoxia imitative agent on ANGPTL7 expression in differentiated adipocytes (3T3-L1). (A) Whole cell lysates were separated on 8% SDS-PAGE and analyzed by Western blotting. Expression of ANGPTL7 showed an increase after 12hrs, 24hrs that was significant at 48hrs. (B) Protein quantification of ANGPTL7 that was normalized to β-actin showing statistical significance (p-value < 0.05) at 48hrs. Data represent mean ± SEM and it’s a representative of three independent measurements. *: p-value < 0.05.