Tyrosine hydroxylase localization in mice adipose tissue under obesity

Obesity is dramatically increasing worldwide, and is associated with an increased risk of cardiovascular disease and metabolic disorders. Adipose tissue (AT) is now recognized as a major endocrine and secretory organ, producing a variety of adipokines. Recently, AT was described as a producer of catecholamines (CA), major regulators of lipolysis, and also affecting the differentiation and proliferation of adipocytes. This study aimed to investigate the localization of tyrosine hydroxylase (TH), the rate limiting enzyme in CA synthesis, in adipocytes under an obese state. Two groups of male and female C57BL/6 mice with about 22-23 g were used. During 12 or 16 weeks, one group was fed with a high-fat diet (HFD, 45% lipids, 20% proteins and 35% carbohydrates) and the other (control group) with a standard diet (SD, 13% lipids, 20% proteins and 67% carbohydrates). After 12 and 16 weeks, mice were respectively overweighed or obese (15% and 45% of weight gain, respectively). All the animals were sacrificed and tissues collected after diet intervention. The number of adipocytes producing TH and cellular localization of this enzyme were evaluated by immunhistochemistry. All images were analyzed and processed using NIS-Elements BR 3.2, Nikon, and ImageJ software. A two-way analysis of variance (ANOVA) was used to compare differences between diets and degree of body fat (overweight or obesity) for subcutaneous AT (SAT) and visceral AT (VAT). Normality of data distribution was assessed using the Shapiro-Wilk test. To compare VAT with SAT, a paired sample t-test was used. In adipocytes, TH was found in the perinuclear space and along the cytoplasmatic membrane. In VAT, the number of TH positive adipocytes decreased significantly from 29±3% to 25±3% respectively in SD (n=6) and HFD (n=13) groups (p=0.016). In SAT, although not achieving a statistical meaning, the same tendency was observed: 30±2% in SD (n=7) to 27±3% in HFD (n=13) groups (p=0.078). Globally, SAT presented a higher percentage of TH positive adipocytes comparing to VAT (respectively 28±3% and 26±3%; n=19; p=0.030), regardless the fat mass degree or the type of diet. The lower number of adipocytes expressing TH in visceral adipose tissue suggests that local CA production in this tissue may be involved in visceral adipocyte dysfunction underlying obesity.