The adaptor protein p62 is involved in RANKL-induced autophagy and osteoclastogenesis

SAPHO, autophagy, IL-1, FoxO1, and Propionibacterium (Cutibacterium) acnes

Overt infection by Propionibacterium acnes is lacking in many SAPHO syndromes, and antibiotics have only a transient and incomplete effect, either in SAPHO syndrome or acne. As several auto-inflammatory bone disorders sharing overproduction of IL-1β can mimic SAPHO, this syndrome could partly depend on genetically encoded overproduction of IL-1β. However, cyclic intracellular infections, mostly by P. acnes, can contribute to the enhanced IL-1β release by some skin cells, and probably by bone cells. P. acnes is indeed a powerful trigger of NLRP3-inflammasome activation and IL-1β, leading to osteitis and enhanced mesenchymal cells differentiation in osteoblasts. Recent advances in the understanding of acne suggest that first steps of this disorder are not driven by P. acnes, but by a relative deficiency of FoxO1 within the nucleus of sebaceous cells. A similar defect of FoXO1 in bone cells should also be sought in SAPHO, since repression of FoxO1 gene is found in lesional psoriasis skin, and is associated with an increased number of osteoblasts and high bone mass in mice. FoxO1 selectively promotes IL-1β production, so that its downregulation could help some P. acnes t escape innate immunity and persist in a latent state in bone cells, including mesenchymal stem cells. However, P. acnes itself possibly contributes to FoxO1 downregulation, like H. pylori infection which induces nuclear inactivation of FoxO1 in human gastric cells to slow down autophagic clearance. As bisphosphonates, which often improve SAPHO syndromes, enhance autophagy, it may be worth testing whether their combination with antibiotics is synergistic in SAPHO syndromes.

Role of p62 in the suppression of inflammatory cytokine production by adiponectin in macrophages: Involvement of autophagy and p21/Nrf2 axis

Adiponectin possesses potent anti-inflammatory properties. p62, an adaptor protein composed of multi-functional domain, is known to play a role in controlling inflammatory responses. In the present study, we examined the role of p62 in suppressing inflammatory cytokines produced by globular adiponectin (gAcrp) and the potential underlying mechanisms in macrophages. We demonstrated that gAcrp significantly increased p62 expression. Knockdown of p62 abrogated the suppressive effects of gAcrp on LPS-stimulated TNF-α and IL-1β expression and TRAF6/p38 MAPK pathway, indicating that p62 signaling is critical for suppressing inflammatory cytokines production by gAcrp. We next examined the role of p62 in gAcrp-induced autophagy activation, because autophagy has been shown to play a pivotal role in suppressing TNF-α. Herein, we observed that gene silencing of p62 prevented gAcrp-induced increases in autophagy-related genes and autophagosome formation. In addition, we found that Nrf2 knockdown prevented gAcrp-induced p62 expression, and p21 knockdown prevented Nrf2 induction, suggesting the role of p21/Nrf2 axis in gAcrp-induced p62 expression. Taken together, these findings imply that p62 signaling plays a crucial role in suppressing inflammatory cytokine production by globular adiponectin in macrophages, at least in part, through autophagy induction. Furthermore, the p21/Nrf2 signaling cascade contributes to p62 induction by globular adiponectin.

Natural uranium impairs the differentiation and the resorbing function of osteoclasts

BACKGROUND

Uranium is a naturally occurring radionuclide ubiquitously present in the environment. The skeleton is the main site of uranium long-term accumulation. While it has been shown that natural uranium is able to perturb bone metabolism through its chemical toxicity, its impact on bone resorption by osteoclasts has been poorly explored. Here, we examined for the first time in vitro effects of natural uranium on osteoclasts.

METHODS

The effects of uranium on the RAW 264.7 monocyte/macrophage mouse cell line and primary murine osteoclastic cells were characterized by biochemical, molecular and functional analyses.

RESULTS

We observed a cytotoxicity effect of uranium on osteoclast precursors. Uranium concentrations in the μM range are able to inhibit osteoclast formation, mature osteoclast survival and mineral resorption but don't affect the expression of the osteoclast gene markers Nfatc1, Dc-stamp, Ctsk, Acp5, Atp6v0a3 or Atp6v0d2 in RAW 274.7 cells. Instead, we observed that uranium induces a dose-dependent accumulation of SQSTM1/p62 during osteoclastogenesis.

CONCLUSIONS

We show here that uranium impairs osteoclast formation and function in vitro. The decrease in available precursor cells, as well as the reduced viability of mature osteoclasts appears to account for these effects of uranium. The SQSTM1/p62 level increase observed in response to uranium exposure is of particular interest since this protein is a known regulator of osteoclast formation. A tempting hypothesis discussed herein is that SQSTM1/p62 dysregulation contributes to uranium effects on osteoclastogenesis.

GENERAL SIGNIFICANCE

We describe cellular and molecular effects of uranium that potentially affect bone homeostasis.

Autophagy in the pathogenesis of ankylosing spondylitis

The pathogenesis of ankylosing spondylitis (AS) is not well understood, and treatment options have met with limited success. Autophagy is a highly conserved mechanism of controlled digestion of damaged organelles within a cell. It helps in the maintenance of cellular homeostasis. The process of autophagy requires the formation of an isolation membrane. They form double-membraned vesicles called "autophagosomes" that engulf a portion of the cytoplasm. Beyond the role in maintenance of cellular homeostasis, autophagy has been demonstrated as one of the most remarkable tools employed by the host cellular defense against bacteria invasion. Autophagy also affects the immune system and thus is implicated in several rheumatic disease processes. In this article, we explore the potential role of autophagy in the pathogenesis of AS.

P62: An emerging oncotarget for osteolytic metastasis

Bone metastasis occurs in the majority of late-stage tumors with poor prognosis. It is mainly classified as osteoblastic metastasis and osteolytic metastasis. The pathogenesis of osteolytic metastasis is a “vicious cycle” between tumor cells and bone cells (primarily the osteoclasts), which is mediated by secretory factors. The P62 adapter protein is a versatile multitasker between tumor cells and bone cells. The overexpression of P62 has been detected among a variety of tumors, playing positive roles in both tumorigenesis and metastasis. Moreover, P62 is an important modulator of the osteoclastogenesis pathway. Therefore, the ability of P62 to modulate tumors and osteoclasts suggests that it may be a feasible oncotarget for bone metastasis, especially for osteolytic metastasis. Recent research has shown that a P62 DNA vaccine triggered effective anti-tumor, anti-metastatic and anti-osteoporotic activities. Growing lines of evidence point to P62 as an emerging oncotarget for osteolytic metastasis. In this review, we outline the different roles of P62 in tumor cells and osteoclasts, focusing on the P62-related signaling pathway in key steps of osteolytic metastasis, including tumorigenesis, metastasis and osteoclastogenesis. Finally, we discuss the newest observations on P62 as an oncotarget for osteolytic metastasis treatment.

Production and Functional Characterization of Murine Osteoclasts Differentiated from ER-Hoxb8-Immortalized Myeloid Progenitor Cells

In vitro differentiation into functional osteoclasts is routinely achieved by incubation of embryonic stem cells, induced pluripotent stem cells, or primary as well as cryopreserved spleen and bone marrow-derived cells with soluble receptor activator of nuclear factor kappa-B ligand and macrophage colony-stimulating factor. Additionally, osteoclasts can be derived from co-cultures with osteoblasts or by direct administration of soluble receptor activator of nuclear factor kappa-B ligand to RAW 264.7 macrophage lineage cells. However, despite their benefits for osteoclast-associated research, these different methods have several drawbacks with respect to differentiation yields, time and animal consumption, storage life of progenitor cells or the limited potential for genetic manipulation of osteoclast precursors. In the present study, we therefore established a novel protocol for the differentiation of osteoclasts from murine ER-Hoxb8-immortalized myeloid stem cells. We isolated and immortalized bone marrow cells from wild type and genetically manipulated mouse lines, optimized protocols for osteoclast differentiation and compared these cells to osteoclasts derived from conventional sources. In vitro generated ER-Hoxb8 osteoclasts displayed typical osteoclast characteristics such as multi-nucleation, tartrate-resistant acid phosphatase staining of supernatants and cells, F-actin ring formation and bone resorption activity. Furthermore, the osteoclast differentiation time course was traced on a gene expression level. Increased expression of osteoclast-specific genes and decreased expression of stem cell marker genes during differentiation of osteoclasts from ER-Hoxb8-immortalized myeloid progenitor cells were detected by gene array and confirmed by semi-quantitative and quantitative RT-PCR approaches. In summary, we established a novel method for the quantitative production of murine bona fide osteoclasts from ER-Hoxb8 stem cells generated from wild type or genetically manipulated mouse lines. These cells represent a standardized and theoretically unlimited source for osteoclast-associated research projects.

Recent insights into the role of autophagy in the pathogenesis of rheumatoid arthritis

Autophagy appears to play a dual role in eukaryotic cells. It manifests cytoprotective effects through the regulation of catabolic processes and the clearance of pathogens; however, a correlation between autophagy and the pathogenesis of autoimmune/autoinflammatory conditions has recently been described. Autophagy has emerged as a mediator in the pathogenesis of RA. Autophagy may regulate apoptosis resistance and hyperplasia in synovial fibroblasts, promote osteoclastogenesis and stimulate osteoclast-mediated bone resorption through the delivery of citrullinated peptides to MHC compartments, which results in the activation of the innate and adaptive immune response, thereby resulting in RA. Given the likely importance of autophagy in the pathogenesis of RA, here we reviewed the detailed mechanisms concerning the pathogenicity of autophagy and autophagy proteins in RA.