S-adenosylmethionine regulates cytoplasmic HuR via AMP-activated kinase.
OBJECTIVE
After liver injury, hepatic S-adenosylmethionine (SAM) content decreases, and the blockage this molecule imposes on hepatocyte proliferation is released, facilitating liver regeneration. This activity of SAM is important for normal liver function because mice deficient in hepatic SAM display abnormal liver regeneration and develop hepatocellular carcinoma. How SAM regulates hepatocyte growth is unclear, but because SAM blocks hepatocyte growth factor (HGF)-induced cyclin D1 expression and DNA synthesis without affecting HGF-induced extracellular signal-regulated kinase phosphorylation, the mitogen-activated protein kinase (MAPK) pathway is probably not the target.


METHODS
The effects of SAM on AMPK, HuR localization were assessed in rat hepatocytes after HGF, AICAR, and SAM treatment.


RESULTS
We show here that HGF and 5-aminoimidazole-4-carboxamide-riboside (AICAR), an activator of AMP-activated protein kinase (AMPK), induce the phosphorylation of AMPK in hepatocytes and that SAM blocks this process. We also show that HGF- and AICAR-induced AMPK activation stimulate the transport from nucleus to cytoplasm of HuR, an RNA-binding protein that increases the half-life of target mRNA such as cyclin A2, and that SAM blocks this process. We found that, in hepatocytes, AICAR increases HuR binding to cyclin A2 messenger RNA (mRNA) as well as the expression and stability of this mRNA and that SAM blocks these events. Consistently, we found that AICAR induces hepatocyte proliferation and that SAM blocks this effect. Finally, we found that liver AMPK phosphorylation, cytoplasmic HuR, and binding of HuR to HuR-target mRNA and the steady-state levels of these mRNA are increased in knockout mice deficient in hepatic SAM.


CONCLUSIONS
Our results yield novel insights about the mechanism by which SAM inhibits cell-cycle progression in the liver.
Martínez-Chantar
María L
ML
CIC bioGUNE, Technological Park of Bizkaia, Bizkaia, Spain.
Vázquez-Chantada
Mercedes
M
Garnacho
Marta
M
Latasa
M Ujue
MU
Varela-Rey
Marta
M
Dotor
Javier
J
Santamaria
Monica
M
Martínez-Cruz
Luis A
LA
Parada
Luis A
LA
Lu
Shelly C
SC
Mato
José M
JM
D055372:AMP-Activated Protein Kinases; D000620:Aminoimidazole Carboxamide / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D000954:Antigens, Surface / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D002453:Cell Cycle / Q000187:drug effects; D002478:Cells, Cultured; D003593:Cytoplasm / Q000378:metabolism*; D051959:ELAV Proteins; D000067780:ELAV-Like Protein 1; D015870:Gene Expression / Q000187:drug effects; D017228:Hepatocyte Growth Factor / Q000494:pharmacology; D022781:Hepatocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D015151:Immunoblotting; D047468:Immunoprecipitation; D066298:In Vitro Techniques; D008297:Male; D009097:Multienzyme Complexes / Q000494:pharmacology*; D010766:Phosphorylation / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases / Q000494:pharmacology*; D012333:RNA, Messenger / Q000235:genetics; D016601:RNA-Binding Proteins / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012265:Ribonucleotides / Q000494:pharmacology; D012436:S-Adenosylmethionine / Q000494:pharmacology*
D000954:Antigens, Surface; D051959:ELAV Proteins; D000067780:ELAV-Like Protein 1; C098770:ELAVL1 protein, human; D009097:Multienzyme Complexes; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D012265:Ribonucleotides; D000620:Aminoimidazole Carboxamide; D017228:Hepatocyte Growth Factor; D012436:S-Adenosylmethionine; D017346:Protein Serine-Threonine Kinases; D055372:AMP-Activated Protein Kinases; C031143:AICA ribonucleotide