Vasoactive intestinal peptide increases intracellular cAMP and gonadotropin-alpha gene activity in JEG-3 syncytial trophoblasts. Constraints posed by desensitization.

Expression of the human chorionic gonadotropin (hCG)-alpha gene in placental trophoblasts is markedly stimulated by cAMP, a property preserved in a reporter plasmid containing its cAMP response elements (CREs) linked to the chloramphenicol acetyltransferase coding sequence (CRE alpha CAT). In search of a potential physiologic regulator of hCG gene expression via cAMP, we found that JEG-3 syncytial trophoblast cells have specific binding sites for vasoactive intestinal peptide (VIP) with dissociation constant of 1 nM. VIP maximally increased the transient expression of CRE alpha CAT and the expression of endogenous hCG-alpha mRNA in JEG-3 cells by 4- and 9-fold, respectively. Exposure of JEG-3 cells to 30 nM VIP increased cAMP levels 60-fold after 10-30 min, but cAMP rapidly declined thereafter. As a consequence of this desensitization, the effect of VIP on stimulation of both CRE alpha CAT and endogenous hCG-alpha and hCG-beta mRNA levels more closely resembled that of forskolin or 8-br-cAMP at time points much less than 24 h. Moreover, transient exposure to 8-br-cAMP was much less effective than 24 h of continuous incubation on CRE alpha CAT activity. We conclude that VIP rapidly increases cAMP content and activates hCG-alpha gene expression in JEG-3 cells, but sustained elevations in cAMP are necessary for maximal accumulation of this CRE-regulated gene product.     

Effect of IL-1beta on CRE-dependent gene expression in human airway smooth muscle cells

IL-1beta inhibits isoproterenol (ISO)-induced relaxation of cultured human airway smooth muscle (HASM) cells. The purpose of this study was to determine whether IL-1beta can also suppress ISO-induced cAMP response element (CRE)-dependent gene expression. ISO (10 microM) caused a marked increase in CRE-binding protein (CREB) phosphorylation, which was attenuated by IL-1beta (2 ng/ml). This effect of IL-1beta was abolished by the cyclooxygenase (COX) inhibitor indomethacin. To examine CRE-driven gene expression, we transiently transfected HASM cells with a construct containing CRE upstream of a luciferase reporter gene. ISO (6 h) caused a sixfold increase in luciferase activity. IL-1beta (24 h) alone also increased luciferase activity, although to a lesser extent (2-fold). However, the ability of ISO to elicit luciferase expression was markedly reduced in cells treated with IL-1beta. Indomethacin, the MEK and p38 inhibitors U-0126 and SB-203580, the protein kinase A inhibitor H-89, and dexamethasone each completely abolished the ability of IL-1beta to induce CRE-driven gene expression but only slightly increased the ability of ISO to induce CRE-driven gene expression in IL-1beta-treated cells. IL-1beta also attenuated dibutyryl cAMP-induced CRE-driven gene expression, but not dibutyryl cAMP-induced CREB phosphorylation. Tumor necrosis factor-alpha (10 ng/ml) also attenuated ISO-induced CRE-driven gene expression, even though it was without effect on ISO-induced cAMP formation or ISO-induced CREB phosphorylation. The results suggest that IL-1beta and tumor necrosis factor-alpha may attenuate the ability of beta-agonists to induce expression of genes with CRE in their regulatory regions at least in part through events downstream of CREB phosphorylation.     

Identification and characterization of a 3',5'-cyclic adenosine monophosphate-responsive element in the human corticotropin-releasing hormone gene promoter.

The regulation of human corticotropin-releasing hormone (hCRH) gene promoter activity by inducers of cAMP was investigated by transient transfection with a construct containing the hCRH gene promoter fused to the chloramphenicol acetyltransferase gene. Expression of hCRH-chloramphenicol acetyltransferase was strongly enhanced by forskolin in the neuroblastoma SK-N-MC and choriocarcinoma JAR cell lines. Overexpression of the catalytic subunit of protein kinase A dispensed the need for forskolin, and cotransfection of cAMP-responsive element-binding protein cDNAs enhanced forskolin-dependent expression of the hCRH promoter. Progressive 5'-end deletions of the hCRH promoter delineated a cAMP- responsive region between -226 and -164 base pairs. This fragment contained the sequence TGACGTCA at -221 base pairs, consistent with the consensus motif for a CRE. A homologous oligonucleotide responded to cAMP when cloned in either orientation in front of the thymidine kinase promoter. However, the level of constitutive and inductive cAMP expression was dependent on the cell line and on intrinsic properties of the promoter. Mutation of the wild type CRH-CRE sequence into an AP-1 site (TGAGTCA) completely abolished stimulation by cAMP. In contrast, coexpression of the catalytic subunit of protein kinase A dispensed the need for stimulation with forskolin, which showed that the CRH-CRE oligonucleotide served as a functional equivalent of the native CRE element.     

中国北方人群谷胱甘肽转硫酶P1基因多态性及其与胃癌遗传易感性的关系

为了分析中国北方人群谷胱甘肽转硫酶P1基因(glutathione-S-transferase P1, GSTP1)多态性分布, 同时探讨GSTP1基因多态性及其与幽门螺杆菌(H. pylori)既往感染联合作用对胃癌发病风险的影响, 采用多聚酶链反应-限制性片段长度多态性(PCR-RFLP)技术检测1,612例外周血DNA GSTP1的多态性; 采用ELISA方法检测血清H. pylori IgG。结果显示, (1) 中国北方人群GSTP1基因Val等位基因分布频率为22%, 胃癌高、低发区GSTP1 Val等位基因分布频率有显著性差异(0.23/0.20); (2) 以Ile/Ile基因型为参照组与其他两种基因型比较进行胃癌的风险分析, 结果显示携带Val/Val基因型的个体患胃癌的危险性最大, 其OR为5.588 (3.256 ~ 9.591); 携带Val等位基因的个体患胃癌危险性是非携带Val等位基因个体的1.587倍; (3) 以H. pylori IgG(-)并携带GSTP1基因纯合野生型(Ile/ Ile)的个体为参照, H. pylori IgG(+)并携带纯合多态基因型(Val/Val)的个体患胃癌的风险最高, OR为17.571(6.207 ~ 49.742)。说明GSTP1 Val等位基因的分布存在人群及地区差异。携带GSTP1 Val等位基因的个体胃癌发病风险增高。GSTP1 Val等位基因纯合型与H. pylori感染对于胃癌的发生具有交互作用。