4-Phenylbutyric acid presents therapeutic effect on osteoarthritis via inhibiting cell apoptosis and inflammatory response induced by endoplasmic reticulum stress

GC/MS Fatty Acid Profile of Marine-Derived Actinomycetes from Extreme Environments: Chemotaxonomic Insights and Biotechnological Potential

This study investigated the fatty acids (FA) profile of 54 actinomycete strains isolated from marine sediments collected off the Portugal continental coast, specifically from the Estremadura Spur pockmarks field, by GC/MS. Fatty acid methyl esters (FAMEs) were prepared from the ethyl acetate lipidic extracts of these strains and analyzed by gas chromatography-mass spectrometry (GC/MS), with FA identification performed using the NIST library. The identified FAs varied from C12:0 to C20:0, where 32 distinct FAs were identified, including 7 branched-chain fatty acids (BCFAs), 9 odd-chain fatty acids (OCFAs), 8 monounsaturated fatty acids (MUFAs), 6 saturated fatty acids (SFAs), 1 polyunsaturated fatty acid (PUFA), and 1 cyclic chain fatty acid (CCFA). The average expressed content was BCFA (47.54%), MUFA (28.49%), OCFA (26.93%), and SFA (22.16%), of which i-C16:0, C18:1ω9, and C16:0 were predominant, while PUFA (3.58%) and CCFA (0.41%) were identified as minor components. The identified BCFA were i-C16:0, a-C15:0, i-C15:0, i-C15:1ω6, a-C16:0, a-C14:0, and i-C17:0, which include combined branching and unsaturation and branching and odd. SFAs were present in all species, with C16:0 and C18:0 being the most representative. Rare OCFAs C19:1ω9, C17:1ω7, C15:0, and C17:0 were expressed. PUFA C18:1ω9 was detected; within this class, omega families ω9, ω7, ω6, and ω5 were identified, and no ω3 was detected. The only CCFA was benzene-butanoic acid (benzene-C4:0). These findings highlight the metabolic versatility of actinomycetes, providing valuable insights into microbial chemotaxonomy and offering promising biochemical leads for the development of biofuel, nutraceutical, and antifungal agents. Furthermore, these results underline the diversity and biotechnological potential of FAs in actinomycetes, uncovering their potential to be used as microbial cell factories, and paving the way for innovations in biofuels, pharmaceuticals, and eco-friendly industrial products.

Endoplasmic reticulum stress-mediated apoptosis and autophagy in osteoarthritis: From molecular mechanisms to therapeutic applications

Osteoarthritis (OA) is characterized primarily by the degeneration of articular cartilage, with a high prevalence and disability rate. The functional phenotype of chondrocytes, as the sole cell type within cartilage, is vital for OA progression. Due to the avascular nature of cartilage and its limited regenerative capacity, repair following injury poses significant challenges. Various cellular stressors, including hypoxia, nutrient deprivation, oxidative stress, and collagen mutations, can lead to the accumulation of misfolded proteins in the endoplasmic reticulum (ER), resulting in ER stress (ERS). In response to restore ER homeostasis as well as cellular vitality and function, a series of adaptive mechanisms are triggered, including the unfolded protein response, ER-associated degradation, and ER-phagy. Prolonged or severe ERS may exceed the adaptive capacity of cells, leading to dysregulation in apoptosis and autophagy-key pathogenic factors contributing to chondrocyte damage and OA progression. This review examines the relationship between ERS in OA chondrocytes and both apoptosis and autophagy in order to identify potential therapeutic targets and strategies for prevention and treatment of OA.

Regulation of chondrocyte apoptosis in osteoarthritis by endoplasmic reticulum stress

Osteoarthritis (OA), a common degenerative joint disease, is characterized by the apoptosis of chondrocytes as a primary pathophysiological change, with endoplasmic reticulum stress (ERS) playing a crucial role. It has been demonstrated that an imbalance in endoplasmic reticulum (ER) homeostasis can lead to ERS, activating three cellular adaptive response pathways through the unfolded protein response to restore ER homeostasis. Mild ERS exerts a protective effect on cells, while prolonged ERS that disrupts the self-regulatory balance of the ER activates apoptotic signaling pathways, leading to chondrocyte apoptosis and hastening OA progression. Hence, controlling the ERS signaling pathway and its apoptotic factors has become a critical focus for preventing and treating OA. This review aims to elucidate the key mechanisms of ERS pathway-induced apoptosis, associated targets, and regulatory pathways, offering valuable insights to enhance the mechanistic understanding of OA. It also reviews the mechanisms studied for ERS-related drugs or compounds for the treatment of OA.

Insights into the underlying pathogenesis and therapeutic potential of endoplasmic reticulum stress in degenerative musculoskeletal diseases

Degenerative musculoskeletal diseases are structural and functional failures of the musculoskeletal system, including osteoarthritis, osteoporosis, intervertebral disc degeneration (IVDD), and sarcopenia. As the global population ages, degenerative musculoskeletal diseases are becoming more prevalent. However, the pathogenesis of degenerative musculoskeletal diseases is not fully understood. Previous studies have revealed that endoplasmic reticulum (ER) stress is a stress response that occurs when impairment of the protein folding capacity of the ER leads to the accumulation of misfolded or unfolded proteins in the ER, contributing to degenerative musculoskeletal diseases. By affecting cartilage degeneration, synovitis, meniscal lesion, subchondral bone remodeling of osteoarthritis, bone remodeling and angiogenesis of osteoporosis, nucleus pulposus degeneration, annulus fibrosus rupture, cartilaginous endplate degeneration of IVDD, and sarcopenia, ER stress is involved in the pathogenesis of degenerative musculoskeletal diseases. Preclinical studies have found that regulation of ER stress can delay the progression of multiple degenerative musculoskeletal diseases. These pilot studies provide foundations for further evaluation of the feasibility, efficacy, and safety of ER stress modulators in the treatment of musculoskeletal degenerative diseases in clinical trials. In this review, we have integrated up-to-date research findings of ER stress into the pathogenesis of degenerative musculoskeletal diseases. In a future perspective, we have also discussed possible directions of ER stress in the investigation of degenerative musculoskeletal disease, potential therapeutic strategies for degenerative musculoskeletal diseases using ER stress modulators, as well as underlying challenges and obstacles in bench-to-beside research.

PLXNC1 interference alleviates the inflammatory injury, apoptosis and extracellular matrix degradation of IL-1β-exposed chondrocytes via suppressing GRP78 expression

BACKGROUND

Osteoarthritis (OA) is a frequently encountered debilitating joint disorder. Whether plexin C1 (PLXNC1) is implicated in OA is far from being investigated despite its well-documented pro-inflammatory property in human diseases. The goal of this study is to expound the specific role of PLXNC1 in OA and elaborate the probable action mechanism.

METHODS

Firstly, PLXNC1 expression in the cartilage tissues of patients with OA was examined with GEO database. In interleukin-1beta (IL-1β)-induced OA cell model, RT-qPCR and western blotting tested the expression of PLXNC1, glucose-regulating protein 78 (GRP78) and extracellular matrix (ECM) degradation-related factors. Cell viability and inflammation were respectively judged by CCK-8 assay and RT-qPCR. TUNEL and western blotting estimated cell apoptosis. The potential binding between PLXNC1 and GRP78 was corroborated by Co-IP assay. Western blotting also tested the expression of endoplasmic reticulum stress (ERS)-associated proteins.

RESULTS

As it turned out, PLXNC1 expression was elevated in the cartilage tissues of patients with OA and IL-1β-treated chondrocytes. When PLXNC1 was depleted, the viability injury, inflammation, apoptosis and ECM degradation of chondrocytes exposed to IL-1β were obstructed. Besides, GRP78 bond to PLXNC1 in IL-1β-treated chondrocytes. The ascending GRP78 expression in the chondrocytes exposed to IL-1β was depleted after PLXNC1 was silenced. Meanwhile, the impacts of PLXNC1 deficiency on the viability, inflammatory response, apoptosis, ECM degradation as well as ERS in IL-1β-exposed chondrocytes were abolished by GRP78 up-regulation.

CONCLUSION

In summary, PLXNC1 silencing might interact with and down-regulate GRP78 to mitigate the apoptosis, inflammation, and ECM degradation of IL-1β-insulted chondrocytes in OA.

Nuclear protein-1 is the common link for pathways activated by aging and obesity in chondrocytes: A potential therapeutic target for osteoarthritis

Pathways leading to osteoarthritis (OA) are diverse depending on the risk factors involved; thus, developing OA therapeutics has been challenging. Here we report that nuclear protein-1 (Nupr1), a stress-inducible protein/transcription factor, is activated by pathways associated with obesity and aging in chondrocytes. Treatment of human chondrocytes with free fatty acids (palmitate and oleate; a model for high-fat diet/obesity) induced PERK signaling and increased expression of caspase-3, TRB3, and Nupr1. On the other hand, treatment of chondrocytes with menadione (oxidative stress inducer) induced oxidation of IRE1, activated antioxidant response (higher Nrf2 expression), and increased expression of Nupr1 and matrix metalloproteinases. Experimental OA was induced by destabilization of the medial meniscus (DMM) in the knee joints of Nupr1+/+ and Nupr1-/- mice. Loss of Nupr1 expression reduced the severity of cartilage lesions in this model. Together, our findings suggest that Nupr1 is a common factor activated by signaling pathways activated by obesity (ER stress) and age (oxidative stress) and a potential drug target for OA resulting from various risk factors.

The Role of Regulated Programmed Cell Death in Osteoarthritis: From Pathogenesis to Therapy

Osteoarthritis (OA) is a worldwide chronic disease that can cause severe inflammation to damage the surrounding tissue and cartilage. There are many different factors that can lead to osteoarthritis, but abnormally progressed programmed cell death is one of the most important risk factors that can induce osteoarthritis. Prior studies have demonstrated that programmed cell death, including apoptosis, pyroptosis, necroptosis, ferroptosis, autophagy, and cuproptosis, has a great connection with osteoarthritis. In this paper, we review the role of different types of programmed cell death in the generation and development of OA and how the different signal pathways modulate the different cell death to regulate the development of OA. Additionally, this review provides new insights into the radical treatment of osteoarthritis rather than conservative treatment, such as anti-inflammation drugs or surgical operation.

Organelle-Specific Mechanisms in Crosstalk between Apoptosis and Ferroptosis

Apoptosis has been extensively studied, whereas ferroptosis is a newly discovered form of regulated cell death that involves iron-dependent accumulations of lipid hydroperoxides. While these two cell death mechanisms were initially believed to be mutually exclusive, recent studies have revealed cellular contexts requiring a balanced interaction between them. Numerous subcellular sites and signaling molecules within these sites are involved in both processes, either as modules or switches that allow cells to choose on how to proceed. The close relationships between apoptosis and ferroptosis, as well as the possibility of switching from one to the other, are described in this review. To understand the crosstalk between apoptosis and ferroptosis, various organelle-specific mechanisms must be analyzed and compared. The ability to switch apoptosis to ferroptosis by targeting cellular organelles has a great potential in cancer therapy.

Endoplasmic Reticulum Stress, Oxidative Stress, and Rheumatic Diseases

BACKGROUND

The endoplasmic reticulum (ER) is a multi-functional organelle responsible for cellular homeostasis, protein synthesis, folding and secretion. It has been increasingly recognized that the loss of ER homeostasis plays a central role in the development of autoimmune inflammatory disorders, such as rheumatic diseases. Purpose/Main contents: Here, we review current knowledge of the contribution of ER stress to the pathogenesis of rheumatic diseases, with a focus on rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). We also review the interplay between protein folding and formation of reactive oxygen species (ROS), where ER stress induces oxidative stress (OS), which further aggravates the accumulation of misfolded proteins and oxidation, in a vicious cycle. Intervention studies targeting ER stress and oxidative stress in the context of rheumatic diseases are also reviewed.

CONCLUSIONS

Loss of ER homeostasis is a significant factor in the pathogeneses of RA and SLE. Targeting ER stress, unfolded protein response (UPR) pathways and oxidative stress in these diseases both in vitro and in animal models have shown promising results and deserve further investigation.

4-Phenylbutyric Acid Plus Valproic Acid Exhibits the Therapeutic and Neuroprotective Effects in Acute Seizures Induced by Pentylenetetrazole

Endoplasmic reticulum (ER) stress and apoptosis are implicated in the pathogenesis of epilepsy. Here we examine the effects of valproic acid (VA) plus 4-phenylbutyric acid (4-PBA) on abnormal electrical brain activity, ER stress and apoptosis in acute seizures induced by pentylenetetrazole (PTZ). Forty male rats were randomly divided into five groups, each consisting of 8 rats as follows: Sham, PTZ, VA+PTZ, 4-PBA+PTZ, and VA plus 4-PBA+PTZ. The treated groups received VA, 4-PBA and VA plus 4-PBA by intraperitoneal application for 7 days prior to PTZ-induced seizure. On the 8th day, acute epileptic seizures were induced by PTZ (50 mg/kg, i.p.) injection, except for the sham group. Then, the seizure stage was observed and ECoG activities were recorded during the 30 min. At 24th post seizures, the hippocampus and blood samples were collected for biochemical and histopathological examinations. Administration of VA plus 4-PBA prior to PTZ-induced seizures significantly decreased seizure stage, the duration of generalized tonic-clonic seizure and the total number of spikes as increased the latency to the first myoclonic jerk when compared to the PTZ group. 4-PBA suppressed the increased levels of ER stress markers GRP78 and CHOP in the hippocampus. VA plus 4-PBA treatment before seizures significantly inhibited PTZ-induced elevations of apoptosis-related indicators caspase-3 and caspase-12, and significantly reduced the number of histopathological lesions of the hippocampus region at 24th post seizures. These findings suggest that administration of VA plus 4-PBA prior to PTZ-induced seizures may be involved in the neuroprotective potential of these agents for seizures.

Targeting Cell Death: Pyroptosis, Ferroptosis, Apoptosis and Necroptosis in Osteoarthritis

New research has shown that the development of osteoarthritis (OA) is regulated by different mechanisms of cell death and types of cytokines. Therefore, elucidating the mechanism of action among various cytokines, cell death processes and OA is important towards better understanding the pathogenesis and progression of the disease. This paper reviews the pathogenesis of OA in relation to different types of cytokine-triggered cell death. We describe the cell morphological features and molecular mechanisms of pyroptosis, apoptosis, necroptosis, and ferroptosis, and summarize the current research findings defining the molecular mechanisms of action between different cell death types and OA.

Endoplasmic Reticulum Stress in Osteoarthritis: A Novel Perspective on the Pathogenesis and Treatment

Osteoarthritis (OA), the most common degenerative joint disease, causes an enormous socioeconomic burden due to its disabling properties and high prevalence. Increasing evidence suggests that OA is a whole-joint disease involving cartilage degradation, synovitis, meniscal lesions, and subchondral bone remodeling. Endoplasmic reticulum (ER) stress is the accumulation of misfolded/unfolded proteins in the ER. Recent studies have found that ER stress is involved in the OA pathological changes by influencing the physiological function and survival of chondrocytes, fibroblast-like synoviocytes, synovial macrophages, meniscus cells, osteoblasts, osteoclasts, osteocytes, and bone marrow mesenchymal stem cells. Therefore, ER stress is an attractive and promising target for OA. However, although targeting ER stress has been proven to alleviate OA progression in vitro and in vivo, the treatments for OA remain in preclinical stage and require further investigation.

Sodium phenylbutyrate inhibits Schwann cell inflammation via HDAC and NFκB to promote axonal regeneration and remyelination

Background

Epigenetic regulation by histone deacetylases (HDACs) in Schwann cells (SCs) after injury facilitates them to undergo de- and redifferentiation processes necessary to support various stages of nerve repair. Although de-differentiation activates the synthesis and secretion of inflammatory cytokines by SCs to initiate an immune response during nerve repair, changes in either the timing or duration of prolonged inflammation mediated by SCs can affect later processes associated with repair and regeneration. Limited studies have investigated the regulatory processes through which HDACs in SCs control inflammatory cytokines to provide a favorable environment for peripheral nerve regeneration.

Methods

We employed the HDAC inhibitor (HDACi) sodium phenylbutyrate (PBA) to address this question in an in vitro RT4 SC inflammation model and an in vivo sciatic nerve transection injury model to examine the effects of HDAC inhibition on the expression of pro-inflammatory cytokines. Furthermore, we assessed the outcomes of suppression of extended inflammation on the regenerative potential of nerves by assessing axonal regeneration, remyelination, and reinnervation.

Results

Significant reductions in lipopolysaccharide (LPS)-induced pro-inflammatory cytokine (tumor necrosis factor-α [TNFα]) expression and secretion were observed in vitro following PBA treatment. PBA treatment also affected the transient changes in nuclear factor κB (NFκB)-p65 phosphorylation and translocation in response to LPS induction in RT4 SCs. Similarly, PBA mediated long-term suppressive effects on HDAC3 expression and activity. PBA administration resulted in marked inhibition of pro-inflammatory cytokine secretion at the site of transection injury when compared with that in the hydrogel control group at 6-week post-injury. A conducive microenvironment for axonal regrowth and remyelination was generated by increasing expression levels of protein gene product 9.5 (PGP9.5) and myelin basic protein (MBP) in regenerating nerve tissues. PBA administration increased the relative gastrocnemius muscle weight percentage and maintained the intactness of muscle bundles when compared with those in the hydrogel control group.

Conclusions

Suppressing the lengthened state of inflammation using PBA treatment favors axonal regrowth and remyelination following nerve transection injury. PBA treatment also regulates pro-inflammatory cytokine expression by inhibiting the transcriptional activation of NFκB-p65 and HDAC3 in SCs in vitro.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02273-1.

Increased uptake of 68Ga-DOTA-FAPI-04 in bones and joints: metastases and beyond

PURPOSE

To describe the uptake of ⁶⁸Gallium-labelled fibroblast activation protein inhibitor (⁶⁸Ga-FAPI) in the bones and joints for better understanding of the role of ⁶⁸Ga-FAPI PET in benign and malignant bone lesions and joint diseases.

METHODS

All 129 ⁶⁸Ga-FAPI PET/MR or PET/CT scans from June 1, 2020, to February 20, 2021, performed at our PET center were retrospectively reviewed. Foci of elevated ⁶⁸Ga-FAPI uptake in the bones and joints were identified. All lesions were divided into malignant and benign diseases. Benign lesions included osteofibrous dysplasia, periodontitis, degenerative bone diseases, arthritis, and other inflammatory or trauma-related abnormalities. The number, locations, and SUVmax of all lesions were recorded and analyzed. The detectability of ⁶⁸Ga-FAPI PET and ¹⁸F-FDG PET in patients who had two scans was also compared.

RESULTS

Elevated uptake of ⁶⁸Ga-FAPI in/around the bones and joints was found in 82 cases (63.57%). A total of 295 lesions were identified, including 94 (31.9%) malignant lesions (all were metastases) and 201 (68.1%) benign lesions. The benign lesions consisted of 13 osteofibrous dysplasia, 48 degenerative bone disease, 33 periodontitis, 56 arthritis, and 51 other inflammatory or trauma-related abnormalities. The spine, shoulder joint, alveolar ridge, and pelvis were the most commonly involved locations. Bone metastases were mainly distributed in the spine, pelvis, and ribs. Among benign diseases, periodontitis and arthritis are site-specific. The mean SUVmax of bone metastases was significantly higher than that of benign diseases (7.14 ± 4.33 vs. 3.57 ± 1.60, p < 0.001), but overlap existed. The differences in SUVmax among subgroups of benign diseases were statistically significant (p < 0.001), with much higher uptake in periodontitis (4.45 ± 1.17). ⁶⁸Ga-FAPI PET identified much more lesions than ¹⁸F-FDG PET (104 vs. 48) with higher uptake value.

CONCLUSION

⁶⁸Ga-FAPI accumulated in both bone metastases and some benign diseases of the bones and joints. Although the uptake of ⁶⁸Ga-FAPI was often higher in bone metastases, this finding cannot be used to distinguish between benign and malignant lesions. ⁶⁸Ga-FAPI PET also has the potential to locate and evaluate the extent of both malignant tumor and benign diseases in bones and joints.

TRIAL REGISTRATION

NCT04554719, NCT04605939. Registered September 8, 2020 and October 25, 2020-retrospectively registered, http://clinicaltrails.gov/show/NCT04554719 ; http://clinicaltrails.gov/show/NCT04605939.

A dual-targeting ruthenium nanodrug that inhibits primary tumor growth and lung metastasis via the PARP/ATM pathway

Background

Many studies have found that ruthenium complexes possess unique biochemical characteristics and inhibit tumor growth or metastasis.

Results

Here, we report the novel dual-targeting ruthenium candidate 2b, which has both antitumor and antimetastatic properties and targets tumor sites through the enhanced permeability and retention (EPR) effect and transferrin/transferrin receptor (TF/TFR) interaction. The candidate 2b is composed of ruthenium-complexed carboline acid and four chloride ions. In vitro, 2b triggered DNA cleavage and thus blocked cell cycle progression and induced apoptosis via the PARP/ATM pathway. In vivo,2b inhibited not only Lewis lung cancer (LLC) tumor growth but also lung metastasis. We detected apoptosis and decreased CD31 expression in tumor tissues, and ruthenium accumulated in the primary tumor tissue of C57BL/6 mice implanted with LLC cells.

Conclusions

Thus, we conclude that 2b targets tumors, inhibits tumor growth and prevents lung metastasis.

Targeting cellular stress in vitro improves osteoblast homeostasis, matrix collagen content and mineralization in two murine models of osteogenesis imperfecta

Most cases of dominantly inherited osteogenesis imperfecta (OI) are caused by glycine substitutions in the triple helical domain of type I collagen α chains, which delay collagen folding, and cause the synthesis of collagen triple helical molecules with abnormal structure and post-translational modification. A variable extent of mutant collagen ER retention and other secondary mutation effects perturb osteoblast homeostasis and impair bone matrix quality. Amelioration of OI osteoblast homeostasis could be beneficial both to osteoblast anabolic activity and to the content of the extracellular matrix they deposit. Therefore, the effect of the chemical chaperone 4-phenylbutyrate (4-PBA) on cell homeostasis, collagen trafficking, matrix production and mineralization was investigated in primary osteoblasts from two murine models of moderate OI, Col1a1+/G349C and Col1a2+/G610C. At the cellular level, 4-PBA prevented intracellular accumulation of collagen and increased protein secretion, reducing aggregates within the mutant cells and normalizing ER morphology. At the extracellular level, increased collagen incorporation into matrix, associated with more mature collagen fibrils, was observed in osteoblasts from both models. 4-PBA also promoted OI osteoblast mineral deposition by increasing alkaline phosphatase expression and activity. Targeting osteoblast stress with 4-PBA improved both cellular and matrix abnormalities in culture, supporting further in vivo studies of its effect on bone tissue composition, strength and mineralization as a potential treatment for classical OI.

Inhibition of Endoplasmic Reticulum Stress by 4-Phenyl Butyric Acid Presents Therapeutic Effects on Periodontitis: Experimental Studies In Vitro and in Rats

This study investigated the probable mechanisms of endoplasmic reticulum (ER) stress involved in periodontitis in vitro and in vivo. We isolated periodontal ligament stem cells from periodontitis patients and healthy controls (P-PDLSCs and H-PDLSCs). To further simulate the periodontal microenvironment in patients, lipopolysaccharide (LPS) was used to treat H-PDLSCs. The results showed that periodontitis-related inflammation gave rise to the upregulated expression levels of ER stress representative genes including GRP78, PERK, ATF4, and CHOP. In contrast, the treatment of 4-phenyl butyric acid (4-PBA) remarkably suppressed ER stress and supported cell viability. The increased secretion of proinflammatory factors like TNF-α, IL-1β, and IL-6 and the activation of NF-κB pathway were also attenuated by 4-PBA treatment. Moreover, 4-PBA treatment restored the impaired osteogenic differentiation ability of PDLSCs, as demonstrated by the upregulated expression levels of Runx2 and OCN as well as the enhanced Alizarin red staining. Local administration of 4-PBA could rescue alveolar bone resorption of LPS-induced periodontitis rats. Thus, our findings suggested ER stress might act as a promising therapeutic target against periodontitis.

Endoplasmic reticulum stress, cell death and tumor: Association between endoplasmic reticulum stress and the apoptosis pathway in tumors (Review)

External and internal stimuli are often involved in the pathogenesis of tumors, and the deterioration of endoplasmic reticulum (ER) function within cells is also an important etiological factor of tumorigenesis resulting in the impairment of the endoplasmic reticulum, which is termed ER stress. The ER is an organelle that serves a crucial role in the process of protein synthesis and maturation, and also acts as a reservoir of calcium to maintain intracellular Ca2+ homeostasis. ER stress has been revealed to serve a critical role in tumorigenesis. In the present review, the association between ER stress‑related pathways and tumor cell apoptosis is examined. Primarily, the role of ER stress in tumor cell apoptosis is discussed, and it is stipulated that ER stress, induced by drugs both directly and indirectly, promotes tumor cell apoptosis.

Review: ER stress-induced cell death in osteoarthritic cartilage

In cartilage, chondrocytes are responsible for the biogenesis and maintenance of the extracellular matrix (ECM) composed of proteins, glycoproteins and proteoglycans. Various cellular stresses, such as hypoxia, nutrient deprivation, oxidative stress or the accumulation of advanced glycation end products (AGEs) during aging, but also translational errors or mutations in cartilage components or chaperone proteins affect the synthesis and secretion of ECM proteins, causing protein aggregates to accumulate in the endoplasmic reticulum (ER). This condition, referred to as ER stress, interferes with cartilage cell homeostasis and initiates the unfolded protein response (UPR), a rescue mechanism to regain cell viability and function. Chronic or irreversible ER stress, however, triggers UPR-initiated cell death. Due to unresolved ER stress in chondrocytes, diseases of the skeletal system, such as chondrodysplasias, arise. ER stress has also been identified as a contributing factor to the pathogenesis of cartilage degeneration processes such as osteoarthritis (OA). This review provides current knowledge about the biogenesis of ECM components in chondrocytes, describes possible causes for the impairment of involved processes and focuses on the ER stress-induced cell death in articular cartilage during OA. Targeting of the ER stress itself or intervention in UPR signaling to reduce death of chondrocytes may be promising for future osteoarthritis therapy.

Tissue plasminogen activator attenuates outflow facility reduction in mouse model of juvenile open angle glaucoma

Tissue plasminogen activator (tPA) has been shown to prevent steroid-induced reduction in aqueous humor outflow facility via an upregulation in matrix metalloproteinase (Mmp) expression. The purpose of this study was to determine whether tPA can rescue outflow facility reduction in the Tg-MYOC^Y437H mouse model, which replicates human juvenile open angle glaucoma. Outflow facility was measured in Tg-MYOC^Y437H mice following: periocular steroid exposure and intraocular protein treatment with enzymatically active or enzymatically inactive tPA. Effects of tPA on outflow facility were compared to those of animals treated with topical sodium phenylbutarate (PBA), a modulator of endoplasmic reticulum stress. Gene expression of fibrinolytic pathway components (Plat, Plau, and Pai-1) and matrix metalloproteinases (Mmp-2, -9, and -13) was determined in angle ring tissues containing the trabecular meshwork. Tg-MYOC^Y437H mice did not display further outflow facility reduction following steroid exposure. Enzymatically active and enzymatically inactive tPA were equally effective in attenuating outflow facility reduction in Tg-MYOC^Y437H mice and caused enhanced expression of matrix metalloproteinases (Mmp-9 and Mmp-13). tPA was equally effective to topical PBA treatment in ameliorating outflow facility reduction in Tg-MYOC^Y437H mice. Both treatments were associated with an upregulation in Mmp-9 expression while tPA also upregulated Mmp-13 expression. tPA increases the expression of matrix metalloproteinases and may cause extracellular matrix remodeling at the trabecular meshwork, which results in reversal of outflow facility reduction in Tg-MYOC^Y437H mice.

Silica nanoparticles induce unfolded protein reaction mediated apoptosis in spermatocyte cells

With increasing air pollution, silica nanoparticles (SiNPs), as a main inorganic member of PM_2.5, have gained increasing attention to its reproductive toxicity. Most existing studies focused on the acute exposure, while data regarding the chronic effect of SiNPs on reproduction is limited. Therefore, this study was designed to evaluate the chronic toxicity of SiNPs on spermatocyte cells. The cells were continuously exposed to SiNPs for 1, 10, 20 and 30 generations at dose of 5 μg/ml SiNPs for 24 h per generation after attachment. The results showed that with the increasing generations of the exposure, SiNPs decreased the viability of spermatocyte cells, induced apoptosis and increased the level of reactive oxygen species in spermatocyte cells. Moreover, SiNPs increased the protein expression of GRP-78, p-PERK, IRE1α, ATF6 and Cleaved caspase-3 in spermatocyte cells, suggesting that SiNPs improved unfolded protein response (UPR) and apoptosis. The present results indicated that the long-term and low-dose exposure to SiNPs could induce apoptosis by triggering ROS-mediated UPR in spermatocyte cells.

Celastrol ameliorates endoplasmic stress-mediated apoptosis of osteoarthritis via regulating ATF-6/CHOP signalling pathway

Objectives

Osteoarthritis (OA) is a common degenerative joint disease with the pathological features of the reduced cartilage cellularity. Celastrol, a compound from Tripterygium wilfordii, exerted therapeutic effects on arthritis, but the potential mechanism remains unclear.

Methods

Tunicamycin was used to establish a model of OA in vitro, and ACLT surgery model in rats was applied to verify the mechanism. Chondrocytes were isolated from the knee articular cartilage of rabbit. MTT and flow cytometry assay were used to detect cell viability and apoptosis rate. Haematoxylin–eosin staining was used to assess for the histopathological changes. The activity and expression of apoptosis-related factors and ERs (endoplasmic reticulum stress)-related factors were detected by ELISA, WB, PCR and IHC, respectively.

Key findings

Celastrol exhibited significant enhancement on cell viability and reduced the rate of apoptosis in Tm-exposed chondrocytes. Celastrol reduced enzyme activity and protein expression of caspase-3, caspase-6 and caspase-9, decreased Bip, Atf6, Chop and Xbp-1 expression both at protein and mRNA levels. Celastrol showed a more significant effect on cell apoptosis rate and mRNA expression in the combination with 4-PBA.

Conclusions

This study reveals that celastrol may prevent OA by inhibiting the ERs-mediated apoptosis. All these might supply beneficial hints for celastrol on OA treatment.

Ablation of PKM2 ameliorated ER stress-induced apoptosis and associated inflammation response in IL-1β-treated chondrocytes via blocking Rspo2-mediated Wnt/β-catenin signaling

Endoplasmic reticulum (ER) stress and the related apoptosis and inflammation damage play key roles in osteoarthritis development. The aim of the present work was to investigate the exact role and potential underlying mechanism of pyruvate kinase M2 (PKM2) in rat chondrocytes exposed to interleukin-Iβ (IL-1β). We observed that IL-1β stimulation resulted in an apparent enhancement in PKM2 expression. Additionally, loss of PKM2 evidently ascended cell viability in response to IL-1β exposure. Simultaneously, elimination of PKM2 manifestly repressed IL-1β-stimulated chondrocyte apoptosis, concomitant with attenuated in the proapoptotic protein markers Bax and cleaved caspase-3, and elevated the antiapoptotic protein Bcl-2. In the meanwhile, knockdown of PKM2 ameliorated ER stress in IL-1β-treated chondrocytes, as evidenced by reduced expression of the ER stress-associated proteins GRP78, CHOP, and cleaved caspase-12. Furthermore, PKM2 silencing protected chondrocytes against IL-1β-triggered inflammatory response, as reflected by the downregulated release of proinflammatory mediators, including tumor necrosis factor-α, IL-6, inducible nitric oxide synthase, cyclooxygenase-2, and prostaglandin E2, as well as decreased nitric oxide generation. More important, abrogating PKM2 expression caused a marked decline in Rspo2 expression, and subsequently blocked Wnt/β-catenin signaling. Mechanistically, the Wnt/β-catenin signaling activator Licl effectively impeded the beneficial effects of PKM2 ablation on IL-1β-stimulated apoptosis and inflammatory response. These findings collectively implicated that PKM2 inhibition protected against ER stress-mediated cell apoptosis and inflammatory injury in rat chondrocytes stimulated with IL-1β by inactivating Rspo2-mediated Wnt/β-catenin pathway, and may represented a novel therapeutic target for osteoarthritis.

4-Phenylbutyric Acid Reduces Endoplasmic Reticulum Stress in Chondrocytes That Is Caused by Loss of the Protein Disulfide Isomerase ERp57

Objective

The integrity of cartilage depends on the correct synthesis of extracellular matrix (ECM) components. In case of insufficient folding of proteins in the endoplasmic reticulum (ER) of chondrocytes, ECM proteins aggregate, ER stress evolves, and the unfolded protein response (UPR) is initiated. By this mechanism, chondrocytes relieve the stress condition or initiate cell death by apoptosis. Especially persistent ER stress has emerged as a pathogenic mechanism in cartilage diseases, such as chondrodysplasias and osteoarthritis. As pharmacological intervention is not available yet, it is of great interest to understand cartilage ER stress in detail and to develop therapeutics to intervene.

Methods

ERp57-deficient chondrocytes were generated by CRISPR/Cas9-induced KO. ER stress and autophagy were studied on mRNA and protein level as well as by transmission electron microscopy (TEM) in chondrocyte micromass or cartilage explant cultures of ERp57 KO mice. Thapsigargin (Tg), an inhibitor of the ER-residing Ca²⁺-ATPase, and 4-Phenylbutyric acid (4-PBA), a small molecular chemical chaperone, were applied to induce or inhibit ER stress.

Results

Our data reveal that the loss of the protein disulfide isomerase ERp57 is sufficient to induce ER stress in chondrocytes. 4-PBA efficiently diffuses into cartilage explant cultures and diminishes excessive ER stress in chondrocytes dose dependently, no matter if it is induced by ERp57 KO or stimulation with Tg.

Conclusion

ER-stress-related diseases have different sources; therefore, various targets for therapeutic treatment exist. In the future, 4-PBA may be used alone or in combination with other drugs for the treatment of ER-stress-related skeletal disorders in patients.