The role of intermittent hypoxia on adipogenesis : in vivo and in vitro studies on pathogenic mechanisms linking obstructive sleep apnea to metabolic disorders

Abstract

Obstructive sleep apnea (OSA), characterized by intermittent hypoxia (IH), is a potential risk factor for metabolic disorders. In the exploration of this link, the understanding of adipose tissue (AT) functions from initial energy storage to pleiotropic participation in dynamic metabolic regulation is important, especially in the context of OSA as it is highly associated with obesity. The adipogenic capacity of depot-specific adipose tissue has physiological and pathophysiological significance in the regulation of AT function. The impact of IH on adipocyte differentiation is not known. Therefore, the work in this thesis investigated the effects of various intensities of IH exposure on depot-specific adipogenic capacity. To investigate the influence of IH exposure on adipogenic capacity, SD rats were exposed to low-frequency IH for 6 weeks. IH-regulated adipogenesis of subcutaneous adipose tissue (SAT), demonstrated by the up-regulation of adipogenic transcription factors and differentiated markers, but not of visceral adipose tissue (VAT) was found. In line with the in vivo observation, IH exposure accelerated adipogenic differentiation of human subcutaneous pre-adipocytes (HPAs) in vitro, which was accompanied by the activation of IGF-1R/AKT pathway, supporting that low-frequency IH exposure could facilitate adipogenesis of SAT. Given the strong association of OSA and obesity, the impact of two different frequencies of IH exposures on adipogenic differentiation was compared in a diet-induced obese mouse model. Low-frequency (IH10) exposure accelerated adipogenic differentiation of subcutaneous (SUB)-SVF but suppressed adipogenic differentiation of visceral (VIS)-SVF. On the other hand, high-frequency (IH60) exposure suppressed adipogenic differentiation in both SUB-SVF and VIS-SVF. In addition, the elevated IL-6 release was abolished in IH10-exposed but not IH60- exposed SUB-SVF after adipogenic differentiation. Our results suggest that low-frequency (IH10) may exert aprotective role whilst high-frequency (IH60) may play a pathologic role in a fat depot-specific manner for adipogenesis and obesity-associated pro-inflammatory status in AT. Macrophage polarization is a process when macrophage expresses different functional programs in response to micro environmental signals. As preferential M1macrophage polarization is involved in the regulation of adipogenesis, the role of macrophage polarization in IH-regulated adipogenic differentiation was further explored. After induction of differentiation in vitro, adipogenic capacity of SUB-SVF from IH-exposed mice was significantly down-regulated, whilst an inhibitory effect of IH was not observed in VIS-SVF. Furthermore, IH exposure increased infiltration preferentially of M1 macrophages with the reduction of M2 phenotype and also caused hypertrophic adipocytes in SAT but not in VAT. In support of the involvement of macrophage polarization in IH-regulated subcutaneous adipogenesis, conditioned-medium from IH-exposed RAW264.7 (M1 macrophage dominant) cell cultures in vitro also significantly inhibited the adipogenesis of SUB-SVF.