Immune regulation of adipogenesis

The Adipose tissue (AT) contains adipocytes and a number of other cell types, summarized as the stromal-vascular cell fraction (SVF). The SVF includes endothelial cells, various immune cells and adipocyte progenitors. In the course of obesity, adipose tissue undergoes remodeling, which is characterized primarily by AT expansion and immune cell accumulation. In the course of long caloric excess, new adipocytes emerge from the differentiation of adipocyte progenitors and contribute to AT expansion.

Adipocyte progenitors / precursors (preadipocytes, APs) represent a relatively small population of immature cells that reside in perivascular regions of the AT and can differentiate into mature fat cells. Apart from adipogenic differentiation, adipocyte progenitors can also give rise to myofibroblasts upon transforming growth factor beta 1 (TGF-β1) and platelet-derived growth factor (PDGF) stimulation derived from inflammatory cells, hence driving AT fibrosis, which is a form of AT dysfunction. Several studies have described the existence of a population of pro-fibrotic progenitors that expands during obesity and positively correlates with the degree of AT fibrosis.

Depending on the microenvironment-derived cues, adipocyte progenitors in the white AT may differentiate into white or beige adipocytes or acquire a fibroblastic phenotype. In obesity, macrophages in the AT acquire a pro-inflammatory phenotype and secrete a plethora of pro-inflammatory factors, including tumor necrosis factor (TNF), interleukin 1 (IL-1) and 6 (IL-6). A crosstalk between immune cells and adipocyte progenitors, can orchestrate the differentiation potential of APs in the lean and obese state.

Interleukin 1 beta (IL-1β) is one of the major cytokines produced by pro-inflammatory macrophages. It has been shown to at least in parts contribute to the development of insulin resistance in both preadipocytes and mature fat cells. Single-cell transcriptome analysis of mouse AT revealed that the IL-1β receptor IL-1R1 is predominantly expressed by mesenchymal stem cells, including adipocyte progenitors. Therefore, IL-1β can regulate the crosstalk between inflammatory macrophages and adipocyte progenitors in the context of obesity, although this has not been sufficiently studied. Importantly, IL-1β is involved in the induction of trained immunity. Trained immunity may also alter the function of non-immune cells. In the present project, we focus on IL-1β-dependent inflammatory regulation of the fate of adipocyte progenitors.

First, we confirmed IL-1R1 expression on primary adipocyte progenitors by FACS and qPCR. Indeed, a substantial portion of pre-adipocytes express the IL-1β receptor. For experiments we used FACS-sorted primary adipocyte progenitors (APs) from subcutaneous adipose tissue (defined as CD45^- CD31^- PDGFRα⁺ Sca-1⁺). 24 hrs treatment of APs with IL-1β resulted in a strong upregulation of pro-inflammatory genes (IL-6, TNFα, CCL2, CCL20, and CXCL-1) at the transcriptional level. The expression of genes related to fibrosis and cell proliferation remained unchanged. IL-1β treatment downregulated expression of the master regulator of adipogenesis, PPARγ, in primary progenitors.

In order to investigate the effect of IL-1β on the differentiation potential of primary adipocyte progenitors, they were also subjected to the adipogenic differentiation cocktail (contains IBMX, insulin, dexamethasone, rosiglitazone). Undifferentiated cells were treated with IL-1β for 24 hrs, followed by differentiation for 3 days in a differentiation medium and a maturation medium (contains insulin only) for 5 more days (until day 8). Gene expression analysis revealed that early exposure of APs to IL-1β led to significant downregulation of genes related to adipogenesis (PPARγ, C/EBPα, adiponectin, SREBP-1c), lipid uptake (FABP4, CD36, PLIN1), glucose metabolism and uptake (ChREBP, Glut4, and IRS-1) in the differentiated state. The expression of pro-inflammatory markers induced by IL-1β remained increased in differentiated fat cells. Nile Red staining confirmed the reduced lipid accumulation in adipocytes, which were exposed to IL-1β in the undifferentiated state. Our finding suggests that early exposure of pre-adipocytes to IL-1β can modulate their adipogenic potential and affect their cell fate decision. Apart from influencing white adipogenic differentiation of primary AP, IL-1β can also have an effect of beige and myofibroblastic differentiation, which requires further investigation. We would like to address the mechanisms of the inflammatory “memory” in adipocyte progenitors upon IL-1β treatment using unbiased methods, such as RNA sequencing and proteomics analysis. Adiponectin-cre IL-1R1 flox/flox mice, which have a deletion of IL-1R1 specifically in adipocyte progenitors, will help us verify the identified mechanisms in vivo.

Publications:

Metabolic inflammation as an instigator of fibrosis during non-alcoholic fatty liver disease.  A. Katsarou, I.I. Moustakas, I. Pyrina, P. Lembessis, M. Koutsilieris, A. Chatzigeorgiou. World J Gastroenterol. 2020;26:1993-2011.

Fate of Adipose Progenitor Cells in Obesity-Related Chronic Inflammation.  I. Pyrina, K.J. Chung, Z. Michailidou, M. Koutsilieris, T. Chavakis, A. Chatzigeorgiou. Front Cell Dev Biol. 2020;8:644.

Robo4-mediated pancreatic endothelial integrity decreases inflammation and islet destruction in autoimmune diabetes.  M. Troullinaki, L.S. Chen, A. Witt, I. Pyrina, J. Phieler, I. Kourtzelis, J. Chmelar, D. Sprott, B. Gercken, M. Koutsilieris, T. Chavakis, A. Chatzigeorgiou. FASEB J. 2020;34:3336-3346.

Hepatic Senescence Accompanies the Development of NAFLD in Non-Aged Mice Independently of Obesity.  I.I. Moustakas, A. Katsarou, A.I. Legaki, I. Pyrina, K. Ntostoglou, A.M. Papatheodoridi, B. Gercken, I.S. Pateras, V.G. Gorgoulis, M. Koutsilieris, T. Chavakis, A. Chatzigeorgiou. Int J Mol Sci. 2021;22:3446. 

The RNA binding protein HuR is a gatekeeper of liver homeostasis.  P. Subramanian, S. Gargani, A. Palladini, M. Chatzimike, M. Grzybek, M. Peitzsch, A.D. Papanastasiou, I. Pyrina, V. Ntafis, B. Gercken, M. Lesche, A. Petzold, A. Sinha, M. Nati, V.R. Thangapandi, I. Kourtzelis, M. Andreadou, A. Witt, A. Dahl, R. Burkhardt, R. Haase, A.M.J. Domingues, I. Henry, N. Zamboni, P. Mirtschink, K.J.Chung, J. Hampe, Ü. Coskun, D.L. Kontoyiannis, T. Chavakis. Hepatology. 2021. Epub ahead of print.