SIK2 can be activated by deprivation of nutrition and it inhibits expression of lipogenic genes in adipocytes.
OBJECTIVE
Determine the biological function of SIK2 in 3T3-L1 adipocytes.


METHODS
3T3-L1 adipocytes were treated with serum-free, serum/glucose-free, or complete Dulbecco's Modified Eagle's Medium (DMEM) for 15 min, or 10 microg/ml oligomycin in serum-free DMEM for 15 min, or different doses of 5-amino-4-imidazolecarboxamide (AICAR) for different periods of time in serum-free DMEM. After treatment, SIK2 kinase activity was measured by examining phosphorylation of a peptide substrate. SIK2 was overexpressed and knocked down in 3T3-L1 CAR adipocytes using adenovirus-mediated gene transfer for the assessment of lipogenic gene expression and triglyceride content. Electroporation was used to transiently transfect 3T3-L1 adipocytes with fatty acid synthase (FAS) promoter-driven luciferase construct to evaluate the effect of SIK2 on FAS transcription.


RESULTS
In 3T3-L1 adipocytes, SIK2 can be activated by nutrient deprivation, inhibition of ATP synthesis, and treatment of AICAR. Overexpression of SIK2 repressed the expression of lipogenic genes, including FAS, acetyl CoA carboxylase 2 (ACC2), and stearoyl CoA desaturase 1 (SCD1), and reduced triglyceride content. Reduction of endogenous SIK2 expression through RNA interference increased the expression of FAS, ACC2, and SCD1. This effect is independent of adenosine monophosphate-activated protein kinase (AMPK) since neither phosphorylation state nor protein level of AMPKalpha1 and AMPKalpha2 was affected by SIK2 overexpression. SIK2 inhibits the expression of FAS-promoter driven luciferase reporter gene, and this effect can be reversed by overexpression of constitutively active sterol regulatory element binding protein-1 (SREBP-1). Furthermore, SIK2 also reduces the nuclear translocation of endogenous SREBP-1.


CONCLUSIONS
Our results support a role for SIK2 in adipocyte energy metabolism. SIK2 may function similarly to AMPK for turning off lipogenesis in low-energy state.
Du
Jing
J
Brown Medical School, Department of Medicine, Hallett Center for Diabetes and Endocrinology, Providence, RI, USA.
Chen
Qiu
Q
Takemori
Hiroshi
H
Xu
Haiyan
H
D041721:3T3-L1 Cells; D055372:AMP-Activated Protein Kinases; D000103:Acetyl-CoA Carboxylase / Q000235:genetics / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D017667:Adipocytes / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D000620:Aminoimidazole Carboxamide / Q000494:pharmacology; D000818:Animals; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation; D018274:Electroporation; D004789:Enzyme Activation; D020536:Enzyme Activators / Q000494:pharmacology; D064429:Fatty Acid Synthases / Q000235:genetics / Q000378:metabolism; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D017930:Genes, Reporter; D005947:Glucose / Q000172:deficiency / Q000378:metabolism; D050155:Lipogenesis / Q000187:drug effects / Q000235:genetics*; D051379:Mice; D009097:Multienzyme Complexes / Q000378:metabolism; D010766:Phosphorylation; D011401:Promoter Regions, Genetic; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000378:metabolism; D013230:Stearoyl-CoA Desaturase / Q000235:genetics / Q000378:metabolism; D051780:Sterol Regulatory Element Binding Protein 1 / Q000378:metabolism; D013997:Time Factors; D014162:Transfection; D014280:Triglycerides / Q000378:metabolism
D020536:Enzyme Activators; D009097:Multienzyme Complexes; D034741:RNA, Small Interfering; D051780:Sterol Regulatory Element Binding Protein 1; D014280:Triglycerides; D000620:Aminoimidazole Carboxamide; D000255:Adenosine Triphosphate; C490333:Scd1 protein, mouse; D013230:Stearoyl-CoA Desaturase; D064429:Fatty Acid Synthases; C474808:salt-inducible kinase-2, mouse; D017346:Protein Serine-Threonine Kinases; D055372:AMP-Activated Protein Kinases; C498191:Acacb protein, mouse; D000103:Acetyl-CoA Carboxylase; D005947:Glucose