Factors controlling brown adipose tissue development

Weight-loss with activation of brown fat: Suspect pheochromocytoma

INTRODUCTION

Excess catecholamine stimulates heat production in brown adipose tissue (BAT). Activation of BAT can be detected in patients presenting pheochromocytoma.

CASE STUDY

A 58-year-old female patient sought medical advice due to 13 kg weight loss over 2 years accompanied by sweating and high blood pressure. Thoracic-abdominal-pelvic CT-scan revealed a solid 40 mm mass in the left adrenal compartment with peri-adrenal nodules and a solid 80 mm mass at the lower end of the right kidney. ¹⁸FDG-PET scan exhibited intense uptake in the supraclavicular, intercostal, mediastinal, peri-renal, mesenteric, iliac and inguinal spaces. Renal tumor with locoregional infiltration and remote metastases was initially considered. Diagnosis of pheochromocytoma was subsequently confirmed by a 10-fold increase in urinary catecholamine, metanephrine and normetanephrine levels. Left adrenalectomy confirmed the diagnosis of pheochromocytoma, with 3 lymph-node metastases in the adjacent adipose tissue surrounded by brown fat. The patient was clinically asymptomatic with normal blood pressure at 3 months post-surgery. A weight gain of 6 kg was recorded, with normalisation of catecholamines/metanephrine/normetanephrine levels. Bilateral peri-renal infiltration (including the right renal mass) disappeared on CT-scan, and TEP-18-FDG no longer showed hypermetabolism. Recurrent mediastinal metastases were diagnosed 6 months after surgery.

CONCLUSION

Brown fat activation may mislead diagnosis of pheochromocytoma, suggesting multi-metastatic extra-adrenal tumor, if clinicians are not aware of it.

Cell-cycle arrest in mature adipocytes impairs BAT development but not WAT browning, and reduces adaptive thermogenesis in mice

We previously reported brown adipocytes can proliferate even after differentiation. To test the involvement of mature adipocyte proliferation in cell number control in fat tissue, we generated transgenic (Tg) mice over-expressing cell-cycle inhibitory protein p27 specifically in adipocytes, using the aP2 promoter. While there was no apparent difference in white adipose tissue (WAT) between wild-type (WT) and Tg mice, the amount of brown adipose tissue (BAT) was much smaller in Tg mice. Although BAT showed a normal cellular morphology, Tg mice had lower content of uncoupling protein 1 (UCP1) as a whole, and attenuated cold exposure- or β3-adrenergic receptor (AR) agonist-induced thermogenesis, with a decrease in the number of mature brown adipocytes expressing proliferation markers. An agonist for the β3-AR failed to increase the number of proliferating brown adipocytes, UCP1 content in BAT, and oxygen consumption in Tg mice, although the induction and the function of beige adipocytes in inguinal WAT from Tg mice were similar to WT mice. These results show that brown adipocyte proliferation significantly contributes to BAT development and adaptive thermogenesis in mice, but not to induction of beige adipocytes.

Intense FDG activity in the brown adipose tissue in omental and mesenteric regions in a patient with malignant pheochromocytoma

A 39-year-old man who had a history of surgical excision of left adrenal pheochromocytoma 3 years ago underwent an FDG PET/CT scan to evaluate possible metastatic/recurrent disease. In addition to multiple FDG-avid lesions typical of hypermetabolic malignant disease and the FDG uptake in regions rich in brown adipose tissue, there was also intense FDG activity in the omental and mesenteric regions, which are not common locations of elevated FDG activity. On the repeat FDG PET/CT scan 3 days later after the patient was prepared with propranolol, the omental and mesenteric FDG activity was diminished.

S22153, a melatonin antagonist, dissociates different aspects of photoperiodic responses in Syrian hamsters

In the Syrian hamster, short photoperiod (SP) induces changes in several physiological functions (body mass, reproduction, hibernation), and these responses involve the pineal hormone melatonin. The present study investigated the effects of a melatonin antagonist, S22153, on photoperiodic adaptation of male Syrian hamster. When constantly released from subcutaneous implants, S22153 had no effect on body or testes masses of animals kept in long photoperiod. S22153 decreased the total hibernation duration observed in animals exposed to SP and low temperature. The decrease in hibernation duration was due to a marked reduction in the number and duration of hypothermic bouts. Moreover, S22153 significantly inhibited the increase of interscapular brown adipose tissue (BAT) mass induced by SP. However, neither the gonadal atrophy nor the body mass increase induced by SP were affected by S22153. These results show that S22153 affects only part of the physiological changes controlled by SP and cold. Whether the decreases in BAT mass and hibernation duration are linked still remains an open question.

Increased energy expenditure, decreased adiposity, and tissue-specific insulin sensitivity in protein-tyrosine phosphatase 1B-deficient mice

Protein-tyrosine phosphatase 1B (PTP-1B) is a major protein-tyrosine phosphatase that has been implicated in the regulation of insulin action, as well as in other signal transduction pathways. To investigate the role of PTP-1B in vivo, we generated homozygotic PTP-1B-null mice by targeted gene disruption. PTP-1B-deficient mice have remarkably low adiposity and are protected from diet-induced obesity. Decreased adiposity is due to a marked reduction in fat cell mass without a decrease in adipocyte number. Leanness in PTP-1B-deficient mice is accompanied by increased basal metabolic rate and total energy expenditure, without marked alteration of uncoupling protein mRNA expression. In addition, insulin-stimulated whole-body glucose disposal is enhanced significantly in PTP-1B-deficient animals, as shown by hyperinsulinemic-euglycemic clamp studies. Remarkably, increased insulin sensitivity in PTP-1B-deficient mice is tissue specific, as insulin-stimulated glucose uptake is elevated in skeletal muscle, whereas adipose tissue is unaffected. Our results identify PTP-1B as a major regulator of energy balance, insulin sensitivity, and body fat stores in vivo.

Molecular cloning and functional characterization of the promoter region of the human uncoupling protein-2 gene

As a member of the uncoupling protein family, UCP2 is ubiquitously expressed in rodents and humans, implicating a major role in thermogenesis. To analyze promoter function and regulatory motifs involved in the transcriptional regulation of UCP2 gene expression, 3.3 kb of 5'-flanking region of the human UCP2 (hUCP2) gene have been cloned. Sequence analysis showed that the promoter region of hUCP2 lacks a classical TATA or CAAT box, however, appeared GC-rich resulting in the presence of several Sp-1 motifs and Ap-1/-2 binding sites near the transcription initiation site. Functional characterization of human UCP2 promoter-CAT fusion constructs in transient expression assays showed that minimal promoter activity was observed within 65 bp upstream of the transcriptional start site (+1). 75 bp further upstream (from nt -141 to -66) a strong cis-acting regulatory element (or enhancer) was identified, which significantly enhanced basal promoter activity. The regulation of human UCP2 gene expression involves complex interactions among positive and negative regulatory elements distributed over a minimum of 3.3 kb of the promoter region.

Induction of functional uncoupling protein in guinea pigs infused with noradrenaline. Studies with isolated brown adipocytes

Continuous infusion of noradrenaline over the interscapular brown fat of guinea pigs maintained at thermoneutrality (28-32 degrees C) induces changes similar to those after cold-adaptation. (1) Multilocular fat droplets appear within the brown adipocytes. (2) The number of mitochondria per adipocyte and the total number of adipocytes both increase. (3) Noradrenaline addition to isolated adipocytes causes near maximal uncontrolled respiration. (4) The cells become more sensitive to fatty acid-induced uncoupling. (5) The tissue-specific uncoupling protein per mg of mitochondrial protein is increased 5-fold. Specific alpha- and beta-agonists were also chronically infused. (6) Separate infusion of phenylephrine or isoprenaline was not able to stimulate mitochondriogenesis or hyperplasia. (7) Adipocytes from these animals could not be uncoupled by acute noradrenaline. (8) Simultaneous chronic infusion of phenylephrine and isoprenaline reproduced the effects of chronic noradrenaline infusion.