Low‐Frequency Intermittent Hypoxia Promotes Subcutaneous Adipogenic Differentiation

Figures

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  Figure 1. Intermittent hypoxia (IH) accelerated subcutaneous adipose tissue (SAT) but not visceral adipose tissue (VAT) adipogenesis. The gene expression levels of adipogenic markers and regulators in depot-specific adipose tissue after 6-week IH exposure. The mRNA expressions of FABP4 (a), PPARγ (b), CEBPα (c), and GLUT4 (d) were measured by real-time PCR. The adiponectin (e) levels released into conditioned medium were measured with ELISA. Protein expressions of adipogenic markers and regulators in depot-specific adipose tissue after IH exposure (f–i). The protein expressions of FABP4 (f and g) and CHOP (h and i) in VAT and SAT were detected by Western blots. N = 5–6. Bars: mean ± SEM. ∗ P < 0.05 and ∗∗ P < 0.01.

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  Figure 2. Protein expression of insulin growth factor-1 receptor (IGF-1R) in both subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) after intermittent hypoxia (IH) exposure. The protein expression of IGF-1R was detected in SAT (a) and VAT (b) via Western blots. N = 5–6. Bars: mean ± SEM. ∗ P < 0.05.

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  Figure 3. Intermittent hypoxia (IH) caused smaller size of adipocytes with an increase in number in subcutaneous adipose tissue (SAT) but not in visceral adipose tissue (VAT). (a–d) Photomicrographs of adipocytes from rat adipose tissue (hematoxylin-eosin staining, ×20 magnification): (a) and (c) SAT and VAT of IN group, respectively; (b) and (d) SAT and VAT of IH group, respectively, SAT but not VAT with smaller size of adipocytes and the elevation of number of adipocytes, suggesting the presence of hyperplasia.

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  Figure 4. Intermittent hypoxia (IH) exposure promoted human primary subcutaneous preadipocyte (HPA) adipogenesis. Oil Red O staining was carried out for the identification of differentiation (a) (N = 4). One differentiated cycle was defined as 3-day differentiation and 2-day maintenance. The expressions of differentiation-associated markers during the conversion of HPAs into adipocytes (b–f). The mRNA expressions of FABP4 (b), GLUT4 (c), CEBPα (d), and CEBPδ (e) were quantified using real-time PCR. The protein expression of PPARγ (f) was measured via Western blots. The results were expressed as mean ± SEM as fold change with respect to control (IN) at cycle 2. (N = 5). Bars: mean ± SEM. ∗ P < 0.05, ∗∗ P < 0.01, and ∗∗∗ P < 0.001.

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  Figure 5. Insulin growth factor-1 receptor (IGF-1R)/Akt signaling pathway in human primary subcutaneous preadipocytes (HPAs) and differentiated HPAs after intermittent normoxia (IN) or intermittent hypoxia (IH) exposure. The protein expressions of IGF-1Rβ and phosphorylated protein p-Akt (Ser 473) in HPAs (a–c) and in differentiated HPAs (d–f) were examined by Western blots. N = 4. Bars: mean ± SEM. ∗ P < 0.05.

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  Figure 6. The Akt signaling and adipogenic differentiation in the absence or presence of a selective insulin growth factor-1 receptor (IGF-1R) kinase inhibitor NVP-AEW541. The protein expressions of p-Akt (Ser 473) and total Akt were detected by Western blots (a). The phase-contrast picture showed oily droplets after adipogenic differentiation of HPAs in IN and IH condition (b). The protein expressions of adipogenic markers FABP4 (c) and PPARγ (d) were measured by Western blots. The results were expressed as mean ± SEM as fold change with respect to control (IN). N = 5. Bars: mean ± SEM. ∗ P < 0.05 and ∗∗ P < 0.01.

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  Figure 7. The release of proinflammatory mediators during human primary subcutaneous preadipocyte (HPA) differential process. Levels of proinflammatory mediators MCP-1 (a) and IL-6 (b) in conditioned media were detected by ELISA. N = 6. Bars: mean ± SEM. ∗ P < 0.05 and ∗∗ P < 0.01.

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  Figure 8. Schematic diagram summarizing the role of intermittent hypoxia (IH) in adipogenesis in in vivo and in vitro model. Low-frequency IH exposure could activate insulin growth factor-1 receptor (IGF-1R)/Akt to promote adipogenic differentiation of subcutaneous preadipocytes. In addition, the enhanced inflammation elicited by IH exposure could be another possible mechanism responsible for intensified adipogenesis of subcutaneous adiposity. The accelerative subcutaneous adiposity may be a compensatory response to IH-stimulated metabolic disorders.