Yong-Ling Liu

Expression of the Functional Leptin Receptor mRNA in Pancreatic Islets and Direct Inhibitory Action of Leptin on Insulin Secretion

Leptin, encoded for by the mouse ob gene, regulates feeding behavior and energy metabolism. Its receptor (Ob-R) is encoded by the mouse diabetic (db) gene and is mutated in the db/db mouse so that it lacks the cytoplasmic domain. We show that the full-length leptin receptor (Ob-Rb), which is believed to transmit the leptin signal, is expressed in pancreatic islets of ob/ob and wild-type mice, as well as in hypothalamus, liver, kidney, spleen, and heart. Recombinant leptin inhibited basal insulin release in the perfused pancreas preparation from ob/ob mice but not in that from Zucker fa/fa rats. Leptin (1–100 nmol/l) also produced a dose-dependent inhibition of glucose-stimulated insulin secretion by isolated islets from ob/ob mice. In contrast, leptin at maximum effective concentration (100 nmol/l) did not inhibit glucose-stimulated insulin secretion by islets from db/db mice. These results provide evidence that a functional leptin receptor is present in pancreatic islets and suggest that leptin overproduction, particularly from abdominal adipose tissue, may modify directly both basal and glucose-stimulated insulin secretion.

Leptin Action in Intestinal Cells

The adipocyte hormone leptin activates signal transducer and activator of transcription 3 (STAT3) in the hypothalamus, mediating increased satiety and increased energy expenditure. To date, leptin-mediated activation of the STAT pathwayin vivo has not been established in tissues other than hypothalamus. We now describe leptin receptor expression and in vivo signaling in discrete regions of the mouse gastrointestinal tract. Expression of the functional isoform leptin receptor (OB-Rb) is restricted to the jejunum and is readily detected by RT-PCR in isolated enterocytes from this site. Intravenous injection of leptin rapidly induced nuclear STAT5 DNA binding activity in jejunum of +/+ and obese (ob/ob) mice but had no effect in the diabetic (db/db) mouse that lacks the OB-Rb isoform. In addition, an induction of the immediate-early gene c-fos is observed in jejunum in vivo. Leptin-mediated induction of a number of immediate-early genes and activation of STAT3 and STAT5 in a human model of small intestine epithelium, CACO-2 cells, corroborate this effect. Furthermore, intravenous leptin administration caused a significant 2-fold reduction in the apolipoprotein AIV transcript levels in jejunum 90 min after a fat load. Our results suggest that the epithelium of jejunum is a direct target of leptin action, and this activity is dependent on the presence of OB-Rb. Lack of leptin or resistance to leptin action in this site may contribute to obesity and its related syndromes by directly affecting lipid handling.

Leptin inhibits glycogen synthesis in the isolated soleus muscle of obese (ob/ob) mice

The ob gene product, leptin, causes significant and dose‐dependent inhibition of basal and insulin‐stimulated glycogen synthesis in isolated soleus muscle from ob/ob mice, and a smaller, non‐significant inhibition in muscle from wild‐type mice. Leptin had no inhibitory effect on glycogen synthesis in soleus muscle from the diabetic (db/db) mice, which lack the functional leptin receptor. The full‐length leptin receptor (Ob‐Rb), is expressed in soleus muscle of both ob/ob and wild‐type mice, however with no detectable differences in expression level. These results suggest that hyperleptinaemia may attenuate insulin action on glucose storage in skeletal muscle.

Biphasic effects of the β-adrenoceptor agonist, BRL 37344, on glucose utilization in rat isolated skeletal muscle

1 The effects of the selective β3‐adrenoceptor agonist, BRL 37344 (BRL) on glucose uptake and phosphorylation (i.e. glucose utilization; GU) and glycogen synthesis in rat isolated soleus and extensor digitorium longus (EDL) muscle preparations in vitro were investigated by use of 2‐deoxy‐[3H]‐ glucose (GU) and [U‐14C]‐glucose (glycogen synthesis). 2 Low concentrations of BRL (10−11‐10−9M) significantly increased GU, with maximal increases of 30% in soleus and 24% in EDL at 10−11M. Neither the selective β1‐adrenoceptor antagonist, atenolol (10−8‐10−6M), nor the selective β2‐adrenoceptor antagonist, ICI 118551 (10−8 − 10−6M) had any effect on the stimulation of GU induced by 10−11M BRL. 3 High concentrations of BRL (10−6 − 10−5M) caused significant inhibition (up to 30%) of GU in both soleus and EDL muscles. The inhibition at 10−6M BRL was blocked completely by 10−6 and 10−7M ICI 118551 in soleus, and by 10−6‐10−8M ICI 118551 in EDL; atenolol (10−8 − 106M) had no effect. 4 Another selective β3‐adrenoceptor agonist, CL 316,243, also caused a significant stimulation of muscle GU, with maximal increases of 43% at 10−9M in soleus and 45% at 10−10M in EDL. The stimulation of GU declined with further increases in the concentration of CL 316,243, but no inhibition of GU was seen, even at the highest concentration (10−5M) tested. 5 BRL at 10−5M inhibited completely insulin‐stimulated glycogen synthesis in both soleus and EDL, but this inhibitory effect of BRL was abolished by 10−6M ICI 118551. BRL at 10−11M (with or without 10−6M ICI 118551) had no effect on insulin‐stimulated glycogen synthesis. 6 It is concluded that: (i) low (nM) concentrations of BRL; (iii) inhibition of GU via β2‐adrenoceptor activation is associated with inhibition of glycogen synthesis, possibly due to activation of glycogenolysis; (iv) the opposing effects of β2‐adrenoceptor and atypical β‐adrenoceptor activation on GU suggest that in skeletal muscle these adrenoceptors are linked to different post‐receptor pathways.

Retinoid X receptor agonists have anti-obesity effects and improve insulin sensitivity in Zucker fa/fa rats

OBJECTIVE: To investigate whether retinoid X receptor agonists act as insulin sensitizers and compare their effects with that of thiazolidinedione BRL 49653 in obese Zucker rats.DESIGN: In two independent studies, obese Zucker rats were dosed orally once daily for 14 days with one of the following treatments: LG 100268 (20 mg/kg), LG 100324 (20 mg/kg), BRL 49653 (3 mg/kg) or vehicle.MEASUREMENTS: Daily food intake and body weight gain, blood glucose, plasma and pancreatic insulin, whole body glucose disposal (by euglycaemic–hyperinsulinaemic clamp) and tissue glucose utilization.RESULTS: The retinoid X receptor agonists (rexinoids) LG 100268 and LG 100324 caused a reduction in the food intake of obese Zucker rats relative to controls and to rats receiving BRL 49653. The two rexinoids also produced a marked decrease in the body weight gain, whereas the growth rate of rats treated with BRL 49653 tended to increase. Both rexinoids and BRL 49653 reduced the plasma insulin concentration of fed rats. LG 100268 and LG 100324 also significantly lowered blood glucose concentrations after 1 week of treatment. The 5 h fasted plasma insulin concentration was significantly lower in the rexinoid-treated groups and the terminal insulin level (at the end of the clamp) tended to be lower in all treated groups compared with animals given the dosing vehicle. However, pancreatic insulin content was not affected by any of the treatments. Under euglycaemic–hyperinsulinaemic clamp conditions, there were no significant differences in the rate of hepatic glucose output and whole body glucose disposal, except that, in experiment 1, BRL 49653 caused significant increase in the glucose infusion rate and muscle glucose utilization. In experiment 2, a similar glucose infusion rate to the controls was achieved in all treatment groups but the steady-state insulin concentration in the treated animals was only about 50% of that in the control animals, despite the fact that all rats received a similar insulin infusion concentration. This suggests that both the rexinoids and BRL 49653 increased insulin clearance.CONCLUSIONS: Chronic administration of retinoid X receptor agonists LG 100268 and LG 100324 to Zucker fa/fa rats reduces food intake and body weight gain, lowers plasma insulin concentrations while maintaining normoglycaemia, indicating an improvement of insulin sensitivity.