Another milestone in the field is the
Another milestone in the field is the de novo discovery of EPAC2 and EPAC1 specific inhibitors through fluorescence-based HTS assays. Due to the excellent EPAC/PKA selectivity of ptc124 , it has been widely applied as a useful chemical probe to discriminate EPAC related signaling pathways and biological functions. In addition, compound and its analogues exert excellent PK and toxicity profiles and are promising lead compounds for therapeutic applications. Nevertheless, more extensive chemical optimizations are imperative to improve the potency (ideally submicromolar to nanomolar IC) and the isoform-selectivity (EPAC1 EPAC2) of this class of EPAC inhibitors. Therefore, EPAC-isoform specific agonists or antagonists with high potency and selectivity are still in urgent need, and to this end, modern drug discovery approaches (e.g. HTS, fragment-based drug discovery, , and computer-aided drug design may pave the way. Of note, EPAC1 and EPAC2 may play significant and distinct biological roles in a variety of human diseases, especially in cancer, inflammation, cardiovascular diseases, CNS disorders (e.g. drug addiction, and pain), infections and diabetes. Hence, potent EPAC agonists or antagonists with high EPAC1 or EPAC2-isoform selectivity, as well as ideal pharmacokinetic profiles are highly appreciated for further clinical development towards a viable therapeutic strategy for various human diseases.
This work was supported by grants R01 GM106218, R01 GM066170, and R01 AI111464 from the National Institutes of Health, John Sealy Memorial Endowment Fund, and the Center for Addiction Research (CAR) at UTMB.
Introduction High incidences of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) have been noted, but therapies are still lacking . Lipopolysaccharide (LPS) is a major component of gram-negative bacteria and is widely used to induce and investigate the molecular mechanisms of acute inflammatory injury of the lung . As shown in our previous studies, an intratracheal instillation of LPS activates alveolar macrophages, resulting in the production of early-response cytokines, such as tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and IL-6. LPS then induces a strong pro-inflammatory cascade that eventually causes acute damage to capillary and alveolar epithelial cells . Clinical and basic studies have revealed beneficial effects of elevated intracellular cAMP levels in various pathological settings, such as pulmonary edema  and ischemia-reperfusion (I-R) injury in isolated blood-perfused rabbit lungs . In previous studies of ALI, cAMP was shown to exert protective effects on various animal models, such as Escherichia coli-induced ALI in guinea pigs , protamine-induced ALI in isolated rat lungs , endotoxin-induced lung injury in rats , and pulmonary air embolism-induced lung injury in sheep . Protein kinase A (PKA) and exchange protein directly activated by cAMP (Epac), a guanine nucleotide exchange factor, are two principal effectors of cAMP, are expressed in a wide range of tissues and control diverse biological functions. According to recent evidence, Epac and PKA might play independent, synergistic, or opposite roles in specific cellular functions . The development of the specific Epac activator and cAMP analog 8-(4-chlorophenylthio)-2′-O-methyl-adenosine-3′,5′-cyclic mono-phosphate (8CPT) and the selective PKA activator N6-benzoyl-adenosine-3′,5′-cyclic monophosphate (6Bnz)  enable the separate study of PKA- and Epac-mediated pathways. Therefore, in this study, we initially used in vitro models of LPS-induced lung injury to determine the effects of Epac and PKA and confirmed that an Epac activator plays a more important role in LPS-induced production of mediators of the early inflammatory response, such as TNF-α. We then used the Epac activator 8CPT by intratracheal instillation of LPS to induced lung injury in mice. The results suggested that 8CPT decreases the production of early-response cytokines, such as TNF-α, IL-1β, IL-6, and keratinocyte-derived cytokine (KC). Moreover, 8CPT improves lung injury and decreases microvascular leakage; the mechanism involves Rac1/2 but not the mitogen-activated protein kinase (MAPK) pathway. In summary, Epac plays an important role in modulating LPS-induced ALI. Epac activation reduces inflammation and microvascular permeability; therefore, a Epac activator represents a novel choice for the early therapy of ALI or ARDS.