The adipose tissue is a central metabolic organ in the regulation of whole-body energy homeostasis. The white adipose tissue functions as a key energy reservoir for other organs, whereas the brown adipose tissue accumulates lipids for cold-induced adaptive thermogenesis. Adipose tissues secrete various hormones, cytokines, and metabolites (termed as adipokines) that control systemic energy balance by regulating appetitive signals from the central nerve system as well as metabolic activity in peripheral tissues. In response to changes in the nutritional status, the adipose tissue undergoes dynamic remodeling, including quantitative and qualitative alterations in adipose tissue-resident cells. A growing body of evidence indicates that adipose tissue remodeling in obesity is closely associated with adipose tissue function. Changes in the number and size of the adipocytes affect the microenvironment of expanded fat tissues, accompanied by alterations in adipokine secretion, adipocyte death, local hypoxia, and fatty acid fluxes. Concurrently, stromal vascular cells in the adipose tissue, including immune cells, are involved in numerous adaptive processes, such as dead adipocyte clearance, adipogenesis, and angiogenesis, all of which are dysregulated in obese adipose tissue remodeling. Chronic overnutrition triggers uncontrolled inflammatory responses, leading to systemic low-grade inflammation and metabolic disorders, such as insulin resistance. This review will discuss current mechanistic understandings of adipose tissue remodeling processes in adaptive energy homeostasis and pathological remodeling of adipose tissue in connection with immune response.

Adipocyte differentiation is an important component of obesity and other metabolic diseases. This process is strongly inhibited by many mitogens and oncogenes. Several growth factors that inhibit fat cell differentiation caused mitogen-activated protein (MAP) kinase-mediated phosphorylation of the dominant adipogenic transcription factor peroxisome proliferator-activated receptor γ (PPARγ) and reduction of its transcriptional activity. Expression of PPARγ with a nonphosphorylatable mutation at this site (serine-112) yielded cells with increased sensitivity to ligand-induced adipogenesis and resistance to inhibition of differentiation by mitogens. These results indicate that covalent modification of PPARγ by serum and growth factors is a major regulator of the balance between cell growth and differentiation in the adipose cell lineage.

Adipocyte determination and differentiation-dependent factor 1 (ADD1) is a member of the basic helix-loop--helix leucine zipper (bHLH-LZ) family of transcription factors that binds to two distinct DNA sequences and has been associated with both adipocyte development and cholesterol homeostasis (where it has been termed SREBP1). To investigate the biological role of ADD1, we expressed wild-type and dominant negative forms of this protein with retroviral vectors in preadipocytes and nonadipogenic cells. A dominant-negative form of ADD1 with a point mutation in the DNA-binding domain sharply represses the differentiation of 3T3-L1 cells as observed morphologically or by the expression of adipocyte-specific mRNAs. When NIH-3T3 cells ectopically expressing ADD1 are cultured under hormonal conditions not favoring differentiation, they do not overtly differentiate but still activate expression of mRNAs for fatty acid synthase (FAS) and lipoprotein lipase (LPL), two key genes that regulate fatty acid metabolism. Under culture conditions permissive for differentiation including a PPAR activator, 15%-20% of the cells expressing ADD1 undergo adipogenesis while 2%-3% of cells containing a control vector differentiate. Simultaneous expression of ADD1 with PPAR-y increases the transcriptional activity of this adipogenic nuclear hormone receptor, suggesting involvement of ADD1 in this pathway. These data indicate that ADD1 plays an important role in fat cell gene expression and differentiation, and suggest that it may function by augmenting a step in PPAR3,-mediated transcription.

The ability to regulate specific genes of energy metabolism in response to fasting and feeding is an important adaptation allowing survival of intermittent food supplies. However, little is known about transcription factors involved in such responses in higher organisms. We show here that gene expression in adipose tissue for adipocyte determination differentiation dependent factor (ADD) 1/sterol regulatory element binding protein (SREBP) 1, a basic-helix-loop-helix protein that has a dual DNA-binding specificity, is reduced dramatically upon fasting and elevated upon refeeding; this parallels closely the regulation of two adipose cell genes that are crucial in energy homeostasis, fatty acid synthetase (FAS) and leptin. This elevation of ADD1/SREBP1, leptin, and FAS that is induced by feeding in vivo is mimicked by exposure of cultured adipocytes to insulin, the classic hormone of the fed state. We also show that the promoters for both leptin and FAS are transactivated by ADD1/SREBP1. A mutation in the basic domain of ADD1/SREBP1 that allows E-box binding but destroys sterol regulatory element-1 binding prevents leptin gene transactivation but has no effect on the increase in FAS promoter function. Molecular dissection of the FAS promoter shows that most if not all of this action of ADD1/SREBP1 is through an E-box motif at -64 to -59, contained with a sequence identified previously as the major insulin response element of this gene. These results indicate that ADD1/SREBP1 is a key transcription factor linking changes in nutritional status and insulin levels to the expression of certain genes that regulate systemic energy metabolism.