Paradoxical regulation of Sp1 transcription factor by glucagon.
Insulin is a potent regulator of Sp1 transcription factor. To examine if glucagon, which usually antagonizes insulin, regulates Sp1, we assessed the levels of Sp1 by Western blotting from H-411E cells exposed to glucagon with or without insulin. Glucagon alone (1.5 x 10(-9) to 1.5 x 10(-5) M) stimulated Sp1 accumulation but inhibited insulin's (10,000 microU/ml) stimulatory effect on Sp1. We also assessed the effect of TNF-alpha, wortmannin, a PI3K inhibitor, and cAMP-dependent protein kinase inhibitor on Sp1 accumulation. While TNF-alpha (5 ng/ml) blocked insulin-stimulated Sp1, it failed to block stimulation of Sp1 by glucagon (1.5 x 10(-5) M). Similarly, wortmannin inhibited insulin- but not glucagon-stimulated Sp1, whereas protein kinase inhibitor had an opposite effect. Thus, insulin acts primarily via PI3K, and glucagon apparently stimulates through a cAMP-dependent pathway. Insulin increased the staining intensity of Sp1 seen exclusively in the nuclei of H-411E cells. Sp1 was demonstrable in both nucleus and cytoplasm after glucagon treatment. Finally, as judged by immunoblotting to specific antibody, insulin but not glucagon, stimulated O-glycosylation of Sp1. Thus, unique signaling mechanisms mediate the response of Sp1 to glucagon in the presence or absence of insulin.
Keembiyehetty
Chithra N
CN
Veterans Affairs Medical Center, Research Services, University of Tennessee Health Science Center, Memphis, Tennessee 38104, USA.
Candelaria
Rosalind P
RP
Majumdar
Gipsy
G
Raghow
Rajendra
R
Martinez-Hernandez
Antonio
A
Solomon
Solomon S
SS
D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D002147:Calmodulin / Q000096:biosynthesis / Q000235:genetics; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D005786:Gene Expression Regulation / Q000187:drug effects*; D005934:Glucagon / Q000494:pharmacology*; D007004:Hypoglycemic Agents / Q000494:pharmacology; D015151:Immunoblotting; D007150:Immunohistochemistry; D007328:Insulin / Q000494:pharmacology; D007329:Insulin Antagonists / Q000494:pharmacology; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D051381:Rats; D011956:Receptors, Cell Surface / Q000235:genetics; D015398:Signal Transduction / Q000187:drug effects; D016329:Sp1 Transcription Factor / Q000096:biosynthesis* / Q000235:genetics; D013997:Time Factors; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology; D000077191:Wortmannin
D000730:Androstadienes; D002147:Calmodulin; D007004:Hypoglycemic Agents; D007328:Insulin; D007329:Insulin Antagonists; D012333:RNA, Messenger; D011956:Receptors, Cell Surface; D016329:Sp1 Transcription Factor; D014409:Tumor Necrosis Factor-alpha; D005934:Glucagon; D000242:Cyclic AMP; D000077191:Wortmannin