Identification of cAMP‐responsive substrates of SIK2 in adipocytes

Abstract

Salt‐inducible kinase (SIK) 2 is an AMP‐activated protein kinase (AMPK)‐related kinase and is highly expressed in adipose tissue. SIK2 activity is regulated by LKB1‐phosphorylation of a conserved residue in the activation (T)‐loop. In adipocytes, SIK2 is phosphorylated at Ser358 upon cAMP‐induction, leading to its binding to 14‐3‐3 proteins and re‐localisation into the cytosol. This regulation may restrict its ability to interact with potential substrates and the aim of our study was to investigate if histone deacetylase ‐4 (HDAC4), CREB regulated transcription co‐activator 2 (CRTC2) and ‐3, all regulated by cAMP, are substrates of SIK2 in adipocytes. Adenovirus‐mediated over‐expression of wild type SIK2 in adipocytes increased the phosphorylation of all proposed substrates, compared to kinase inactive SIK2 and GFP‐transduced cells. In addition, HDAC4, CRTC2 and ‐3 were found to co‐immunoprecipitate with HA‐SIK2, with a reduction in binding upon cAMP‐elevation for all but HDAC4. After siRNA‐mediated silencing of SIK2, the phosphorylation of CRTCs was reduced. Ongoing experiments address the role of Ser358 phosphorylation of SIK2 in the regulation of HDAC4, CRTC2 and ‐3 in adipocytes. These proteins, which we suggest are substrates of SIK2 in adipocytes, are known to be involved in the control of metabolic gene expression, supporting a role for SIK2 in adipocyte metabolism. This work was mainly supported by the Swedish Research Council, The Craaford Foundation and the Novo Nordisk Foundation.