Oxytocin reduces adipose tissue inflammation in obese mice

Abstract

Background: Obesity and adipose tissue expansion is characterized by a chronic state of systemic inflammation that contributes to disease. The neuropeptide, oxytocin, working through its receptor has been shown to attenuate inflammation in sepsis, wound healing, and cardiovascular disease. The current study examined the effects of chronic oxytocin infusions on adipose tissue inflammation in a murine model of obesity, the leptin receptor-deficient (db/db) mouse.

Methods: The effect of obesity on oxytocin receptor protein and mRNA expression in adipose tissue was evaluated by Western blotting and real-time polymerase chain reaction. Mice were implanted with osmotic minipumps filled with oxytocin or vehicle for 8 weeks. At study endpoint adipose tissue inflammation was assessed by measurement of cytokine and adipokine mRNA tissue levels, adipocyte size and macrophage infiltration via histopathology, and plasma levels of adiponectin and serum amyloid A as markers of systemic inflammation.

Results: The expression of adipose tissue oxytocin receptor was increased in obese db/db mice compared to lean controls. In adipose tissue oxytocin infusion reduced adipocyte size, macrophage infiltration, IL-6 and TNFα mRNA expression, and increased the expression of the anti-inflammatory adipokine, adiponectin. In plasma, oxytocin infusion reduced the level of serum amyloid A, a marker of systemic inflammation, and increased circulating adiponectin.

Conclusions: In an animal model of obesity and diabetes chronic oxytocin treatment led to a reduction in visceral adipose tissue inflammation and plasma markers of systemic inflammation, which may play a role in disease progression.

Keywords: Adipose tissue; Anti-inflammation; Obese mouse model; Obesity; Oxytocin.

Fig. 1

Obesity modulates OXTR expression in epididymal fat in lean and obese mice. OXTR mRNA expression relative to a control gene, 18S rRNA (Panel a, n = 9 per group; p < 0.05 comparing lean and db/db mice), Panels b and c; Distribution of Oxtr and adiponectin (Adipoq) mRNA, expression in total tissue, adipocyte fraction and stromal vascular fraction (SVF). Data were expressed relative to mRNA level in the total tissue for each respective group (n = 4 for lean, n = 4 for db/db). Oxtr and Adipoq expression were significantly greater in in the adipocyte fraction compared to the SVF fraction (both p < 0.05). Western Blot of OXTR from adipose tissue membrane fractions (Panel d). Molecular weight in kilodaltons (kDa) is indicated on the left. Densitometric quantification of OXTR protein for 67 and 43 kDa bands (Panel e) Data were expressed as mean ± SEM. * indicates p < 0.05.

Fig. 2

Body weight (Panel a) and epididymal fat weight (Panel b) in vehicle Control (n = 8) or OXT treated (n = 10) db/db mice after 8 weeks of treatment and in age matched lean control (n = 9) mice. Data were expressed as mean ± SEM. * indicates p < 0.05

Fig. 3

Effects of OXT infusion on adipocyte size, lipolysis and adipose tissue macrophage infiltration. (Panel a) Adipocyte size in epididymal tissues of Vehicle (n = 8) or OXT-treated db/db mice (n = 10) and lean control mice (n = 6). (Panel b) Lipolysis of triglycerides in isolated adipocytes from db/db mice incubated with increasing concentration of OXT for 30 min evaluated by release of free fatty acids. Adipocytes isolated from 3 or 4 mice were pooled and assayed in triplicate at each OXT dose. Data represent the mean for each dose from three separate experiments. OXT significantly increased lipolysis in a dose dependent manner; p = 0.003 for OXT dose and * indicates p < 0.01 compared to no oxytocin at all doses by post-hoc analysis (Panel c). Macrophage infiltration into adipose tissue of Vehicle (n = 8) or OXT-treated db/db mice (n = 10) and lean control mice (n = 6) assessed by immunohistochemistry for F4/80 antigen expression. (Panel d) Densities of crown-like structures in epididymal fat pads of db/db mice treated with vehicle (n = 8) or OXT (n = 9) and lean controls (n = 6) expressed as number per microscopic field (average of 5 fields per mouse tissue). (Panel e). Representative photomicrographs of F4/80 immunostained epididymal fat from vehicle and OXT treated db/db mice and lean control mice, bar = 50 μm. Data were expressed as mean ± SEM for each respective group.

Fig. 4

Adipose tissue mRNA expression of cytokines and adiponectin are altered in oxytocin-treated obese mice compared to vehicle control and lean mice. There was a significant decrease in mRNA expression for IL-6 (Panel a), TNF-α (Panel b), and in the macrophage marker, F4/80 (Panel C) relative to vehicle controls and lean mice. In contrast, oxytocin treatment led to an increase in the mRNA expression of adiponectin (Panel D), an anti-inflammatory adipokine. Data were expressed relative to 18S rRNA level for each respective group (n = 8 for vehicle, n = 10 for oxytocin-treated, and n = 7or 8 for lean)

Fig. 5

Oxytocin treatment altered plasma biomarkers of inflammation. (Panel a) Inflammatory biomarker, serum amyloid A (SAA) was attenuated in oxytocin-treated obese mice relative to vehicle controls. (Panel b) There was an increase in the anti-inflammatory plasma marker, adiponectin compared to vehicle controls. Data were expressed as mean ± SEM for each respective group (n = 4 for vehicle, n = 9 for oxytocin-treated, and n = 7 for lean)