Suppressor of cytokine signaling-3 is a glucagon-inducible inhibitor of PKA activity and gluconeogenic gene expression in hepatocytes.
SOCS3 is a cytokine-inducible negative regulator of cytokine receptor signaling. Recently, SOCS3 was shown to be induced by a cAMP-dependent pathway involving exchange protein directly activated by cAMP (Epac). We observed in livers of fasted mice that Socs3 mRNA was increased 4-fold compared with refed mice, suggesting a physiologic role for SOCS3 in the fasted state that may involve glucagon and Epac. Treating primary hepatocytes with glucagon resulted in a 4-fold increase in Socs3 mRNA levels. The Epac-selective cAMP analog 8-4-(chlorophenylthio)-2'-O-methyladenosine-3',5'-monophosphate, acetoxymethyl ester (cpTOME) increased Socs3 expression comparably. In gain-of-function studies, adenoviral expression of SOCS3 in primary hepatocytes caused a 50% decrease in 8-br-cAMP-dependent PKA phosphorylation of the transcription factor CREB. Induction of the gluconeogenic genes Ppargc1a, Pck1, and G6pc by glucagon or 8-br-cAMP was suppressed nearly 50%. In loss-of-function studies, hepatocytes from liver-specific SOCS3 knock-out mice responded to 8-br-cAMP with a 200% greater increase in Ppargc1a and Pck1 expression, and a 30% increase in G6pc expression, relative to wild-type cells. Suppression of SOCS3 by shRNA in hepatocytes resulted in a 60% increase in cAMP-dependent G6pc and Pck1 expression relative to control cells. SOCS3 expression also inhibited cAMP-dependent phosphorylation of the IP3 receptor but did not inhibit nuclear localization of the catalytic subunit of PKA. Using an in vitro kinase assay, cAMP-dependent PKA activity was reduced by 80% in hepatocytes expressing ectopic SOCS3. These data indicate that cAMP activates both the PKA and Epac pathways with induction of SOCS3 by the Epac pathway negatively regulating the PKA pathway.
Gaudy
Allison M
AM
Department of Pharmacology and Physiology, University of Rochester Medical Center, Rochester, New York 14642, USA.
Clementi
Alicia H
AH
Campbell
Jean S
JS
Smrcka
Alan V
AV
Mooney
Robert A
RA
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000378:metabolism; D000818:Animals; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004952:Esters / Q000737:chemistry*; D005934:Glucagon / Q000378:metabolism*; D005943:Gluconeogenesis; D022781:Hepatocytes / Q000378:metabolism*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D012333:RNA, Messenger / Q000378:metabolism; D034741:RNA, Small Interfering / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D000071223:Suppressor of Cytokine Signaling 3 Protein; D050826:Suppressor of Cytokine Signaling Proteins / Q000378:metabolism*
C555650:8-(4-chlorophenylthio)-2'-O-methyladenosine-3',5'-monophosphate acetoxymethyl ester; D004952:Esters; D012333:RNA, Messenger; D034741:RNA, Small Interfering; C491411:Socs3 protein, mouse; D000071223:Suppressor of Cytokine Signaling 3 Protein; D050826:Suppressor of Cytokine Signaling Proteins; D015124:8-Bromo Cyclic Adenosine Monophosphate; D005934:Glucagon; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases