All-trans-retinoic acid (RA), one of the active metabolites of vitamin A, can increase the expression of uncoupling protein-1 (UCP1) gene. To determine whether RA stimulates brown adipose tissue (BAT) thermogenesis and modulates leptin gene expression in vivo, 6-month-old, vitamin-A sufficient, F344 x BN rats were administered a single dose of RA (7.5 mg/kg, i.p.) or the beta 3-adrenergic receptor (beta 3AR) specific agonist, CGP 12177 (0.75 mg/kg). Levels of UCP1 mRNA in BAT and leptin mRNA in perirenal white adipose tissue (WAT) were examined 5 h after treatment. mRNA levels of lipoprotein lipase (LPL) were also examined in BAT and perirenal WAT. Administration of CGP 12177 caused the expected increase in UCP1 mRNA levels. RA treatment also significantly increased UCP1 mRNA levels but to a lesser extent than CGP 12177. In contrast, there was no acute effect of RA on whole body oxygen consumption, one measure of BAT thermogenesis. Both CGP 12177 and RA treatment decreased levels of leptin mRNA to a similar extent. RA treatment had no effect on mRNA levels of LPL in BAT or perirenal WAT. There were no changes in total DNA content, total protein content, or in the levels of beta-actin mRNA in either BAT or perirenal WAT upon administration of RA or CGP 12177. Thus, the acute effects of RA paralleled the effects of the beta 3AR specific agonist, CGP 12177, on UCP1 and leptin gene expression. This involvement of RA in positive regulation of UCP1 mRNA and negative regulation of leptin mRNA suggests a contrasting role for RA in energy homeostasis.

The objectives of this study were to determine if reduced long-form leptin receptor (ObRb) expression in diet-induced obese (DIO) animals is associated with deficits in maximal leptin signaling and, secondly, to establish the effects of short-term caloric restriction (CR) on ObRb expression and function. Groups of DIO and life-long chow-fed (CHOW) F344xBN male rats, aged 6 months, were given an i.c.v. injection containing 2 micro g leptin or artificial cerebrospinal fluid (ACSF) vehicle. Leptin induced a >6-fold increase in STAT3 phosphorylation in CHOW rats, but less than 2-fold increase in DIO. Reduced maximal leptin-stimulated STAT3 phosphorylation in DIO rats was coupled with a decline in both ObRb expression and protein. At this point, subgroups of DIO and CHOW animals underwent CR for 30 days and were then tested for acute leptin responsiveness. CR resulted in a 45 and 85% increase respectively in leptin-stimulated STAT3 phosphorylation in CHOW and DIO animals. Similarly, CR increased ObRb expression and protein in both CHOW and DIO animals. To explore the role of leptin in regulating ObRb expression, we reversibly overexpressed leptin in the hypothalamus and found that ObRb mRNA inversely follows central leptin expression. By enhancing both ObRb expression and signaling capacity, CR may enhance leptin responsiveness in leptin-resistant DIO animals.

To determine the effects of food restriction and leptin administration on several transcripts involved in energy homeostasis, we examined leptin, uncoupling proteins (UCP) 1, 2 and 3, lipoprotein lipase (LPL), beta3-adrenergic receptors (beta3AR) and hormone-sensitive lipase (HSL) mRNA levels in brown adipose tissue (BAT) and epididymal (EWAT) and perirenal (PWAT) white adipose tissue in three groups of rats. The groups were administered leptin for 1 week, or had food restricted to the amount of food consumed by the leptin-treated animals, or had free access to food. Leptin administration increased serum leptin concentrations 50-fold and decreased food consumption by 43%, whereas serum insulin and corticosterone concentrations were unchanged. Leptin increased LPL mRNA by 80%, UCP1 mRNA twofold, and UCP3 mRNA levels by 62% in BAT, and increased UCP2 mRNA levels twofold in EWAT. In contrast, UCP2 mRNA levels were unchanged in PWAT and BAT. In WAT from food-restricted rats, leptin gene expression was diminished by 40% compared with those fed ad libitum. With leptin administration, there was a further 50% decrease in leptin expression. LPL mRNA levels were decreased by food restriction but not by leptin in WAT, whereas beta3AR and HSL mRNA levels were unchanged with either food restriction or leptin treatment. The present study indicates that leptin increases the gene expression of UCP2 in EWAT and that of UCP1, UCP3 and LPL in BAT, whereas reduced food consumption but not leptin, decreases LPL expression in WAT. In addition, with leptin administration there is a decrease in leptin gene expression in WAT, independent of food intake and serum insulin and corticosterone concentrations.

We recently reported that the leptin-induced increase in uncoupling protein 1 (UCP1) mRNA in brown adipose tissue (BAT) is prevented by the denervation of BAT. We also reported that retinoic acid (RA) increases UCP1 mRNA in BAT. To extend these finding to UCP2 and UCP3 in BAT, we examined UCP2 and UCP3 mRNA after unilateral denervation of BAT, as well as after leptin, beta(3)-adrenergic agonist, RA, and glucocorticoid administration to rats. UCP3 mRNA was 20% less in the denervated compared with the intact BAT, whereas UCP2 mRNA was unchanged with denervation. The beta(3)-adrenergic agonist, CGP-12177 (0.75 mg/kg), increased UPC3 mRNA by 40% in the innervated and by 85% in the denervated BAT. Leptin (0.9 mg/day for 3 days) increased both UCP2 and UCP3 mRNA by 30% in the innervated and, surprisingly, in the denervated BAT. RA (7.5 mg/kg) increased UCP1 mRNA but decreased UCP2 and UCP3 mRNA by 50%, whereas methylprednisolone (65 mg/kg, two doses 24 h apart) suppressed all three uncoupling proteins by greater than 60%. The present findings indicate that: sympathetic innervation is necessary to maintain basal levels of UCP3 mRNA; beta(3)-adrenergic agonist stimulation induces UCP3 mRNA; leptin induces UCP2 and UCP3 mRNA and this induction is not dependent on sympathetic innervation; RA increases UCP1 but decreases UCP2 and UCP3 mRNA; and methylprednisolone suppresses UCP1, UCP2, and UCP3 mRNA equally. These data suggest that there are distinct patterns of regulation between UCP1, UCP2, and UCP3, and there may be at least two modes by which leptin could modulate thermogenesis in BAT; first, by increasing sympathetic stimulation of BAT and induction of UCP1 mRNA and, secondly, by increasing UCP2 and UCP3 mRNA by a mechanism independent of sympathetic stimulation.

The effects of the chronic activation of the central melanocortin (MC) system by melanotan II (MTII) were assessed in chow-fed (CH) and high-fat (HF) diet-induced obese (DIO) Sprague-Dawley rats. Six-day central infusion of MTII (1 nmol/day) reduced body weight and visceral adiposity compared with ad libitum-fed control and pair-fed groups and markedly suppressed caloric intake in both CH and DIO rats. The anorexic response to MTII was similar in DIO relative to CH rats. MTII induced a sustained increase in oxygen consumption in DIO but a delayed response in CH rats. In both diet groups, MTII reduced serum insulin and cholesterol levels compared with controls. HF feeding increased brown adipose tissue (BAT) uncoupling protein 1 (UCP1) by over twofold, and UCP1 levels were further elevated in MTII-treated CH and DIO rats. MTII lowered acetyl-CoA carboxylase expression and prevented the reduction in muscle-type carnitine palmitoyltransferase I mRNA by pair-feeding in the muscle of DIO rats. Compared with CH controls, hypothalamic MC3 and MC4 receptor expression levels were reduced in DIO controls. This study has demonstrated that, despite reduced hypothalamic MC3/MC4 receptor expression, anorexic and thermogenic responses to MTII are unabated with an initial augmentation of energy expenditure in DIO versus CH rats. The HF-induced up-regulation of UCP1 in BAT may contribute to the immediate increase in MTII-stimulated thermogenesis in DIO rats. MTII also increased fat catabolism in the muscle of DIO rats and improved glucose and cholesterol metabolism in both groups.

Leptin is a peripheral immunoenhancing reagent that directly activates splenic lymphocytes in mice. We found that a 48 h fast in rats resulted in a decrease in serum leptin that was accompanied by a lower delayed-type hypersensitivity (DTH) response. Peripheral leptin replacement completely restored this response in fasted animals. We employed a recombinant adeno-associated virus (rAAV) system to deliver leptin gene directly into rat brain to assess the effect of sustained long-term central expression of leptin on immune responses. The rAAV-leptin rats had elevated central leptin over the 60 day duration of the experiment, whereas body fat and circulating leptin fell to near zero levels. The DTH response was significantly reduced by 10-20% in rats receiving rAAV-leptin compared with the control rats, and the difference was maintained for over 50 h. When the rats undergoing rAAV-leptin gene therapy were given either murine recombinant leptin or PBS s.c., rats receiving leptin had a 17% higher DTH response than rats receiving PBS. The isolated splenocytes from the former group also proliferated 34% more in vitro in response to the mitogen concanavalin A as compared with the latter group. These results suggest that peripheral leptin has a dominant role in maintaining T-cell-mediated immune responses in rats, and central leptin is unable to compensate for the immunosuppression associated with peripheral hypoleptinemia. Furthermore, preservation of normal cell-mediated immune responses does not require fat tissue as along as serum leptin levels are maintained.