Role of the exchange protein directly activated by cyclic adenosine 5'-monophosphate (Epac) pathway in regulating proglucagon gene expression in intestinal endocrine L cells.
Although proglucagon gene expression and the synthesis of proglucagon encoded peptide hormones could be activated by protein kinase A (PKA) activators such as forskolin/3-isobutyl-1-methylxanthine (IBMX) and cholera toxin, whether the activation is entirely attributed to PKA has not been previously examined. We found that forskolin/IBMX also activate ERK1/2 phosphorylation in intestinal and pancreatic proglucagon-producing cell lines. The MEK inhibitors PD98059 and U0126 were found to repress the expression of proglucagon promoter as well as endogenous proglucagon mRNA in two intestinal proglucagon-producing cell lines and to block the stimulatory effect of forskolin/IBMX on proglucagon mRNA expression. The repressive effect of the PKA-specific inhibitors H-89 and KT-5720, however, was either not observable or much less potent. Forskolin could activate ERK1/2 phosphorylation and proglucagon gene transcription on its own, whereas forskolin plus IBMX are required to effectively activate the PKA pathway in the proglucagon-producing cells. Exchange protein directly activated by cyclic AMP 2 (Epac2, or cAMP-binding guanine nucleotide exchange factor-2) was found to be expressed in gut and pancreatic proglucagon-producing cell lines, whereas the Epac-pathway-specific cAMP analog, 8-pMeOPT-2'O-Me-cAMP, effectively stimulated ERK1/2 phosphorylation as well as proglucagon mRNA expression. We therefore suggest that cAMP at least partially regulates proglucagon gene expression via the Epac-Ras/Rap-Raf-MEK-ERK signaling pathway.
Lotfi
Shamim
S
Division of Cell and Molecular Biology, Toronto General Research Institute, University Health Network, 67 College Street, Toronto, Ontario, Canada M5G 2M1.
Li
Zhihong
Z
Sun
Jane
J
Zuo
Yang
Y
Lam
Patrick P L
PP
Kang
Youhou
Y
Rahimi
Mehdi
M
Islam
Diana
D
Wang
Peixiang
P
Gaisano
Herbert Y
HY
Jin
Tianru
T
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000818:Animals; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism; D019858:Enteroendocrine Cells / Q000166:cytology / Q000502:physiology*; D048049:Extracellular Signal-Regulated MAP Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism*; D020935:MAP Kinase Signaling System / Q000502:physiology; D051379:Mice; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D010766:Phosphorylation; D052336:Proglucagon / Q000235:genetics*; D011401:Promoter Regions, Genetic / Q000235:genetics; D012333:RNA, Messenger / Q000378:metabolism; D014158:Transcription, Genetic / Q000502:physiology
D002352:Carrier Proteins; D020662:Guanine Nucleotide Exchange Factors; D010726:Phosphodiesterase Inhibitors; D012333:RNA, Messenger; C486707:Rapgef4 protein, mouse; D005576:Colforsin; D052336:Proglucagon; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D048049:Extracellular Signal-Regulated MAP Kinases; D015056:1-Methyl-3-isobutylxanthine