Metformin stimulates IGFBP-2 gene expression through PPARalpha in diabetic states

In biological fluids, the insulin-like growth factor binding protein (IGFBP) family proteins can complex with both IGF-I and II, and are regulators of IGF actions on metabolism and growth1,2. Currently, there are six mammalian IGFBPs designated IGFBP-1–6 have been characterized3 and newly IGFBP-7 was identified as a member of the IGFBP superfamily4. The primary function of the IGFBPs is to restrict the bioavailability of IGF-1 in target tissues5. Among the IGFBPs, IGFBP-2 modulates IGF-1 bioactivity by interacting with IGF-16. IGFBP-2 is highly expressed in the liver, adipocytes, and central nervous system, and is involved in metabolic homeostasis, insulin resistance, diabetes, and obesity7,8,9. Moreover, IGFBP-2 is suggested to be used as a marker protein for metabolic dysfunction10,11. IGFBP-2 is the abundant in blood, and has been shown to play a role in preventing insulin resistance and diet-associated obesity in mice9. However, the regulatory mechanism of IGFBP-2 expression and its clinical relevance to diabetic states in mice and humans remain poorly understood. Sirtuin 1 (Sirt1), an NAD+-dependent protein deacetylase, is involved in controlling glucose, lipid homeostasis, aging, inflammation, and circadian regulation of metabolism and cellular processes12. It regulates metabolic homeostasis by deacetylating crucial transcriptional factors such as peroxisome proliferator-activated receptor α (PPARα), PPARγ, farnesoid X receptor (FXR), liver X receptor α (LXRα), PPARγ coactivator 1-α (PGC-1α), and p5313. PPARα belongs to the nuclear receptor superfamily; it functions as a transcription factor and plays crucial roles in the regulation of varieties of metabolic dysfunctions related to inflammatory response, glucose and lipid metabolism14. PPARα is expressed in various tissues including the liver, adipose tissue, heart, kidney, and intestine14,15. PPARα is activated during fasting or by ligands; PPARα induces fatty acid oxidation and gluconeogenesis16. In addition, PPARα is known to suppress inflammation and to preserve insulin sensitivity17. It also interacts with the retinoid X receptor, and the resulting heterodimer promotes the transcriptional activation of target genes by binding to the consensus PPAR response element (PPRE) on target gene promoters18. Further, PPARα regulates cancer metabolism by attenuating IGF-1R signaling and Akt phosphorylation in various cancer cells19,20. However, a potential link between the Sirt1-PPARα axis and IGF-1 signaling system has not been elucidated yet. The antidiabetic drug, metformin, is widely used for the treatment of type 2 diabetes. Metformin attenuates hepatic glucose production and triglycerides accumulation by ameliorating hyperglycemia and fatty oxidation in the liver21,22,23. Metformin also improves hepatic dysfunction by stimulating liver kinase B-1, which promotes the expression of AMP-activated protein kinase (AMPK) in the liver24. Because AMPK functions as a potential intracellular energy sensor and a master regulator of metabolic homeostasis, understanding the mechanisms of its activation by various physiological stimuli or therapeutic drugs and several hormones including adiponectin and leptin25 are of utmost importance in developing anti-diabetic drugs. In this study, we elucidated the potential role of IGFBP-2 in diet-induced obesity or diabetic mice. Our results demonstrated that metformin controls Igfbp-2 gene transcription through the AMPK-Sirt1-PPARα signaling pathway. Moreover, we showed that regulation of the Sirt1-PPARα-IGFBP-2 signaling cascade by AMPK activator represents a novel pathway that could be applied to ameliorate metabolic syndromes by controlling IGF-1 homeostasis.