Effect of cadmium on Rho GTPases signal transduction during osteoclast differentiation

The therapeutic mechanism of Compound Lurong Jiangu Capsule for the treatment of cadmium-induced osteoporosis: network pharmacology and experimental verification

Background

Among bone diseases, osteoporosis-like skeleton, such as trabecular thinning, fracture and so on, is the main pathological change of cadmium-induced osteoporosis(Cd-OP), accompanied by brittle bone and increased fracture rate. However, the mechanism underlying cadmium-induced osteoporosis has remained elusive. Compound Lurong Jiangu Capsule (CLJC) is an experienced formula for the treatment of bone diseases, which has the effect of tonifying kidney and strengthening bones, promoting blood circulation and relieving pain.

Objective

Network pharmacology and molecular docking technology combined with experiments were used to investigate the potential mechanism of CLJC in treating Cd-OP.

Method

The active compounds and corresponding targets of each herb in CLJC were searched in the TCMSP and BATMAN-TCM databases. The DisGeNet, OMIM, and GeneCards databases searched for Cd-OP targets. The relationship between both of them was visualized by establishing an herb-compound-target network using Cytoscape 3.9.1 software. Gene ontology (GO), and Kyoto encyclopedia of genes and genomes (KEGG) pathway enrichment analyses were performed after determining the intersection of the targets from CLJC and Cd-OP. What's more, molecular docking was performed to validate the results. All of them were aim to obtain hud signaling pathways for further study. Finally, BAX, BCL-2, and CASPASE-3 were screened and selected for further experiments, which included bone imaging and reconstruction analysis (Micro-CT), hematoxylin-eosin Staining (HE), and western blot (WB).

Results

106 common targets from CLJC and Cd-OP targets were identified. KEGG pathway analysis suggested that multiple signaling pathways, such as the pathways in cancer, may play roles in treatment. Verification of the molecular docking was successful. Here we showed that Cd-OP displayed Tb.Th and Tb.N significantly reduced and even broke, irregular proliferation of bone cortex, uneven and loose trabecular bone arrangement, changed in apoptosis-related proteins, such as significant upregulation of CASPASE-3, BAX protein and significant downregulation of BCL-2 protein in vivo, while CLJC rescued these phenotypes.

Conclusion

This study revealed that CLJC can reduce the expression of apoptosis-related proteins, and multiple components and multiple targets inhibit Cd-OP through apoptosis signaling pathway.

Prolonged Cadmium Exposure and Osteoclastogenesis: A Mechanistic Mouse and in Vitro Study

BACKGROUND

Cadmium (Cd) is a highly toxic and widespread environmental oxidative stressor that causes a myriad of health problems, including osteoporosis and bone damage. Although nuclear factor erythroid 2-related factor 2 (NRF2) and its Cap 'n' Collar and basic region Leucine Zipper (CNC-bZIP) family member nuclear factor erythroid 2-related factor 1 (NRF1) coordinate various stress responses by regulating the transcription of a variety of antioxidant and cytoprotective genes, they play distinct roles in bone metabolism and remodeling. However, the precise roles of both transcription factors in bone loss induced by prolonged Cd exposure remain unclear.

OBJECTIVES

We aimed to understand the molecular mechanisms underlying Cd-induced bone loss, focusing mainly on the roles of NRF2 and NRF1 in osteoclastogenesis provoked by Cd.

METHODS

Male wild-type (WT), global Nrf2-knockout (N r f 2 - / - ) and myeloid-specific Nrf2 knockout [Nrf2(M)-KO] mice were administered Cd (50 or 100  ppm ) via drinking water for 8 or 16 wk, followed by micro-computed tomography, histological analyses, and plasma biochemical testing. Osteoclastogenesis was evaluated using bone marrow-derived osteoclast progenitor cells (BM-OPCs) and RAW 264.7 cells in the presence of Cd (10 or 20  nM ) with a combination of genetic and chemical modulations targeting NRF2 and NRF1.

RESULTS

Compared with relevant control mice, global N r f 2 - / - or Nrf2(M)-KO mice showed exacerbated bone loss and augmented osteoclast activity following exposure to 100  ppm Cd in drinking water for up to 16 wk. In vitro osteoclastogenic analyses suggested that Nrf2-deficient BM-OPCs and RAW 264.7 cells responded more robustly to low levels of Cd (up to 20  nM ) with regard to osteoclast differentiation compared with WT cells. Further mechanistic studies supported a compensatory up-regulation of long isoform of NRF1 (L-NRF1) and subsequent induction of nuclear factor of activated T cells, cytoplasmic, calcineurin dependent 1 (NFATc1) as the key molecular events in the Nrf2 deficiency-worsened and Cd-provoked osteoclastogenesis. L-Nrf1 silenced (via lentiviral means) Nrf2-knockdown (KD) RAW cells exposed to Cd showed dramatically different NFATc1 and subsequent osteoclastogenesis outcomes compared with the cells of Nrf2-KD alone exposed to Cd, suggesting a mitigating effect of the Nrf1 silencing. In addition, suppression of reactive oxygen species by exogenous antioxidants N -acetyl-l-cysteine (2  mM ) and mitoquinone mesylate (MitoQ; 0.2 μ M ) mitigated the L-NRF1-associated effects on NFATc1-driven osteoclastogenesis outcomes in Cd-exposed Nrf2-KD cells.

CONCLUSIONS

This in vivo and in vitro study supported the authors' hypothesis that Cd exposure caused bone loss, in which NRF2 and L-NRF1 responded to Cd and osteoclastogenic stimuli in a cooperative, but contradictive, manner to coordinate Nfatc1 expression, osteoclastogenesis and thus bone homeostasis. Our study suggests a novel strategy targeting NRF2 and L-NRF1 to prevent and treat the bone toxicity of Cd. https://doi.org/10.1289/EHP13849.

METTL14 represses osteoclast formation to ameliorate osteoporosis via enhancing GPX4 mRNA stability

Excessive bone resorption by osteoclasts results in the development of multiple bone disorders including osteoporosis. This study aimed to explore the biological function of methyltransferase-like14 (METTL14) in osteoclast formation, as well as its related mechanisms. Expression levels of METTL14, GPX4 and osteoclast-related proteins TRAP, NFATc1, c-Fos were detected by qRT-PCR and Western blotting. The osteoporosis model was established in mice by bilateral ovariectomy (OVX). Bone histomorphology was determined by micro-CT and H&E staining. NFATc1 expression in bone tissues was determined by immunohistochemical staining. Proliferation of primary bone marrow macrophages cells (BMMs) was assessed by MTT assay. Osteoclast formation was observed by TRAP staining. The regulatory mechanism was evaluated by RNA methylation quantification assay, MeRIP-qPCR, dual luciferase reporter assay, and RIP, respectively. METTL14 was down-regulated in the serum samples of postmenopausal osteoporotic women, which was positively associated with bone mineral density (BMD). Osteoclast formation was promoted in OVX-treated METTL14+/- mice as compared with wild-type littermates. Conversely, METTL14 overexpression repressed RANKL-induced osteoclast differentiation of BMMs. Mechanistically, METTL14-mediated m6A modification post-transcriptionally stabilized glutathione peroxidase 4 (GPX4), with the assistance of Hu-Antigen R (HuR). Finally, GPX4 depletion-mediated osteoclast formation in BMMs could be counteracted by METTL14 or HuR overexpression. Collectively, METTL14 inhibits osteoclastogenesis and bone resorption via enhancing GPX4 stability through an m6A-HuR dependent mechanism. Therefore, targeting METTL14 might be a novel promising treatment strategy for osteoporosis.

Osteoclastogenesis Requires Primary Cilia Disassembly and Can Be Inhibited by Promoting Primary Cilia Formation Pharmacologically

The primary cilium is a solitary, sensory organelle with many roles in bone development, maintenance, and function. In the osteogenic cell lineage, including skeletal stem cells, osteoblasts, and osteocytes, the primary cilium plays a vital role in the regulation of bone formation, and this has made it a promising pharmaceutical target to maintain bone health. While the role of the primary cilium in the osteogenic cell lineage has been increasingly characterized, little is known about the potential impact of targeting the cilium in relation to osteoclasts, a hematopoietic cell responsible for bone resorption. The objective of this study was to determine whether osteoclasts have a primary cilium and to investigate whether or not the primary cilium of macrophages, osteoclast precursors, serves a functional role in osteoclast formation. Using immunocytochemistry, we showed the macrophages have a primary cilium, while osteoclasts lack this organelle. Furthermore, we increased macrophage primary cilia incidence and length using fenoldopam mesylate and found that cells undergoing such treatment showed a significant decrease in the expression of osteoclast markers tartrate-resistant acid phosphatase, cathepsin K, and c-Fos, as well as decreased osteoclast formation. This work is the first to show that macrophage primary cilia resorption may be a necessary step for osteoclast differentiation. Since primary cilia and preosteoclasts are responsive to fluid flow, we applied fluid flow at magnitudes present in the bone marrow to differentiating cells and found that osteoclastic gene expression by macrophages was not affected by fluid flow mechanical stimulation, suggesting that the role of the primary cilium in osteoclastogenesis is not a mechanosensory one. The primary cilium has been suggested to play a role in bone formation, and our findings indicate that it may also present a means to regulate bone resorption, presenting a dual benefit of developing ciliary-targeted pharmaceuticals for bone disease.

Cadmium accelerates autophagy of osteocytes by inhibiting the PI3K/AKT/mTOR signaling pathway

Cadmium (Cd) can damage bone cells and cause osteoporosis. Osteocytes are the most numerous bone cells and also important target cells for Cd-induced osteotoxic damage. Autophagy plays important role in the progression of osteoporosis. However, osteocyte autophagy in Cd-induced bone injury is not well characterized. Thus, we established a Cd-induced bone injury model in BALB/c mice and a cellular damage model in MLO-Y4 cells. Aqueous Cd exposure for 16 months showed an increase in plasma alkaline phosphatase (ALP) activity and increase in urine calcium (Ca) and phosphorus (P) concentrations in vivo. Moreover, expression level of autophagy-related microtubule-associated protein 1A/1B-light chain 3 II (LC3II) and autophagy-related 5 (ATG5) proteins were induced, and the expression of sequestosome-1 (p62) was reduced, along with Cd-induced trabecular bone damage. In addition, Cd inhibited the phosphorylation of mammalian target of rapamycin (mTOR), protein kinase B (AKT), and phosphatidylinositol 3-kinase (PI3K). In vitro, 80 μM Cd concentrations exposure upregulated LC3II protein expression, and downregulated of p62 protein expression. Similarly, we found that treatment with 80 μM Cd resulted in a reduction in the phosphorylation levels of mTOR, AKT, and PI3K. Further experiments revealed that addition of rapamycin, an autophagy inducer, enhanced autophagy and alleviated the Cd-induced damage to MLO-Y4 cells. The findings of our study reveal for the first time that Cd causes damage to both bone and osteocytes, as well as induces autophagy in osteocytes and inhibits PI3K/AKT/mTOR signaling, which could be a protective mechanism against Cd-induced bone injury.