on February 23, 2006
Published online before print February 23, 2006, doi: 10.1161/01.ATV.0000214981.58499.32
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Submitted on October 25, 2005
Accepted on February 8, 2006
Homocysteine Activates cAMP-response Element Binding Protein in HepG2 through cAMP/PKA Signaling Pathway
Connie W.H. Woo ;
From the Department of Animal Science, Department of Physiology, University of Manitoba, Canadian Centre for Agri-Food Research in Medicine, St. Boniface Hospital Research Centre, Winnipeg, Canada.
* To whom correspondence should be addressed. E-mail: karmino{at}sbrc.ca.
Objective--Hyperhomocystinemia is an independent risk factor for cardiovascular disorders. Our previous studies demonstrated that hyperhomocystinemia not only elicited inflammatory responses in the vascular endothelium but also induced fatty liver and hypercholesterolemia via transcriptional regulation. One of the transcription factors activated in the liver during hyperhomocystinemia was cAMP-response element binding protein (CREB). CREB regulates the expression of many genes including those involved in lipid and glucose metabolism. In this study, we investigated the molecular mechanism by which Hcy activated CREB in rat liver and in hepatocytes (HepG2).
Method and Results--Hyperhomocystinemia was induced in rats by feeding high-methionine diet for 4 weeks. There was a significant increase in hepatic cAMP levels, protein kinase A (PKA) activity and an activation of CREB. Incubation of HepG2 cells with Hcy (50 to 100 µmol/L) significantly enhanced CREB phosphorylation and subsequently increased CREB/DNA binding activity. PKA was activated in Hcy-treated cells as a result of increased cellular cAMP level. Inhibition of adenylyl cyclase not only reduced the intracellular cAMP levels elevated by Hcy treatment but also inhibited PKA activation and prevented Hcy-induced CREB phosphorylation.
Conclusion--These results suggest that the cAMP/PKA signaling pathway plays an important role in mediating Hcy-induced CREB activation in hepatocyte.
Key words: cAMP • CREB • homocysteine (HMG)-coenzyme A (CoA) reductase • PKA
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