Leptin expression and action: new experimental paradigms.

The identification of the ob gene through positional cloning (1) and the demonstration that its protein product leptin reverses the obesity–diabetes syndrome in obyob mice (2–4) have ushered in an exciting new era in nutritional physiology and obesity research. Preeminent among a rapidly expanding list of molecules whose discovery has fundamentally advanced our understanding of the regulation of appetite and energy expenditure, leptin deserves special status. With the realization that adipocytes produce a hormone that acts through discrete receptors (5, 6) on distant targets to create a feedback loop for body weight regulation, our understanding of the pathophysiology of obesity has entered the ‘‘endocrine era.’’ Although numerous endocrine abnormalities have been identified in association with obesity, endocrine insights into the etiology and therapy of obesity were lacking until leptin appeared on the scene. Now, laboratories race to measure leptin secretion and levels in disorders of body weight, and to characterize the sites and mechanisms of leptin action, much as insulin secretion and action have been studied for many years in obesity and obesity-linked non-insulin-dependent diabetes mellitus. Indeed, interesting parallels between research on insulin and leptin continue to emerge. As is the case with insulin and its receptor in diabetes, mutations of leptin and its receptor are rare in human obesity (7, 8) (none have yet been described), and most obese individuals have higher levels of serum-immunoreactive leptin than do nonobese individuals, raising the specter of ‘‘leptin resistance’’ (9, 10). Although absolutely increased compared with lean individuals, leptin levels may be relatively lower as a function of body fat content in some obese individuals than in others, and individuals with relatively ‘‘low leptin’’ may be more likely to gain weight over subsequent years (11). It will be interesting to determine whether these lower levels are a fixed attribute of certain individuals or, analagous to the pathophysiology of insulin secretion in developing non-insulindependent diabetes, leptin production diminishes over time. These findings increase the importance of elucidating the factors that determine leptin expression in adipocytes, and the mechanism of leptin action in target cells. This issue of the Proceedings has two papers that address these opposite poles of leptin biology. The paper by Mandrup et al. (12) establishes a new system for defining the regulation of leptin expression by adipocytes, and the paper by Shimabukuro et al. (13) raises a provocative new hypothesis regarding the nature of leptin actions on metabolic pathways. In this commentary, I will attempt to place these findings in the broader perspective of leptin biology two and one-half years after the initial discovery of leptin. Several aspects of regulated expression of leptin are worthy of note. The first involves the tissue-specific expression of leptin. Leptin is strongly expressed in white adipose tissue, and is absent or expressed at extremely low levels in other tissues. Although expression of leptin mRNA has been described in the closely related brown adipose tissue (BAT; refs. 14–16), most of this expression may be due to contaminating white adipocytes (17). Because white adipocytes are primarily involved in energy storage, for which leptin expression is the measure, whereas brown adipocytes subserve regulated energy dissipation, it is not surprising that these two tissues have divergent capacities for leptin expression. Similarly, uncoupling protein (UCP; ref. 18), the gene most fundamental to brown adipocyte thermogenic function (19), is expressed exclusively in BAT. On the other hand, leptin may be the first gene described to be expressed in white adipose tissue (WAT) but to a limited extent or not at all in BAT, a fact that creates new opportunities to evaluate the molecular basis for tissuespecific gene expression in these tissues. A second issue relates to the mechanism for regulated expression of leptin within WAT in physiology and disease. In experimental animals and man that were studied in the fed state, leptin expression and levels generally increase in parallel to adipose stores (9, 20), in agreement with the proposed role of leptin as a readout signal of adipocyte triglyceride stores. The mechanism for this tight coupling between triglyceride stores and leptin expression and secretion remains obscure. Many studies have observed a correlation between insulin and leptin levels (21), but this is most often explained by leptin and insulin each covarying with obesity. Insulin however has been found to be capable of increasing leptin expression (22) and levels (23) under some circumstances, and the idea that insulin may be a controlling factor over leptin expression has been suggested. Although possible, dominant control of leptin expression by an exogenous factor such as insulin would seem to diminish the rationale for the adipocyte being in the feedback loop in the first place. On the other hand, leptin expression and levels fall rapidly with starvation (14, 24), and this suppression is disproportionate to the fall in adipocyte energy stores. The fall in leptin appears to be central to the neuroendocrine adaptation to starvation (25), and could be the primary purpose for which leptin evolved. Falling insulin may be a key regulatory signal for the suppression of leptin expression with starvation (24). Other positive regulators include glucocorticoids at high doses (26, 27) and certain cytokines (28, 29), and negative regulators include beta adrenergic agonists or cAMP (27, 30). Despite these external influences, it is likely that cell-autonomous factors are the major links between adipocyte size and leptin gene expression. Which intracellular metabolites, signaling molecules, or transcription factors provide the necessary link is as yet unclear. Like many other adipocyte genes, the ob gene promoter is positively regulated through a functional binding site for CEBPa (31–34). In contrast, thiazoladinedione agonists for PPARg transcription factors (35, 36) suppress leptin expression in vitro and in vivo in rodents (37–40), and this may involve, at least in part, a functional antagonism between CEBPa and PPARg on the leptin promoter (41). The study of leptin gene expression has been hampered by the absence of a suitable in vitro test system. Preadipocyte cell lines that differentiate in culture have been used extensively to characterize cis and trans factors that regulate adipocyte gene Copyright q 1997 by THE NATIONAL ACADEMY OF SCIENCES OF THE USA 0027-8424y97y944242-4$2.00y0 PNAS is available online at http:yywww.pnas.org. Abbreviations: CNS, central nervous system; FFA, free fatty acids.