Leptin Deficiency Causes Insulin Resistance Induced by Uncontrolled Diabetes

OBJECTIVE

Depletion of body fat stores during uncontrolled, insulin-deficient diabetes (uDM) results in markedly reduced plasma leptin levels. This study investigated the role of leptin deficiency in the genesis of severe insulin resistance and related metabolic and neuroendocrine derangements induced by uDM.

RESEARCH DESIGN AND METHODS

Adult male Wistar rats remained nondiabetic or were injected with the β-cell toxin, streptozotocin (STZ) to induce uDM and subsequently underwent subcutaneous implantation of an osmotic minipump containing either vehicle or leptin at a dose (150 μg/kg/day) designed to replace leptin at nondiabetic plasma levels. To control for leptin effects on food intake, another group of STZ-injected animals were pair fed to the intake of those receiving leptin. Food intake, body weight, and blood glucose levels were measured daily, with body composition and indirect calorimetry performed on day 11, and an insulin tolerance test to measure insulin sensitivity performed on day 16. Plasma hormone and substrate levels, hepatic gluconeogenic gene expression, and measures of tissue insulin signal transduction were also measured.

RESULTS

Physiologic leptin replacement prevented insulin resistance in uDM via a mechanism unrelated to changes in food intake or body weight. This effect was associated with reduced total body fat and hepatic triglyceride content, preservation of lean mass, and improved insulin signal transduction via the insulin receptor substrate–phosphatidylinositol-3-hydroxy kinase pathway in the liver, but not in skeletal muscle or adipose tissue. Although physiologic leptin replacement lowered blood glucose levels only slightly, it fully normalized elevated plasma glucagon and corticosterone levels and reversed the increased hepatic expression of gluconeogenic enzymes characteristic of rats with uDM.

CONCLUSIONS

We conclude that leptin deficiency plays a key role in the pathogenesis of severe insulin resistance and related endocrine disorders in uDM. Treatment of diabetes in humans may benefit from correction of leptin deficiency as well as insulin deficiency.Recent evidence implicates leptin not only in the regulation of energy balance but also in glucose homeostasis as well. In addition to hyperphagia and obesity, insulin resistance is a prominent feature of animal models characterized by reduced leptin signaling (1), and leptin administration improves insulin sensitivity and glucose metabolism in these models (2,3) independently of its effects on energy homeostasis (4). Investigation into the role of leptin in glucose metabolism has focused largely on genetic models of impaired leptin signaling (e.g., leptin-deficient ob/ob mice), whereas other studies have used pharmacologic doses of leptin (1–6). In this study we investigated the physiologic role of leptin in glucose metabolism by determining the contribution made by leptin deficiency to the severe insulin resistance and associated metabolic and endocrine dysfunction characteristic of uncontrolled, insulin-deficient diabetes (uDM).Severe leptin deficiency is a well-documented consequence of uDM that occurs after destruction of insulin-secreting β-cells (7,8). Because insulin is required for the synthesis and storage of triglyceride in adipose tissue, weight gain cannot occur in uDM, and the associated loss of body fat is accompanied by markedly reduced plasma leptin levels. This effect, in turn, is implicated in the mechanism whereby uDM increases food intake (9), because exogenous leptin administration at doses that prevent a fall in plasma leptin levels also prevent hyperphagia in uDM (8). Another feature of uDM in humans is progressive, severe insulin resistance (10–12), an effect also observed in streptozotocin (STZ)-induced diabetes in rats (13). Although insulin deficiency clearly underlies hyperglycemia and weight loss in uDM, the contribution of markedly reduced plasma leptin levels to insulin resistance and related metabolic and endocrine derangements in this setting remains to be determined. Because plasma levels of leptin as well as of insulin are normalized by insulin treatment of STZ-induced diabetes, at least some of the beneficial effects that have been ascribed to insulin treatment could result from restoring leptin action to normal (7). Indeed, hyperleptinemia generated either by pharmacologic administration of leptin (5) or with adenoviral gene therapy (14) ameliorates hyperglycemia and associated increases of plasma glucagon levels in STZ-induced diabetes, despite persistently low insulin levels. These data raise the possibility that deficient endogenous leptin signaling may underlie at least some manifestations of uDM.Based on these considerations, we sought to determine the extent to which deficiency of endogenous leptin contributes to insulin resistance and related endocrine dysfunction in STZ-induced diabetes. To accomplish this goal, we subcutaneously infused either vehicle or leptin at a dose that prevents leptin deficiency in rats with STZ-induced uDM. We found that physiologic leptin replacement prevented the development of insulin resistance in uDM via a mechanism independent of its effects on food intake and body weight. Moreover, this leptin effect was associated with normalization of elevated plasma levels of glucagon and corticosterone, with the reversal of increased hepatic expression of the gluconeogenic genes, glucose-6-phosphatase (G6Pase), and phosphoenolpyruvate kinase (Pepck), and with improved insulin signal transduction via the insulin receptor substrate–phosphatidylinositol-3-hydroxy kinase (IRS-PI3K) pathway in the liver, but not in skeletal muscle or adipose tissue. In comparison, physiologic leptin replacement only modestly reduced hyperglycemia in STZ-induced diabetic rats and did not alter the potent upregulation of hepatic Igfbp2 mRNA levels previously reported (15). Taken together, these data suggest that reduced leptin levels contribute to the progressive, severe insulin resistance characteristic of uDM via a mechanism that appears to predominantly involve the liver.