Toll-like receptor-4 mediated inflammation is involved in the cardiometabolic alterations induced by intermittent hypoxia

Abstract

Objective: Intermittent hypoxia (IH) is a major component of sleep apnea syndrome as its cardiometabolic complications have been mainly attributed to IH. The pathophysiology is still poorly understood but there are some similarities with the obesity-associated cardiometabolic complications. As the latter results from inflammation involving toll-like receptor-4 (TLR4) signaling, we assessed this pathway in the cardiometabolic consequences of IH.

Methods: Lean adult male TLR4-deficient (TLR4(-/-)) mice and their controls (C57BL/6 mice) were exposed to either IH (FiO2 21-5%, 1 min cycle, 8 h/day) or air (normoxic mice) for 4 weeks. Animals were assessed at 1-week exposure for insulin tolerance test and after 4-week exposure for morphological and inflammatory changes of the epididymal fat and thoracic aorta.

Results: IH induced insulin resistance, morphological and inflammatory changes of the epididymal fat (smaller pads and adipocytes, higher release of TNF-α and IL-6) and aorta (larger intima-media thickness and higher NFκB-p50 activity). All these alterations were prevented by TLR4 deletion.

Conclusion: IH induces metabolic and vascular alterations that involve TLR4 mediated inflammation. These results confirm the important role of inflammation in the cardiometabolic consequences of IH and suggest that targeting TLR4/NFκB pathway could represent a further therapeutic option for sleep apnea patients.

Figure 1

TLR4 signaling is involved in IH-induced epididymal fat alterations. Morphological and inflammatory changes of epididymal fat pads were studied in C57BL/6 and TLR4^−/− mice exposed to intermittent hypoxia (IH) or normoxia (N) for 4 weeks. (a) Measurements of weight of bilateral epididymal fat pads (n = 13–15 per group), (b) mice body weights (n = 13–15 per group), (c) adipocyte size (n = 4–6 per group), and (d) representative photographs of adipose tissue remodeling. Inflammation was studied through the release of TNF-α (e) and IL-6 (f) (n = 6–8 per group). ^* P < 0.05 IH/C57BL/6 versus N/C57BL/6.

Figure 2

TLR4 signaling is involved in IH-induced insulin resistance. Glucose variation during the 90 minutes of the intraperitoneal insulin tolerance test (IpITT) in C57BL/6 or TLR4^−/− mice exposed to 1 week of intermittent hypoxia (IH) or normoxia (N) (a). IpITT presented separately for C57BL/6 (b) and TLR4^−/− mice (c). For each group, lowest blood glucose level (nadir) during the 90 minutes of the IpITT (d). ^* P < 0.05 IH/C57BL/6 versus N/C57BL/6, n = 13–15 per group.

Figure 3

TLR4 signaling is involved in IH-induced vascular remodeling. Morphometric and inflammatory remodeling of aorta was assessed in C57BL/6 and TLR4^−/− mice exposed to 4 weeks of intermittent hypoxia (IH) or normoxia (N). (a) Aortic intima-media thickness quantification, (b) representative photographs of aorta remodeling (wall thickness represented by white double-headed arrows), (c) quantification of activated NFκB (NFκB-p50 activity) in aorta (n = 8–11 per group). (d) Plasma levels of total cholesterol in C57BL/6 and TLR4^−/− mice exposed to 4 weeks of IH or N, n = 12–16 per group. ^* P < 0.05 IH/C57BL/6 versus N/C57BL/6.