Hétérogénéité et plasticité cellulaires des tissus adipeux

The nervous system and adipose tissues: a tale of dialogues

White, beige, and brown adipose tissues play a crucial role in maintaining energy homeostasis. Due to the heterogeneous and diffuse nature of fat pads, this balance requires a fine and coordinated control of many actors and therefore permanent dialogues between these tissues and the central nervous system. For about two decades, many studies have been devoted to describe the neuro-anatomical and functional complexity involved to ensure this dialogue. Thus, if it is now clearly demonstrated that there is an efferent sympathetic innervation of different fat depots controlling plasticity as well as metabolic functions of the fat pad, the crucial role of sensory innervation capable of detecting local signals informing the central nervous system of the metabolic state of the relevant pads is much more recent. The purpose of this review is to provide the current state of knowledge on this subject.

Human adipose cells as candidates in defense and tissue remodeling phenomena

Macrophages are part of the immunity defense mechanism via oxidative burst and phagocytosis. They are also involved in tissue remodeling via cytokine secretion and apoptotic body clearance. Previously, we demonstrated that adipose cells and macrophages share some of their features and functions. Our aim was to further test this hypothesis in humans. We first demonstrated that human preadipocytes exhibit phagocytosis of yeast, this effect being specific compared to another fibroblastic cell type, the skin fibroblast. Furthermore, as in rodents, human preadipocytes exhibit anti-microbial activity. Finally, for the first time, it was shown that these cells were able to phagocyte apoptotic lymphocytes. Altogether, these data suggest an active involvement of fat cells in host defense and tissue remodeling, which might play an important role at the level of the whole organism due to the large amount of adipose tissue. This gives support for some observations linking obesity or cachexia to immunological disorders.

Multilocular fat cells in WAT of CL-316243-treated rats derive directly from white adipocytes

Multilocular, mitochondria-rich adipocytes appear in white adipose tissue (WAT) of rats treated with the beta3-adrenoceptor agonist, CL-316243 (CL). Objectives were to determine whether these multilocular adipocytes derived from cells that already existed in the WAT or from proliferation of precursor cells and whether new mitochondria contained in them were typical brown adipocyte mitochondria. Use of 5-bromodeoxyuridine to identify cells that had undergone mitosis during the CL treatment showed that most multilocular cells derived from cells already present in the WAT. Morphological techniques showed that at least a subpopulation of unilocular adipocytes underwent conversion to multilocular mitochondria-rich adipocytes. A small proportion of multilocular adipocytes ( approximately 8%) was positive for UCP1 by immunohistochemistry. Biochemical techniques showed that mitochondrial protein recovered from WAT increased 10-fold and protein isolated from brown adipose tissue (BAT) doubled in CL-treated rats. Stained gels showed a different protein composition of new mitochondria isolated from WAT from that of mitochondria isolated from BAT. Western blotting showed new mitochondria in WAT to contain both UCP1, but at a much lower concentration than in BAT mitochondria, and UCP3, at a higher concentration than that in BAT mitochondria. We hypothesize that multilocular adipocytes present at 7 days of CL treatment have two origins. First, most come from convertible unilocular adipocytes that become multilocular and make many mitochondria that contain UCP3. Second, some come from a cell that gives rise to more typical brown adipocytes that express UCP1.

Hypertrophy of brown adipocytes in brown and white adipose tissues and reversal of diet-induced obesity in rats treated with a beta3-adrenoceptor agonist

In a previous study, we demonstrated that chronic treatment with a new beta3-adrenoceptor agonist, CL 316,243 [disodium (R,R)-5-[2-[[2-(3-chlorophenyl)-2-hydroxyethyl]-amino]propyl]-1,3-ben zodioxazole-2,2-dicarboxylate], promoted thermogenesis, caused the appearance of multilocular adipocytes in white adipose tissue (WAT), and retarded development of obesity in young rats eating a high-fat diet (Himms-Hagen et al., Am J Physiol 266: R1371-R1382, 1994). Objectives of the present study were to find out whether CL 316,243 could reverse established diet-induced obesity in rats and to identify the multilocular adipocytes that appeared in WAT. Infusion of CL 316,243 (1 mg/kg/day) reduced abdominal fat, with a decrease in enlarged adipocyte size but no loss of white adipocytes. The resting metabolic rate increased by 40-45%, but food intake was not altered. Abundant densely stained multilocular brown adipocytes expressing uncoupling protein (UCP) appeared in retroperitoneal WAT, in which a marked increase in protein content occurred. UCP content of interscapular brown adipose tissue (BAT) was also increased markedly. We suggest that the substantial increase in the resting metabolic rate induced by CL 316,243 occurs in brown adipocytes in both BAT and WAT. The origin of the brown adipocytes that appeared in WAT is uncertain. They may have been small brown preadipocytes, expressing beta3-adrenoceptors but with few mitochondria and little or no UCP, that were induced to hypertrophy by the beta3-agonist.