Aortic carboxypeptidase-like protein enhances adipose tissue stromal progenitor differentiation into myofibroblasts and is upregulated in fibrotic white adipose tissue

AEBP1 or ACLP, which is the key factor in inflammation and fibrosis?

Adipocyte enhancer-binding protein 1 (AEBP1) and Aortic carboxypeptidase-like protein (ACLP) are two protein isoforms produced by the AEBP1 gene. AEBP1, originally discovered in preadipocytes, functions as a transcriptional repressor and is involved in promoting inflammation, proliferation, and migration through various signaling pathways. ACLP is an extracellular matrix protein linked to Ehlers-Danlos syndrome, a genetic disorder characterized by defective connective tissue development. Structurally, AEBP1 and ACLP share many similarities, and both participate in critical physiological or pathological processes, such as cancer and fibrosis, by influencing pathways like NK-κB, WNT, and TGF-β. In recent years, research on AEBP1 and ACLP has expanded to include major organs such as the brain, kidneys, and lungs, with a particular focus on the cardiovascular system, where they show potential as novel drug targets. However, most studies do not clearly distinguish between AEBP1 and ACLP. For instance, AEBP1 is implicated in myocardial fibrosis in hypertrophic cardiomyopathy models, whereas ACLP is associated with fibrosis in other organs. Additionally, literature on the relationship between AEBP1 and fibrosis is often contradictory. Clarifying the distinct roles of AEBP1 and ACLP and their different functions in various cell types would greatly benefit further research. Current research suggests that the AEBP1 gene encodes two proteins, AEBP1 and ACLP, which have been reported to exhibit distinct functions in different studies. However, many studies do not differentiate between these two proteins, potentially leading to misconceptions. Therefore, we have conducted a comprehensive review of the existing literature to elucidate the functions of the AEBP1 gene and its encoded proteins in detail.

Inactive metallopeptidase homologs: the secret lives of pseudopeptidases

Inactive enzyme homologs, or pseudoenzymes, are proteins, found within most enzyme families, that are incapable of performing catalysis. Rather than catalysis, they are involved in protein-protein interactions, sometimes regulating the activity of their active enzyme cousins, or scaffolding protein complexes. Pseudoenzymes found within metallopeptidase families likewise perform these functions. Pseudoenzymes within the M14 carboxypeptidase family interact with collagens within the extracellular space, while pseudopeptidase members of the M12 "a disintegrin and metalloprotease" (ADAM) family either discard their pseudopeptidase domains as unnecessary for their roles in sperm maturation or utilize surface loops to enable assembly of key complexes at neuronal synapses. Other metallopeptidase families contain pseudopeptidases involved in protein synthesis at the ribosome and protein import into organelles, sometimes using their pseudo-active sites for these interactions. Although the functions of these pseudopeptidases have been challenging to study, ongoing work is teasing out the secret lives of these proteins.

Dysregulated AEBP1 and COLEC12 Genes in Late-Onset Alzheimer's Disease: Insights from Brain Cortex and Peripheral Blood Analysis

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by memory and cognitive impairment, often accompanied by alterations in mood, confusion, and, ultimately, a state of acute mental disturbance. The cerebral cortex is considered a promising area for investigating the underlying causes of AD by analyzing transcriptional patterns, which could be complemented by investigating blood samples obtained from patients. We analyzed the RNA expression profiles of three distinct areas of the brain cortex, including the frontal cortex (FC), temporal cortex (TC), and entorhinal cortex (EC) in patients with AD. Functional enrichment analysis was performed on the differentially expressed genes (DEGs) across the three regions. The two genes with the most significant expression changes in the EC region were selected for assessing mRNA expression levels in the peripheral blood of late-onset AD patients using quantitative PCR (qPCR). We identified eight shared DEGs in these regions, including AEBP1 and COLEC12, which exhibited prominent changes in expression. Functional enrichment analysis uncovered a significant association of these DEGs with the transforming growth factor-β (TGF-β) signaling pathway and processes related to angiogenesis. Importantly, we established a robust connection between the up-regulation of AEBP1 and COLEC12 in both the brain and peripheral blood. Furthermore, we have demonstrated the potential of AEBP1 and COLEC12 genes as effective diagnostic tools for distinguishing between late-onset AD patients and healthy controls. This study unveils the intricate interplay between AEBP1 and COLEC12 in AD and underscores their potential as markers for disease detection and monitoring.

Identification of central genes for endometriosis through integration of single-cell RNA sequencing and bulk RNA sequencing analysis

This study aimed to identify the key genes involved in the development of endometriosis and construct an accurate predictive model to provide new directions for the diagnosis and treatment of endometriosis. Using bioinformatics analysis, we employed the single-cell cell communication method to identify the key cell subtypes. By combining chip data and integrating differential analysis, WGCNA analysis, and the least absolute shrinkage and selection operator (LASSO) model, key genes were identified for immune infiltration and functional enrichment analyses. Cell communication analysis identified tissue stem cells as the key subtype. Differential analysis revealed 1879 differentially expressed genes, whereas WGCNA identified 357 module genes. The LASSO model further selects 4 key genes: Adipocyte Enhancer Binding Protein 1(AEBP1), MBNL1, GREM1, and DES. All 4 key genes showed significant correlations with immune cell content. Moreover, these genes were significantly expressed in single cells. The predictive model demonstrated good diagnostic performance. Through scRNA-seq, WGCNA, and LASSO methodologies, DES, GREM1, MBNL1, and AEBP1 emerged as crucial core genes linked to tissue stem cell markers in endometriosis. These genes have promising applications as diagnostic markers and therapeutic targets for endometriosis.

Single-cell transcriptomics analysis of proliferative diabetic retinopathy fibrovascular membranes reveals AEBP1 as fibrogenesis modulator

The management of preretinal fibrovascular membranes, a devastating complication of advanced diabetic retinopathy (DR), remains challenging. We characterized the molecular profile of cell populations in these fibrovascular membranes to identify potentially new therapeutic targets. Preretinal fibrovascular membranes were surgically removed from patients and submitted for single-cell RNA-Seq (scRNA-Seq). Differential gene expression was implemented to define the transcriptomics profile of these cells and revealed the presence of endothelial, inflammatory, and stromal cells. Endothelial cell reclustering identified subclusters characterized by noncanonical transcriptomics profile and active angiogenesis. Deeper investigation of the inflammatory cells showed a subcluster of macrophages expressing proangiogenic cytokines, presumably contributing to angiogenesis. The stromal cell cluster included a pericyte-myofibroblast transdifferentiating subcluster, indicating the involvement of pericytes in fibrogenesis. Differentially expressed gene analysis showed that Adipocyte Enhancer-binding Protein 1, AEBP1, was significantly upregulated in myofibroblast clusters, suggesting that this molecule may have a role in transformation. Cell culture experiments with human retinal pericytes (HRP) in high-glucose condition confirmed the molecular transformation of pericytes toward myofibroblastic lineage. AEBP1 siRNA transfection in HRP reduced the expression of profibrotic markers in high glucose. In conclusion, AEBP1 signaling modulates pericyte-myofibroblast transformation, suggesting that targeting AEBP1 could prevent scar tissue formation in advanced DR.

Loss of myeloid lipoprotein lipase exacerbates adipose tissue fibrosis with collagen VI deposition and hyperlipidemia in leptin-deficient obese mice

During obesity, tissue macrophages increase in number and become proinflammatory, thereby contributing to metabolic dysfunction. Lipoprotein lipase (LPL), which hydrolyzes triglyceride in lipoproteins, is secreted by macrophages. However, the role of macrophage-derived LPL in adipose tissue remodeling and lipoprotein metabolism is largely unknown. To clarify these issues, we crossed leptin-deficient Lep^ob/ob mice with mice lacking the Lpl gene in myeloid cells (Lpl^m−/m−) to generate Lpl^m−/m−;Lep^ob/ob mice. We found the weight of perigonadal white adipose tissue (WAT) was increased in Lpl^m−/m−;Lep^ob/ob mice compared with Lep^ob/ob mice due to substantial accumulation of both adipose tissue macrophages and collagen that surrounded necrotic adipocytes. In the fibrotic epidydimal WAT of Lpl^m−/m−;Lep^ob/ob mice, we observed an increase in collagen VI and high mobility group box 1, while α-smooth muscle cell actin, a marker of myofibroblasts, was almost undetectable, suggesting that the adipocytes were the major source of the collagens. Furthermore, the adipose tissue macrophages from Lpl^m−/m−;Lep^ob/ob mice showed increased expression of genes related to fibrosis and inflammation. In addition, we determined Lpl^m−/m−;Lep^ob/ob mice were more hypertriglyceridemic than Lep^ob/ob mice. Lpl^m−/m−;Lep^ob/ob mice also showed slower weight gain than Lep^ob/ob mice, which was primarily due to reduced food intake. In conclusion, we discovered that the loss of myeloid Lpl led to extensive fibrosis of perigonadal WAT and hypertriglyceridemia. In addition to illustrating an important role of macrophage LPL in regulation of circulating triglyceride levels, these data show that macrophage LPL protects against fibrosis in obese adipose tissues.

Extracellular vesicle-derived AEBP1 mRNA as a novel candidate biomarker for diabetic kidney disease

BACKGROUND

A novel and improved methodology is still required for the diagnosis of diabetic kidney disease (DKD). The aim of the present study was to identify novel biomarkers using extracellular vesicle (EV)-derived mRNA based on kidney tissue microarray data.

METHODS

Candidate genes were identified by intersecting the differentially expressed genes (DEGs) and eGFR-correlated genes using the GEO datasets GSE30528 and GSE96804, followed by clinical parameter correlation and diagnostic efficacy assessment.

RESULTS

Fifteen intersecting genes, including 8 positively correlated genes, B3GALT2, CDH10, MIR3916, NELL1, OCLM, PRKAR2B, TREM1 and USP46, and 7 negatively correlated genes, AEBP1, CDH6, HSD17B2, LUM, MS4A4A, PTN and RASSF9, were confirmed. The expression level assessment results revealed significantly increased levels of AEBP1 in DKD-derived EVs compared to those in T2DM and control EVs. Correlation analysis revealed that AEBP1 levels were positively correlated with Cr, 24-h urine protein and serum CYC and negatively correlated with eGFR and LDL, and good diagnostic efficacy for DKD was also found using AEBP1 levels to differentiate DKD patients from T2DM patients or controls.

CONCLUSIONS

Our results confirmed that the AEBP1 level from plasma EVs could differentiate DKD patients from T2DM patients and control subjects and was a good indication of the function of multiple critical clinical parameters. The AEBP1 level of EVs may serve as a novel and efficacious biomarker for DKD diagnosis.

Aortic carboxypeptidase-like protein regulates vascular adventitial progenitor and fibroblast differentiation through myocardin related transcription factor A

The vascular adventitia contains numerous cell types including fibroblasts, adipocytes, inflammatory cells, and progenitors embedded within a complex extracellular matrix (ECM) network. In response to vascular injury, adventitial progenitors and fibroblasts become activated and exhibit increased proliferative capacity and differentiate into contractile cells that remodel the ECM. These processes can lead to vascular fibrosis and disease progression. Our previous work established that the ECM protein aortic carboxypeptidase-like protein (ACLP) promotes fibrotic remodeling in the lung and is activated by vascular injury. It is currently unknown what controls vascular adventitial cell differentiation and if ACLP has a role in this process. Using purified mouse aortic adventitia Sca1+ progenitors, ACLP repressed stem cell markers (CD34, KLF4) and upregulated smooth muscle actin (SMA) and collagen I expression. ACLP enhanced myocardin-related transcription factor A (MRTFA) activity in adventitial cells by promoting MRTFA nuclear translocation. Sca1 cells from MRTFA-null mice exhibited reduced SMA and collagen expression induced by ACLP, indicating Sca1 cell differentiation is regulated in part by the ACLP-MRTFA axis. We determined that ACLP induced vessel contraction and increased adventitial collagen in an explant model. Collectively these studies identified ACLP as a mediator of adventitial cellular differentiation, which may result in pathological vessel remodeling.

Transcriptional Regulation in Non-Alcoholic Fatty Liver Disease

Obesity is the primary risk factor for the pathogenesis of non-alcoholic fatty liver disease (NAFLD), the worldwide prevalence of which continues to increase dramatically. The liver plays a pivotal role in the maintenance of whole-body lipid and glucose homeostasis. This is mainly mediated by the transcriptional activation of hepatic pathways that promote glucose and lipid production or utilization in response to the nutritional state of the body. However, in the setting of chronic excessive nutrition, the dysregulation of hepatic transcriptional machinery promotes lipid accumulation, inflammation, metabolic stress, and fibrosis, which culminate in NAFLD. In this review, we provide our current understanding of the transcription factors that have been linked to the pathogenesis and progression of NAFLD. Using publicly available transcriptomic data, we outline the altered activity of transcription factors among humans with NAFLD. By expanding this analysis to common experimental mouse models of NAFLD, we outline the relevance of mouse models to the human pathophysiology at the transcriptional level.

Mechanisms of aortic carboxypeptidase-like protein secretion and identification of an intracellularly retained variant associated with Ehlers-Danlos syndrome

Aortic carboxypeptidase-like protein (ACLP) is a collagen-binding extracellular matrix protein that has important roles in wound healing and fibrosis. ACLP contains thrombospondin repeats, a collagen-binding discoidin domain, and a catalytically inactive metallocarboxypeptidase domain. Recently, mutations in the ACLP-encoding gene, AE-binding protein 1 (AEBP1), have been discovered, leading to the identification of a new variant of Ehlers-Danlos syndrome causing connective tissue disruptions in multiple organs. Currently, little is known about the mechanisms of ACLP secretion or the role of post-translational modifications in these processes. We show here that the secreted form of ACLP contains N-linked glycosylation and that inhibition of glycosylation results in its intracellular retention. Using site-directed mutagenesis, we determined that glycosylation of Asn-471 and Asn-1030 is necessary for ACLP secretion and identified a specific N-terminal proteolytic ACLP fragment. To determine the contribution of secreted ACLP to extracellular matrix mechanical properties, we generated and mechanically tested wet-spun collagen ACLP composite fibers, finding that ACLP enhances the modulus (or stiffness), toughness, and tensile strength of the fibers. Some AEBP1 mutations were null alleles, whereas others resulted in expressed proteins. We tested the hypothesis that a recently discovered 40-amino acid mutation and insertion in the ACLP discoidin domain regulates collagen binding and assembly. Interestingly, we found that this protein variant is retained intracellularly and induces endoplasmic reticulum stress identified with an XBP1-based endoplasmic reticulum stress reporter. Our findings highlight the importance of N-linked glycosylation of ACLP for its secretion and contribute to our understanding of ACLP-dependent disease pathologies.

The Adipocyte Acquires a Fibroblast-Like Transcriptional Signature in Response to a High Fat Diet

Visceral white adipose tissue (vWAT) expands and undergoes extensive remodeling during diet-induced obesity. Much is known about the contribution of various stromal vascular cells to the remodeling process, but less is known of the changes that occur within the adipocyte as it becomes progressively dysfunctional. Here, we performed a transcriptome analysis of isolated vWAT adipocytes to assess global pathway changes occurring in response to a chronic high fat diet (HFD). The data demonstrate that the adipocyte responds to the HFD by adopting a fibroblast-like phenotype, characterized by enhanced expression of ECM, focal adhesion and cytoskeletal genes and suppression of many adipocyte programs most notably those associated with mitochondria. This study reveals that during obesity the adipocyte progressively becomes metabolically dysfunctional due to its acquisition of fibrogenic functions. We propose that mechano-responsive transcription factors such as MRTFA and SRF contribute to both upregulation of morphological genes as well as suppression of mitochondrial programs.

Bi-allelic AEBP1 mutations in two patients with Ehlers-Danlos syndrome

The Ehlers-Danlos syndromes (EDSs) are a clinically and molecularly diverse group of heritable connective tissue disorders caused by defects in a wide range of genes. Recently, bi-allelic loss-of-function mutations in the adipocyte enhancer-binding protein 1 (AEBP1) gene were reported in three families with an autosomal recessive EDS-like condition characterized by thin and hyperextensible skin, poor wound healing with prominent atrophic scarring, joint hypermobility and osteoporosis. Using whole exome sequencing, we identified novel bi-allelic AEBP1 variants in two unrelated adult patients, previously diagnosed with an undefined EDS type, which shows important clinical resemblance to several other EDS subtypes. Our patients present with similar cutaneous and musculoskeletal features as the previously reported patients. They also show unreported clinical features, including pectus deformity, premature aged appearance, sparse and frizzled hair, fatigue and pain. AEBP1 is ubiquitously expressed and encodes the secreted aortic carboxypeptidase-like protein (ACLP) that can bind fibrillar collagens and assist in collagen polymerization. Transmission electron microscopy studies on the patients' skin biopsies show ultrastructural alterations in collagen fibril diameter and appearance, underscoring an important role for ACLP in collagen fibril organization. This report further expands the clinical, molecular and ultrastructural spectrum associated with AEBP1 defects and highlights the complex and variable phenotype associated with this new EDS variant.

Identification of a Paracrine Signaling Mechanism Linking CD34high Progenitors to the Regulation of Visceral Fat Expansion and Remodeling

Accumulation of visceral (VIS) is a predictor of metabolic disorders and insulin resistance. This is due in part to the limited capacity of VIS fat to buffer lipids allowing them to deposit in insulin-sensitive tissues. Mechanisms underlying selective hypertrophic growth and tissue remodeling properties of VIS fat are not well understood. We identified subsets of adipose progenitors (APs) unique to VIS fat with differential Cd34 expression and adipogenic capacity. VIS low (Cd34 low) APs are adipogenic, whereas VIS high (Cd34 high) APs are not. Furthermore, VIS high APs inhibit adipogenic differentiation of SUB and VIS low APs in vitro through the secretion of soluble inhibitory factor(s). The number of VIS high APs increased with adipose tissue expansion, and their abundance in vivo caused hypertrophic growth, fibrosis, inflammation, and metabolic dysfunction. This study unveils the presence of APs unique to VIS fat involved in the paracrine regulation of adipogenesis and tissue remodeling.

AEBP1 expression increases with severity of fibrosis in NASH and is regulated by glucose, palmitate, and miR-372-3p

Factors governing the development of liver fibrosis in nonalcoholic steatohepatitis (NASH) are only partially understood. We recently identified adipocyte enhancer binding protein 1 (AEBP1) as a member of a core set of dysregulated fibrosis-specific genes in human NASH. Here we sought to investigate the relationship between AEBP1 and hepatic fibrosis. We confirmed that hepatic AEBP1 expression is elevated in fibrosis compared to lobular inflammation, steatosis, and normal liver, and increases with worsening fibrosis in NASH patients. AEBP1 expression was upregulated 5.8-fold in activated hepatic stellate cells and downregulated during chemical and contact induction of biological quiescence. In LX-2 and HepG2 cells treated with high glucose (25 mM), AEBP1 expression increased over 7-fold compared to normal glucose conditions. In response to treatment with either fructose or palmitate, AEBP1 expression in primary human hepatocytes increased 2.4-fold or 9.6-fold, but was upregulated 55.8-fold in the presence of fructose and palmitate together. AEBP1 knockdown resulted in decreased expression of nine genes previously identified to be part of a predicted AEBP1-associated NASH co-regulatory network and confirmed to be upregulated in fibrotic tissue. We identified binding sites for two miRNAs known to be downregulated in NASH fibrosis, miR-372-3p and miR-373-3p in the AEBP1 3' untranslated region. Both miRNAs functionally interacted with AEBP1 to regulate its expression. These findings indicate a novel AEBP1-mediated pathway in the pathogenesis of hepatic fibrosis in NASH.

Impaired Glucocorticoid Suppression of TGFβ Signaling in Human Omental Adipose Tissues Limits Adipogenesis and May Promote Fibrosis

Visceral obesity is associated with insulin resistance and higher risk of type 2 diabetes and metabolic diseases. A limited ability of adipose tissues to remodel through the recruitment and differentiation of adipose stem cells (ASCs) is associated with adipose tissue inflammation and fibrosis and the metabolic syndrome. We show that the lower adipogenesis of omental (Om) compared with abdominal subcutaneous (Abdsc) ASCs was associated with greater secretion of TGFβ ligands that acted in an autocrine/paracrine loop to activate SMAD2 and suppress adipogenesis. Inhibition of TGFβ signaling rescued Om ASC differentiation. In Abdsc ASCs, low concentrations of dexamethasone suppressed TGFβ signaling and enhanced adipogenesis, at least in part by increasing TGFBR3 protein that can sequester TGFβ ligands. Om ASCs were resistant to these dexamethasone effects; recombinant TGFBR3 increased their differentiation. Pericellular fibrosis, a hallmark of dysfunctional adipose tissue, was greater in Om and correlated with higher level of tissue TGFβ signaling activity and lower ASC differentiation. We conclude that glucocorticoids restrain cell-autonomous TGFβ signaling in ASCs to facilitate adipogenesis and healthy remodeling in Abdsc and these processes are impaired in Om. Therapies directed at overcoming glucocorticoid resistance in visceral adipose tissue may improve remodeling and help prevent metabolic complications of visceral obesity.