DRG2 Depletion Promotes Endothelial Cell Senescence and Vascular Endothelial Dysfunction

Role of endosomal RANKL-LGR4 signaling during osteoclast differentiation

Leucine-rich repeat-containing G-protein-coupled receptor 4 (LGR4, also known as GPR48) is a membrane receptor that negatively regulates the RANK signaling cascade during osteoclastogenesis. Traditionally, cell signaling and endocytic membrane trafficking via membrane receptors have been considered distinct processes; however, they are now recognized to be closely and bidirectionally linked. The present study investigated the difference between membrane-bound and endosomal LGR4 signaling and whether the LGR4 signaling pathway influences RANK-RANKL signaling during RANKL-induced osteoclastogenesis. We used CRISPR-Cas9 to create LGR4 conditional knock-out (CKO) in RAW 264.7 cells and Drg2 knockout (KO) in mice to study the impacts of LGR4 and DRG2 on osteoclastogenesis. LGR4 was endocytosed into endosomes after binding to RANKL in RAW 264.7 s osteoclast precursor cells. Within the early endosomes, internalized LGR4 activates LGR4-RANKL signaling. When bound to RANKL, LGR4 is endocytosed and localized in the RAB5-positive endosomes. In Lgr4 CKO RAW 264.7 cells, early endosome signaling was increased and the inhibitory phosphorylation of GSK-3β was decreased, both in the whole lysate and endosome fraction. RANKL treatment increased nuclear translocation of NFATC1 in Lgr4 CKO RAW 264.7 cells and Drg2 KO mice. Overall, our results suggested that RANKL-LGR4 signaling is regulated by membrane-to-endosomal trafficking during osteoclastogenesis. KEY MESSAGES: Bone resorption by osteoclasts is essential for bone homeostasis and remodeling. However, the mechanisms underlying the regulation of osteoclastogenesis are not yet fully understood. The present study investigated the difference between membrane-bound and endosomal LGR4 signaling, and whether the LGR4 signaling pathway influences RANK-RANKL signaling during RANKL-induced osteoclastogenesis. Our results suggested that RANKL-LGR4 signaling is regulated by membrane-to-endosomal trafficking during osteoclastogenesis.

Elevated reactive oxygen species can drive the alternative lengthening of telomeres pathway in ATRX-null cancers

The alternative lengthening of telomeres (ALT) pathway is a telomerase-independent mechanism for immortalization in cancer cells and is commonly activated in low-grade and high-grade glioma, as well as osteosarcoma. The ALT pathway can be activated under various conditions and has often been shown to include mutational loss of ATRX. However, this is insufficient in isolation and so other cellular event must also be implicated. It has been shown that excessive accumulation of DNA:RNA hybrid structures (R-loops) and/or formation of DNA-protein crosslinks (DPCs) can be other important driving factors. The underlying cellular events leading to R-loop and DPC formation in ALT cancer cells to date remain unclear. Here, we demonstrate that excessive cellular reactive oxygen species (ROS) is an important causative factor in the evolution of ALT-telomere maintenance in ATRX-deficient glioma. We identified three sources of elevated ROS in ALT-positive gliomas: co-mutation of SETD2, downregulation of DRG2, and hypoxic tumour microenvironment. We demonstrate that elevated ROS leads to accumulation of R-loops and, crucially, resolution of R-loops by the enzyme RNase H1 prevents ALT pathway activity in cells exposed to elevated ROS. Further, we found a possible causal link between the formation of R-loops and the accumulation of DPCs, in particular, formation of TOP1 complexes covalently linked to DNA (Top1cc). We also demonstrate that elevation of ROS can trigger over-activity of the ALT pathway in osteosarcoma and glioma cell lines, resulting in excessive DNA damage and cell death. This work presents important mechanistic insights into the endogenous origin of excessive R-loops and DPCs in ALT-positive cancers, as well as highlighting potential novel therapeutic approaches in these difficult-to-treat cancer types.

DRG2 levels in prostate cancer cell lines predict response to PARP inhibitor during docetaxel treatment

PURPOSE

Developmentally regulated GTP-binding protein 2 (DRG2) regulates microtubule dynamics and G2/M arrest during docetaxel treatment. Poly ADP-ribose polymerase (PARP) acts as an important repair system for DNA damage caused by docetaxel treatment. This study investigated whether DRG2 expression affects response to PARP inhibitors (olaparib) using prostate cancer cell lines PC3, DU145, LNCaP-FGC, and LNCaP-LN3.

MATERIALS AND METHODS

The cell viability and DRG2 expression levels were assessed using colorimetric-based cell viability assay and western blot. Cells were transfected with DRG2 siRNA, and pcDNA6/V5-DRG2 was used to overexpress DRG2. Flow cytometry was applied for cell cycle assay and apoptosis analysis using the Annexing V cell death assay.

RESULTS

The expression of DRG2 was highest in LNCaP-LN3 and lowest in DU145 cells. Expressions of p53 in PC3, DU145, and the two LNCaP cell lines were null-type, high-expression, and medium-expression, respectively. In PC3 (DRG2 high, p53 null) cells, docetaxel increased G2/M arrest without apoptosis; however, subsequent treatment with olaparib promoted apoptosis. In DU145 and LNCaP-FGC (DRG2 low), docetaxel increased sub-G1 but not G2/M arrest and induced apoptosis, whereas olaparib had no additional effect. In LNCaP-LN3 (DRG2 high, p53 wild-type), docetaxel increased sub-G1 and G2/M arrest, furthermore olaparib enhanced cell death. Docetaxel and olaparib combination treatment had a slight effect on DRG2 knockdown PC3, but increased apoptosis in DRG2-overexpressed DU145 cells.

CONCLUSIONS

DRG2 and p53 expressions play an important role in prostate cancer cell lines treated with docetaxel, and DRG2 levels can predict the response to PARP inhibitors.

Hypoxia Combined With Interleukin-17 Regulates Hypoxia-Inducible Factor-1α/Endothelial Nitric Oxide Synthase Expression in Pulmonary Artery Endothelial Cells

The pathogenesis of chronic thromboembolic pulmonary hypertension may be multifactorial and requires further studies. We explored alterations in pulmonary artery endothelial cells under the hypoxic and elevated interleukin-17 conditions that are commonly present in patients with chronic thromboembolic pulmonary hypertension. We measured the serum interleukin-17 levels in 10 chronic thromboembolic pulmonary hypertension patients and 10 healthy control persons. The expressions and localisations of hypoxia-inducible factor-1α and endothelial nitric oxide synthase were detected in tissues. The levels of hypoxia-inducible factor-1α, endothelial nitric oxide synthase, nitric oxide, and reactive oxygen species in cultured pulmonary artery endothelial cells were examined under hypoxia and/or interleukin-17 treatment. The serum interleukin-17 level was increased in chronic thromboembolic pulmonary hypertension patients. Hypoxia-inducible factor-1α was increased, and endothelial nitric oxide synthase was decreased in chronic thromboembolic pulmonary hypertension pulmonary vascular tissue. After receiving the hypoxia combined with interleukin-17 treatment, pulmonary artery endothelial cells showed increased levels of hypoxia-inducible factor-1α and phospho-endothelial nitric oxide synthase (Thr495) (p = 0.001 and 0.063, respectively) and a decreased level of endothelial nitric oxide synthase (p < 0.001). In addition, the nitric oxide level was significantly decreased (p = 0.001), whereas the reactive oxygen species level was insignificantly increased in pulmonary artery endothelial cells. Chronic thromboembolic pulmonary hypertension patients might experience increased inflammation and hypoxia due to dysregulation of the hypoxia-inducible factor-1α/endothelial nitric synthase pathway in pulmonary artery endothelial cells under inflammation and hypoxia, contributing to the pathogenesis of chronic thromboembolic pulmonary hypertension.

Menthol-Modified Quercetin Liposomes with Brain-Targeting Function for the Treatment of Senescent Alzheimer's Disease

Oxidative stress and neuroinflammation in the aging brain are correlated with the development of neurodegenerative diseases, such as Alzheimer's disease (AD). The blood-brain barrier (BBB) poses a significant challenge to the effective delivery of therapeutics for AD. Prior research has demonstrated that menthol (Men) can augment the permeability of the BBB. Consequently, in the current study, we modified Men on the surface of liposomes to construct menthol-modified quercetin liposomes (Men-Qu-Lips), designed to cross the BBB and enhance quercetin (Qu) concentration in the brain for improved therapeutic efficacy. The experimental findings indicate that Men-Qu-Lips exhibited good encapsulation efficiency and stability, successfully crossed the BBB, improved oxidative stress and neuroinflammation in the brains of aged mice, protected neurons, and enhanced their learning and memory abilities.

Age-related secretion of grancalcin by macrophages induces skeletal stem/progenitor cell senescence during fracture healing

Skeletal stem/progenitor cell (SSPC) senescence is a major cause of decreased bone regenerative potential with aging, but the causes of SSPC senescence remain unclear. In this study, we revealed that macrophages in calluses secrete prosenescent factors, including grancalcin (GCA), during aging, which triggers SSPC senescence and impairs fracture healing. Local injection of human rGCA in young mice induced SSPC senescence and delayed fracture repair. Genetic deletion of Gca in monocytes/macrophages was sufficient to rejuvenate fracture repair in aged mice and alleviate SSPC senescence. Mechanistically, GCA binds to the plexin-B2 receptor and activates Arg2-mediated mitochondrial dysfunction, resulting in cellular senescence. Depletion of Plxnb2 in SSPCs impaired fracture healing. Administration of GCA-neutralizing antibody enhanced fracture healing in aged mice. Thus, our study revealed that senescent macrophages within calluses secrete GCA to trigger SSPC secondary senescence, and GCA neutralization represents a promising therapy for nonunion or delayed union in elderly individuals.

Endothelial Senescence in Neurological Diseases

Endothelial cells, which are highly dynamic cells essential to the vascular network, play an indispensable role in maintaining the normal function of the body. Several lines of evidence indicate that the phenotype associated with senescent endothelial cells causes or promotes some neurological disorders. In this review, we first discuss the phenotypic changes associated with endothelial cell senescence; subsequently, we provide an overview of the molecular mechanisms of endothelial cell senescence and its relationship with neurological disorders. For refractory neurological diseases such as stroke and atherosclerosis, we intend to provide some valid clues and new directions for clinical treatment options.

Factors and Pathways Modulating Endothelial Cell Senescence in Vascular Aging

Aging causes a progressive decline in the structure and function of organs. With advancing age, an accumulation of senescent endothelial cells (ECs) contributes to the risk of developing vascular dysfunction and cardiovascular diseases, including hypertension, diabetes, atherosclerosis, and neurodegeneration. Senescent ECs undergo phenotypic changes that alter the pattern of expressed proteins, as well as their morphologies and functions, and have been linked to vascular impairments, such as aortic stiffness, enhanced inflammation, and dysregulated vascular tone. Numerous molecules and pathways, including sirtuins, Klotho, RAAS, IGFBP, NRF2, and mTOR, have been implicated in promoting EC senescence. This review summarizes the molecular players and signaling pathways driving EC senescence and identifies targets with possible therapeutic value in age-related vascular diseases.