Adaptive thermogenesis is an important component of energy homeostasis and a metabolic defense against obesity. We have cloned a novel transcriptional coactivator of nuclear receptors, termed PGC-1, from a brown fat cDNA library. PGC-1 mRNA expression is dramatically elevated upon cold exposure of mice in both brown fat and skeletal muscle, key thermogenic tissues. PGC-1 greatly increases the transcriptional activity of PPARgamma and the thyroid hormone receptor on the uncoupling protein (UCP-1) promoter. Ectopic expression of PGC-1 in white adipose cells activates expression of UCP-1 and key mitochondrial enzymes of the respiratory chain, and increases the cellular content of mitochondrial DNA. These results indicate that PGC-1 plays a key role in linking nuclear receptors to the transcriptional program of adaptive thermogenesis.

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.

Adipose differentiation is an important part of the energy homeostasis system of higher organisms. Recent data have suggested that this process is controlled by an interplay of transcription factors including PPAR␥, the C͞EBPs, and ADD1͞SREBP1. Although these factors interact functionally to initiate the program of differentiation, there are no data concerning specific mechanisms of interaction. We show here that the expression of ADD1͞SREBP1 specifically increases the activity of PPAR␥ but not other isoforms, PPAR␣, or PPAR␦. This activation occurs through the ligandbinding domain of PPAR␥ when it is fused to the DNA-binding domain of Gal4. The stimulation of PPAR␥ by ADD1͞SREBP1 does not require coexpression in the same cells; supernatants from cultures that express ADD1͞SREBP1 augment the transcriptional activity of PPAR␥. Finally, we demonstrate directly that cells expressing ADD1͞SREBP1 produce and secrete lipid molecule(s) that bind directly to PPAR␥, displacing the binding of radioactive thiazolidinedione ligands. These data establish that ADD1͞SREBP1 can control the production of endogenous ligand(s) for PPAR␥ and suggest a mechanism for coordinating the actions of these adipogenic factors.The adipocyte is a highly specialized cell type that plays an important role in energy homeostasis. The primary role of fat cells is to store triglycerides in periods of energy excess and to release this energy during nutritional deprivation. Adipocyte differentiation is a complex process accompanied by coordinated changes in cell morphology, hormone sensitivity, and gene expression. These changes result from the action of several transcription factors that influence these processes, including PPAR␥, the C͞EBPs, and ADD1͞SREBP1 (ref. 1 and references therein). These factors do not work completely independently but interact functionally in several important ways.PPAR␥ is a member of the nuclear hormone receptor family that is expressed preferentially in adipose tissue (2). It functions as an obligate heterodimer with the retinoid X receptor (RXR) to bind to target sequences termed DR-1 sites (2). Activation of PPAR␥ by ligands is sufficient to stimulate adipose differentiation in many types of fibroblastic cells (2-4). Ligands for this receptor include the antidiabetic thiazolidinedione (TZD) drugs, 15-deoxy-⌬ 12,14 -prostaglandin J2 (15-deoxy-PGJ2), and certain polyunsaturated fatty acids (5-7). The biological ligands now known bind to PPAR␥ with relatively low affinity (K d ϭ 2-50 M), compared with the affinity (typically K d Ͻ 1 nM) for most bona fide ligands for nuclear receptors. Whether the known PPAR␥ ligands represent endogenous activators for this receptor is not clear.The C͞EBP family also appears to be very important in adipogenesis. Ectopic expression of C͞EBP␤ or C͞EBP␣ can promote the differentiation of fibroblastic cells (8-13). There is evidence that C͞EBP␤ does this by increasing the expression of PPAR␥, whereas C͞EBP␣ appears to be able to powerfully synergize with PPAR␥ to promote differentiat...