Iron down-regulates leptin by suppressing protein O-GlcNAc modification in adipocytes, resulting in decreased levels of O-glycosylated CREB

Abstract

We previously reported that iron down-regulates transcription of the leptin gene by increasing occupancy of phosphorylated cAMP response element-binding protein (pCREB) at two sites in the leptin gene promoter. Several nutrient-sensing pathways including O-GlcNAcylation also regulate leptin. We therefore investigated whether O-glycosylation plays a role in ironand CREB-mediated regulation of leptin. We found that high iron decreases protein O-GlcNAcylation both in cultured 3T3-L1 adipocytes and in mice fed high-iron diets and downregulates leptin mRNA and protein levels. Glucosamine treatment, which bypasses the rate-limiting step in the synthesis of substrate for glycosylation, increased both O-GlcNAc and leptin, whereas inhibition of O-glycosyltransferase (OGT) decreased O-GlcNAc and leptin. The increased leptin levels induced by glucosamine were susceptible to the inhibition by iron, but in the case of OGT inhibition, iron did not further decrease leptin. Mice with deletion of the O-GlcNAcase gene, either via whole-body heterozygous deletion or through adipocyte-targeted homozygous deletion, exhibited increased O-GlcNAc levels in adipose tissue and increased leptin levels that were inhibited by iron. Of note, iron increased the occupancy of pCREB and decreased the occupancy of O-GlcNAcylated CREB on the leptin promoter. These patterns observed in our experimental models suggest that iron exerts its effects on leptin by decreasing O-glycosylation and not by increasing protein deglycosylation and that neither O-GlcNAcase nor OGT mRNA and protein levels are affected by iron. We conclude that iron down-regulates leptin by decreasing CREB glycosylation, resulting in increased CREB phosphorylation and leptin promoter occupancy by pCREB.

Relationship between Energy Balance and Circulating Levels of Hepcidin and Ferritin in the Fasted and Postprandial States

“…The iron-induced cellular changes in leptin triggered changes that led to rodent feeding behaviour where mice fed on high-iron chow had decreased leptin and increased food intake [26]. Although the precise mechanism of the link between iron and CREB-activation is unknown, iron has been shown to cause CREB-activation through decreasing levels of protein O-glycosylation (a nutrient-dependent mechanism of leptin regulation) [27]. Given that CREB is a key factor in the transition of fuel utilisation from feeding to fasting (leading to a gradual decrease of leptin) [51] and taking into account the evidence that hepcidin is involved in gluconeogenic sensing in the fasted state (by upregulation through PPARGC1A/CREBH potentiating reduced levels of circulating iron) [52], leptin could be a key mediator of the cross-talk between iron homeostasis and energy metabolism.…”

“…Dietary iron supplementation has been found to increase appetite and deplete leptin levels in rodents [26]. Further, increased iron stores (i.e., serum ferritin) were demonstrated to down-regulate leptin expression directly [26,27], and ghrelin was found to be involved in the regulation of the expression of ferroportin 1 (the transmembrane protein that allows entry and exit of iron into cells) [28]. Human studies have mainly focused on people with disorders of iron metabolism.…”

CREB-associated glycosylation and function in human disease

“…In this study, researchers discovered that high iron intake changed the CREB occupancy pattern, increasing the occupancy of p-CREB and decreasing the occupancy of O-GlcNAcylated CREB on the leptin promoter. 46…”

Protein O-GlcNAcylation in Metabolic Modulation of Skeletal Muscle: A Bright but Long Way to Go

“…O -GlcNAcylation of S6 kinase beta1 inhibits its phosphorylation and mTORC1 signaling, thereby causing insulin resistance [ 133 ]. In addition, O -GlcNAcylation of cAMP-response element binding protein (CREB) crosstalk with its phosphorylation [ 134 , 135 ]. O -GlcNAcylation of CREB-regulated transcription coactivator 2 at Ser 70 and Ser 171 inhibits the phosphorylation of these two sites, thereby facilitating their nuclear translocation and binding to CREB [ 136 ].…”