Mesoporous Bioglasses Enriched with Bioactive Agents for Bone Repair, with a Special Highlight of María Vallet-Regí's Contribution.

Throughout her impressive scientific career, Prof. María Vallet-Regí opened various research lines aimed at designing new bioceramics, including mesoporous bioactive glasses for bone tissue engineering applications. These bioactive glasses can be considered a spin-off of silica mesoporous materials because they are designed with a similar technical approach. Mesoporous glasses in addition to SiO2 contain significant amounts of other oxides, particularly CaO and P2O5 and therefore, they exhibit quite different properties and clinical applications than mesoporous silica compounds. Both materials exhibit ordered mesoporous structures with a very narrow pore size distribution that are achieved by using surfactants during their synthesis. The characteristics of mesoporous glasses made them suitable to be enriched with various osteogenic agents, namely inorganic ions and biopeptides as well as mesenchymal cells. In the present review, we summarize the evolution of mesoporous bioactive glasses research for bone repair, with a special highlight on the impact of Prof. María Vallet-Regí´s contribution to the field.

Recombinant Proteins-Based Strategies in Bone Tissue Engineering.

The increase in fracture rates and/or problems associated with missing bones due to accidents or various pathologies generates socio-health problems with a very high impact. Tissue engineering aims to offer some kind of strategy to promote the repair of damaged tissue or its restoration as close as possible to the original tissue. Among the alternatives proposed by this specialty, the development of scaffolds obtained from recombinant proteins is of special importance. Furthermore, science and technology have advanced to obtain recombinant chimera's proteins. This review aims to offer a synthetic description of the latest and most outstanding advances made with these types of scaffolds, particularly emphasizing the main recombinant proteins that can be used to construct scaffolds in their own right, i.e., not only to impregnate them, but also to make scaffolds from their complex structure, with the purpose of being considered in bone regenerative medicine in the near future.

Urinary excretion of parathyroid hormone-related protein correlates with renal function in control rats and rats with cisplatin nephrotoxicity.

Parathyroid hormone-related protein (PTHrP) and its receptor are abundantly expressed throughout the renal parenchyma, where PTHrP exerts a modulatory action on renal function. PTHrP upregulation is a common event associated with the mechanism of renal injury and repair. However, no study has yet explored the putative excretion of PTHrP in urine, including its potential relationship with renal function. In the present study, we tested this hypothesis by studying the well-known rat model of acute renal injury induced by the chemotherapeutic agent cisplatin. Using Western blot analysis, we could detect a single protein band, corresponding to intact PTHrP, in the urine of both control and cisplatin-injected rats, whose levels were significantly higher in the latter group. PTHrP was detected in rat urine by dot blot, and its quantification with two specific ELISA kits showed that, compared with control rats, those treated with cisplatin displayed a significant increase in urinary PTHrP (expressed as the PTHrP-to-creatinine ratio or 24-h excretion). In addition, a positive correlation between urinary PTHrP excretion and serum creatinine was found in these animals. In conclusion, our data demonstrate that PTHrP is excreted in rat urine and that this excretion is higher with the decrease of renal function. This suggests that urinary PTHrP levels might be a renal function marker.

Molecular evidence of field cancerization initiated by diabetes in colon cancer patients.

The potential involvement of type 2 diabetes mellitus (T2DM) as a risk factor for colon cancer (CC) has been previously reported. While several clinical studies show a higher incidence of CC and a lower survival rate in diabetics, others report no association. Our own experience indicates that diabetes does not seem to worsen the prognosis once the tumor is present. Despite this controversy, there are no wide-spectrum molecular studies that delve into the impact of T2DM-related mechanisms in colon carcinogenesis. Here, we present a transcriptomic and proteomic profiling of paired tumor and normal colon mucosa samples in a cohort of 42 CC patients, 23 of which have T2DM. We used gene set enrichment and network approaches to extract relevant pathways in diabetics, referenced them to current knowledge, and tested them using in vitro techniques. Through our transcriptomics approach, we identified an unexpected overlap of pathways overrepresented in diabetics compared to nondiabetics, in both tumor and normal mucosa, including diabetes-related metabolic and signaling processes. Proteomic approaches highlighted several cancer-related signaling routes in diabetics found only in normal mucosa, not in tumors. An integration of the transcriptome and proteome analyses suggested the deregulation of key pathways related to colon carcinogenesis which converged on tumor initiation axis TEAD/YAP-TAZ as a potential initiator of the process. In vitro studies confirmed upregulation of this pathway in nontumor colon cells under high-glucose conditions. In conclusion, T2DM associates with deregulation of cancer-related processes in normal colon mucosa adjacent to tissue which has undergone a malignant transformation. These data support that in diabetic patients, the local microenvironment in normal colon mucosa may be a factor driving field cancerization promoting carcinogenesis. Our results set a new framework to study links between diabetes and colon cancer, including a new role of the TEAD/YAP-TAZ complex as a potential driver.

Osteogenic Effect of ZnO-Mesoporous Glasses Loaded with Osteostatin.

Mesoporous Bioactive Glasses (MBGs) are a family of bioceramics widely investigated for their putative clinical use as scaffolds for bone regeneration. Their outstanding textural properties allow for high bioactivity when compared with other bioactive materials. Moreover, their great pore volumes allow these glasses to be loaded with a wide range of biomolecules to stimulate new bone formation. In this study, an MBG with a composition, in mol%, of 80% SiO2⁻15% CaO⁻5% P2O5 (Blank, BL) was compared with two analogous glasses containing 4% and 5% of ZnO (4ZN and 5ZN) before and after impregnation with osteostatin, a C-terminal peptide from a parathyroid hormone-related protein (PTHrP107-111). Zn2+ ions were included in the glass for their bone growth stimulator properties, whereas osteostatin was added for its osteogenic properties. Glasses were characterized, and their cytocompatibility investigated, in pre-osteoblastic MC3T3-E1 cell cultures. The simultaneous additions of osteostatin and Zn2+ ions provoked enhanced MC3T3-E1 cell viability and a higher differentiation capacity, compared with either raw BL or MBGs supplemented only with osteostatin or Zn2+. These in vitro results show that osteostatin enhances the osteogenic effect of Zn2+-enriched glasses, suggesting the potential of this combined approach in bone tissue engineering applications.

Unexpected Bone Formation Produced by RANKL Blockade.

Denosumab (Dmab) is a humanized monoclonal antibody that blocks RANKL (receptor activator for nuclear factor κB ligand), thereby exerting a potent bone antiresorptive action. Dmab treatment leads to a dramatic and sustained increase in bone mass through mechanisms that are currently under debate. It is also a matter of controversy whether this potent action of Dmab could lead to intrabone dystrophic mineralization. Recent research has uncovered a possible anabolic role of Dmab involving RANKL-dependent reverse signaling in osteoblasts, and that bone marrow adipocytes can modulate osteoclastogenesis through the production of RANKL. We comment here on potential pathways which might account for the anabolic action of Dmab. The impact of this proposed mechanism needs to be addressed in further research.

Colon cancer modulation by a diabetic environment: A single institutional experience.

BACKGROUND: Multiple observational studies suggest an increased risk of colon cancer in patients with diabetes mellitus (DM). This can theoretically be the result of an influence of the diabetic environment on carcinogenesis or the tumor biologic behavior. AIM: To gain insight into the influence of a diabetic environment on colon cancer characteristics and outcomes. MATERIAL AND METHODS: Retrospective analysis of clinical records in an academic tertiary care hospital with detailed analysis of 81 diabetic patients diagnosed of colon cancer matched with 79 non-diabetic colon cancer patients. The impact of streptozotocin-induced diabetes on the growth of colon cancer xenografts was studied in mice. RESULTS: The incidence of DM in 1,137 patients with colorectal cancer was 16%. The diabetic colon cancer cases and non-diabetic colon cancer controls were well matched for demographic and clinical variables. The ECOG Scale Performance Status was higher (worse) in diabetics (ECOG ≥1, 29.1% of controls vs 46.9% of diabetics, p = 0.02), but no significant differences were observed in tumor grade, adjuvant therapy, tumor site, lymphovascular invasion, stage, recurrence, death or cancer-related death. Moreover, no differences in tumor variables were observed between patients treated or not with metformin. In the xenograft model, tumor growth and histopathological characteristics did not differ between diabetic and nondiabetic animals. CONCLUSION: Our findings point towards a mild or negligible effect of the diabetes environment on colon cancer behavior, once cancer has already developed.

High glucose alters the secretome of mechanically stimulated osteocyte-like cells affecting osteoclast precursor recruitment and differentiation.

Diabetes mellitus (DM) induces bone deterioration, while mechanical stimulation promotes osteocyte-driven bone formation. We aimed to evaluate the interaction of acute exposure (24 h) to high glucose (HG) with both the pro-survival effect conferred to osteocytic MLO-Y4 cells and osteoblastic MC3T3-E1 cells by mechanical stimulation and the interaction of these cells with osteoclast precursor RAW264.7 cells. We found that 24 h of HG (25 mM) pre-exposure prevented both cell survival and ERK and ß-catenin nuclear translocation upon mechanical stimulation by fluid flow (FF) (10 min) in both MLO-Y4 and MC3T3-E1 cells. However, migration of RAW 264.7 cells was inhibited by MLO-Y4 cell-conditioned medium (CM), but not by MC3T3-E1 cell-CM, with HG or FF. This inhibitory effect was associated with consistent changes in VEGF, RANTES, MIP-1α, MIP-1ß MCP-1, and GM-CSF in MLO-Y4 cell-CM. RAW264.7 proliferation was inhibited by MLO-Y4 CM under static or HG conditions, but it increased by FF-CM with or without HG. In addition, both FF and HG abrogated the capacity of RAW 264.7 cells to differentiate into osteoclasts, but in a different manner. Thus, HG-CM in static condition allowed formation of osteoclast-like cells, which were unable to resorb hydroxyapatite. In contrast, FF-CM prevented osteoclastogenesis even in HG condition. Moreover, HG did not affect basal RANKL or IL-6 secretion or their inhibition induced by FF in MLO-Y4 cells. In conclusion, this in vitro study demonstrates that HG exerts disparate effects on osteocyte mechanotransduction, and provides a novel mechanism by which DM disturbs skeletal metabolism through altered osteocyte-osteoclast communication.

2017 update on the relationship between diabetes and colorectal cancer: epidemiology, potential molecular mechanisms and therapeutic implications.

Worldwide deaths from diabetes mellitus (DM) and colorectal cancer increased by 90% and 57%, respectively, over the past 20 years. The risk of colorectal cancer was estimated to be 27% higher in patients with type 2 DM than in non-diabetic controls. However, there are potential confounders, information from lower income countries is scarce, across the globe there is no correlation between DM prevalence and colorectal cancer incidence and the association has evolved over time, suggesting the impact of additional environmental factors. The clinical relevance of these associations depends on understanding the mechanism involved. Although evidence is limited, insulin use has been associated with increased and metformin with decreased incidence of colorectal cancer. In addition, colorectal cancer shares some cellular and molecular pathways with diabetes target organ damage, exemplified by diabetic kidney disease. These include epithelial cell injury, activation of inflammation and Wnt/ß-catenin pathways and iron homeostasis defects, among others. Indeed, some drugs have undergone clinical trials for both cancer and diabetic kidney disease. Genome-wide association studies have identified diabetes-associated genes (e.g. TCF7L2) that may also contribute to colorectal cancer. We review the epidemiological evidence, potential pathophysiological mechanisms and therapeutic implications of the association between DM and colorectal cancer. Further studies should clarify the worldwide association between DM and colorectal cancer, strengthen the biological plausibility of a cause-and-effect relationship through characterization of the molecular pathways involved, search for specific molecular signatures of colorectal cancer under diabetic conditions, and eventually explore DM-specific strategies to prevent or treat colorectal cancer.

Parathyroid Hormone-Related Protein Protects Osteoblastic Cells From Oxidative Stress by Activation of MKP1 Phosphatase.

Oxidative damage is an important contributor to the morphological and functional changes in osteoporotic bone. Aging increases the levels of reactive oxygen species (ROS) that cause oxidative stress and induce osteoblast apoptosis. ROS modify several signaling responses, including mitogen-activated protein kinase (MAPK) activation, related to cell survival. Both parathyroid hormone (PTH) and its bone counterpart, PTH-related protein (PTHrP), can regulate MAPK activation by modulating MAPK phosphatase-1 (MKP1). Thus, we hypothesized that PTHrP might protect osteoblasts from ROS-induced apoptosis by targeting MKP1. In osteoblastic MC3T3-E1 and MG-63 cells, H2 O2 triggered p38, JNK, ERK and p66Shc phosphorylation, and cell apoptosis. Meanwhile, PTHrP (1-37) rapidly but transiently increased ERK and Akt phosphorylation without affecting p38, JNK, or p66Shc activation. H2 O2 -induced p38 and ERK phosphorylation and apoptosis were both decreased by pre-treatment with specific kinase inhibitors or PTHrP (1-37) in both osteoblastic cell types. These dephosphorylating and prosurvival actions of PTHrP (1-37) were prevented by a phosphatase inhibitor cocktail, the phosphatase MKP1 inhibitor sanguinarine or a MKP1 siRNA. PTHrP (1-37) promptly enhanced MKP1 protein and gene expression and MKP1-dependent catalase activity in osteoblastic cells. Furthermore, exposure to PTHrP (1-37) adsorbed in an implanted hydroxyapatite-based ceramic into a tibial defect in aging rats increased MKP1 and catalase gene expression in the healing bone area. Our findings demonstrate that PTHrP counteracts the pro-apoptotic actions of ROS by a mechanism dependent on MKP1-induced dephosphorylation of MAPKs in osteoblasts. J. Cell. Physiol. 232: 785-796, 2017. © 2016 Wiley Periodicals, Inc.

Anti-senescence and Anti-inflammatory Effects of the C-terminal Moiety of PTHrP Peptides in OA Osteoblasts.

Osteoarthritis (OA) is characterized by degenerative changes in the whole joint leading to physical disability in the elderly population. This condition is associated with altered bone metabolism in subchondral areas suggesting that therapeutic strategies aimed at modifying bone cell metabolism may be of interest. We have investigated the effects of several parathyroid hormone-related protein (PTHrP)-derived peptides (1-37): (N-terminal), (107-111) and (107-139) (C-terminal) on senescence features induced by inflammatory stress in human OA osteoblasts. Incubation of these primary cells with interleukin(IL)-1ß led to an increased expression of senescence markers senescence-associated-ß-galactosidase activity, γH2AX foci, p16, p21, p53, and caveolin-1. PTHrP (107-111) and PTHrP (107-139) significantly reduced all these parameters. Both peptides decreased the production of IL-6 and prostaglandin E2 which was the consequence of cyclo-oxygenase-2 downregulation. PTHrP (107-139) also reduced tumor necrosis factor-α release. These anti-inflammatory effects would be related to the reduction of nuclear factor-κB activation by both peptides and activator protein-1 by PTHrP (107-139). The three PTHrP peptides favored osteoblastic function although the C-terminal domain of PTHrP was more efficient than its N-terminal domain. Our data support an anti-senescence and anti-inflammatory role for the C-terminal moiety of PTHrP with potential applications in chronic inflammatory conditions such as OA.

Molecular mechanisms in bone mechanotransduction.

Bone is one of the most adaptable tissues in the body as it is continuously subjected to load bearing. In fact, mechanical loading is an important regulator of bone mass. The skeleton adjusts to load by changing its mass, shape and microarchitecture, depending on the magnitude of the strain. Mechanical stimulation is necessary for the development of the skeleton, whereas in adults physiological levels of strain help maintain bone mass by reducing bone resorption. On the other hand, an excessive level of strain or bone disuse induces bone loss. Osteocytes are long-lived cells comprising more than 90% of bone cellularity, which are embedded in the bone matrix forming a functional syncytium extending to the bone surface. These cells are considered to be the main bone cells responsible for translating mechanical strain into regulatory signals for osteoblasts and osteoclasts, leading to adapting bone responses to environmental changes. In this review, we discuss the complexity and well-orchestrated events that occur in bone mechanotransduction, focusing on osteocyte viability as an important biological response in this respect. Elucidation of the molecular mechanisms of bone mechanotransduction and the key role of osteocytes is opening new avenues for the treatment of bone loss-related diseases.

Oxidation inhibits PTH receptor signaling and trafficking.

Reactive Oxygen Species (ROS) increase during aging, potentially affecting many tissues including brain, heart, and bone. ROS alter signaling pathways and constitute potential therapeutic targets to limit oxidative damaging effects in aging-associated diseases. Parathyroid hormone receptors (PTHR) are widely expressed and PTH is the only anabolic therapy for osteoporosis. The effects of oxidative stress on PTHR signaling and trafficking have not been elucidated. Here, we used Fluorescence Resonance Energy Transfer (FRET)-based cAMP, ERK, and calcium fluorescent biosensors to analyze the effects of ROS on PTHR signaling and trafficking by live-cell imaging. PTHR internalization and recycling were measured in HEK-293 cells stably transfected with HA-PTHR. PTH increased cAMP production, ERK phosphorylation, and elevated intracellular calcium. Pre-incubation with H2O2 reduced all PTH-dependent signaling pathways. These inhibitory effects were not a result of PTH oxidation since PTH incubated with H2O2 triggered similar responses. PTH promoted internalization and recycling of the PTHR. Both events were significantly reduced by H2O2 pre-incubation. These findings highlight the role of oxidation on PTHR signaling and trafficking, and suggest the relevance of ROS as a putative target in diseases associated with oxidative stress such as age-related osteoporosis.

PTHrP-Derived Peptides Restore Bone Mass and Strength in Diabetic Mice: Additive Effect of Mechanical Loading.

There is an unmet need to understand the mechanisms underlying skeletal deterioration in diabetes mellitus (DM) and to develop therapeutic approaches to treat bone fragility in diabetic patients. We demonstrate herein that mice with type 1 DM induced by streptozotocin exhibited low bone mass, inferior mechanical and material properties, increased bone resorption, decreased bone formation, increased apoptosis of osteocytes, and increased expression of the osteocyte-derived bone formation inhibitor Sost/sclerostin. Further, short treatment of diabetic mice with parathyroid hormone related protein (PTHrP)-derived peptides corrected these changes to levels undistinguishable from non-diabetic mice. In addition, diabetic mice exhibited reduced bone formation in response to mechanical stimulation, which was corrected by treatment with the PTHrP peptides, and higher prevalence of apoptotic osteocytes, which was reduced by loading or by the PTHrP peptides alone and reversed by a combination of loading and PTHrP peptide treatment. In vitro experiments demonstrated that the PTHrP peptides or mechanical stimulation by fluid flow activated the survival kinases ERKs and induced nuclear translocation of the canonical Wnt signaling mediator ß-catenin, and prevented the increase in osteocytic cell apoptosis induced by high glucose. Thus, PTHrP-derived peptides cross-talk with mechanical signaling pathways to reverse skeletal deterioration induced by DM in mice. These findings suggest a crucial role of osteocytes in the harmful effects of diabetes on bone and raise the possibility of targeting these cells as a novel approach to treat skeletal deterioration in diabetes. Moreover, our study suggests the potential therapeutic efficacy of combined pharmacological and mechanical stimuli to promote bone accrual and maintenance in diabetic subjects. © 2016 American Society for Bone and Mineral Research.

Local delivery of parathyroid hormone-related protein-derived peptides coated onto a hydroxyapatite-based implant enhances bone regeneration in old and diabetic rats.

Diabetes mellitus (DM) and aging are associated with bone fragility and increased fracture risk. Both (1-37) N- and (107-111) C-terminal parathyroid hormone-related protein (PTHrP) exhibit osteogenic properties. We here aimed to evaluate and compare the efficacy of either PTHrP (1-37) or PTHrP (107-111) loaded into gelatin-glutaraldehyde-coated hydroxyapatite (HA-Gel) foams to improve bone repair of a transcortical tibial defect in aging rats with or without DM, induced by streptozotocin injection at birth. Diabetic old rats showed bone structural deterioration compared to their age-matched controls. Histological and µ-computerized tomography studies showed incomplete bone repair at 4 weeks after implantation of unloaded Ha-Gel foams in the transcortical tibial defects, mainly in old rats with DM. However, enhanced defect healing, as shown by an increase of bone volume/tissue volume and trabecular and cortical thickness and decreased trabecular separation, occurred in the presence of either PTHrP peptide in the implants in old rats with or without DM. This was accompanied by newly formed bone tissue around the osteointegrated HA-Gel implant and increased gene expression of osteocalcin and vascular endothelial growth factor (bone formation and angiogenic markers, respectively), and decreased expression of Sost gene, a negative regulator of bone formation, in the healing bone area. Our findings suggest that local delivery of PTHrP (1-37) or PTHrP (107-111) from a degradable implant is an attractive strategy to improve bone regeneration in aged and diabetic subjects. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 2060-2070, 2016.

Parathyroid Hormone-Related Protein Analogs as Osteoporosis Therapies.

The only bone anabolic agent currently available for osteoporosis treatment is parathyroid hormone (PTH)-either its N-terminal 1-34 fragment or the whole molecule of 1-84 aminoacids-whose intermittent administration stimulates new bone formation by targeting osteoblastogenesis and osteoblast survival. PTH-related protein (PTHrP) is an abundant factor in bone which shows N-terminal homology with PTH and thus exhibits high affinity for the same PTH type 1 receptor in osteoblasts. Therefore, it is not surprising that intermittently administered N-terminal PTHrP peptides induce bone anabolism in animals and humans. Furthermore, the C-terminal region of PTHrP also elicits osteogenic features in vitro in osteoblastic cells and in various animal models of osteoporosis. In this review, we discuss the current concepts about the cellular and molecular mechanisms whereby PTHrP may induce anabolic actions in bone. Pre-clinical studies and clinical data using N-terminal PTHrP analogs are also summarized, pointing to PTHrP as a promising alternative to current bone anabolic therapies.

Effects of bleaching on osteoclast activity and their modulation by osteostatin and fibroblast growth factor 2.

HYPOTHESIS: Dental bleaching with H2O2 is a common daily practice in dentistry to correct discoloration of anterior teeth. The aim of this study has been to determine whether this treatment of human teeth affects growth, differentiation and activity of osteoclast-like cells, as well as the putative modulatory action of osteostatin and fibroblast growth factor 2 (FGF-2). EXPERIMENTS: Previously to the in vitro assays, structural, physical-chemical and morphological features of teeth after bleaching were studied. Osteoclast-like cells were cultured on human dentin disks, pre-treated or not with 38% H2O2 bleaching gel, in the presence or absence of osteostatin (100 nM) or FGF-2 (1 ng/ml). Cell proliferation and viability, intracellular content of reactive oxygen species (ROS), pro-inflammatory cytokine (IL-6 and TNFα) secretion and resorption activity were evaluated. FINDINGS: Bleaching treatment failed to affect either the structural or the chemical features of both enamel and dentin, except for slight morphological changes, increased porosity in the most superficial parts (enamel), and a moderate increase in the wettability degree. In this scenario, bleaching produced an increased osteoclast-like cell proliferation but decreased cell viability and cytokine secretion, while it augmented resorption activity on dentin. The presence of either osteostatin or FGF-2 reduced the osteoclast-like cell proliferation induced by bleaching. FGF-2 enhanced ROS content, whereas osteostatin decreased ROS but increased TNFα secretion. The bleaching effect on resorption activity was increased by osteostatin, but this effect was less evident with FGF-2. CONCLUSIONS: These findings further confirm the deleterious effects of tooth bleaching by affecting osteoclast growth and function as well as different modulatory actions of osteostatin and FGF-2.

Adverse Effects of Diabetes Mellitus on the Skeleton of Aging Mice.

In the present study, the possibility that a diabetic (DM) status might worsen age-related bone deterioration was explored in mice. Male CD-1 mice aged 2 (young control group) or 16 months, nondiabetic or made diabetic by streptozotocin injections, were used. DM induced a decrease in bone volume, trabecular number, and eroded surface, and in mineral apposition and bone formation rates, but an increased trabecular separation, in L1-L3 vertebrae of aged mice. Three-point bending and reference point indentation tests showed slight changes pointing to increased frailty and brittleness in the mouse tibia of diabetic old mice. DM was related to a decreased expression of both vascular endothelial growth factor and its receptor 2, which paralleled that of femoral vasculature, and increased expression of the pro-adipogenic gene peroxisome proliferator-activated receptor γ and adipocyte number, without affecting ß-catenin pathway in old mouse bone. Concomitant DM in old mice failed to affect total glutathione levels or activity of main anti-oxidative stress enzymes, although xanthine oxidase was slightly increased, in the bone marrow, but increased the senescence marker caveolin-1 gene. In conclusion, DM worsens bone alterations of aged mice, related to decreased bone turnover and bone vasculature and increased senescence, independently of the anti-oxidative stress machinery.

Oxidative stress, autophagy, epigenetic changes and regulation by miRNAs as potential therapeutic targets in osteoarthritis.

Aging is a natural process characterized by the declining ability of the different organs and tissues to respond to stress, increasing homeostatic imbalance and risk of disease. Osteoarthritis (OA) is a multifactorial disease in which cartilage degradation is a central feature. Aging is the main risk factor for OA. In OA cartilage, a decrease in the number of chondrocytes and in their ability to regenerate the extracellular matrix and adequately respond to stress has been described. OA chondrocytes show a senescence secretory phenotype (SSP) consisting on the overproduction of cytokines (interleukins 1 and 6), growth factors (e.g., epidermal growth factor) and matrix metalloproteinases (MMP) (e.g., MMP-3, MMP-13). Reactive Oxygen Species (ROS) play a major role in the induction of the SSP. In chondrocytes, an increase in ROS production leads to hyper-peroxidation, protein carbonylation and DNA damage which alter chondrocyte function. ROS overproduction also induces changes in metabolic pathways such as PI3K-Akt and ERK. Autophagy is a key mechanism for maintaining cell homeostasis by adjusting cell metabolism to nutrient supply and removing damaged organelles. In cartilage, aging-related loss of autophagy leads to cell death and OA, while stimulation of autophagy exerts protective effects on cartilage deterioration. Aging also interferes with epigenetic mechanisms such as activity of histone acetylases that control the pattern of DNA methylation, and induces up- or down-regulation of microRNAs expression. A deeper knowledge of the mechanisms involved in chondrocyte aging could identify potential targets for the treatment of OA, a prevalent and therapeutic-orphan disease.