Introduction

Osteoarthritis (OA) affects at least 27 million Americans, and is the leading cause of disability in the United States. OA is characterized by the breakdown and loss of articular cartilage, which leads to chronic pain during joint movement. The cause of this disease is not known, but aging is the most influential risk factor for developing OA. Whereas 7.6% of the 18- to 44-year-old age group, and 29.8% of the 45- to 64-year-old age group report doctor-diagnosed arthritis, 50% of persons ages 65 years or older are diagnosed with arthritis [1]. Although aging is generally not viewed as the cause of OA, aging events within articular chondrocytes may predispose the joint to damage when exposed to mechanical loads. This review characterizes age-related changes in the articular chondrocyte, discusses the known molecular mechanisms underlying the effects of chondrocyte aging, and then concludes with how an aged chondrocyte can increase the risk of developing OA.

Age-Related Changes in Articular Chondrocytes

The articular cartilage consists of an extracellular matrix composed primarily of type II collagen and proteoglycans, and one cell type—the chondrocyte. The primary role of the chondrocyte is to maintain cartilage homeostasis, in part through the production of extracellular matrix components. With age, chondrocytes exhibit features consistent with a senescent phenotype, including telomere shortening and increased senescence-associated β-galactosidase activity (Table 1) [2, 3]. These age-related changes impair the ability of chondrocytes to maintain the surrounding extracellular matrix. Accordingly, in aged chondrocytes, synthetic activity decreases and the proteoglycans produced are smaller and more irregular [4, 5].

Table 1 Molecular events in articular chondrocytes with aging
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Chondrocyte synthetic activity is regulated by anabolic growth factors [6]. Aged chondrocytes exhibit a reduced responsiveness to growth factors such as insulin-like growth factor-1 (IGF-1) [7, 8], osteogenic protein-1 (OP-1) or bone morphogenic protein-7 [9], and transforming growth factor-β (TGF-β) [10, 11]. For example, TGF-β stimulates proteoglycan synthesis in young animals, but this effect is impaired in old mice [10, 12]. It is hypothesized that age-related alterations in the TGF-β signaling pathway trigger chondrocytes to leave their normally quiescent state to an autolytic phenotype, leading to degradation of the cartilage extracellular matrix [13].

Factors Affecting Articular Chondrocyte Aging

The accumulation of advanced glycation end products (AGEs), increased reactive oxygen species (ROS) production, and age-related changes in joint tissues are several factors that affect chondrocyte aging (Fig. 1). One feature of aging is AGE accumulation in many tissues, including the articular cartilage [14]. AGEs are produced through a nonenzymatic reaction between reducing sugars and free amino groups of proteins, lipids, or nucleic acids [15•]. AGEs are formed within the body, and are also derived from cooking techniques that involve “browning” foods [15•]. Excessive levels of AGEs in the body are pathogenic, and its effects include increased production of oxidative stress and inflammation [16]. In chondrocytes, AGEs increase production of inflammatory cytokine tumor necrosis factor-α (TNF-α) and inflammatory mediators prostaglandin E2 and nitric oxide, suppress collagen II production, and stimulate expression of degradative enzymes matrix metalloproteinases (MMPs) and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) [17•, 18, 19]. AGE accumulation also has adverse effects on the cartilage extracellular matrix. AGEs increase collagen crosslinking, which increase tissue stiffness, make cartilage more brittle, and increase susceptibility of cartilage to mechanical failure [20–22]. Although not reported in chondrocytes, AGEs also induce production of ROS in cells such as murine hepatic stellate cells and bone marrow mesenchymal stem cells [23, 24].

Fig. 1
figure 1

Chondrocyte aging and cartilage destruction. Age-related changes within the cartilage extracellular matrix and surrounding joint tissues initiate a cascade of events within the articular chondrocyte that lead to cartilage destruction and susceptibility for the development of osteoarthritis. ADAMTS a disintegrin and metalloproteinase with thrombospondin motifs, MMPs matrix metalloproteinases

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ROS play roles in many physiologic processes but have the potential to cause oxidative damage of protein, lipid, and DNA [25]. Mechanisms of oxidative stress suppression include upregulation of antioxidant proteins, such as the ROS-scavenging peroxidases, and enzymes that reverse oxidative damage [26]. However, a loss of reduction-oxidation (redox) homeostasis is linked to degenerative conditions such as Alzheimer’s disease and cancer [27]. Human articular chondrocytes actively produce ROS, and increased levels of ROS were reported in cartilage of old versus young rats [28–31]. Furthermore, the cartilage of old rats exhibited a significant decline in the activity of antioxidant catalase [31]. This redox imbalance may be caused by an age-related decline in the activity and number of mitochondria, which play roles in protecting cells from the harmful effects of ROS [32•]. A consequence of this increase in oxidative stress is DNA damage and telomere shortening, leading to a decline in matrix production, chondrocyte senescence, and apoptosis [33–35]. Increased levels of ROS also upregulate proinflammatory cytokines and MMPs, factors that play a role in the cartilage degradation process [36].

Aging-related changes in joint components also contribute to changes in the chondrocyte. Subchondral bone softening, which occurs during age-related osteoporosis [37], has been predicted to alter the biomechanics of the tibiofemoral joint by increasing maximum tensile strains in the cartilage and magnitudes of joint contact pressure [38]. A decline in quadriceps strength in the elderly population may be another factor responsible for altered joint loading patterns as a consequence of joint laxity [39]. These nonphysiologic loads exerted on the chondrocyte will lead to increased catabolic signaling and cartilage tissue breakdown [40].

Molecular Mechanisms of Chondrocyte Aging

As cellular senescence is a classic feature of chondrocyte aging, several studies have explored mechanisms regulating chondrocyte senescence (Table 1). Shimada et al. used a senescence-accelerated mouse (SAM) to explore the regulation of p21, a molecular marker of senescence [41], by growth arrest and DNA damage-inducible (GADD)45β and CCAAT/enhancer binding protein β (C/EBPβ) [42•]. The GADD45 family of proteins is associated with prosurvival functions in hematopoietic cells [43], and C/EBPβ belongs to a family of transcription factors involved in chondrocyte differentiation [44]. Expectedly, higher expression of senescence-associated β-galactosidase was detected in SAM when compared with controls [42•]. At 58 weeks of age, more chondrocytes in SAM expressed both GADD45β and C/EBPβ. Furthermore, the effect of GADD45β and C/EBPβ on p21 transactivation was determined using a luciferase reporter construct driven by a p21 promoter sequence. The overexpression of GADD45β and C/EBPβ alone slightly increased p21 promoter activity, but the overexpression of both GADD45β and C/EBPβ had a synergistic effect on promoting p21 transcription, suggesting the interactions of GADD45β and C/EBPβ may play important roles in chondrocyte senescence and aging [42•].

Other mediators of cellular senescence include TRF (telomeric repeat binding factor), XRCC5 (x-ray repair complementing defective repair in Chinese hamster cells 5), and SIRT1 (sirtuin 1). TRF1 and TRF2 are telomeric proteins that function to form and maintain telomere structure [45, 46]. XRCC5 is involved in repairing DNA double-strand breaks [47] and SIRT1 is a negative regulator of p53 and prevents growth arrest, senescence, and apoptosis [48]. Oxidative stress in human chondrocytes induced senescence and accelerated telomere shortening [49•]. After acute oxidative insult, TRF1, TRF2, XRCC5, and SIRT1 were upregulated in the early passages of human chondrocytes, but upregulated to a lesser extent in late passages of chondrocytes [49•]. This suggests that TRF proteins, XRCC5, and SIRT1 enable young chondrocytes to cope with oxidative stress by preventing DNA damage accumulation and telomere shortening. Consistently, aged chondrocytes with lower induction levels of these regulatory proteins have a reduced tolerance to oxidative challenge, and the accumulation of DNA damage may trigger chondrocyte senescence.

Membrane protein caveolin-1 is also involved in senescence. Expression of caveolin proteins is increased in the tissues of aged rats [50] and the overexpression of caveolin-1 leads to a senescent phenotype, likely through the p53/p21 pathway [51]. In addition, angiogenic growth factor (AGF) treatment of human chondrocytes downregulated interleukin-1β (IL-1β)–induced caveolin-1 expression and prevented chondrocyte replicative lifespan shortening [52]. Inhibition of p42/p44 mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) abolished the effect of AGF on caveolin-1, suggesting the AGF inhibition of IL-1β–induced chondrocyte aging is mediated, at least in part, by p42/44 MAPK and PI3K [52].

The extent of cell death is unclear in aged populations, but studies suggest there are age-related increases in chondrocyte apoptosis [53, 54], and molecular mechanisms regulating chondrocyte survival have begun to be elucidated (Table 1). IGF-I and OP-1, in addition to their roles in promoting matrix synthesis, contribute to increased survival in normal and OA chondrocytes, suggesting decline of these factors may contribute to age-related cell death [55]. Protein kinase CK2 is a ubiquitously expressed protein with roles in cell growth, proliferation, and apoptosis [56]. Decreased activity of CK2 is reported in chondrocytes of aged rats compared with young controls, and downregulation of CK2 facilitates TNF-α–induced chondrocyte death [57]. Nonhistone chromatin protein high-mobility group box (HMGB) protein 2 is expressed in the superficial zone of mature human chondrocytes, and its expression declines with age. Six-month-old HMGB2−/− mice exhibited reduced cellularity attributable to increased cell death and significant proteoglycan loss, suggesting HMGB2 plays a critical role in the survival of superficial zone chondrocytes [58]. A novel mediator of chondrocyte fate determination might be CBP/p300-interacting transactivator with ED-rich tail 2 (CITED2). Age-related decreases in CITED2 expression have been reported in rat tendon-derived stem/progenitor cells, and were correlated with decreases in cell proliferation and increased cell cycle arrest [59]. CITED2 is expressed in chondrocytes, and has been reported to play a critical role in maintaining cartilage homeostasis through the suppression of MMPs [60, 61]. It will be interesting to investigate whether CITED2 plays a role in cell fate determination in chondrocyte aging.

Chondrocyte Aging and the Development of OA

Aging is not generally viewed as the initiating factor for the development of OA. However, age-related changes within the chondrocyte, including cellular senescence and a reduced responsiveness to growth factors, and external factors affecting chondrocyte aging, such as AGE accumulation and oxidative stress may work in combination to disrupt cartilage homeostasis (Table 1). These changes will make the cartilage matrix more vulnerable to damage and lead to the onset of OA (Fig. 1).

OA is a progressive joint disease that is characterized by cartilage destruction, and affects the structural and functional integrity of the bone and other joint tissues. OA is the most common of all joint diseases, affecting an estimated 15% of the US population [62]. Risk factors for OA include old age, joint trauma, obesity, and heritable genetic factors [63]. The diagnosis of OA is generally performed radiographically and defined by bony changes such as joint space narrowing and osteophyte development [64]. OA commonly affects the knee, hip, and hand joints, and clinical symptoms of OA include joint pain, stiffness, and swelling [65].

The onset of OA is characterized by increased cell proliferation resulting in the formation of chondrocyte clusters and increased synthesis of irregular matrix components including collagens and proteoglycans [66–68]. With OA progression, there is excessive catabolic activity leading to an imbalance of cartilage homeostasis and cartilage matrix breakdown. These catabolic events are mediated largely by proinflammatory cytokines and mediators, MMPs, and ADAMTS [40]. Of note, many characteristics of an aged chondrocyte parallel changes observed in early OA, which might explain why age is highly correlated with the development of OA [69••].

Conclusions

Articular chondrocyte aging is influenced by many systemic and local factors that shift the cell toward a senescent phenotype and initiate catabolic signaling pathways. Together, these factors may contribute to the onset of OA and joint tissue breakdown. Because increased AGE and ROS accumulation are major factors affecting chondrocyte aging, anti-AGE and antioxidant therapies may yield beneficial effects. For example, increased physical fitness is correlated with lower levels of oxidative stress and moderate exercise decreases advanced glycation in small animals, but whether exercise affects levels of AGE and ROS in cartilage is unclear [70, 71]. Therefore, a better understanding of the mechanisms underlying the chondrocyte aging process and its effects on the cartilage extracellular matrix will lead to the development of novel therapeutic strategies to slow or reverse age-related joint degeneration.