The Role of Endoplasmic Reticulum in Lipotoxicity during Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD) Pathogenesis

Abstract

Perturbations in lipid and protein homeostasis induce endoplasmic reticulum (ER) stress in metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as nonalcoholic fatty liver disease. Lipotoxic and proteotoxic stress can activate the unfolded protein response (UPR) transducers: inositol requiring enzyme1α, PKR-like ER kinase, and activating transcription factor 6α. Collectively, these pathways induce expression of genes that encode functions to resolve the protein folding defect and ER stress by increasing the protein folding capacity of the ER and degradation of misfolded proteins. The ER is also intimately connected with lipid metabolism, including de novo ceramide synthesis, phospholipid and cholesterol synthesis, and lipid droplet formation. Following their activation, the UPR transducers also regulate lipogenic pathways in the liver. With persistent ER stress, cellular adaptation fails, resulting in hepatocyte apoptosis, a pathological marker of liver disease. In addition to the ER-nucleus signaling activated by the UPR, the ER can interact with other organelles via membrane contact sites. Modulating intracellular communication between ER and endosomes, lipid droplets, and mitochondria to restore ER homeostasis could have therapeutic efficacy in ameliorating liver disease. Recent studies have also demonstrated that cells can convey ER stress by the release of extracellular vesicles. This review discusses lipotoxic ER stress and the central role of the ER in communicating ER stress to other intracellular organelles in MASLD pathogenesis.

Figure 1

The three unfolded protein response transducers. Inositol-Requiring Enzyme 1α (IRE1α), PKR-like ER kinase (PERK), and activating transcription factor 6α (ATF6α) are the three endoplasmic reticulum (ER) stress sensors that trigger a transcriptional program termed the unfolded protein response (UPR). GRP78/BiP is an ER chaperone that is associated with all three transducers and inhibits them under normal physiological conditions. When ER stress or misfolded proteins accumulate, BiP dissociates and allow the initiation of downstream signaling. IRE1α pathway: ER stress induces IRE1α homodimerization and autophosphorylation, which triggers its RNase activity to splice XBP1. As a transcription factor, X-box binding proteins 1 (XBP1s) activates genes related to the UPR, ERAD, and chaperones. PERK pathway: The activated PERK phosphorylates the alpha subunit of eIF2a, which attenuates protein translation to reduce the burden of misfolded proteins. Phosphorylated eIF2 up-regulates ATF4, which increases proapoptotic CHOP and UPR genes. ATF6α pathway: ATF6α is cleaved by site-1 and site-2 proteases (S1P and S2P) in the Golgi apparatus to produce ATF6N. ATF6N further initiates the transcription of its target UPR genes in the nucleus. All these pathways collectively aim to improve the protein-folding capacity and decrease the protein-folding burden by shutting down translation and degrading the ER-bound mRNAs. When this adaptive response fails, upregulated UPR signaling induces apoptosis. This figure was generated using BioRender.com (Toronto, ON, Canada).

Figure 2

Intersection of the unfolded protein response transducers, lipotoxicity, and lipid metabolism. Inositol-requiring enzyme 1α (IRE1α), PKR-like ER kinase (PERK), and activating transcription factor 6α (ATF6α) are implicated in lipotoxicity and regulation of lipid metabolism. The IRE1α-XBP1 axis is important in regulating several aspects of lipid homeostasis including very low density lipoprotein (VLDL) lipidation, lipolysis, and de novo lipogenesis. Hyperactivation of IRE1α results in regulated IRE1α-dependent decay of RNA (RIDD), which degrades several mRNAs and microRNAs that regulate lipid metabolism. S-nitrosylation of IRE1α can occur in obesity, leading to a reduction in IRE1α-mediated XBP1 processing. PERK pathway regulates SREBP activation by inhibition of the translation of INSIG and also by ATF4 mediated effects on lipid synthesis pathways. ATF6α can inhibit SREBP activation and can directly also activate phospholipid biosynthesis and fatty acid oxidation. This figure was generated using BioRender.com (Toronto, ON, Canada). LD, lipid droplets; TAG, triacylglycerol.

Figure 3

Tethering proteins at endoplasmic reticulum (ER) membrane contact sites with other organelles. Representative illustration of the membrane contact sites (MCSs) of the ER with mitochondria, endosomes, and lipid droplet (LD). ER-Lipid droplet: Three proteins highlighted are Seipin, Sorting Nexin 14 (SNX14), and Rab18. ER-Mitochondria-Associated Membrane: ER membrane protein IP3R1 interacts with proteins on the mitochondrial membrane such as VDAC1 for lipid transport and GRP75 for calcium transport. PTPI51 on the mitochondrial membrane interacts with MOSPD2, VAP-B, ORP, and PDC8 in the ER membrane. ER-Endosomes: Annexin A1 and its ligand S100A11 in the endosomal compartment interact with the phosphatase PTP1B for EGFR down-regulation. ORP1L, STARD3, and STARD3NL in the endosomal compartment interacts with VAP-A on ER membranes to aid cholesterol transfer from ER to the endosomes. This figure was generated using BioRender.com (Toronto, ON, Canada).