Epac信号分子与哮喘关系研究进展

支气管哮喘是临床常见的气道慢性炎症性疾病,以气道高反应性、慢性气道炎症和气道重塑为病理生理特征,其发病机制目前尚未完全阐明。环磷酸腺苷(cAMP)是体内重要的第二信使,参与调控机体新陈代谢、细胞钙信号传导、细胞生长与分化、凋亡等多种病理生理过程。长期以来,蛋白激酶A (PKA)被认为是介导cAMP生物学效应的唯一下游信号分子。但新近发现的新型cAMP靶分子——cAMP直接激活的交换蛋白(Epac)的发现打破了这一说法。大量研究证实,Epac可单独或协同PKA介导cAMP的多种生物学效应。本文就Epac在支气管哮喘发病中的作用及其可能机制作一综述,为寻找哮喘治疗新靶点奠定基础。     

由cAMP激活的鸟苷酸交换蛋白（Epac）调控内皮细胞表面膜联蛋白A2的表达

目的 本研究旨在探讨内皮细胞内Epac1是否参与调控细胞表面ANXA2的表达。方法 本研究细胞实验以人静脉内皮细胞（HUVECs）为细胞模型,采用原子动力学检测方法,结合免疫荧光,分别从原子及蛋白水平验证Epac1是否参与细胞表面ANXA2表达的调控作用。分别提取15只Epac1基因敲除小鼠和15只C57BL6野生型小鼠血浆,检测每组样品ANXA2的表达水平。结果 （1）免疫荧光测得处理组（ESI09）相对荧光强度为17.59 ± 0.69,处理组（ESI09）相对荧光强度为6.32 ± 0.32,两组间数据结果有差异且有统计学意义（P<0.01）;原子动力显微镜法测得处理组（ESI09）测得力值为35113.07 ± 9866.65 pN,对照组（DMSO）测得力值为106277.70 ± 14233.77 pN,两组间数据结果有差异且有统计学意义（P<0.01）;由以上数据可得药物性抑制内皮细胞Epac的功能使其表面ANXA2表达量下调;（2）蛋白印迹法及酶联免疫吸附法测得处理组（ESI09）和对照组（DMSO）内皮细胞分泌ANXA2有差异,药物性抑制内皮细胞Epac1功能使内皮细胞胞外分泌ANXA2能力下降低;（3）酶联免疫吸附法测得Epac1基因敲除小鼠组血浆ANXA2平均含量为138.81 ± 38.93 ng/ml,显著低于野生型小鼠组血浆ANXA2平均含量281.46 ± 49.66 ng/ml（p<0.05）。结论 Epac1参与调控内皮细胞表面ANXA2的表达。     

Roles of cAMP signalling in insulin granule exocytosis

Insulin secretion is regulated by a series of complex events generated by various intracellular signals including Ca²⁺, ATP, cAMP and phospholipid-derived signals. Glucose-stimulated insulin secretion is the principal mode of insulin secretion, and the mechanism potentiating the secretion is critical for physiological responses. Among the various intracellular signals involved, cAMP is particularly important for amplifying insulin secretion. Recently, glucagon-like peptide-1 (GLP-1) analogues and dipeptidyl peptidase-IV (DPP-IV) inhibitors have been developed as new antidiabetic drugs. These drugs all act through cAMP signalling in pancreatic β-cells. Until recently, cAMP was generally thought to potentiate insulin secretion through protein kinase A (PKA) phosphorylation of proteins associated with the secretory process. However, it is now known that in addition to PKA, cAMP has other targets such as Epac (also referred to as cAMP-GEF). The variety of the effects mediated by cAMP signalling may be linked to cAMP compartmentation in the pancreatic β-cells.     

Sulphonylurea receptor‐1, sulphonylureas and amplification of insulin secretion by Epac activation in β cells

Amplification of insulin secretion by cyclic AMP involves activation of protein kinase A (PKA) and Epac2 in pancreatic β cells. Recent hypotheses suggest that sulphonylurea receptor‐1 (SUR1), the regulatory subunit of ATP‐sensitive potassium channels, is implicated in Epac2 effects and that direct activation of Epac2 by hypoglycaemic sulphonylureas contributes to the stimulation of insulin secretion by these drugs. In the present experiments, using islets from Sur1KO mice, we show that dibutyryl‐cAMP and membrane‐permeant selective activators of Epac or PKA normally amplify insulin secretion in β cells lacking SUR1. In contrast to Epac activator, sulphonylureas (glibenclamide and tolbutamide) did not increase insulin secretion in Sur1KO islets, as would be expected if they were activating Epac2 directly. Furthermore, glibenclamide and tolbutamide did not augment the amplification of insulin secretion produced by Epac activator or dibutyryl‐cAMP. Collectively, the results show that SUR1 is dispensable for amplification of insulin secretion by Epac2 activation and that direct activation of Epac2 is unimportant for the action of therapeutic concentrations of sulphonylureas in β cells.     

cAMP inhibits modulation of airway smooth muscle phenotype via the exchange protein activated by cAMP (Epac) and protein kinase A

BACKGROUND AND PURPOSE

Changes in airway smooth muscle (ASM) phenotype may contribute to the pathogenesis of airway disease. Platelet-derived growth factor (PDGF) switches ASM from a contractile to a proliferative, hypo-contractile phenotype, a process requiring activation of extracellular signal-regulated kinase (ERK) and p70^S6 Kinase (p70^S6K). The effects of cAMP-elevating agents on these processes is unknown. Here, we investigated the effects of cAMP elevation by prostaglandin E₂ (PGE₂) and the activation of the cAMP effectors, protein kinase A (PKA) and exchange protein activated by cAMP (Epac) on PDGF-induced phenotype switching in bovine tracheal smooth muscle (BTSM).

EXPERIMENTAL APPROACH

Effects of long-term treatment with the PGE₂ analogue 16,16-dimethyl-PGE₂, the selective Epac activator, 8-pCPT-2′-O-Me-cAMP and the selective PKA activator, 6-Bnz-cAMP were assessed on the induction of a hypo-contractile, proliferative BTSM phenotype and on activation of ERK and p70^S6K, both induced by PDGF.

KEY RESULTS

Treatment with 16,16-dimethyl-PGE₂ inhibited PDGF-induced proliferation of BTSM cells and maintained BTSM strip contractility and contractile protein expression in the presence of PDGF. Activation of both Epac and PKA similarly prevented PDGF-induced phenotype switching and PDGF-induced activation of ERK. Interestingly, only PKA activation resulted in inhibition of PDGF-induced phosphorylation of p70^S6K.

CONCLUSIONS AND IMPLICATIONS

Our data indicate for the first time that both Epac and PKA regulated switching of ASM phenotype via differential inhibition of ERK and p70^S6K pathways. These findings suggest that cAMP elevation may be beneficial in the treatment of long-term changes in airway disease.     

Epac inhibits migration and proliferation of human prostate carcinoma cells

Background:

It was recently found that cAMP mediates protein kinase A-independent effects through Epac proteins. The aim of this study was to investigate the role of Epac in migration and proliferation of prostate carcinoma cells.

Methods:

The effect of Epac activation was determined by [³H]thymidine incorporation and scratch assays in PC-3 and DU 145 cells. Furthermore, cytoskeletal integrity was analysed by phalloidin staining. The participation of intracellular Epac effectors such as mitogen-activated protein (MAP) kinases, Rap1- and Rho-GTPases was determined by immunoblotting and pull-down assay.

Results:

The specific Epac activator 8-pCPT-2′-O-Me-cAMP (8-pCPT) interfered with cytoskeletal integrity, reduced DNA synthesis, and migration. Although 8-pCPT activated Rap1, it inhibited MAP kinase signalling and RhoA activation. These findings were translated into functional effects such as inhibition of mitogenesis, cytoskeletal integrity, and migration.

Conclusion:

In human prostate carcinoma cells, Epac inhibits proliferative and migratory responses likely because of inhibition of MAP kinase and RhoA signalling pathways. Therefore, Epac might represent an attractive therapeutic target in the treatment of prostate cancer.     

The role of Epac proteins,novel cAMP mediators,in the regulation of immune,lung and neuronal function

Chronic degenerative inflammatory diseases, such as chronic obstructive pulmonary disease and Alzheimer''s dementia, afflict millions of people around the world, causing death and debilitation. Despite the global impact of these diseases, there have been few innovative breakthroughs into their cause, treatment or cure. As with many debilitating disorders, chronic degenerative inflammatory diseases may be associated with defective or dysfunctional responses to second messengers, such as cyclic adenosinemonophosphate (cAMP). The identification of the cAMP-activated guanine nucleotide exchange factors for Ras-like GTPases, Epac1 (also known as cAMP-GEF-I) and Epac2 (also known as cAMP-GEF-II), profoundly altered the prevailing assumptions concerning cAMP signalling, which until then had been solely associated with protein kinase A (PKA). Studies of the molecular mechanisms of Epac-related signalling have demonstrated that these novel cAMP sensors regulate many physiological processes either alone and/or in concert with PKA. These include calcium handling, cardiac and smooth muscle contraction, learning and memory, cell proliferation and differentiation, apoptosis, and inflammation. The diverse signalling properties of cAMP might be explained by spatio-temporal compartmentalization, as well as A-kinase anchoring proteins, which seem to coordinate Epac signalling networks. Future research should focus on the Epac-regulated dynamics of cAMP, and, hopefully, the development of compounds that specifically interfere with the Epac signalling system in order to determine the precise significance of Epac proteins in chronic degenerative inflammatory disorders.