Restoration of the ER stress response protein TDAG51 in hepatocytes mitigates NAFLD in mice.

Endoplasmic reticulum stress is associated with insulin resistance and the development of nonalcoholic fatty liver disease. Deficiency of the endoplasmic reticulum stress response T-cell death-associated gene 51 (TDAG51) (TDAG51-/-) in mice promotes the development of high-fat diet (HFD)-induced obesity, fatty liver, and hepatic insulin resistance. However, whether this effect is due specifically to hepatic TDAG51 deficiency is unknown. Here, we report that hepatic TDAG51 protein levels are consistently reduced in multiple mouse models of liver steatosis and injury as well as in liver biopsies from patients with liver disease compared to normal controls. Delivery of a liver-specific adeno-associated virus (AAV) increased hepatic expression of a TDAG51-GFP fusion protein in WT, TDAG51-/-, and leptin-deficient (ob/ob) mice. Restoration of hepatic TDAG51 protein was sufficient to increase insulin sensitivity while reducing body weight and fatty liver in HFD fed TDAG51-/- mice and in ob/ob mice. TDAG51-/- mice expressing ectopic TDAG51 display improved Akt (Ser473) phosphorylation, post-insulin stimulation. HFD-fed TDAG51-/- mice treated with AAV-TDAG51-GFP displayed reduced lipogenic gene expression, increased beta-oxidation and lowered hepatic and serum triglycerides, findings consistent with reduced liver weight. Further, AAV-TDAG51-GFP-treated TDAG51-/- mice exhibited reduced hepatic precursor and cleaved sterol regulatory-element binding proteins (SREBP-1 and SREBP-2). In vitro studies confirmed the lipid-lowering effect of TDAG51 overexpression in oleic acid-treated Huh7 cells. These studies suggest that maintaining hepatic TDAG51 protein levels represents a viable therapeutic approach for the treatment of obesity and insulin resistance associated with nonalcoholic fatty liver disease.

Reduced plasma GDF10 levels are positively associated with cholesterol impairment and childhood obesity.

Childhood obesity is a global health concern affecting over 150 million children worldwide, with projections of a rise to 206 million by 2025. Understanding the mechanisms underlying this epidemic is crucial for developing effective interventions. In this study, we investigated circulating levels of Growth Differentiation Factor 10 (GDF10), a novel regulator of adipogenesis. Previous studies report diminished circulating GDF10 levels contribute to obesity and hepatic steatosis in mice. To further understand the role of plasma GDF10 in childhood obesity, a prospective case-control study was conducted. Using an enzyme-linked immunosorbent assay, plasma GDF10 levels were measured in children aged 5-17 years of age with normal (n = 36) and increased (n = 56) body mass index (BMI). Subsequently, plasma GDF10 levels were compared to various cardio-metabolic parameters. Children with increased BMI exhibit significantly lower levels of plasma GDF10 compared to children with normal BMI (p < 0.05). This study not only supports previous mouse data but is the first to report that lower levels of GDF10 is associated with childhood obesity, providing an important human connection for the relevance of GDF10 in obesity. Furthermore, this study revealed a significant correlation between low plasma GDF10 levels and elevated LDL-cholesterol and total cholesterol levels dependent on BMI (95% CI, p < 0.05). This study supports the hypothesis that children with obesity display lower plasma levels of GDF10, which correlates with elevated cholesterol levels. These insights shed light on potential mechanisms contributing to childhood obesity and may lead to future therapeutic interventions targeting GDF10 to mitigate adverse effects of adipogenesis in cardiometabolic health.

Myeloid-specific deletion of activating transcription factor 6 alpha increases CD11b+ macrophage subpopulations and aggravates lung fibrosis.

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, fibrotic interstitial lung disease of unknown etiology. The accumulation of macrophages is associated with disease pathogenesis. The unfolded protein response (UPR) has been linked to macrophage activation in pulmonary fibrosis. To date, the impact of activating transcription factor 6 alpha (ATF6α), one of the UPR mediators, on the composition and function of pulmonary macrophage subpopulations during lung injury and fibrogenesis is not fully understood. We began by examining the expression of Atf6α in IPF patients' lung single-cell RNA sequencing dataset, archived surgical lung specimens, and CD14+ circulating monocytes. To assess the impact of ATF6α on pulmonary macrophage composition and pro-fibrotic function during tissue remodeling, we conducted an in vivo myeloid-specific deletion of Atf6α. Flow cytometric assessments of pulmonary macrophages were carried out in C57BL/6 and myeloid specific ATF6α-deficient mice in the context of bleomycin-induced lung injury. Our results demonstrated that Atf6α mRNA was expressed in pro-fibrotic macrophages found in the lung of a patient with IPF and in CD14+ circulating monocytes obtained from blood of a patient with IPF. After bleomycin administration, the myeloid-specific deletion of Atf6α altered the pulmonary macrophage composition, expanding CD11b+ subpopulations with dual polarized CD38+ CD206+ expressing macrophages. Compositional changes were associated with an aggravation of fibrogenesis including increased myofibroblast and collagen deposition. A further mechanistic ex vivo investigation revealed that ATF6α was required for CHOP induction and the death of bone marrow-derived macrophages. Overall, our findings suggest a detrimental role for the ATF6α-deficient CD11b+ macrophages which had altered function during lung injury and fibrosis.

Inhibitory Antibodies against PCSK9 Reduce Surface CD36 and Mitigate Diet-Induced Renal Lipotoxicity.

Background: PCSK9 modulates the uptake of circulating lipids through a range of receptors, including the low-density lipoprotein receptor (LDLR) and CD36. In the kidney, CD36 is known to contribute to renal injury through pro-inflammatory and -fibrotic pathways. In this study, we sought to investigate the role of PCSK9 in modulating renal lipid accumulation and injury through CD36 using a high fat diet (HFD)-induced murine model. Methods: The effect of PCSK9 on the expression of CD36 and intracellular accumulation of lipid was examined in cultured renal cells and in the kidneys of male C57BL/6J mice. The effect of these findings was subsequently explored in a model of HFD-induced renal injury in Pcsk9 -/- and Pcsk9 +/+ littermate control mice on a C57BL/6J background. Results: In the absence of PCSK9, we observed heightened CD36 expression levels, which increased free fatty acid (FFA) uptake in cultured renal tubular cells. As a result, PCSK9 deficiency was associated with an increase in long-chain saturated FFA-induced ER stress. Consistent with these observations, Pcsk9-/- mice fed a HFD displayed elevated ER stress, inflammation, fibrosis, and renal injury relative to HFD-fed control mice. In contrast to Pcsk9-/- mice, pretreatment of WT C57BL/6J mice with evolocumab, an anti-PCSK9 monoclonal antibody (mAb) that binds to and inhibits the function of circulating PCSK9, protected against HFD-induced renal injury in association with reducing cell surface CD36 expression on renal epithelia. Conclusions: We report that circulating PCSK9 modulates renal lipid uptake in a manner dependent on renal CD36. In the context of increased dietary fat consumption, the absence of circulating PCSK9 may promote renal lipid accumulation and subsequent renal injury. However, although the administration of evolocumab blocks the interaction of PCSK9 with the LDLR, this evolocumab/PCSK9 complex can still bind CD36, thereby protecting against HFD-induced renal lipotoxicity.

Scratching the Surface-An Overview of the Roles of Cell Surface GRP78 in Cancer.

The 78 kDa glucose-regulated protein (GRP78) is considered an endoplasmic reticulum (ER)-resident molecular chaperone that plays a crucial role in protein folding homeostasis by regulating the unfolded protein response (UPR) and inducing numerous proapoptotic and autophagic pathways within the eukaryotic cell. However, in cancer cells, GRP78 has also been shown to migrate from the ER lumen to the cell surface, playing a role in several cellular pathways that promote tumor growth and cancer cell progression. There is another insidious consequence elicited by cell surface GRP78 (csGRP78) on cancer cells: the accumulation of csGRP78 represents a novel neoantigen leading to the production of anti-GRP78 autoantibodies that can bind csGRP78 and further amplify these cellular pathways to enhance cell growth and mitigate apoptotic cell death. This review examines the current body of literature that delineates the mechanisms by which ER-resident GRP78 localizes to the cell surface and its consequences, as well as potential therapeutics that target csGRP78 and block its interaction with anti-GRP78 autoantibodies, thereby inhibiting further amplification of cancer cell progression.

TDAG51 induces renal interstitial fibrosis through modulation of TGF-ß receptor 1 in chronic kidney disease.

Chronic kidney disease (CKD) is characterized by the gradual loss of renal function and is a major public health concern. Risk factors for CKD include hypertension and proteinuria, both of which are associated with endoplasmic reticulum (ER) stress. ER stress-induced TDAG51 protein expression is increased at an early time point in mice with CKD. Based on these findings, wild-type and TDAG51 knock-out (TDKO) mice were used in an angiotensin II/deoxycorticosterone acetate/salt model of CKD. Both wild-type and TDKO mice developed hypertension, increased proteinuria and albuminuria, glomerular injury, and tubular damage. However, TDKO mice were protected from apoptosis and renal interstitial fibrosis. Human proximal tubular cells were used to demonstrate that TDAG51 expression induces apoptosis through a CHOP-dependent mechanism. Further, a mouse model of intrinsic acute kidney injury demonstrated that CHOP is required for ER stress-mediated apoptosis. Renal fibroblasts were used to demonstrate that TGF-ß induces collagen production through an IRE1-dependent mechanism; cells treated with a TGF-ß receptor 1 inhibitor prevented XBP1 splicing, a downstream consequence of IRE1 activation. Interestingly, TDKO mice express significantly less TGF-ß receptor 1, thus, preventing TGF-ß-mediated XBP1 splicing. In conclusion, TDAG51 induces apoptosis in the kidney through a CHOP-dependent mechanism, while contributing to renal interstitial fibrosis through a TGF-ß-IRE1-XBP1 pathway.

FK506-Binding Protein 13 Expression Is Upregulated in Interstitial Lung Disease and Correlated with Clinical Severity. A Potentially Protective Role.

Pulmonary fibrosis is a progressive lung disease characterized by myofibroblast accumulation and excessive extracellular matrix deposition. We sought to investigate the role of FKBP13 (13-kD FK506-binding protein), an endoplasmic reticulum-resident molecular chaperone, in various forms of pulmonary fibrosis. We first characterized the gene and protein expression of FKBP13 in lung biopsy specimens from 24 patients with idiopathic pulmonary fibrosis and 17 control subjects. FKBP13 expression was found to be elevated in the fibrotic regions of idiopathic pulmonary fibrosis lung tissues and correlated with declining forced vital capacity and dyspnea severity. FKBP13 expression was also increased in lung biopsy specimens of patients with hypersensitivity pneumonitis, rheumatoid arthritis, and sarcoidosis-associated interstitial lung disease. We next evaluated the role of this protein using FKBP13-/- mice in a bleomycin model of pulmonary fibrosis. Animals were assessed for lung function and histopathology at different stages of lung injury including the inflammatory (Day 7), fibrotic (Day 21), and resolution (Day 50) phases. FKBP13-/- mice showed increased infiltration of inflammatory cells and cytokines at Day 7, increased lung elastance and fibrosis at Day 21, and impaired resolution of fibrosis at Day 50. These changes were associated with an increased number of cells that stained positive for TUNEL and cleaved caspase 3 in the FKBP13-/- lungs, indicating a heightened cellular sensitivity to bleomycin. Our findings suggest that FKBP13 is a potential biomarker for severity of interstitial lung diseases and that it has a biologically relevant role in protecting mice against bleomycin-induced injury, inflammation, and fibrosis.

Protein Misfolding and Endoplasmic Reticulum Stress in Chronic Lung Disease: Will Cell-Specific Targeting Be the Key to the Cure?

Chronic lung disease accounts for a significant global burden with respect to death, disability, and health-care costs. Due to the heterogeneous nature and limited treatment options for these diseases, it is imperative that the cellular and molecular mechanisms underlying the disease pathophysiology are further understood. The lung is a complex organ with a diverse cell population, and each cell type will likely have different roles in disease initiation, progression, and resolution. The effectiveness of a given therapeutic agent may depend on the net effect on each of these cell types. Over the past decade, it has been established that endoplasmic reticulum stress and the unfolded protein response are involved in the development of several chronic lung diseases. These conserved cellular pathways are important for maintaining cellular proteostasis, but their aberrant activation can result in pathology. This review discusses the current understanding of endoplasmic reticulum stress and the unfolded protein response at the cellular level in the development and progression of various chronic lung diseases. We highlight the need for increased understanding of the specific cellular contributions of unfolded protein response activation to these pathologies and suggest that the development of cell-specific targeted therapies is likely required to further decrease disease progression and to promote resolution of chronic lung disease.

Pharmacologic inhibition of S1P attenuates ATF6 expression, causes ER stress and contributes to apoptotic cell death.

Mammalian cells express unique transcription factors embedded in the endoplasmic reticulum (ER) membrane, such as the sterol regulatory element-binding proteins (SREBPs), that promote de novo lipogenesis. Upon their release from the ER, the SREBPs require proteolytic activation in the Golgi by site-1-protease (S1P). As such, inhibition of S1P, using compounds such as PF-429242 (PF), reduces cholesterol synthesis and may represent a new strategy for the management of dyslipidemia. In addition to the SREBPs, the unfolded protein response (UPR) transducer, known as the activating transcription factor 6 (ATF6), is another ER membrane-bound transcription factor that requires S1P-mediated activation. ATF6 regulates ER protein folding capacity by promoting the expression of ER chaperones such as the 78-kDa glucose-regulated protein (GRP78). ER-resident chaperones like GRP78 prevent and/or resolve ER polypeptide accumulation and subsequent ER stress-induced UPR activation by folding nascent polypeptides. Here we report that pharmacological inhibition of S1P reduced the expression of ATF6 and GRP78 and induced the activation of UPR transducers inositol-requiring enzyme-1α (IRE1α) and protein kinase RNA-like ER kinase (PERK). As a consequence, S1P inhibition also increased the susceptibility of cells to ER stress-induced cell death. Our findings suggest that S1P plays a crucial role in the regulation of ER folding capacity and also identifies a compensatory cross-talk between UPR transducers in order to maintain adequate ER chaperone expression and activity.