Insulin and leptin revisited: adiposity signals with overlapping physiological and intracellular signaling capabilities

Neural systems controlling food intake and energy balance in the modern world

PURPOSE OF REVIEW

Obesity continues to increase around the globe, and creates a major health problem because of its comorbidities such as diabetes and cardiovascular disease. Considering that the major cause of most human obesities is the modern lifestyle in a rapidly changing environment, this suggests that the battle is between brain areas controlling internal metabolic homeostasis and those dealing with cognitive and emotional processing of external information. Thus, this review is intended to bridge this apparent neural dichotomy and highlight possible ways of interactions between these homeostatic and nonhomeostatic systems.

RECENT FINDINGS

Rapid progress has been made in identifying the molecular mechanisms underlying the neural circuits regulating food intake and energy balance. Specific populations of peptidergic neurons in the medial hypothalamus can be considered metabolic integrators sensing both short and long-term availability of fuels. These cell groups in turn connect with various other brain regions to orchestrate adaptive responses through changes in food intake, as well as endocrine and autonomic responses. Alternatively, the impact of the environmental changes on ingestive behavior have been documented, and many of the underlying cortico-limbic pathways are starting to be identified. It has been proposed that a switch from instinctual control of energy balance to a more cognitive control will be necessary to stop the obesity epidemic.

SUMMARY

The realization that there is nothing wrong with the homeostatic regulatory system in common obesity suggests that a systematic investigation of pathways that link the 'cognitive' with the 'metabolic' brain might be a more promising area of research. Such an investigation would reveal new molecular and cellular mechanisms providing new drug targets, leading to behavioral therapies that are directed more towards the causes of common obesity.

Morphine induces desensitization of insulin receptor signaling

Morphine analgesia is mediated principally by the micro -opioid receptor (MOR). Since morphine and other opiates have been shown to influence glucose homeostasis, we investigated the hypothesis of direct cross talk between the MOR and the insulin receptor (IR) signaling cascades. We show that prolonged morphine exposure of cell lines expressing endogenous or transfected MOR, IR, and the insulin substrate 1 (IRS-1) protein specifically desensitizes IR signaling to Akt and ERK cascades. Morphine caused serine phosphorylation of the IR and impaired the formation of the signaling complex among the IR, Shc, and Grb2. Morphine also resulted in IRS-1 phosphorylation at serine 612 and reduced tyrosine phosphorylation at the YMXM p85-binding motifs, weakening the association of the IRS-1/p85 phosphatidylinositol 3-kinase complex. However, the IRS-1/Grb2 complex was unaffected by chronic morphine treatment. These results suggest that morphine attenuates IR signaling to Akt by disrupting the IRS-1-p85 interaction but inhibits signaling to ERK by disruption of the complex among the IR, Shc, and Grb2. Finally, we show that systemic morphine induced IRS-1 phosphorylation at Ser612 in the hypothalamus and hippocampus of wild type, but not MOR knockout, mice. Our results demonstrate that opiates can inhibit insulin signaling through direct cross talk between the downstream signaling pathways of the MOR and the IR.

Minireview: The adipocyte--at the crossroads of energy homeostasis, inflammation, and atherosclerosis

Adipose tissue evolved to efficiently store energy for times of caloric restriction. The large caloric excess common in many Western diets has negated the need for this thrifty function, leaving adipose tissue ill-equipped to handle this increased load. An excess of adipose tissue increases risk for a number of conditions including coronary artery disease, hypertension, dyslipidemias, type 2 diabetes, and even cancer. Indeed, the ability of the adipocyte to function properly when engorged with lipid can lead to lipid accumulation in other tissues, reducing their ability to function and respond normally. The role of adipose tissue as an endocrine organ capable of secreting a number of adipose tissue-specific or enriched hormones, known as adipokines, is gaining appreciation. The normal balance of these adipose tissue secretory proteins is perturbed in obesity. Paradoxically, the lack of normal adipose tissue, as seen in cases of lipodystrophy and lipoatrophy, is also associated with pathologic sequelae similar to what is seen with obesity. The pathologic findings associated with lack of adipose tissue, largely due to inability to properly store lipids, may also be due to a lack of adipokines. In this review, we highlight the role of adipose tissue as an endocrine organ focusing on some of the recent advances in the identification and pharmacological characterization of adipokines as well as their regulation in the context of obesity and insulin-resistant states.