Obesity Is a Positive Modulator of IL-6R and IL-6 Expression in the Subcutaneous Adipose Tissue: Significance for Metabolic Inflammation

Abstract

The role of IL-6R/IL-6 axis in metabolic inflammation remains controversial. We determined the changes in adipose tissue expression of IL-6R and IL-6 in obese, overweight, and lean non-diabetic individuals. Subcutaneous adipose tissue biopsies were collected from 33 obese, 22 overweight, and 10 lean individuals and the expression of IL-6R, IL-6, TNF-α, MCP-1, IP-10, CD11b, CD163, and CD68 was detected by immunohistochemistry; results were also confirmed by real-time RT-PCR and confocal microscopy. The data were compared using unpaired t-test and the dependence between two variables was assessed by Pearson's correlation test. Obese individuals showed higher IL-6R expression (103.8±4.807) in the adipose tissue as compared with lean/overweight (68.06±4.179) subjects (P<0.0001). The elevated IL-6R expression correlated positively with body mass index (BMI) (r=0.80 P<0.0001) and percent body fat (r=0.69 P=0.003). The increased IL-6R expression in obesity was also confirmed by RT-PCR (Obese: 3.921±0.712 fold; Lean/Overweight: 2.191±0.445 fold; P=0.0453) and confocal microscopy. IL-6 expression was also enhanced in obese adipose tissue (127.0±15.91) as compared with lean/overweight (86.69±5.25) individuals (P=0.03) which correlated positively with BMI (r=0.58 P=0.008). IL-6 mRNA expression was concordantly higher in obese (16.60±2.214 fold) versus lean/overweight (9.376±1.656 fold) individuals (P=0.0108). These changes in the IL-6R/IL-6 expression correlated positively with the adipose tissue expression of CD11b (IL-6R r=0.44 P=0.063; IL-6 r=0.77 P<0.0001), CD163 (IL-6R r=0.45 P=0.045; IL-6 r=0.55 P=0.013), TNF-α (IL-6R r=0.73 P=0.0003; IL-6 r=0.60 P=0.008), MCP-1 (IL-6R r=0.61 P=0.005; IL-6 r=0.63 P=0.004) and IP-10 (IL-6R r=0.41 P=0.08; IL-6 r=0.50 P=0.026). It was, therefore, concluded that obesity was a positive modulator of IL-6R and IL-6 expression in the adipose tissue which might be a contributory mechanism to induce metabolic inflammation.

Fig 1. Adipose tissue expression of IL-6R is enhanced in obesity.

The subcutaneous adipose tissue samples were obtained by surgical biopsy from 10 lean/ overweight (BMI = 20.285 to 29.456) and 10 obese (BMI = 31.752 to 38.218) non-diabetic individuals and protein expression (intensity) of IL-6R was measured by immunohistochemistry (IHC). Group means were compared using unpaired t-test and all P-values ≤0.05 were considered statistically significant. The data show that (A) IL-6R expression was significantly elevated in the adipose tissue samples from obese (103.8±4.807) as compared with lean/ overweight (68.06±4.179) individuals (P<0.0001). (B) The increase in IL-6R expression correlated positively with body mass index (BMI) (r = 0.80, P<0.0001) and (C) percent body fat (PBF) (r = 0.69, P = 0.002). (D) The representative IHC photomicrographs (100× magnification) of IL-6R staining intensity (arrows) in the adipose tissue samples from lean, overweight, and obese individuals, 3 each, are shown. Similar photomicrographs at a wider field view (20× magnification) are shown in supporting S1 Fig.

Fig 2. IL-6R expression changes are validated by confocal microscopy and real time RT-PCR.

(A) The increased IL-6R protein expression in the adipose tissue samples of obese and overweight individuals as compared with lean individuals was also confirmed by confocal microscopy. (B) Real-time RT-PCR was used to assess the IL-6R gene expression in the adipose tissue samples from lean, overweight, and obese individuals. The data obtained confirmed the significantly increased IL-6R gene expression in obese adipose tissues (3.921±0.712 fold) as compared with lean/ overweight (2.191±0.445 fold) adipose tissue samples (P = 0.0453). (C) A strong positive correlation was found between IL-6R gene and protein expression in the adipose tissues (r = 0.63, P = 0.02).

Fig 3. IL-6 expression is also elevated in the obese adipose tissues.

(A) Adipose tissue expression of the IL-6R ligand i.e. IL-6 expression was also determined by immunohistochemistry (IHC) and was found to be significantly elevated in the adipose tissues from obese (127.0±15.91) as compared with overweight and lean (86.69±5.25) individuals (P = 0.03). (B) The increase in IL-6 expression correlated positively with body mass index (BMI) (r = 0.58, P = 0.008); (C) while the correlation with percent body fat (PBF) was found to be non-significant (r = 0.39, P = 0.14). (D) The representative IHC photomicrographs (100× magnification) of IL-6 staining intensity (arrows) in the adipose tissue samples from lean, overweight, and obese individuals, 3 each, are shown. Similar photomicrographs at a wider field view (20× magnification) are shown in supporting S2 Fig.

Fig 4. IL-6 expression changes are confirmed by confocal microscopy and real time RT-PCR.

(A) The increased IL-6 protein expression in the adipose tissues of obese and overweight individuals as compared with lean subjects was confirmed by confocal microscopy. (B) Real-time RT-PCR data also confirmed the increased IL-6 gene expression in obese adipose tissues (16.60±2.214 fold) as compared with lean/ overweight (9.376±1.656 fold) adipose tissue samples (P = 0.0108).

Fig 5. Enhanced expression of monocytes/ macrophage markers in the obese adipose tissue.

The protein expression (intensity) of monocyte/ macrophage markers was detected by immunohistochemistry (IHC) in the adipose tissue samples from lean, overweigh, and obese individuals, 10 each. As shown by representative IHC photomicrographs (100× magnification), expression of (A) CD11b, (B) CD163, and (C) CD68 was found to be markedly elevated in overweight and obese adipose tissue samples as compared with lean samples.

Fig 6. Adipose tissue IL-6R/IL-6 expression relates with the local expression of monocyte/ macrophage markers.

The immunohistochemistry (IHC) data show a positive correlation of IL-6R with (A) CD11b (r = 0.44, P = 0.053) and (B) CD163 (r = 0.45, P = 0.045); and also of IL-6 with (D) CD11b (r = 0.77, P<0.0001) and (E) CD163 (r = 0.55, P = 0.013). However, the correlations of CD68 with (C) IL-6R (r = -0.34, P = 0.14) and (F) IL-6 were found to be non-significant (r = -0.24, P = 0.31).

Fig 7. Adipose tissue IL-6R/IL-6 expression correlates with the local expression of signature inflammatory mediators.

The protein expression (intensity shown by arrows) of TNF-α, MCP-1, and IP-10 was detected in the adipose tissue samples from lean, overweigh, and obese individuals by immunohistochemistry (IHC). As revealed by representative (IHC) photomicrographs (100× magnification), expression of (A) TNF-α, (B) MCP-1, and (C) IP-10 was found to be markedly increased in overweight and obese adipose tissue samples as compared with lean tissue samples.

Fig 8. Adipose tissue expression of the signature inflammatory mediators correlates with body mass index (BMI) as well as with IL-6R/IL-6 expression.

The adipose tissue expression of TNF-α, MCP-1, IP-10, IL-6R, and IL-6 was assessed by immunohistochemistry (IHC). The IHC data show a positive correlation of body mass index (BMI) with (A) TNF-α (r = 0.70, P = 0.0007), (B) MCP-1 (r = 0.63, P = 0.004), and (C) IP-10 (r = 0.75, P = 0.0002). Besides, the adipose tissue expression of IL-6R related directly with local expression of (D) TNF-α (r = 0.73, P = 0.0003), (E) MCP-1 (r = 0.61, P = 0.005), and (F) IP-10 (r = 0.41, P = 0.08). Similarly, IL-6 expression also correlated with the adipose tissue expression of (G) TNF-α (r = 0.60, P = 0.008), (H) MCP-1 (r = 0.63, P = 0.004), and (I) IP-10 (r = 0.50, P = 0.026).