Abstract
The regulation of hepatic gluconeogenesis is of great significance to improve insulin resistance and benefit diabetes therapy. cAMP-Regulated Transcriptional Co-activator 2 (CRTC2) plays a key role in regulating hepatic gluconeogenesis through controlling the expression of gluconeogenic rate-limiting enzymes such as glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK). Recently, salt-inducible kinase 1 (SIK1) has been identified to play an important role in glucose metabolism disorders, but whether and how SIK1 regulates the CTRC2 signaling in liver cells under high glucose conditions has rarely been intensively elucidated. Here, we show that high glucose stimulation resulted in time-dependent down-regulated expression of SIK1, phosphorylated SIK1 at T182 site, and phosphorylated CRTC2 at S171 site, as well as upregulated expression of total CRTC2 and its downstream targets G6Pase and PEPCK in the human liver cell line HepG2. The nuclear expression levels of SIK1 and CRTC2 were time-dependently upregulated upon high glucose challenge, which was accompanied by enhanced cytoplasm-to-nucleus translocation of SIK1. Manipulation of SIK1 activity using plasmid-mediated SIK1 over-expression and the use of the SIKs inhibitor HG-9-91-01 confirmed that SIK1 regulated the CRTC2 signaling pathway in HepG2 cells. Furthermore, in mouse primary hepatocytes, high glucose exposure down-regulated SIK1 expression, and promoted SIK1 nuclear accumulation. While HG-9-91-01 treatment suppressed SIK1 expression and released the inhibitory effects of SIK1 on the expressions of key molecules involved in the CRTC2 signaling pathway, additional ectopic expression of SIK1 using adenovirus infection reversed the impacts of HG-9-91-01 on the expressions of these molecules in mouse hepatocytes. Therefore, SIK1 regulates CRTC2-mediated gluconeogenesis signaling pathway under both physiological and high glucose-induced pathological conditions. The modulation of the SIK1-CRTC2 signaling axis could provide an attractive means for treating diabetes.
Highlights
Gluconeogenesis is the process of glucose synthesis using non-sugar precursors and is critical in maintaining stable human blood glucose levels in humans
In this study, using the human liver HepG2 cell line and mouse primary hepatocytes as in vitro cell models, we examined the changes in salt-inducible kinase 1 (SIK1) expression and phosphorylation, as well as the expression levels of key molecules involved in the cAMP-Regulated Transcriptional Co-activator 2 (CRTC2) signaling pathway, in response to high glucose exposure
We first evaluated the impact of high glucose on SIK1 expression in HepG2 cells by Quantitative Reverse Transcription-Polymerase Chain Reaction (qRT-PCR) and immunoblotting
Summary
Gluconeogenesis is the process of glucose synthesis using non-sugar precursors and is critical in maintaining stable human blood glucose levels in humans. An abnormal increase of gluconeogenesis in the liver significantly promotes diabetic hyperglycemia [1]. Excessive liver glucose output represents a major inducer of hyperglycemia in type 2 diabetes mellitus (T2DM), and its inhibition can reduce blood glucose levels in patients with T2DM [4, 5]. The amount of glucose output by the liver due to gluconeogenesis accounts for about 65% of the total blood glucose levels [6]. The regulation of gluconeogenesis is of great significance to stabilize blood glucose, improve insulin resistance, and even manage diabetes mellitus and other disorders of glucose metabolism
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