Ghrelin, an endogenous ligand for growth hormone secretagogue (GHS) receptor originally isolated from the stomach, occurs in the hypothalamic arcuate nucleus and may play a role in energy homeostasis. Synthetic GHSs have activated the hypothalamic arcuate neurons containing neuropeptide Y (NPY), suggesting the involvement of NPY in some of ghrelin actions. This study was designed to elucidate the role of ghrelin in the regulation of food intake. A single intracerebroventricular (ICV) injection of ghrelin (5-5,000 ng/rat) caused a significant and dose-related increase in cumulative food intake in rats. Ghrelin (500 ng/rat) was also effective in growth hormone-deficient spontaneous dwarf rats. Hypothalamic NPY mRNA expression was increased in rats that received a single ICV injection of ghrelin (500 ng/rat) (~160% of that in vehicle-treated groups, P < 0.05). The ghrelin's orexigenic effect was abolished dose-dependently by ICV co-injection of NPY Y1 receptor antagonist (10-30 µg/rat). The leptininduced inhibition of food intake was reversed by ICV co-injection of ghrelin in a dose-dependent manner (5-500 ng/rat). Leptin reduced hypothalamic NPY mRNA expression by 35% (P < 0.05), which was abolished by ICV co-injection of ghrelin (500 ng/rat). This study provides evidence that ghrelin is an orexigenic peptide that antagonizes leptin action through the activation of hypothalamic NPY/Y1 receptor pathway.

Lipoatrophic diabetes is caused by a deficiency of adipose tissue and is characterized by severe insulin resistance, hypoleptinemia, and hyperphagia. The A-ZIP/F-1 mouse (A-ZIPTg/؉) is a model of severe lipoatrophic diabetes and is insulin resistant, hypoleptinemic, hyperphagic, and shows severe hepatic steatosis. We have also produced transgenic "skinny" mice that have hepatic overexpression of leptin (LepTg/؉) and no adipocyte triglyceride stores, and are hypophagic and show increased insulin sensitivity. To explore the pathophysiological and therapeutic roles of leptin in lipoatrophic diabetes, we crossed LepTg/؉ and A-ZIPTg/؉ mice, producing doubly transgenic mice (LepTg/؉:A-ZIPTg/؉) virtually lacking adipose tissue but having greatly elevated leptin levels. The LepTg/؉:A-ZIPTg/؉ mice were hypophagic and showed improved hepatic steatosis. Glucose and insulin tolerance tests revealed increased insulin sensitivity, comparable to LepTg/؉ mice. These effects were stable over at least 6 months of age. Pairfeeding the A-ZIPTg/؉ mice to the amount of food consumed by LepTg/؉:A-ZIPTg/؉ mice did not improve their insulin resistance, diabetes, or hepatic steatosis, demonstrating that the beneficial effects of leptin were not due to the decreased food intake. Continuous leptin administration that elevates plasma leptin concentrations to those of LepTg/؉:A-ZIPTg/؉ mice also effectively improved hepatic steatosis and the disorder of glucose and lipid metabolism in A-ZIP/F-1 mice. These data demonstrate that leptin can improve the insulin resistance and diabetes of a mouse model of severe lipoatrophic diabetes, suggesting that leptin may be therapeutically useful in the long-term treatment of lipoatrophic diabetes.

A number of studies have demonstrated that insulin resistance in the skeletal muscle plays a pivotal role in the insulin resistance associated with obesity and type 2 diabetes. A decrease in GLUT4 translocation from the intracellular pool to the plasma membranes in skeletal muscles has been implicated as a possible cause of insulin resistance. Herein, we examined the effects of an insulin-sensitizing drug, troglitazone (TGZ), on glucose uptake and the translocation of GLUT4 in L6 myotubes. The prolonged exposure (24 h) of L6 myotubes to TGZ (10 ؊5 mol/l) caused a substantial increase in the 2-deoxy-[ 3 H]D-glucose (2-DG) uptake without changing the total amount of the glucose transporters GLUT4, GLUT1, and GLUT3. The TGZ-induced 2-DG uptake was completely abolished by cytochalasin-B (10 mol/l). The ability of TGZ to translocate GLUT4 from light microsomes to the crude plasma membranes was greater than that of insulin. Both cycloheximide treatment (3.5 ؋ 10 ؊6 mol/l) and the removal of TGZ by washing reversed the 2-DG uptake to the basal level. Moreover, insulin did not enhance the TGZ-induced 2-DG uptake additively. The TGZ-induced 2-DG uptake was only partially reversed by wortmannin to 80%, and TGZ did not change the expression and the phosphorylation of protein kinase B; the expression of protein kinase C (PKC)-, PKC-␤2, and PKC-; or 5AMP-activated protein kinase activity. ␣-Tocopherol, which has a molecular structure similar to that of TGZ, did not increase 2-DG uptake. We conclude that the glucose transport in L6 myotubes exposed to TGZ for 24 h is the result of an increased translocation of GLUT4. The present results imply that the effects of troglitazone on GLUT4 translocation may include a new mechanism for improving glucose transport in skeletal muscle.

Impairment of insulin action (insulin resistance) is frequently observed in Type II (non-insulin-dependent) diabetes mellitus as well as in wasting syndromes including neoplastic, inflammatory and chronically infectious diseases [1,2]. Previous studies suggest that inflammatory cytokines, such as tumour necrosis factor-a (TNF-a) [3], interleukin-1 (IL-1) [4], , leukaemia inhibitory factor (LIF) [6] and transforming growth factor-b (TGF-b) [7] are involved in the development of insulin resistance. Clinical studies have shown that TNF-a and IL-6 concentrations are increased in the sera of patients with several cancers and infectious diseases, in which insulin action is impaired [8]. Moreover, infusion of rats Diabetologia (1999) Abstract Aims/hypothesis. Previous studies show that inflammatory cytokines play a part in the development of insulin resistance. Thiazolidinediones were developed as insulin-sensitizing drugs and are ligands for the peroxisome proliferator-activated receptorg (PPARg). We hypothesized that the anti-diabetic mechanism of thiazolidinediones depends on the quantity of PPARg in the insulin resistant state in which inflammatory cytokines play a part. Methods. We isolated rat PPARg1 and g2 cDNAs and examined effects of various cytokines and thiazolidinediones on PPARg mRNA expression in rat mature adipocytes. Results. Various inflammatory cytokines, such as tumour necrosis factor-a (TNF-a), interleukin-1a (IL1a), IL-1b, IL-6 and leukaemia inhibitory factor decreased PPARg mRNA expression. In addition, hydrogen peroxide, lysophosphatidylcholine or phorbol 12-myristate 13-acetate also decreased the expression of PPARg. The suppression of PPARg mRNA expression caused by 10 nmol/l of TNF-a was reversed 60 % and 55 % by treatment with 10 ±4 mol/l of troglitazone and 10 ±4 mol/l of pioglitazone, respectively. The suppression of glucose transporter 4 mRNA expression caused by TNF-a was also reversed by thiazolidinediones. Associated with the change of PPARg mRNA expression, troglitazone improved glucose uptake suppressed by TNF-a. Conclusion/interpretation. Our study suggests that inflammatory cytokines could be factors that regulate PPARg expression for possible modulation of insulin resistance. In addition, we speculate that the regulation of PPARg mRNA expression may contribute to the anti-diabetic mechanism of thiazolidinediones. [Diabetologia (1999) 42: 702±710]