Obesity is the result of numerous, interacting behavioral, physiological, and biochemical factors. One increasingly important factor is the generation of additional fat cells, or adipocytes, in response to excess feeding and/or large increases in body fat composition. The generation of new adipocytes is controlled by several "adipocyte-specific" transcription factors that regulate preadipocyte proliferation and adipogenesis. Generally these adipocyte-specific factors are expressed only following the induction of adipogenesis. The transcription factor(s) that are involved in initiating adipocyte differentiation have not been identified. Here we demonstrate that the transcription factor, CREB, is constitutively expressed in preadipocytes and throughout the differentiation process and that CREB is stimulated by conventional differentiation-inducing agents such as insulin, dexamethasone, and dibutyryl cAMP. Stably transfected 3T3-L1 preadipocytes were generated in which we could induce the expression of either a constitutively active CREB (VP16-CREB) or a dominantnegative CREB (KCREB). Inducible expression of VP16-CREB alone was sufficient to initiate adipogenesis as determined by triacylglycerol storage, cell morphology, and the expression of two adipocyte marker genes, peroxisome proliferator activated receptor gamma 2, and fatty acid binding protein. Alternatively, KCREB alone blocked adipogenesis in cells treated with conventional differentiation-inducing agents. These data indicate that activation of CREB was necessary and sufficient to induce adipogenesis. Finally, CREB was shown to bind to putative CRE sequences in the promoters of several adipocyte-specific genes. These data firmly establish CREB as a primary regulator of adipogenesis and suggest that CREB may play similar roles in other cells and tissues.Excess body fat, or obesity, is a major health concern in the United States and other developed nations. It has been estimated that 26% of Americans are overweight (2), with 5 to 14% of men and 7 to 24% of women considered obese depending on the definition employed (2,5,6,12,22,45,57). Similar or even higher estimates for the prevalence of obesity have been reported in other countries (42). Obesity contributes to an increased rate of mortality (20) by virtue of its role in the development of cardiovascular disease, diabetes, pulmonary dysfunction, and gallstones (5, 10, 12).Weight gain and obesity occur when energy intake by an individual exceeds the rate of energy expenditure (23). Energy intake and expenditure are in turn determined by multiple, interacting factors ranging from dietary composition and feeding and exercise habits to physiologic factors and biochemical pathways that modulate lipid and overall energy metabolism (58). At the cellular level obesity was originally considered a hypertrophic disease resulting from an increase in fat cell size or volume (30). However, several studies have demonstrated a hyperplastic component to obesity. For example, sequential biopsies in children indicate that f...