Reduced gene expression of sirtuins and active AMPK levels in children and adolescents with obesity and insulin resistance

Summary

Background

Sirtuins, including SIRT1 and SIRT2, are longevity-associated deacetylase enzymes that modulate metabolic homeostasis in response to the cellular energy state. Adenosine monophosphate activated protein kinase (AMPK) and SIRT1 are interrelated and share several common target pathways. This study aimed to evaluate the SIRT1 and SIRT2 gene expression in peripheral blood mononuclear cells (PBMCs) as well as plasma levels of AMPK, in obese children and adolescents.

Materials and methods

Participants included 60 children and adolescents (30 obese and 30 age- and gender-matched control subjects). Real-time polymerase chain reaction (PCR) was used to assess the SIRT1 and SIRT2 gene expression in PBMCs. Serum phospho-AMPK and insulin were measured using enzyme-linked immunosorbent assay (ELISA), and insulin resistance (IR) was calculated by the Homeostasis Model of Assessment of Insulin Resistance (HOMA-IR). Glucose and lipid profile were also measured.

Results

SIRT1 gene expression and phospho-AMPK plasma levels were significantly diminished in obese subjects compared to the control group, and both SIRT1 and SIRT2 were significantly lower in obese children with IR compared to those without IR. SIRT1 expression revealed significant negative correlations with body mass index and waist circumference as well as insulin and HOMA-IR and a positive correlation with AMPK. SIRT2 negatively correlated with SIRT1 and positively correlated with high density lipoprotein-cholesterol (HDL-C).

Conclusion

SIRT1 and SIRT2 expression and AMPK levels decrease in children with obesity and IR. Targeting SIRT1 can be valuable in preventing obesity-associated IR in childhood and adolescence.

Introduction

Changes in human lifestyle in the last century, have led to an imbalance between the caloric intake and energy expenditure and the resulting additional energy is stored in adipose tissue, causing obesity [1]. Recently, the increasing prevalence of obesity in children and adolescents forms one of the major concerns of public health issues [2]. Obesity in children is associated with reduced longevity due to increased risk of glucose intolerance, type 2 diabetes, and hypertension in early adulthood [3].

Sirtuins (silent information regulator), which belong to histone deacetylase III family of enzymes are longevity-associated enzymes that increase the life span and are induced by caloric restriction [4]. These enzymes are known as major contributors in the metabolic regulation of the cell by participating in the crucial pathways including lipid metabolism and insulin secretion. Additionally, sirtuins are vital in cell survival, cell senescence, and deoxyribonucleic acid repair [5]. They have the ability to control redox reactions, and thus, are effective against oxidative damage caused by pathological states such as obesity, type 2 diabetes, and cardiovascular diseases [6].

Seven types of sirtuins can be found in mammals, namely SIRT1–SIRT7, with SIRT1 being the most studied [7], [8], [9]. SIRT1 is an NAD-dependent enzyme that is located in the nucleus and is crucial in homeostasis of glucose and lipid metabolism as well as insulin sensitivity [10]. It increases the fatty acid oxidation and prevents lipogenesis by activating the LKB1/AMPK signaling pathway. Therefore, SIRT1 is identified as an essential regulatory sensor of nutrient availability and as a key parameter in obesity prevention [11].

SIRT2, which is primarily located in the cytoplasm, is involved in cell cycle regulation [12]. Less data is available regarding its role in metabolism; however, it has been demonstrated that SIRT2 deacetylates FOXO1 and inhibits adipogenesis and lipid accumulation in 3T3-L1 adipocytes [11].

One of the main elements in energy control, regulation of appetite, fat cell differentiation and cellular stress control is a serine/threonine kinase named AMP activated protein kinase (AMPK) [13]. The activity of AMPK depends on the AMP/ATP ratio, and increased AMP concentration activates this enzyme [13]. The role of AMPK as an energy sensor has made it a potential target in managing childhood obesity [13].

SIRT1 activates AMPK via liver kinase B1 (LKB1), which is deacetylated by SIRT1 and translocated to the cytoplasm where it can phosphorylate and activate AMPK. Alternatively, AMPK activates nicotinamide phosphoribosyltransferase (NAMPT), the key enzyme for biosynthesis of NAD. Thus, AMPK provides SIRT1 substrate and increases SIRT1 activity [14].

The central role of sirtuins and AMPK in metabolic homeostasis necessitates their evaluation in particular, in young children; however, these parameters and their relationship with childhood obesity have not been investigated to date. Therefore, in this study we evaluated the gene expression of SIRT1 and SIRT2 and AMPK plasma levels in obese children and adolescents.