Introduction
The salt-inducible kinases (SIKs)—SIK1, SIK2 and SIK3—are related to AMP-activated protein kinase (AMPK), a master regulator of cellular and whole body energy homeostasis [
1]. AMPK and AMPK-related kinases share sequence homology in their kinase domains [
2] and are all activated by liver kinase B1 (LKB1) [
3]. SIK1 was first identified in the adrenal glands of rats fed a high salt diet [
4] and has since been described in several cell types [
5‐
8]. SIK2 is highly expressed in adipose tissue and increases during adipocyte differentiation [
9‐
11], while SIK3 displays a more ubiquitous expression [
12].
Together SIKs control diverse cellular processes including the regulation of glucose and lipid metabolism in rodent liver [
13‐
18] and adipose tissue [
10,
19,
20]. Recently, it was shown that mice with global SIK2 deficiency display multiple defects in adipocyte metabolism [
20]. Moreover, a previous study described increased expression and activity of SIK2 in white adipose tissue (WAT) from obese
db/db mice [
9], suggesting a role for SIK2 in obesity and diabetes. SIK3 has also been linked to metabolism, and
Sik3
-/- mice display disturbed lipid and glucose homeostasis [
14,
17]. Furthermore, genetic variations in
SIK3 have been associated with dyslipidaemia and obesity in humans [
21].
SIKs regulate gene expression by controlling the phosphorylation of transcriptional regulators, such as class II histone deacetylases (HDACs) [
6,
22] and cAMP-response element binding protein (CREB)-regulated transcription co-activators (CRTCs) [
23,
24]. This has, so far, mainly been studied in the liver but we recently demonstrated that HDAC4, CRTC2 and CRTC3 are direct substrates of SIK2 in rodent adipocytes [
19]. In addition to the activating phosphorylation by LKB1, SIK2 is phosphorylated at several residues—Ser343, Ser358, Thr484 and Ser587—in response to cAMP/protein kinase A (PKA)-signalling in adipocytes, resulting in a subcellular relocalisation of SIK2 without altering kinase activity [
19,
25]. Similarly, SIK3 is phosphorylated at several residues in response to cAMP/PKA in adipocytes [
26].
The biological role of SIK2 in adipocytes is not fully understood and has almost exclusively been studied in rodents. Based on these studies, SIK2 appears to be required for GLUT4 expression and glucose uptake in adipocytes [
19,
20]. Therefore, the aim of our study was to investigate the expression of SIKs, in particular SIK2, in adipose tissue from healthy and obese or insulin-resistant humans, and to analyse the importance of SIKs for glucose uptake in human adipocytes.
Discussion
This study is the first to demonstrate SIK expression and function in human adipose tissue. We have shown that SIK2 and SIK3 are markedly downregulated in adipose tissue from obese or insulin-resistant humans (independently of BMI or age for SIK2) and that the expression is regulated in response to weight change and inflammation (TNF-α). Moreover, SIKs promote insulin signalling, GLUT4 translocation to the plasma membrane and glucose uptake in adipocytes.
Our study demonstrates that interspecies differences exist in the regulation of SIK2 expression and activity in WAT. In contrast to what was previously reported in obese mice [
9], we found that SIK2 expression (mRNA and protein) and activity in adipose tissue and adipocytes were downregulated in human obesity. As adipose tissue inflammation is a key feature of obesity and insulin resistance [
35,
36], we hypothesised that SIK expression is regulated by the inflammatory cytokine TNF-α. Indeed, both SIK2 and SIK3 were rapidly downregulated by TNF-α. The molecular mechanisms mediating transcriptional regulation of SIK2 and SIK3 remain to be elucidated. However, the rapid decrease in gene and protein expression indicates that the effect of TNF-α is probably direct, and not secondary to TNF-induced changes in the expression of other genes. Since
SIK2 and
SIK3 expression was lower in insulin-resistant individuals it is also possible that a functional insulin response is needed for SIK transcription.
A critical question is how the reduced SIK2 and SIK3 expression in obesity impacts adipose tissue physiology. Previous studies in rodents have proposed that SIKs regulate glucose uptake in adipocytes [
19,
20]. In our study, we demonstrate an important role for SIK isoforms in promoting basal and insulin-stimulated glucose uptake also in human adipocytes, using both genetic (siRNA) and pharmacological (pan-SIK inhibition) approaches. Silencing of SIK2 induced compensatory upregulation of SIK1 and SIK3, leading to only a marginal reduction in total SIK activity, making it difficult to conclude on the individual role of SIK2. Given the low abundance of
SIK3 relative to
SIK2 in human adipocytes, at first we anticipated that the functional contribution of SIK3 would be much smaller than that of SIK2. However, kinase activity measurements and si
SIK3 treatment revealed that there is a significant level of SIK3 activity in human adipose tissue that contributes to the positive effect of SIKs on glucose uptake. We were not able to achieve silencing of SIK1 at protein level, making any conclusions about the role of SIK1 uncertain.
When exploring mechanisms for the regulation of glucose uptake by SIKs we noted that, in contrast to murine adipocytes [
19,
20], SIK isoforms do not promote GLUT expression in human adipocytes. Thus, our data suggest that the positive effect of SIKs on glucose uptake in human adipocytes is not likely to be mediated by altered protein levels of glucose transporters. However, the rate of glucose uptake is ultimately dependent on the number of transporters present on the cell surface and not the overall cellular levels. Accordingly, our data suggest that SIKs promote plasma membrane localisation of GLUT4 in adipocytes. The upstream mechanism involves positive effects of SIKs on the insulin-induced phosphorylation and activation of PKB/Akt. The fact that the effect of SIK inhibition on GLUT4 localisation was smaller than that on glucose uptake and detected only at a sub-maximal dose of insulin could be a result of methodological (snapshot vs cumulative assay) and species (rat vs human) differences. Moreover, since GLUT4 translocation and glucose uptake were also reduced by SIK inhibition in the basal state we do not rule out the possibility that additional mechanisms may contribute. The fact that the ability of insulin to induce these processes (stimulation index, fold change) was not altered even though the phosphorylation of PKB/Akt was markedly blunted indicates that the effect of SIKs on basal glucose uptake and GLUT4 localisation is probably mediated by a distinct, yet unknown, mechanism.
An important question to answer in future studies is if the differential expression of SIKs in adipose tissue plays a causal role in the development of obesity or insulin resistance in vivo. Mice with global deficiency of SIK2 displayed no weight phenotype [
20], arguing against a causal relationship between SIK2 downregulation and obesity. However, it is quite possible that some effects of SIK2 deficiency are masked by compensatory mechanisms due to embryonic loss of the protein or isoform redundancy. Although reduced PKB/Akt activation might not fully explain the effects we observed on GLUT4 translocation and glucose uptake in cells, these results per se, as well as the strong negative association of
SIK2 expression with HOMA-IR, suggest that downregulation of
SIK2 and
SIK3 in obesity might contribute to the development of insulin resistance in vivo—at least in adipose tissue. In line with this,
Sik2
-/- mice showed some degree of insulin resistance in their adipose tissue [
20]. Considering the low expression level of
SIK1 in human adipose tissue, we are not sure of the physiological relevance of the differential expression of this isoform in obesity. Previous studies have demonstrated that
Sik1 is upregulated in skeletal muscle, liver and adipose tissue of obese mice [
9,
40] and this has been linked to the development of insulin resistance [
40]. Given the compensatory upregulation of SIK1 that we observed when silencing SIK2 in adipocytes, it is possible that increased
SIK1 expression in adipose tissue from obese individuals is secondary to downregulation of
SIK2 in these individuals.
In summary, we have demonstrated that SIK2 and SIK3 are downregulated in human obesity and insulin resistance. Furthermore, SIKs promote glucose uptake in human adipocytes, at least partly through direct mechanisms. In future studies it will be important to identify molecular targets of SIK2 and SIK3 that could be involved in the regulation of PKB/Akt phosphorylation and GLUT trafficking.
Acknowledgements
The authors thank E. Banke (Insulin Signal Transduction, Lund University, Sweden) for assistance in preparing human specimens for protein expression analysis, G. Åström and E. Dungner (Lipid Laboratory, Karolinska Institutet, Sweden) for excellent technical help with human adipocyte cell cultures and mRNA expression analysis, and M. Lindahl (Glucose Transport and Protein Trafficking, Lund University, Sweden) for excellent technical help with isolation of rat adipocytes. K. Clark (MRC Protein Phosphorylation Unit, University of Dundee, UK) is acknowledged for kindly sharing HG-9-91-01. K. Sakamoto (Diabetes & Circadian Rhythms, Nestlé Institute of Health Sciences, Lausanne, Switzerland) and S.W. Cushman (National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, USA) are acknowledged for kindly sharing pSIK2 Ser343 and GLUT1 antibodies, respectively.
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