The role of creatine metabolism in human white adipose tissue

<p dir="ltr">White adipose tissue (WAT) is the main organ to store energy surplus in the form of fat. In obesity, chronic caloric overconsumption overwhelms the storage capacity of WAT leading to the enlargement of existing adipocytes and spillover of fatty acids into the bloodstream. This promotes ectopic fat accumulation in peripheral organs such as the liver and skeletal muscle, which in turn the development of drives insulin resistance and type 2 diabetes. At the tissue level, increased adipocyte size is associated with alterations in intracellular gene/protein expression and metabolite levels, leading to changes in reactive oxygen species (ROS) production, endoplasmic reticulum (ER) stress, and chronic low-grade inflammation, accompanied by pro-inflammatory immune cell infiltration.</p><p dir="ltr">Creatine is a very well-known supplement used by athletes and is present in many food sources, as well as being synthesized within our body. Recent studies have highlighted the importance of creatine metabolism in brown adipose tissue, emphasizing its role in managing energy expenditure, and insulin resistance. Creatine kinases are enzymes responsible for energy transport between mitochondria and cytoplasm through the cycling of creatine and phosphocreatine, generating ADP and ATP. One specific creatine kinase isoform, creatine kinase brain isoform (CKB), plays a central role in brown adipose tissue thermogenesis. Alterations in the expression of CKB impair thermogenesis, promote weight gain, and worsen metabolic parameters in rodents. However, less is known regarding the role of creatine and CKB in WAT biology. Therefore, the aim of this thesis is to deepen our understanding of creatine metabolism in human WAT and how this pathway is linked to obesity.</p><p dir="ltr">In Study I, we examined creatine metabolism in subjects with and without obesity. Consistent with in vivo studies in mice, we identified CKB as one of the most downregulated genes in obesity. This alteration was associated with the accumulation of phosphocreatine within adipocytes. Mechanistically, this hypermetabolic state led to the inhibition of the catabolic sensor AMP-activated protein kinase (AMPK) and promoted the production of C-C Motif Chemokine Ligand 2 (CCL2), with consequent immune cell infiltration into WAT.</p><p dir="ltr">After studying CKB in human WAT, we next focused on the mechanisms underlying its downregulation in obesity. In Study II, we identified that ER stress, specifically via inositol-requiring enzyme 1 (IRE1) - X-box binding protein 1s (XBP1s) - DNA (cytosine-5)-methyltransferase 3A (DNMT3A) axis, drove hypermethylation in the proximal promoter area of CKB. These findings were also corroborated in humans where, methylome and gene expression data, displayed an inverse correlation between proximal promoter area of CKB and its mRNA levels.</p><p dir="ltr">In our previous studies, we noticed an upregulation of creatine kinases (CKB and CKMT2) during human white adipocyte formation de novo. In Study III, we therefore investigated the effects of creatine kinase depletion on adipocyte differentiation. Early depletion of CKB in differentiating human white adipocytes rewired intracellular metabolism driven by upregulated AKT signaling, promoting ChREBP expression via increased glycolytic rate. This led to increased expression of genes involved in de novo lipogenesis and promoted lipid accumulation both in vitro and in adipocyte-specific Ckb knockout mice.</p><p dir="ltr">In summary, through studies in human adipocytes and mouse knockout models, this thesis provides new insights into the role of WAT creatine metabolism in regulating energy homeostasis, inflammation and insulin sensitivity.</p><h3>List of scientific papers</h3><p dir="ltr">I. Maqdasy S, Lecoutre S, <b>Renzi G,</b> Frendo-Cumbo S, Rizo-Roca D, Moritz T, Juvany M, Hodek O, Gao H, Couchet M, Witting M, Kerr A, Bergo MO, Choudhury RP, Aouadi M, Zierath JR, Krook A, Mejhert N, Ryden M. Impaired phosphocreatine metabolism in white adipocytes promotes inflammation. Nat Metab. 2022 Feb;4(2):190- 202. <a href="https://doi.org/10.1038/s42255-022-00525-9">https://doi.org/10.1038/s42255-022-00525-9</a></p><p dir="ltr">II. <b>Renzi G,</b> Vlassakev I, Hansen M, Higos R, Lecoutre S, Elmastas M, Hodek O, Moritz T, Alaeddine LM, Frendo-Cumbo S, Dahlman I, Kerr A, Maqdasy S, Mejhert N, Rydén M. Epigenetic suppression of creatine kinase B in adipocytes links endoplasmic reticulum stress to obesity-associated inflammation. Mol Metab. 2024 Dec 13;92:102082. <a href="https://doi.org/10.1016/j.molmet.2024.102082">https://doi.org/10.1016/j.molmet.2024.102082</a></p><p dir="ltr">III. <b>Renzi G,</b> Higos R, Vlassakev I, Abdoul Akim B, Hmeadi-Muhmmad O, Hansen M, Merabtene F, Rouault C, Hodek O, Massier L, Antonny B, Genevieve M, Rahbani JF, Lecoutre S and Maqdasy S. Creatine kinase B regulates glycolysis and de novo lipogenesis pathway to Control Lipid accumulation during adipogenesis. [Submitted]</p>