Leptin Signaling Pathway Part 1

Leptin: The Beginning Identified by Jeffrey Friedman in Circulating peptide hormone secreted almost exclusively by adipocytes. Regulates adipose mass through effects on food intake and energy expenditure in the brain. Clinical phenotypes of leptin deficiency include hyperphagia, severe obesity, hypogonadism, and impaired immunity. Has a critical role in regulation of reproductive and immune function in humans.

Parabiosis Experiments Used to determine that there was a circulating hormone responsible for controlling food intake and adipose tissue mass.

Ob/Ob Mouse Model a mutation at amino acid 105 that results in a truncation of the leptin protein. – Administration of the recombinant leptin reduced the food intake and body weight and corrected all of their neuroendocrine and metabolic abnormalities. – Suggests that leptin acts as a signal from adipose to brain regarding quantity of fat tissue stored. Flier and Ahima showed that reduced leptin acts as a signal of nutritional deprivation. – Resulting in an increase in food intake, decreased energy expenditure, and suppression of the reproductive cycle.

Db/Db Mouse Model a deletion of the signaling form of the leptin receptor. – Unresponsive to endogenous or exogenous leptin. – Maintains a normal level of leptin in the blood.

Leptin’s Effect Leptin acts via its receptor in the hypothalamus which is a part of the arcuate nucleus (ARH) Tranduces peripheral signals into neuronal responses. – Activates an appetite suppressing pathway mediated by neurons producing: Pro-opiomelanocortin (POMC) to  melanocyte stimulating hormone Cocaine and amphetamine related transcript – Inhibits an appetite inducing pathway mediated by neurons producing: Neuropeptide Y Agouti related protein

Regulation of Energy Intake Leptin-deficient animals exhibit intense hyperphagia with food-seeking behavior and aggressive behavior when food is denied. They have increased hunger and impaired satiety. – leptin treatment results in normalization of hyperphagia. Leptin-deficient subjects like all foods. After 7days of leptin treatment, they discriminate between foods they like and dislike.

Why not leptin treatment? Very few humans have mutations effecting the production of leptin. (5-10%) Most are leptin resistant. (90-95%) – Clinical trials have shown that obese people do lose weight in a dose dependent manner. – Highly variable in humans due to the difference in leptin resistance. Leptin could benefit some obese individuals but is not likely to significantly alleviate obesity.

Leptin Resistance? Leptin fails to reach brain or failure in leptin receptor signaling cascade. In humans it is still up for debate. – Rosenbaum showed that people put on calorie restriction for 6-10 weeks given a low dose of leptin normalizes appetite and energy expenditure. B6 on HFD show leptin resistance in 3 stages. 1.Gain weight but show normal response to exogenous leptin 2.Show peripheral leptin insensitivity, expressed by changes in food intake and body weight or by lack of activation of signal transducer and activator of transcription (STAT)-3, however these mice retain the capacity to respond to central leptin injection. 3.Develop central leptin resistance and do not show changes in food intake and body weight in response to exogenous leptin.

Energy Expenditure and Fat Oxidation Leptin’s effect is not the result of its appetite effects alone. – Food restricted Ob mice show smaller decreases in body weight and adipose deposits than leptin treated Ob mice. – Leptin treatment causes specific loss of fat mass where food restriction causes loss of both lean and fat mass. – Food restriction also causes a decrease in energy expenditure.

Fatty Liver Ob mice develop massively enlarged livers engorged with lipid, along with depositing lipid in adipose, muscle and other peripheral tissues. – Build up of lipid in nonadipose site contributes to many of the health consequences of obesity such as insulin resistance and NAFLD – After 12 days of leptin treatment the livers are histologically indistinguishable from wild-type. – Interestingly, our Btbr Ob animals do not have a fatty liver. Why?

Leptin mediated weight loss Food restriction leads to a rise in serum free fatty acids. Leptin mediated weight loss is not associated with a rise in free fatty acids or ketones, suggesting a unique mechanism of fatty acid oxidation

Leptin and SCD-1 Cohen and Friedman – Goal: Elucidate the mechanism whereby leptin reduces hepatic lipid content. – Used a gene expression array to select genes whose expression was increased in Ob compared to wild-type and then corrected by leptin administration. – The top gene on their list was SCD-1.

SCD-1 Rate limiting step in the biosynthesis of monounsaturated fats. Make palmitoleic (16:1)and oleic acid(18:1). These are the most abundant fatty acids found in triglycerides, cholesterol esters, and phospholipids. SCD-1 is regulated by SREBP-1 and PUFA.

Leptin’s effect on SCD-1 SCD-1 RNA levels highly elevated in untreated ob livers. SCD-1 RNA levels in leptin treated animals were normalized by 2 days and below lean controls by 4 days. SCD-1 activity was also measured and followed the same pattern as RNA levels.

SCD-1 repression Asebia (ab j / ab j ) are SCD-1 knockout. – Have reduction in body fat relative to littermate controls. – Also a decrease in plasma leptin levels. – A result of leptin treatment is a suppression of hepatic SCD-1 RNA and enzymatic activity. SCD-1 KO/Ob animals resemble leptin treated Ob animals.

SCD-1 KO/Ob Animals Reduction in percent body fat. Increase in lean body mass – Confirmed that reduced adiposity in Ob mice lacking SCD-1 was not the result of a growth or developmental defect. Food intake-ate more than Ob controls and weight less Energy Expenditure-increase oxygen consumption and correction of the hypometabolic Ob state.

SCD-1 and Fatty Liver Livers from double KO were indistinguishable from wild-type mice. TG levels were reduced 3-fold compared to Ob mice. Thus, downregulation of SCD-1 activity plays a major role in leptin-mediated depletion of hepatic lipid.

Lipodystrophy Partial or complete loss of adipose tissue – Associated with hepatic steatosis, insulin resistance, diabetes, and leptin dificiency. Is this due to the absence of adipose tissue as a TG storage depot or to the absence of an adipose- derived factor? The result was due to the reduction in leptin as leptin treatment through injection or fat transplant corrected the hepatic steatosis, diabetes and improve insulin resistance.

SCD-1 deficiency in mice makes them resisitant to both hepatic steatosis and obesity. How? Fates of hepatic fatty acids 1. accumulation in liver 2. packaged into VLDL for transport 3. be oxidized SCD-1 products are required for TG and cholesterol ester synthesis and VLDL production.

Confirmation Tyloxapol (inhibits VLDL hydrolysis) experiments show increased synthesis in Ob mice but a reduction in the ab j /Ob KO animals to the level of control animals. Previously shown reduction in liver TG accumulation. Reduced adiposity and increase energy expenditure in the setting of normal to increase food intake is suggestive of enhanced fatty acid oxidation. SCD-1 KO mice have increased plasma ketone bodies.

Possible Alternative Mechanisms Inhibition of SCD-1 could increase levels of peroxisome proliferator-activated receptor alpha (PPAR  ) which leads to increased peroxisomal fatty acid oxidation. – PPAR  is required for leptin-mediated fatty acid depletion in the liver. Inhibition of SCD-1 may increase activity of AMP-activated protein kinase, which has been shown to stimulate fatty acid oxidation following leptin adminstration.

Citations Leptin:a pivotal regulator of human energy homeostasis – Sadaf Farooqi and Sephen O’Rahilly, The American Journal of Clinical Nutrition Leptin and the Control of Metabolism: Role for Stearoyl-CoA Desaturase-1 (SCD-1) – Paul Cohen and Jeffrey M. Friedman, The Journal of Nutrition Leptin at 14 y of age: an ongoing story – Jeffrey M Friedman, The American Journal of Clinical Nutrition Leptin Suppresses Stearoly-CoA Desaturase 1 by Mechanisms Independent of Insulin and Sterol Regulatory Element-Binding Protein-1c – Sudha B. Biddinger, Makoto Miyazaki, Jeremie Boucher, James M. Ntambi, and C. Ronald Kahn.

Leptin signaling through JAK–STAT and MC4-R Pathway. Leptin binds to its receptor (Ob-Rb) to initiate several signaling pathways, including the Janus kinase (JAK)–signal transducer and activator of transcription (STAT) and melanocortin pathways. JAK–STAT signaling regulates the transcription of numerous proteins, including insulin-like growth factor-I (IGF-I) and suppressor of cytokine signaling (SOCS). SOCS proteins, in turn, suppress the JAK–STAT pathway. Leptin also stimulates melanocortin-4 receptor (MC4-R) signaling, which is antagonized by agouti-related peptide (AgRP).