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 AND PROCEDURES: 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. DISCUSSION: 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.