Submitted on January 12, 2009
Revised on March 27, 2009
Accepted on April 1, 2009
Involvement of SIK2/TORC2 signaling cascade in the regulation of insulin-induced PGC-1
and UCP-1 gene expression in brown adipocytes
Masaaki Muraoka1, Aiko Fukushima1, Say Viengchareun2, Marc Lombès2, Fukuko Kishi3, Akira Miyauchi3, Mariko Kanematsu1, Junko Doi4, Junko Kajimura1, Ryo Nakai1, Tatsuya Uebi1, Mitsuhiro Okamoto5, and Hiroshi Takemori1*
1 National Institute of Biomedical Innovation
2 Univ. Paris-Sud, Faculté de Médecine
3 ProteinExpress Co., Ltd
4 Senri Kinran University
5 Tezukayama University
* To whom correspondence should be addressed. E-mail: takemori{at}nibio.go.jp.
Salt inducible kinase 2 (SIK2) is expressed abundantly in adipose tissues and represses CREB-mediated gene expression by phosphorylating the co-activator TORC2. Phosphorylation at Ser587 of SIK2 diminishes its TORC2 phosphorylation activity. In 3T3-L1 white adipocytes, SIK2 down-regulates lipogenic gene in response to nutritional stresses. To investigate the impact of SIK2 on the function of brown adipose tissue (BAT), we used T37i brown adipocytes, mice with diet-induced obesity and SIK2 mutant (S587A) transgenic mice. When T37i adipocytes were treated with insulin, the levels of PGC-1
and UCP-1 mRNA were increased, and the induction was inhibited by overexpression of SIK2 (S587A) mutant or dominant negative CREB. Insulin enhanced SIK2 phosphorylation at Ser587, which was accompanied by decrease in phospho-TORC2. Similarly, the decrease in the level of SIK2 phosphorylation at Ser587 was observed in the BAT of mice with diet-induced obesity, which was negatively correlated with TORC2 phosphorylation. To confirm the negative correlation between SIK2 phosphorylation at Ser587 and TORC2 phosphorylation in BAT, SIK2 mutant (S587A) was overexpressed in adipose tissues by using the aP2 promoter. The expression of recombinant SIK2 (S587A) was restricted to BAT, and the levels of phospho-TORC2 was elevated in BAT of transgenic mice. Male transgenic mice developed high-fat diet-induced obesity, and their BAT expressed low levels of PGC-1 and UCP-1 mRNA, suggesting that SIK2-TORC2 cascade may be important for the regulation of PGC-1 and UCP-1 gene expression in insulin-signaling in BAT.
