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Adipose Tissue As an Endocrine Organ

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Adipose Tissue As an Endocrine Organ 0021-7557/07/83-05-Supl/S192 Jornal de Pediatria Copyright © 2007 by Sociedade Brasileira de Pediatria ARTIGO DE REVISÃO Adipose tissue as an endocrine organ: from theory to practice O tecido adiposo como órgão endócrino: da teoria à prática Miriam Helena Fonseca-Alaniz1, Julie Takada2, Maria Isabel Cardoso Alonso-Vale3, Fabio Bessa Lima4 Resumo Abstract Objetivos: Mostrar os avanços na pesquisa sobre o papel Objectives: To describe the advances in research into the fisiológico do tecido adiposo branco, ressaltando o seu papel physiological role of white adipose tissue, with emphasis on its endócrino em processos inflamatórios, no comportamento alimentar, endocrinal role in inflammatory processes, feeding behavior, insulin na sensibilização à insulina e na modulação do processo de sensitization and modulation of the atherogenetic process. To deal aterogênese. Abordar o potencial papel do tecido adiposo como with the potential role of adipose tissue as a source of stem cells for fonte de células-tronco para regeneração de tecidos, com especial regeneration of tissues, with special emphasis on adipogenesis and ênfase para a adipogênese e suas conseqüências para a geração de obesidade. its consequences for development of obesity. Fontes dos dados: Informações importantes constantes da Sources: Important information was compiled from the scientific literatura científica foram compiladas de modo a que esta leitura literature in order that this analysis contains an explanatory synthesis contenha uma síntese esclarecedora dos aspectos mencionados of the aspects mentioned above. acima. Summary of the findings: In addition to its classical functions Síntese dos dados: O tecido adiposo possui, além das suas as primary metabolic energy store, meeting energy requirements funções clássicas como principal estoque de energia metabólica, during periods of deprivation by means of lypolisis, adipose tissue suprindo as necessidades energéticas em períodos de carência also has the capacity to synthesize and secrete a variety of hormones mediante a lipólise, a capacidade de sintetizar e secretar vários – the adipokines. These are active in a range of processes, such as hormônios, as adipocinas. Estas agem em diversos processos, como control of nutritional intake (leptin) and control of sensitivity to o controle da ingestão alimentar (leptina) e o controle da insulin and inflammatory processes (TNF-α, IL-6, resistin, visfatin, sensibilidade à insulina e de processos inflamatórios (TNF-α, IL-6, adiponectin). Furthermore, since adipose tissue also contains resistina, visfatina, adiponectina). Além disso, como o tecido adiposo undifferentiated cells, it has the ability to generate new adipocytes, contém também células indiferenciadas, tem a habilidade de gerar regenerating its own tissue (adipogenesis), and also the ability to novos adipócitos, regenerando o próprio tecido (adipogênese), bem give rise to other cells (myoblasts, chondroblasts, osteoblasts), como originar outras células (mioblastos, condroblastos, which has great therapeutic potential in the not-too-distant future. osteoblastos), fato este que tem grande potencial terapêutico em futuro não muito distante. Conclusions: The range of functional possibilities of adipose Conclusão: Amplia-se o leque de possibilidades funcionais do tissue has widened. An understanding of these potentials could tecido adiposo. A compreensão dessas potencialidades pode fazer make this tissue a great ally in the fight against conditions that are deste tecido o grande aliado no combate de moléstias que atualmente currently assuming epidemic proportions (obesity, diabetes mellitus, vêm assumindo proporções epidêmicas (obesidade, diabetes melito, arterial hypertension and arteriosclerosis) and in which adipose hipertensão arterial e arteriosclerose), nas quais o tecido adiposo tissue is still seen as the enemy. ainda é tido como um grande vilão. J Pediatr (Rio J). 2007;83(5 Supl):S192-203: Adipócito, lipogênese, J Pediatr (Rio J). 2007;83(5 Suppl):S192-203: Adipocyte, lipólise, adipocinas, adipogênese. lipogenesis, lypolisis, adipokines adipogenesis. 1. Farmacêutica. Doutora, Instituto de Ciências Biomédicas (ICB), Universidade de São Paulo (USP), São Paulo, SP. 2. Nutricionista. Mestra e Doutoranda, Programa de Fisiologia Humana, Instituto de Ciências Biomédicas (ICB), Universidade de São Paulo (USP), São Paulo, SP. 3. Farmacêutica. Doutora e Pós-doutoranda, Laboratório de Fisiologia do Tecido Adiposo, Departamento de Fisiologia e Biofísica, Instituto de Ciências Biomédi- cas (ICB), Universidade de São Paulo (USP), São Paulo, SP. 4. Professor associado, Departamento de Fisiologia e Biofísica, Instituto de Ciências Biomédicas (ICB), Universidade de São Paulo (USP), São Paulo,SP. Como citar este artigo: Fonseca-Alaniz MH, Takada J, Alonso-Vale MI, Lima FB. Adipose tissue as an endocrine organ: from theory to practice. J Pediatr (Rio J). 2007;83(5 Suppl):S192-203. doi 10.2223/JPED.1709 S192 Tecido adiposo: órgão endócrino - Fonseca-Alaniz MH et al. Jornal de Pediatria - Vol. 83, Nº 5(Supl), 2007 S193 Introdução Nos mamíferos, existem dois tipos de TA: tecido adiposo Aspectos gerais branco (TAB) e tecido adiposo marrom (TAM). Os adipócitos destes apresentam diferenças importantes. O adipócito As espécies animais precisam garantir a sobrevivência sob branco maduro armazena TAG em uma única e grande gota condições inóspitas ou desfavoráveis. Vertebrados em geral lipídica que ocupa a porção central da célula, correspon- e mamíferos em particular,por possuírem tecido adiposo (TA), dendo a 85-90% da massa celular, e desloca citoplasma, conseguem armazenar o excesso de calorias como lipídeos núcleo e demais organelas para a periferia, onde se acomo- (triacilgliceróis – TAG). Estes, que são hidrofóbicos, podem dam dentro de uma fina camada de citosol. Curiosamente, ser estocados em grandes quantidades sem usar água como durante seu desenvolvimento, o adipócito jovem contém múl- solvente e carregam duas vezes mais energia por unidade de tiplas gotículas lipídicas que coalescem para formar uma inclu- massa que os demais nutrientes. são lipídica unitária com o amadurecimento. Embora tenham Sendo o principal reservatório energético, o TA sempre foi volume variável, os adipócitos brancos maduros são células conhecido como reservatório energético e como isolante tér- grandes, centenas a alguns milhares de vezes maiores que mico. Estas duas propriedades, por muito tempo, foram apli- hemácias, fibroblastos e células do sistema imune e podem cadas ao estudo da regulação do metabolismo energético e alterar acentuadamente seu tamanho conforme a quanti- pouca atenção se deu à análise de outras habilidades suas. dade de TAG acumulada4. As pesquisas sobre o TA o encaravam como tendo um padrão único e uniforme de respostas, sem levar em consideração Além dos adipócitos, o TA contém matriz de tecido con- características como localização ou participação em deman- juntivo (fibras colágenas e reticulares), fibras nervosas, das locais desvinculadas de processos mais globais de con- estroma vascular,nódulos linfáticos, células imunes (leucóci- trole metabólico. Por isso, os estudos mais antigos, em sua tos, macrófagos), fibroblastos e pré-adipócitos (células adi- 1 maioria, analisaram respostas metabólicas em amostras de posas indiferenciadas) . uma determinada região, tomando-a como representativa de Tecido adiposo marrom toda a massa adiposa. Esses estudos menosprezaram aspec- tos regionais de ajuste metabólico distorcendo ou ocultando O TAM, especializado na produção de calor (termogê- uma importante dimensão funcional sua. nese), está praticamente ausente em humanos adultos, mas é encontrado em fetos e recém-nascidos. Seu adipócito, com É também verdade que, conforme a origem do TA (por 30-40 µm de diâmetro médio, é menor que o branco (diâme- exemplo, se periepididimal ou retroperitoneal), tro médio de 60-100 µm). Possui várias gotículas lipídicas cito- detectavam-se diferenças na sua capacidade lipolítica ou lipo- plasmáticas de diferentes tamanhos, citoplasma gênica. O tecido periepididimal, em roedores em geral, foi lar- relativamente abundante, núcleo esférico e ligeiramente gamente utilizado como padrão para avaliar respostas frente excêntrico e numerosas mitocôndrias que liberam calor pela às mais diferentes condições, normais ou patológicas. Ques- oxidação de ácidos graxos5. A calorigênese é garantida pela tionamentos sobre a representatividade deste coxim gordu- proteína desacopladora-1 (UCP-1 ou termogenina) locali- roso foram surgindo, indicando que uma revisão detalhada zada na membrana mitocondrial interna, que atua como um sobre o seu papel fisiológico era imperiosa. canal de próton que descarrega o potencial gerado pelo acú- Os adipócitos são as únicas células especializadas e per- mulo de prótons no espaço intermembranoso durante o ciclo feitamente adaptadas para armazenar lipídeos sem que isso de Krebs, desviando-os do complexo F1F0 (ATP sintase), impe- comprometa a sua integridade funcional. Possuem a maqui- dindo a síntese de ATP e permitindo que se dissipe em calor5. naria enzimática necessária para sintetizar ácidos graxos A alta concentração de citocromo oxidase dessas mitocôn- (processo denominado de lipogênese) e estocar TAG em perí- drias contribui para a sua coloração mais escurecida6. odos de oferta abundante de energia e para mobilizá-los pela lipólise quando há déficit calórico (lipólise). O sistema ner- Tecido adiposo branco voso central participa da regulação desses dois processos Embora a sua participação
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