Adipose-specific Expression, Phosphorylation of Ser794 in Insulin Receptor Substrate-1, and Activation in Diabetic Animals of Salt-inducible Kinase-2

Salt-inducible kinase (SIK), first cloned from the adrenal glands of rats fed a high salt diet, is a serine/threonine protein kinase belonging to an AMP-activated protein kinase family. Induced in Y1 cells at an early stage of ACTH stimulation, it regulated the initial steps of steroidogenesis. Here we report the identification of its isoform SIK2. When a green fluorescent protein-fused SIK2 was expressed in 3T3-L1 preadipocytes, it was mostly present in the cytoplasm. When coexpressed in cAMP-responsive element-reporter assay systems, SIK2 could repress the cAMP-responsive element-dependent transcription, although the degree of repression seemed weaker than that by SIK1. SIK2 was specifically expressed in adipose tissues. When 3T3-L1 cells were treated with the adipose differentiation mixture, SIK2 mRNA was induced within 1 h, the time of induction almost coinciding with that of c/EBPbeta mRNA. Coexpressed with human insulin receptor substrate-1 (IRS-1) in COS cells, SIK2 could phosphorylate Ser(794) of human IRS-1. Adenovirus-mediated overexpression of SIK2 in adipocytes elevated the level of phosphorylation at Ser(789), the mouse equivalent of human Ser(794). Moreover, the activity and content of SIK2 were elevated in white adipose tissues of db/db diabetic mice. These results suggest that highly expressed SIK2 in insulin-stimulated adipocytes phosphorylates Ser(794) of IRS-1 and, as a result, might modulate the efficiency of insulin signal transduction, eventually causing the insulin resistance in diabetic animals.