Astrocyte Elevated Gene-1 Cys75 S-Palmitoylation by ZDHHC6 Regulates Its Biological Activity

Spatial transcriptomics unravels palmitoylation and zonation-dependent gene regulation by AEG-1 in mouse liver

Obesity-induced metabolic dysfunction-associated steatohepatitis (MASH) leads to hepatocellular carcinoma (HCC). Astrocyte-elevated gene-1/Metadherin (AEG-1/MTDH) plays a key role in promoting MASH and HCC. AEG-1 is palmitoylated at residue cysteine 75 (Cys75) and a knock-in mouse representing mutated Cys75 to serine (AEG-1-C75S) showed activation of MASH- and HCC-promoting gene signature when compared to wild-type littermates (AEG-1-WT). The liver consists of three zones, periportal, mid-lobular, and pericentral, and zone-specific dysregulated gene expression impairs metabolic homeostasis in the liver, contributing to MASH and HCC. Here, to elucidate how palmitoylation influences AEG-1-mediated gene regulation in regard to hepatic zonation, we performed spatial transcriptomics (ST) in the livers of AEG-1-WT and AEG-1-C75S littermates. ST identified six different clusters in livers and using zone- and cell-type-specific markers we attributed specific zones and cell types to specific clusters. Ingenuity Pathway Analysis (IPA) of differentially expressed genes in each cluster unraveled activation of pro-inflammatory and MASH- and HCC-promoting pathways, mainly in periportal and pericentral hepatocytes, in AEG-1-C75S liver compared to AEG-1-WT. Interestingly, in AEG-1-C75S liver, the mid-lobular zone exhibited widespread inhibition of xenobiotic metabolism pathways and inhibition of PXR/RXR and LXR/RXR activation, versus AEG-1-WT. In conclusion, AEG-1-C75S mutant exhibited zone-specific differential gene expression, which might contribute to metabolic dysfunction and dysregulated drug metabolism leading to MASH and HCC.

Mechanisms and functions of protein S-acylation

Over the past two decades, protein S-acylation (often referred to as S-palmitoylation) has emerged as an important regulator of vital signalling pathways. S-Acylation is a reversible post-translational modification that involves the attachment of a fatty acid to a protein. Maintenance of the equilibrium between protein S-acylation and deacylation has demonstrated profound effects on various cellular processes, including innate immunity, inflammation, glucose metabolism and fat metabolism, as well as on brain and heart function. This Review provides an overview of current understanding of S-acylation and deacylation enzymes, their spatiotemporal regulation by sophisticated multilayered mechanisms, and their influence on protein function, cellular processes and physiological pathways. Furthermore, we examine how disruptions in protein S-acylation are associated with a broad spectrum of diseases from cancer to autoinflammatory disorders and neurological conditions.

The endoplasmic reticulum-localized enzyme zDHHC6 mediates S-acylation of short transmembrane constructs from multiple type I and II membrane proteins

In this study, we investigated the S-acylation of two host cell proteins important for viral infection: TMPRSS2 (transmembrane serine protease 2), which cleaves severe acute respiratory syndrome coronavirus 2 spike to facilitate viral entry, and bone marrow stromal antigen 2, a general viral restriction factor. We found that both proteins were S-acylated by zDHHC6, an S-acyltransferase enzyme localized at the endoplasmic reticulum, in coexpression experiments. Mutagenic analysis revealed that zDHHC6 modifies a single cysteine in each protein, which are in proximity to the transmembrane domains (TMDs). For TMPRSS2, the modified cysteine is positioned two residues into the TMD, whereas the modified cysteine in bone marrow stromal antigen 2 has a cytosolic location two amino acids upstream of the TMD. Cysteine swapping revealed that repositioning the target cysteine of TMPRSS2 further into the TMD substantially reduced S-acylation by zDHHC6. Interestingly, zDHHC6 efficiently S-acylated truncated forms of these proteins that contained only the TMDs and short juxtamembrane regions. The ability of zDHHC6 to modify short TMD sequences was also seen for the transferrin receptor (another type II membrane protein) and for five different type I membrane protein constructs, including cluster of differentiation 4. Collectively, the results of this study show that zDHHC6 can modify diverse membrane proteins (type I and II) and requires only the presence of the TMD and target cysteine for efficient S-acylation. Thus, zDHHC6 may be a broad specificity S-acyltransferase specialized for the modification of a diverse set of transmembrane proteins at the endoplasmic reticulum.

Diverse Roles of Protein Palmitoylation in Cancer Progression, Immunity, Stemness, and Beyond

Protein S-palmitoylation, a type of post-translational modification, refers to the reversible process of attachment of a fatty acyl chain-a 16-carbon palmitate acid-to the specific cysteine residues on target proteins. By adding the lipid chain to proteins, it increases the hydrophobicity of proteins and modulates protein stability, interaction with effector proteins, subcellular localization, and membrane trafficking. Palmitoylation is catalyzed by a group of zinc finger DHHC-containing proteins (ZDHHCs), whereas depalmitoylation is catalyzed by a family of acyl-protein thioesterases. Increasing numbers of oncoproteins and tumor suppressors have been identified to be palmitoylated, and palmitoylation is essential for their functions. Understanding how palmitoylation influences the function of individual proteins, the physiological roles of palmitoylation, and how dysregulated palmitoylation leads to pathological consequences are important drivers of current research in this research field. Further, due to the critical roles in modifying functions of oncoproteins and tumor suppressors, targeting palmitoylation has been used as a candidate therapeutic strategy for cancer treatment. Here, based on recent literatures, we discuss the progress of investigating roles of palmitoylation in regulating cancer progression, immune responses against cancer, and cancer stem cell properties.

Palmitoylation of solute carriers

Post-translational modifications are an important mechanism in the regulation of protein expression, function, and degradation. Well-known post-translational modifications are phosphorylation, glycosylation, and ubiquitination. However, lipid modifications, including myristoylation, prenylation, and palmitoylation, are poorly studied. Since the early 2000s, researchers have become more interested in lipid modifications, especially palmitoylation. The number of articles in PubMed increased from about 350 between 2000 and 2005 to more than 600 annually during the past ten years. S-palmitoylation, where the 16-carbon saturated (C16:0) palmitic acid is added to free cysteine residues of proteins, is a reversible protein modification that can affect the expression, membrane localization, and function of the modified proteins. Various diseases like Huntington's and Alzheimer's disease have been linked to changes in protein palmitoylation. In humans, the addition of palmitic acid is mediated by 23 palmitoyl acyltransferases, also called DHHC proteins. The modification can be reversed by a few thioesterases or hydrolases. Numerous soluble and membrane-attached proteins are known to be palmitoylated, but among the approximately 400 solute carriers that are classified in 66 families, only 15 found in 8 families have so far been documented to be palmitoylated. Among the best-characterized transporters are the glucose transporters GLUT1 (SLC2A1) and GLUT4 (SLC2A4), the three monoamine transporters norepinephrine transporter (NET; SLC6A2), dopamine transporter (DAT; SLC6A3), and serotonin transporter (SERT; SLC6A4), and the sodium-calcium exchanger NCX1 (SLC8A1). While there is evidence from recent proteomics experiments that numerous solute carriers are palmitoylated, no details beyond the 15 transporters covered in this review are available.