Increased mitochondrial respiration of adipocytes from metabolically unhealthy obese compared to healthy obese individuals

Abstract

Among obese subjects, metabolically healthy (MHO) and unhealthy obese (MUHO) subjects exist, the latter being characterized by whole-body insulin resistance, hepatic steatosis, and subclinical inflammation. Insulin resistance and obesity are known to associate with alterations in mitochondrial density, morphology, and function. Therefore, we assessed mitochondrial function in human subcutaneous preadipocytes as well as in differentiated adipocytes derived from well-matched donors. Primary subcutaneous preadipocytes from 4 insulin-resistant (MUHO) versus 4 insulin-sensitive (MHO), non-diabetic, morbidly obese Caucasians (BMI > 40 kg/m²), matched for sex, age, BMI, and percentage of body fat, were differentiated in vitro to adipocytes. Real-time cellular respiration was measured using an XF24 Extracellular Flux Analyzer (Seahorse). Lipolysis was stimulated by forskolin (FSK) treatment. Mitochondrial respiration was fourfold higher in adipocytes versus preadipocytes (p = 1.6*10^-9). In adipocytes, a negative correlation of mitochondrial respiration with donors' insulin sensitivity was shown (p = 0.0008). Correspondingly, in adipocytes of MUHO subjects, an increased basal respiration (p = 0.002), higher proton leak (p = 0.04), elevated ATP production (p = 0.01), increased maximal respiration (p = 0.02), and higher spare respiratory capacity (p = 0.03) were found, compared to MHO. After stimulation with FSK, the differences in ATP production, maximal respiration and spare respiratory capacity were blunted. The differences in mitochondrial respiration between MUHO/MHO were not due to altered mitochondrial content, fuel switch, or lipid metabolism. Thus, despite the insulin resistance of MUHO, we could clearly show an elevated mitochondrial respiration of MUHO adipocytes. We suggest that the higher mitochondrial respiration reflects a compensatory mechanism to cope with insulin resistance and its consequences. Preserving this state of compensation might be an attractive goal for preventing or delaying the transition from insulin resistance to overt diabetes.

Figure 1

Schematic representation of the mitochondrial stress test as adopted from Seahorse (Agilent Technologies) as well as^,. Dotted arrow: optional treatment with 5 µM forskolin (FSK). FCCP: Carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone; MHO: metabolically healthy obese subjects; MUHO: metabolically unhealthy obese subjects. For analysis, non-mitochondrial respiration was subtracted from all results.

Figure 2

Mitochondrial respiration in differentiated adipocytes, n = 4/group; mean ± SEM; panel A—basal respiration; B—ATP production (oligomycin subtracted from basal, see also Fig. 1); C—proton leak (oligomycin); D—maximal respiration (FCCP), n = 3 versus 4; E—spare respiratory capacity (basal subtracted from FCCP).

Figure 3

Mitochondrial respiration in differentiated adipocytes after FSK injection, n = 4/group, if not stated otherwise; mean ± SEM; panel A—basal respiration; B—ATP production (oligomycin subtracted from basal, see also Fig. 1); C—proton leak (oligomycin); D—maximal respiration (FCCP); E—spare respiratory capacity (basal subtracted from FCCP), n = 3 versus 4.

Figure 4

Correlation of ISI_Matsuda and A) basal respiration, B) maximal respiration (n = 7), and C) ATP production in arbitrary units. n = 8, if not stated otherwise. Red line: regression line; analysis by ANOVA.