The roles of NFE2L1 in adipocytes: Structural and mechanistic insight from cell and mouse models

Abstract

Adipocytes play pivotal roles in maintaining energy homeostasis by storing lipids in adipose tissue (AT), regulating the flux of lipids between AT and the circulation in response to the body's energy requirements and secreting a variety of hormones, cytokines and other factors. Proper AT development and function ensure overall metabolic health. Nuclear factor erythroid 2-related factor 1 (NFE2L1, also known as NRF1) belongs to the CNC-bZIP family and plays critical roles in regulating a wide range of essential cellular functions and varies stress responses in many cells and tissues. Human and rodent Nfe2l1 genes can be transcribed into multiple splice variants resulting in various protein isoforms, which may be further modified by a variety of post-translational mechanisms. While the long isoforms of NFE2L1 have been established as master regulators of cellular adaptive antioxidant response and proteasome homeostasis, the exact tissue distribution and physiological function of NFE2L1 isoforms, the short isoforms in particular, are still under intense investigation. With regard to key roles of NFE2L1 in adipocytes, emerging data indicates that deficiency of Nfe2l1 results in aberrant adipogenesis and impaired AT functioning. Intriguingly, a single nucleotide polymorphism (SNP) of the human NFE2L1 gene is associated with obesity. In this review, we summarize the most significant findings regarding the specific roles of the multiple isoforms of NFE2L1 in AT formation and function. We highlight that NFE2L1 plays a fundamental regulatory role in the expression of multiple genes that are crucial to AT metabolism and function and thus could be an important target to improve disease states involving aberrant adipose plasticity and lipid homeostasis.

Keywords: Adipocytes; Adipogenesis; Adipose; Energy metabolism; NFE2L1.

Fig. 1

DNA structure and transcripts of mouse Nfe2l1. Schematic diagram of mouse Nfe2l1 genomic sequence (GeneID: ENSMUSG00000038615) and depiction of different isoforms of Nfe2l1 transcripts. Sequences are from the Ensembl GRCm38. p6 release101-August 2020 (www.ensemble.org). Solid black lines and dashed black lines represent introns and flanking 10 kb length to either side, respectively. White open boxes represent exons. Dark blue and light blue open boxes represent coding regions and untranslated regions, respectively. The numbers in parentheses under each isoform are the gene ID in Ensembl. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2

The structural domains of NFE2L1. (A) Schematic protein structures of different isoforms of mouse NFE2L1. (B) Domain structure and related function of L-NFE2L1. Abbreviations: NTD, N-terminal domain; AD1, acidic domain 1; NST, Asn/Ser/Thr-rich; AD2, acidic domain 2; SR, serine-repeat; Neh6L, Neh6-like; CNC, Cap ‘n’ collar; bZIP, basic-region zipper; CTD, C-terminal domain. Green boxes represent unique sequences different from other isoforms. All the sequences are from the National Center for Biotechnology (www.ncbi.nlm.nih.gov). . (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

Proposed activation pathway of L-NFE2L1. After translation, L-NFE2L1 is inserted into the ER and its NST domain is N-glycosylated so as to become an inactive NFE2L1 glycoprotein. When required for induction by biological cues, the ER-protected transactivation domains of NFE2L1 are dynamically retrotranslocated via the AAA protein p97/VCP and repositioned from the luminal side of ER membranes into the cyto/nucleo-plasmic side, whereupon it undergoes various post-translational modifications, such as N-deglycosylation, O-GlcNAcylation, de-GlcNAcylation and phosphorylation to yield active isoforms (M-NFE2L1). In this process, diverse enzymes like PNGase, O-Linked N-acetylglucosamine transferase (OGT), DDI2, beta-transducin repeats-containing protein (β-TrCP) and ubiquitin ligase HMG-CoA reductase degradation 1 (HRD1) are involved in. There are two distinct (β-TrCP- and HRD1-dependent) degradation mechanisms regulating NFE2L1 protein levels. In the cytoplasm, NFE2L1 is degraded and suppressed by the ER-associated degradation ubiquitin ligase HRD1 and VCP. NFE2L1 is also degraded in the nucleus via β-TrCP-mediated degradation. In cells with insufficient proteasome capacity, active NFE2L1 accumulates and then migrates to the nucleus, where it heterodimerizes with cofactors (small Maf proteins) to bind ARE to induce gene transcription. In contrast, complete proteasomal processing of NFE2L1 may lead to decreased NFE2L1 protein levels and transcriptional activity. FBW7, F-box and WD repeat domain containing 7; GSK3, glycogen sysnthase kinase 3; SKP-1, S-phase kinase-associated protein 1.

Fig. 4

NFE2L1 regulates lipid flux and WAT plasticity. The hypertrophy and hyperplasia of adipocytes affect the physiology of WAT. Metabolic fluxes are the primary determinants of adipocyte hypertrophy, in which lipolysis is the main contributor to lipid efflux, while lipogenesis mainly impacts the influx. Once the balance between influx and efflux of lipid is disrupted, adipocytes will more readily go beyond their maximum volume limits and reach a critical death size, which induces local inflammation. Hyperplasia of adipocytes can provide additional capacity for storage in response to excess lipid which is primarily regulated by PPARγ. The expression of key lipolytic genes is, at least in part, dependent on NFE2L1. Adipocyte-specific deficiency of Nfe2l1 results in reduced lipolytic activity and uncoupled lipid metabolism precipitating adipocyte hypertrophy and inflammation. Rosiglitazone (RGZ)-induced elevation of de novo lipogenesis worsens adipocyte hypertrophy, inflammation and pyroptosis. CL316243, a β3 adrenergic agonist, promotes lipolysis via a post-translational mechanism improving the activity of lipolytic enzymes. Follow CL316243 treatment, this phenotype is improved.