Induction of leptin resistance by activation of cAMP-Epac signaling

Recent advances in the discovery of small molecules targeting exchange proteins directly activated by cAMP (EPAC)

3',5'-Cyclic adenosine monophosphate (cAMP) is a pivotal second messenger that regulates numerous biological processes under physiological and pathological conditions, including cancer, diabetes, heart failure, inflammation, and neurological disorders. In the past, all effects of cAMP were initially believed to be mediated by protein kinase A (PKA) and cyclic nucleotide-regulated ion channels. Since the discovery of exchange proteins directly activated by cyclic adenosine 5'-monophosphate (EPACs) in 1998, accumulating evidence has demonstrated that the net cellular effects of cAMP are also regulated by EPAC. The pursuit of the biological functions of EPAC has benefited from the development and applications of a growing number of pharmacological probes targeting EPACs. In this review, we seek to provide a concise update on recent advances in the development of chemical entities including various membrane-permeable analogues of cAMP and newly discovered EPAC-specific ligands from high throughput assays and hit-to-lead optimizations.

DNA methylation of alternative promoters directs tissue specific expression of Epac2 isoforms

Epac 1 and Epac 2 (Epac1/2; exchange factors directly activated by cAMP) are multidomain proteins that mediate cellular responses upon activation by the signaling molecule cAMP. Epac1 is ubiquitously expressed, whereas Epac2 exhibits a restricted expression pattern. The gene encoding Epac2 gives rise to at least three protein isoforms (Epac2A, Epac2B and Epac2C) that exhibit confined tissue and cell specific expression profiles. Here, we describe alternative promoter usage for the different isoforms of Epac2, and demonstrate that the activity of these promoters depend on the DNA methylation status. Bisulfite sequencing demonstrated that the level of methylation of the promoters in different tissues correlates with Epac2 isoform expression. The presented data indicate that the tissue-specific expression of the Epac2 isoforms is epigenetically regulated, and identify tissue-specific differentially methylated promoter regions within the Epac2 locus that are essential for its transcriptional control.

Role of G-proteins in the effects of leptin on pedunculopontine nucleus neurons

The pedunculopontine nucleus (PPN), the cholinergic arm of the reticular activating system, regulates waking and rapid eye movement sleep. Here, we demonstrate immunohistochemical labeling of the leptin receptor signaling isoform in PPN neurons, and investigated the effects of G-protein modulation and the leptin triple antagonist (TA) on the action of leptin in the PPN. Whole-cell patch clamp recordings were performed in rat brainstem slices from 9 to 17 day old pups. Previous results showed that leptin caused a partial blockade of sodium (I(Na)) and h-current (I(H)) in PPN neurons. TA (100 nM) reduced the blockade of I(Na) (~ 50% reduction) and I(H) (~ 93% reduction) caused by leptin. Intracellular guanosine 5'-[β-thio]diphosphate trilithium salt (a G-protein inhibitor) significantly reduced the effect of leptin on I(Na) (~ 60% reduction) but not on I(H) (~ 25% reduction). Intracellular GTPγS (a G-protein activator) reduced the effect of leptin on both I(Na) (~ 80% reduction) and I(H) (~ 90% reduction). These results suggest that the effects of leptin on the intrinsic properties of PPN neurons are leptin receptor- and G-protein dependent. We also found that leptin enhanced NMDA receptor-mediated responses in single neurons and in the PPN population as a whole, an effect blocked by TA. These experiments further strengthen the association between leptin dysregulation and sleep disturbances. Beck et al. investigated the effects of leptin on the intrinsic properties of neurons from the pedunculopontine nucleus (PPN). Leptin reduced the amplitude of voltage-gated sodium (I(Na)) and hyperpolarization-activated cyclic nucleotide-gated HCN (I(H)) channels. These effects were antagonized by a leptin receptor (OB-R) antagonist and by the G-protein antagonist GDPβ.

Neuroanatomy of melanocortin-4 receptor pathway in the lateral hypothalamic area

The central melanocortin system regulates body energy homeostasis including the melanocortin-4 receptor (MC4R). The lateral hypothalamic area (LHA) receives dense melanocortinergic inputs from the arcuate nucleus of the hypothalamus and regulates multiple processes including food intake, reward behaviors, and autonomic function. By using a mouse line in which green fluorescent protein (GFP) is expressed under control of the MC4R gene promoter, we systemically investigated MC4R signaling in the LHA by combining double immunohistochemistry, electrophysiology, and retrograde tracing techniques. We found that LHA MC4R-GFP neurons coexpress neurotensin as well as the leptin receptor but do not coexpress other peptide neurotransmitters found in the LHA including orexin, melanin-concentrating hormone, and nesfatin-1. Furthermore, electrophysiological recording demonstrated that leptin, but not the MC4R agonist melanotan II, hyperpolarizes the majority of LHA MC4R-GFP neurons in an ATP- sensitive potassium channel-dependent manner. Retrograde tracing revealed that LHA MC4R-GFP neurons do not project to the ventral tegmental area, dorsal raphe nucleus, nucleus accumbens, and spinal cord, and only limited number of neurons project to the nucleus of the solitary tract and parabrachial nucleus. Our findings provide new insights into MC4R signaling in the LHA and its potential implications in homeostatic regulation of body energy balance.

Exchange protein directly activated by cAMP (epac): a multidomain cAMP mediator in the regulation of diverse biological functions

Since the discovery nearly 60 years ago, cAMP is envisioned as one of the most universal and versatile second messengers. The tremendous feature of cAMP to tightly control highly diverse physiologic processes, including calcium homeostasis, metabolism, secretion, muscle contraction, cell fate, and gene transcription, is reflected by the award of five Nobel prizes. The discovery of Epac (exchange protein directly activated by cAMP) has ignited a new surge of cAMP-related research and has depicted novel cAMP properties independent of protein kinase A and cyclic nucleotide-gated channels. The multidomain architecture of Epac determines its activity state and allows cell-type specific protein-protein and protein-lipid interactions that control fine-tuning of pivotal biologic responses through the "old" second messenger cAMP. Compartmentalization of cAMP in space and time, maintained by A-kinase anchoring proteins, phosphodiesterases, and β-arrestins, contributes to the Epac signalosome of small GTPases, phospholipases, mitogen- and lipid-activated kinases, and transcription factors. These novel cAMP sensors seem to implement certain unexpected signaling properties of cAMP and thereby to permit delicate adaptations of biologic responses. Agonists and antagonists selective for Epac are developed and will support further studies on the biologic net outcome of the activation of Epac. This will increase our current knowledge on the pathophysiology of devastating diseases, such as diabetes, cognitive impairment, renal and heart failure, (pulmonary) hypertension, asthma, and chronic obstructive pulmonary disease. Further insights into the cAMP dynamics executed by the Epac signalosome will help to optimize the pharmacological treatment of these diseases.

The circadian clock circuitry and the AHR signaling pathway in physiology and pathology

Life forms populating the Earth must face environmental challenges to assure individual and species survival. The strategies predisposed to maintain organismal homeostasis and grant selective advantage rely on anticipatory phenomena facing periodic modifications, and compensatory phenomena facing unpredictable changes. Biological processes bringing about these responses are respectively driven by the circadian timing system, a complex of biological oscillators entrained to the environmental light/dark cycle, and by regulatory and metabolic networks that precisely direct the body's adjustments to variations of external conditions and internal milieu. A critical role in organismal homeostatic functions is played by the aryl hydrocarbon receptor (AHR) complex, which senses environmental and endogenous compounds, influences metabolic responses controlling phase I/II gene expression, and modulates vital phenomena such as development, inflammation and adaptive immunity. A physiological cross-talk between circadian and AHR signaling pathways has been evidenced. The alteration of AHR signaling pathway deriving from genetic damage with polymorphisms or mutations, or produced by exogenous or endogenous AHR activation, and chronodisruption caused by mismatch between the body's internal clock and geophysical time/social schedules, are capable of triggering pathological mechanisms involved in metabolic, immune-related and neoplastic diseases. On the other hand, the molecular components of the circadian clock circuitry and AHR signaling pathway may represent useful tools for preventive interventions and valuable targets of therapeutic approaches.

Modulation of AgRP-neuronal function by SOCS3 as an initiating event in diet-induced hypothalamic leptin resistance

Chronic consumption of a fat-rich diet leads to attenuation of leptin signaling in hypothalamic neurons, a hallmark feature of cellular leptin resistance. To date, little is known about the temporal and spatial dysregulation of neuronal function under conditions of nutrient excess. We show that agouti-related protein (AgRP)-expressing neurons precede proopiomelanocortin neurons in developing diet-induced cellular leptin resistance. High-fat diet-induced up-regulation of suppressor of cytokine signaling-3 (SOCS3) occurs in AgRP neurons before proopiomelanocortin and other hypothalamic neurons. SOCS3 expression in AgRP neurons increases after 2 d of high-fat feeding, but reduces after switching to a low-fat diet for 1 d. Consistently, transgenic overexpression of SOCS3 in AgRP neurons produces metabolic phenotypes resembling those observed after short-term high-fat feeding. We further show that AgRP neurons are the predominant cell type situated outside the blood-brain barrier in the mediobasal hypothalamus. AgRP neurons are more responsive to low levels of circulating leptin, but they are also more prone to development of leptin resistance in response to a small increase in blood leptin concentrations. Collectively, these results suggest that AgRP neurons are able to sense slight changes in plasma metabolic signals, allowing them to serve as first-line responders to fluctuation of energy intake. Furthermore, modulation of SOCS3 expression in AgRP neurons may play a dynamic and physiological role in metabolic fine tuning in response to short-term changes of nutritional status.

Metabolic transceivers: in tune with the central melanocortin system

The central melanocortin system plays an essential role in the regulation of energy metabolism. Key to this regulation are the responses of neurons expressing proopiomelanocortin (POMC) and agouti-related protein (AgRP) to blood-borne metabolic signals. Recent evidence has demonstrated that POMC and AgRP neurons are not simply mirror opposites of each other in function and responsiveness to metabolic signals, nor are they exclusively first-order neurons. These neurons act as central transceivers, integrating both hormonal and neural signals, and then transmitting this information to peripheral tissues via the autonomic nervous system to coordinate whole-body energy metabolism. This review focuses on most recent developments obtained from rodent studies on the function, metabolic regulation, and circuitry of the central melanocortin system.

Postpartum depression: psychoneuroimmunological underpinnings and treatment

Postpartum depression (PPD) is common, occurring in 10%-15% of women. Due to concerns about teratogenicity of medications in the suckling infant, the treatment of PPD has often been restricted to psychotherapy. We review here the biological underpinnings to PPD, suggesting a powerful role for the tryptophan catabolites, indoleamine 2,3-dixoygenase, serotonin, and autoimmunity in mediating the consequences of immuno-inflammation and oxidative and nitrosative stress. It is suggested that the increased inflammatory potential, the decreases in endogenous anti-inflammatory compounds together with decreased omega-3 poly-unsaturated fatty acids, in the postnatal period cause an inflammatory environment. The latter may result in the utilization of peripheral inflammatory products, especially kynurenine, in driving the central processes producing postnatal depression. The pharmacological treatment of PPD is placed in this context, and recommendations for more refined and safer treatments are made, including the better utilization of the antidepressant, and the anti-inflammatory and antioxidant effects of melatonin.

Enhanced leptin sensitivity, reduced adiposity, and improved glucose homeostasis in mice lacking exchange protein directly activated by cyclic AMP isoform 1

The prototypic second messenger cyclic AMP (cAMP) is essential for controlling cellular metabolism, including glucose and lipid homeostasis. In mammals, the majority of cAMP functions are mediated by cAMP-dependent protein kinase (PKA) and exchange proteins directly activated by cAMP (Epacs). To explore the physiological functions of Epac1, we generated Epac1 knockout mice. Here we report that Epac1 null mutants have reduced white adipose tissue and reduced plasma leptin levels but display heightened leptin sensitivity. Epac1-deficient mice are more resistant to high-fat diet-induced obesity, hyperleptinemia, and glucose intolerance. Furthermore, pharmacological inhibition of Epac by use of an Epac-specific inhibitor reduces plasma leptin levels in vivo and enhances leptin signaling in organotypic hypothalamic slices. Taken together, our results demonstrate that Epac1 plays an important role in regulating adiposity and energy balance.

Isoform-specific antagonists of exchange proteins directly activated by cAMP

The major physiological effects of cAMP in mammalian cells are transduced by two ubiquitously expressed intracellular cAMP receptors, protein kinase A (PKA) and exchange protein directly activated by cAMP (EPAC), as well as cyclic nucleotide-gated ion channels in certain tissues. Although a large number of PKA inhibitors are available, there are no reported EPAC-specific antagonists, despite extensive research efforts. Here we report the identification and characterization of noncyclic nucleotide EPAC antagonists that are exclusively specific for the EPAC2 isoform. These EAPC2-specific antagonists, designated as ESI-05 and ESI-07, inhibit Rap1 activation mediated by EAPC2, but not EPAC1, with high potency in vitro. Moreover, ESI-05 and ESI-07 are capable of suppressing the cAMP-mediated activation of EPAC2, but not EPAC1 and PKA, as monitored in living cells through the use of EPAC- and PKA-based FRET reporters, or by the use of Rap1-GTP pull-down assays. Deuterium exchange mass spectroscopy analysis further reveals that EPAC2-specific inhibitors exert their isoform selectivity through a unique mechanism by binding to a previously undescribed allosteric site: the interface of the two cAMP binding domains, which is not present in the EPAC1 isoform. Isoform-specific EPAC pharmacological probes are highly desired and will be valuable tools for dissecting the biological functions of EPAC proteins and their roles in various disease states.

A novel EPAC-specific inhibitor suppresses pancreatic cancer cell migration and invasion

Exchange protein directly activated by cAMP (EPAC) and cAMP-dependent protein kinase (PKA) are two intracellular receptors that mediate the effects of the prototypic second messenger cAMP. Identifying pharmacological probes for selectively modulating EPAC activity represents a significant unmet need within the research field. Herein, we report the identification and characterization of 3-(5-tert-butyl-isoxazol-3-yl)-2-[(3-chloro-phenyl)-hydrazono]-3-oxo-propionitrile (ESI-09), a novel noncyclic nucleotide EPAC antagonist that is capable of specifically blocking intracellular EPAC-mediated Rap1 activation and Akt phosphorylation, as well as EPAC-mediated insulin secretion in pancreatic β cells. Using this novel EPAC-specific inhibitor, we have probed the functional roles of overexpression of EPAC1 in pancreatic cancer cells. Our studies show that EPAC1 plays an important role in pancreatic cancer cell migration and invasion, and thus represents a potential target for developing novel therapeutic strategies for pancreatic cancer.