Leptin concentrations in the abdominal subcutaneous adipose tissue of patients with anorexia nervosa assessed by in vivo microdialysis

Abstract

Objective

The adipocyte-derived hormone leptin is involved in energy metabolism and body weight regulation. Plasma leptin concentrations are significantly reduced in patients with anorexia nervosa (AN) and with severe malnutrition. Whether reduced plasma leptin is reflected by its decreased production by the adipose tissue is unknown.

Methods

In the present study we measured leptin concentrations locally in the abdominal subcutaneous adipose tissue of 9 female AN patients and 11 healthy controls by in vivo microdialysis.

Results

Adipose tissue free leptin levels were not different in patients with AN compared to controls (2.59±1.99 vs 2.36±0.25 ng/ml, P>0.05). Plasma leptin soluble receptor (sOb-R) levels were significantly higher in patients with AN than in healthy subjects (58.05±38.69 vs 12.79±5.08 U/ml, P<0.01). The area of adipocyte in AN was considerably smaller than in the controls (183±104.01 μm² compared to 2145.8±1003.41).

Conclusions

We conclude that decreased plasma leptin levels in patients with AN are not directly related to dialysate leptin levels in the abdominal subcutaneous adipose tissue.

Introduction

Leptin is a hormone with multiple functions, including predominantly long-term regulation of body weight, energy balance and body temperature by acting as a hunger suppressant signal to the central nervous system [1], [2]. In eating disorders, leptin is involved in the development of its symptoms rather than the pathogenesis itself [3]. Initially thought to be expressed and secreted exclusively by adipocytes, leptin has also been found in other human tissues, including the pituitary, stomach, placenta and mammary gland [4], [5], [6], [7], [8]. Nevertheless, adipose tissue remains its main source responsible for 95% of leptin production [9]. Leptin levels can be influenced by such conditions as satiety, amount of adipose tissue or diurnal cycles. These influences are mediated mostly by sympathetic regulators, insulin, glucocorticoids, and glucose entry into adipocytes [10]. These influences result in great inter- and intrapersonal variations of leptin levels.

Plasma leptin exists in two forms: free and protein-bound, which reflects leptin binding to its soluble receptor (sOb-R), the major leptin binding activity in plasma [11], [12], [13]. Expression of sOb-R and plasma leptin correlates significantly and inversely with age, IGF-I levels, pubertal stage and body composition [14], [15], [16]. In various eating disorders, plasma sOb-R levels were found to vary in the manner opposite to leptin levels [17]. The relationship between bound and free leptin may become clinically relevant in physiological and pathophysiological states that cause rapid changes in total plasma leptin, such as fasting [13]. Leptin as index of adiposity is reduced by caloric restriction and weight loss [18]. It was found that while physical exercise does not induce changes in circulating leptin, fasting reliably affects serum leptin levels [19].

Anorexia nervosa (AN) is a psychiatric disorder characterized by intensive fear of gaining weight, despite the patient's weight being normal or even below normal. This abnormal behavior leads to voluntary reduction of food intake, which results in severe weight and fat loss in affected patients, thus endangering their lives [20]. Hyperactivity often plays a role in developing and maintaining AN and represents an obstacle to weight gain in refeeding [21]. Plasma leptin concentrations are significantly decreased in subjects with AN [22], [23]. It has been suggested that this decrease may reflect fat loss during starvation [22], [24]. Interestingly, weight gain results in significantly increased plasma leptin [22], [23] and decreased plasma sOb-R in AN patients [15]. Leptin levels can also be influenced by lowered insulin levels in AN patients [25], increased activity of sympathetic nervous system (SNS) in abdominal adipose tissue [26] and other factors. Holtkamp et al. [27] found that therapeutically induced weight gain in AN patients accompanied by high serum leptin levels has prognosis of repeated weight loss.

However, all these studies were based on measurement of plasma total and free leptin concentrations but not on measurement of leptin concentrations of local specific adipose tissue. Microdialysis is a powerful and safe technique that allows detection of in vivo local changes in interstitial fluid concentrations of various molecules, including hormones [28], [29], [30], [31]. In previous study [26], we documented increased SNS activity, especially increased norepinephrine (NE) levels in adipose tissue of AN patients, but not in the whole body where the SNS activity was rather decreased. It is known that catecholamines inhibit leptin production [32]. Thus, we hypothesized that the increased NE concentration in the extracellular space of abdominal adipose tissue of AN patients may serve to protect the organism from increased lipolysis by lowering leptin production there. Therefore, we decided to determine leptin levels in interstitial space of adipose tissue in AN patients where the sympathetic activity is alternated.