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

Abstract

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/EBP␤ 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.The lipid metabolism in adipose tissues is under the control of two hormonal signaling pathways; insulin stimulates glucose uptake and lipogenesis, whereas cAMP, generated by exogenous stimuli like adrenalin and glucagon, stimulates lipolysis. If the balance between the two signaling systems becomes lost and the adipose tissues are exposed to hyperinsulinemia for a prolonged time, they gradually become resistant to insulin stimulation (1, 2). The insulin resistance occurring in tissues involved in biological fuel metabolism, such as adipose tissues, liver, and skeletal muscles, would finally cause disorders in energy metabolism of the whole body, such as obesity and type 2 diabetes (3, 4). Insulin receptor substrate (IRS) 1 proteins are key molecules of the insulin-signaling cascade (5); they are phosphorylated on tyrosine residues by the action of insulindependently activated insulin receptor kinase, and the tyrosine-phosphorylated IRS proteins trigger further intracellular cascades. Several investigators recently reported (6, 7) that IRS proteins, under certain non-physiological conditions, were phosphorylated on serine residues. The serine phosphorylation of IRS proteins would modulate the efficiency of the insulinsignaling cascade (8, 9) and eventually render the animals resistant to insulin stimulation (10, 11). Molecular identification of several protein kinases responsible for the serine phosphorylation of IRS proteins has been reported (12-24).Salt-inducible kinase (SIK) was first cloned from ...

PGE2 Induces Macrophage IL-10 Production and a Regulatory-like Phenotype via a Protein Kinase A–SIK–CRTC3 Pathway

“…Our data would indicate that Ser 343 is the critical site on SIK2 for regulation via PKA in macrophages. Ser 358 and Ser 587 in SIK2, or the corresponding Ser 577 site in SIK1, have previously been suggested as PKA sites in other cell types (55)(56)(57). Although we detected these sites in macrophages (Table II), we did not observe any increase in their phosphorylation in response to PGE 2 , indicating that they are unlikely to be involved in our system.…”

Identification of salt-inducible kinase 3 as a novel tumor antigen associated with tumorigenesis of ovarian cancer

“…SIK2, highly expressed in the adipose tissue , is found to inhibit cAMP-response element binding protein-mediated gene expression by phosphorylating transducer of regulated CREB activity 2 (TORC2), thus sequestering TORC2 in the cytoplasm, and preventing cAMP-response element binding proteinmediated gene transcription (Screaton et al, 2004). Insulin receptor substrate-1 is a downstream substrates of SIK2 (Horike et al, 2003), suggesting that SIK2 has a role in the insulin regulation of adipose tissues. As compared with SIK1 and SIK2, the biological functions of SIK3 are entirely obscure, particularly in cancers.…”

Salt-inducible Kinase 3 Signaling Is Important for the Gluconeogenic Programs in Mouse Hepatocytes

“…We isolated SIK1 from the adrenals of rats fed with a high Na ϩ or K ϩ diet (2) and identified SIK2 by its sequence similarity (3). SIK3 was characterized by another group in a wide range analysis (4).…”