Hypoxia, RONS and energy metabolism in articular cartilage

Metabolic Health, Mitochondrial Fitness, Physical Activity, and Cancer

Cancer continues to be a significant global health issue. Traditional genetic-based approaches to understanding and treating cancer have had limited success. Researchers are increasingly exploring the impact of the environment, specifically inflammation and metabolism, on cancer development. Examining the role of mitochondria in this context is crucial for understanding the connections between metabolic health, physical activity, and cancer. This study aimed to review the literature on this topic through a comprehensive narrative review of various databases including MedLine (PubMed), Cochrane (Wiley), Embase, PsychINFO, and CinAhl. The review highlighted the importance of mitochondrial function in overall health and in regulating key events in cancer development, such as apoptosis. The concept of "mitochondrial fitness" emphasizes the crucial role of mitochondria in cell metabolism, particularly their oxidative functions, and how proper function can prevent replication errors and regulate apoptosis. Engaging in high-energy-demanding movement, such as exercise, is a powerful intervention for improving mitochondrial function and increasing resistance to environmental stressors. These findings support the significance of considering the role of the environment, specifically inflammation and metabolism, in cancer development and treatment. Further research is required to fully understand the mechanisms by which physical activity improves mitochondrial function and potentially reduces the risk of cancer.

Time-dependently Appeared Microenvironmental Changes and Mechanism after Cartilage or Joint Damage and the Influences on Cartilage Regeneration

Cartilage and joint damage easily degenerates cartilage and turns into osteoarthritis (OA), which seriously affects human life and work, and has no cure currently. The temporal and spatial changes of multiple microenvironments upon the damage of cartilage and joint are noticed, including the emergences of inflammation, bone remodeling, blood vessels, and nerves, as well as alterations of extracellular and pericellular matrix, oxygen tension, biomechanics, underneath articular cartilage tissues, and pH value. This review summarizes the existing literatures on microenvironmental changes, mechanisms, and their negative effects on cartilage regeneration following cartilage and joint damage. We conclude that time-dependently rebuilding the multiple normal microenvironments of damaged cartilage is the key for cartilage regeneration after systematic studies for the timing and correlations of various microenvironment changes.

Control of the Autophagy Pathway in Osteoarthritis: Key Regulators, Therapeutic Targets and Therapeutic Strategies

Autophagy is involved in different degenerative diseases and it may control epigenetic modifications, metabolic processes, stem cells differentiation as well as apoptosis. Autophagy plays a key role in maintaining the homeostasis of cartilage, the tissue produced by chondrocytes; its impairment has been associated to cartilage dysfunctions such as osteoarthritis (OA). Due to their location in a reduced oxygen context, both differentiating and mature chondrocytes are at risk of premature apoptosis, which can be prevented by autophagy. AutophagomiRNAs, which regulate the autophagic process, have been found differentially expressed in OA. AutophagomiRNAs, as well as other regulatory molecules, may also be useful as therapeutic targets. In this review, we describe and discuss the role of autophagy in OA, focusing mainly on the control of autophagomiRNAs in OA pathogenesis and their potential therapeutic applications.

Adrenomedullin and truncated peptide adrenomedullin(22-52) affect chondrocyte response to apoptotis in vitro: downregulation of FAS protects chondrocyte from cell death

Chondrocyte apoptosis may have a pivotal role in the development of osteoarthritis. Interest has increased in the use of anti-apoptotic compounds to protect against osteoarthritis development. In this work, we investigated the effect of adrenomedullin (AM), a 52 amino-acid hormone peptide, and a 31 amino-acid truncated form, AM(22-52), on chondrocyte apoptosis. Bovine articular chondrocytes (BACs) were cultured under hypoxic conditions to mimic cartilage environment and then treated with Fas ligand (Fas-L) to induce apoptosis. The expression of AM and its calcitonin receptor-like receptor (CLR)/receptor activity-modifying protein (RAMP) (receptor/co-receptor) was assessed by immunostaining. We evaluated the effect of AM and AM(22-52) on Fas-L-induced chondrocyte apoptosis. FAS expression was appreciated by RT-qPCR and immunostainings. The expression of hypoxia-inducible factor 1α (HIF-1α), CLR and one co-receptor (RAMP2) was evidenced. With BACs under hypoxia, cyclic adenosine monophosphate production increased dose-dependently with AM stimulation. AM significantly decreased caspase-3 activity (mean 35% decrease; p = 0.03) as a marker of Fas-L-induced apoptosis. Articular chondrocytes treated with AM showed significantly reduced cell death, along with downregulated Fas expression and production, as compared with AM(22-52). AM decreased articular chondrocyte apoptosis by downregulating a Fas receptor. These findings may pave the way for novel therapeutic approaches in osteoarthritis.

The regulatory effects of miR-138-5p on selenium deficiency-induced chondrocyte apoptosis are mediated by targeting SelM

Apoptosis is a common paradigm of cell death and plays a key role in cartilage damage and selenium (Se) deficiency. Selenoproteins play major roles in determining the biological effects of Se, and are potentially involved in the pathophysiological processes in bone tissue. MicroRNAs (miRNAs) play important roles in cell proliferation, differentiation, apoptosis and tumorigenesis. Based on the preliminary results, the expression of selenoprotein M (SelM) was significantly decreased (69%) in chicken cartilage tissues with Se deficiency, and we subsequently screened and verified that SelM is one of the target genes of miR-138-5p in chicken cartilage using a dual luciferase reporter assay and real-time quantitative PCR (qRT-PCR). The expression of miR-138-5p was increased in response to Se deficiency, and the overexpression of miR-138-5p increased caspase-3, caspase-9, BAX and BAK levels, while the BCL-2 level was decreased, suggesting that miR-138-5p induced apoptosis via the mitochondrial pathway in vivo and in vitro. We explored whether oxidative stress, mitochondrial fission and fusion, and energy metabolism might trigger apoptosis to obtain an understanding of the mechanisms underlying the effects of miR-138-5p on Se deficiency-induced apoptosis in cartilage. The levels of indicators of oxidative stress, mitochondrial dynamics and energy metabolism were changed as well. This study confirmed that SelM is one of the target genes of miR-138-5p, and the overexpression of miR-138-5p induced by Se deficiency triggered oxidative stress, an imbalance in mitochondrial fission and fusion, and energy metabolism dysfunction. Therefore, miR-138-5p is involved in the mitochondrial apoptosis pathway via targeting SelM in chicken chondrocytes.

Deferoxamine Suppresses Collagen Cleavage and Protease, Cytokine, and COL10A1 Expression and Upregulates AMPK and Krebs Cycle Genes in Human Osteoarthritic Cartilage

This study reports the effects of the iron chelator deferoxamine (DFO) on collagen cleavage, inflammation, and chondrocyte hypertrophy in relation to energy metabolism-related gene expression in osteoarthritic (OA) articular cartilage. Full-depth explants of human OA knee articular cartilage from arthroplasty were cultured with exogenous DFO (1–50 μM). Type II collagen cleavage and phospho-adenosine monophosphate-activated protein kinase (pAMPK) concentrations were measured using ELISAs. Gene expression studies employed real-time PCR and included AMPK analyses in PBMCs. In OA explants collagen cleavage was frequently downregulated by 10–50 μM DFO. PCR analysis of 7 OA patient cartilages revealed that 10 μM DFO suppressed expression of MMP-1, MMP-13, IL-1β, and TNFα and a marker of chondrocyte hypertrophy, COL10A1. No changes were observed in the expression of glycolysis-related genes. In contrast, expressions of genes associated with the mitochondrial Krebs cycle (TCA), AMPK, HIF1α, and COL2A1 were upregulated. AMPK gene expression was reduced in OA cartilage and increased in PBMCs from the same patients compared to healthy controls. Our studies demonstrate that DFO is capable of suppressing excessive collagenase-mediated type II collagen cleavage in OA cartilage and reversing phenotypic changes. The concomitant upregulation of proanabolic TCA-related gene expressions points to a potential for availability of energy generating substrates required for matrix repair by end-stage OA chondrocytes. This might normally be prevented by high whole-body energy requirements indicated by elevated AMPK expression in PBMCs of OA patients.

Glucose gradients influence zonal matrix deposition in 3D cartilage constructs

Reproducing the native collagen structure and glycosaminoglycan (GAG) distribution in tissue-engineered cartilage constructs is still a challenge. Articular cartilage has a specific nutrient supply and mechanical environment due to its location and function in the body. Efforts to simulate this native environment have been reported through the use of bioreactor systems. However, few of these devices take into account the existence of gradients over cartilage as a consequence of the nutrient supply by diffusion. We hypothesized that culturing chondrocytes in an environment, in which gradients of nutrients can be mimicked, would induce zonal differentiation. Indeed, we show that glucose gradients facilitating a concentration distribution as low as physiological glucose levels enhanced a zonal chondrogenic capacity similar to the one found in native cartilage. Furthermore, we found that the glucose consumption rates of cultured chondrocytes were higher under physiological glucose concentrations and that GAG production rates were highest in 5 mM glucose. From these findings, we concluded that this condition is better suited for matrix deposition compared to 20 mM glucose standard used in a chondrocyte culture system. Reconsidering the culture conditions in cartilage tissue engineering strategies can lead to cartilaginous constructs that have better mechanical and structural properties, thus holding the potential of further enhancing integration with the host tissue.

Mechanical overloading causes mitochondrial superoxide and SOD2 imbalance in chondrocytes resulting in cartilage degeneration

Mechanical stress and aging are major risk factors of cartilage degeneration. Human studies have previously reported that oxidative damage increased, while SOD2 protein was reciprocally downregulated in osteoarthritic degenerated cartilage. However, it remains unclear whether mitochondrial superoxide imbalance in chondrocytes causes cartilage degeneration. We herein demonstrate that mechanical loading promoted mitochondrial superoxide generation and selective Sod2 downregulation in chondrocytes in vivo and that mitochondrial superoxide inducer also downregulated Sod2 expression in chondrocytes in vitro. A genetically manipulated model revealed that Sod2 deficiency in chondrocytes also resulted in mitochondrial superoxide overproduction and dysfunction, thus leading to cartilage degeneration. Intra-articular injection of a permeable antioxidant effectively suppressed the mechanical loading-induced mitochondrial superoxide generation and cartilage degeneration in mice. Our findings demonstrate that mitochondrial superoxide plays a pivotal role in the development and progression of osteoarthritis, and the mitochondrial superoxide balance may therefore be a promising target for the treatment of cartilage degeneration.

Oxidative stress-induced expression of HSP70 contributes to the inhibitory effect of 15d-PGJ2 on inducible prostaglandin pathway in chondrocytes

The inhibitory effect of 15-deoxy-Δ(12,14)-prostaglandin J2 (15d-PGJ2) on proinflammatory gene expression has been extensively documented and frequently ascribed to its ability to prevent NF-κB pathway activation. We and others have previously demonstrated that it was frequently independent of the peroxisome proliferator activated receptor (PPAR)γ activation. Here, we provide evidence that induction of intracellular heat shock protein (HSP)70 by oxidative stress is an additional regulatory loop supporting the anti-inflammatory effect of 15d-PGJ2 in chondrocytes. Using real-time quantitative PCR and Western blotting, we showed that 15d-PGJ2 stimulated HSP70, but not HSP27 expression while increasing oxidative stress as measured by spectrofluorimetry and confocal spectral imaging. Using N-acetylcysteine (NAC) as an antioxidant, we demonstrated further that oxidative stress was thoroughly responsible for the increased expression of HSP70. Finally, using an HSP70 antisense strategy, we showed that the inhibitory effect of 15d-PGJ2 on IL-1-induced activation of the NF-κB pathway, COX-2 and mPGES-1 expression, and PGE2 synthesis was partly supported by HSP70. These data provide a new anti-inflammatory mechanism to support the PPARγ-independent effect of 15d-PGJ2 in chondrocyte and suggest a possible feedback regulatory loop between oxidative stress and inflammation via intracellular HSP70 up-regulation. This cross talk is consistent with 15d-PGJ2 as a putative negative regulator of the inflammatory reaction.

Hypoxia promotes the production and inhibits the destruction of human articular cartilage

OBJECTIVE

To determine the effects of hypoxia on both anabolic and catabolic pathways of metabolism in human articular cartilage and to elucidate the roles played by hypoxia-inducible factors (HIFs) in these responses.

METHODS

Normal human articular cartilage from a range of donors was obtained at the time of above-the-knee amputations due to sarcomas not involving the joint space. Fresh cartilage tissue explants and isolated cells were subjected to hypoxia and treatment with interleukin-1α. Cell transfections were performed on isolated human chondrocytes.

RESULTS

Using chromatin immunoprecipitation, we found that hypoxia induced cartilage production in human tissue explants through direct binding of HIF-2α to a specific site in the master-regulator gene SOX9. Importantly, hypoxia also suppressed spontaneous and induced destruction of human cartilage in explant culture. We found that anticatabolic responses were predominantly mediated by HIF-1α. Manipulation of the hypoxia-sensing pathway through depletion of HIF-targeting prolyl hydroxylase-containing protein 2 (PHD-2) further enhanced cartilage responses as compared to hypoxia alone. Hypoxic regulation of tissue-specific metabolism similar to that in human cartilage was observed in pig, but not mouse, cartilage.

CONCLUSION

We found that resident chondrocytes in human cartilage are exquisitely adapted to hypoxia and use it to regulate tissue-specific metabolism. Our data revealed that while fundamental regulators, such as SOX9, are key molecules both in mice and humans, the way in which they are controlled can differ. This is all the more important since it is upstream regulators such as this that need to be directly targeted for therapeutic benefit. HIF-specific hydroxylase PHD-2 may represent a relevant target for cartilage repair.

Mitochondrial dysfunction and the inflammatory response

Inflammation has been linked to multiple degenerative and acute diseases as well as the aging process. Moreover, mitochondrial alterations play a central role in these processes. Mitochondria have an important role in pro-inflammatory signaling; similarly, pro-inflammatory mediators may also alter mitochondrial function. Both of these processes increase mitochondrial oxidative stress, promoting a vicious inflammatory cycle. Additionally, damage-associated molecular patterns derived from mitochondria could contribute to inflammasome formation and caspase-1 activation, while alterations in mitochondrial autophagy may cause inflammation. Strategies aimed at controlling excessive oxidative stress within mitochondria may represent both preventive and therapeutic interventions in inflammation.

Autophagy in the immune response to tuberculosis: clinical perspectives

A growing body of evidence points to autophagy as an essential component in the immune response to tuberculosis. Autophagy is a direct mechanism of killing intracellular Mycobacterium tuberculosis and also acts as a modulator of proinflammatory cytokine secretion. In addition, autophagy plays a key role in antigen processing and presentation. Autophagy is modulated by cytokines; it is stimulated by T helper type 1 (Th1) cytokines such as tumour necrosis factor (TNF)-α and interferon (IFN)-γ, and is inhibited by the Th2 cytokines interleukin (IL)-4 and IL-13 and the anti-inflammatory cytokine IL-10. Vitamin D, via cathelicidin, can also induce autophagy, as can Toll-like receptor (TLR)-mediated signals. Autophagy-promoting agents, administered either locally to the lungs or systemically, could have a clinical application as adjunctive treatment of drug-resistant and drug-sensitive tuberculosis. Moreover, vaccines which effectively induce autophagy could be more successful in preventing acquisition or reactivation of latent tuberculosis.