β-ARs, target receptors of catecholamine involved in the SNS signaling, were down-regulated in both OA cartilage and IL-1β-treated chondrocytes compared with normal cartilage and untreated chondrocytes, respectively. In the present study we investigated the role of β-AR signaling in cartilage metabolism using β-adrenergic agonist and antagonists. Stimulation of β-AR facilitated overall anabolic responses by suppressing catabolic mediator expression and increasing cartilage matrix proteins in IL-1β-treated chondrocytes accompanied by down-regulation of ERK/JNK MAPK and NF-kB signaling pathway. On the contrary, blocking of β-AR led to short-term increase of IL-1β-mediated catabolic responses only. The decreased effect of β-blockers on IL-1β-induced GAG release in long-term treatment is likely attributed to alteration in signaling pathways associated with IL-1β and β-AR. These results show that SNS signaling via β-AR is closely related with regulation of cartilage metabolism.
The autonomic nervous system is composed of SNS and parasympathetic nervous system (PNS) that perform opposite actions. The SNS is responsible for the maintenance of homeostasis in response to harmful events leading to fight-or-flight responses [3]. During heart failure (HF), long-term increase in endogenous catecholamines, including NE and epinephrine, due to activation of the SNS plays a role in the progression of HF [14]. This is mediated via β-AR, and β-AR antagonists reduce morbidity and mortality in congestive HF. On the other hand, activation of β-ARs relaxes airway smooth muscle and inhaled β-AR agonists are considered as essential bronchodilator drugs in the treatment of bronchial asthma [15].
Several clinical and animal model studies have investigated the influence of the SNS on the severity of arthritis. In antigen-induced arthritis mouse model, chemical sympathectomy and pharmacological blockade of AR reduced arthritis severity such as joint inflammation and arthritis score [16]. Depletion of catecholamines by sympathectomy and elimination of sensory afferents by administration of capsaicin decreased joint injury in arthritic rats [17]. Patients with RA exhibits milder pain by sympathetic blockade using guanethidine [18]. In murine chondrocyte monolayer culture, stimulation of β2-AR by isoproterenol inhibits Col X and Indian Hedgehog (Ihh) mRNA level through activation of ERK1/2 MAPK [19]. Sox-6 and Col II expression was inhibited by stimulation of β2-AR and this inhibitory effect was suppressed by propranolol, β2-AR antagonist [20], indicating that activation of β-AR signaling has negative effects on matrix protein synthesis in chondrocytes. By contrast, we found that β1-, β2-, and β3-AR expressions were reduced in OA cartilage relative to normal cartilage and pro-inflammatory cytokine IL-1β down-regulated three subtypes of β-AR compared to untreated control. In addition, our data showed that β-AR agonist prevented loss of ECM through decreased expression of pro-catabolic factors and increased expression of anabolic factors. These contradictory results are likely to be due to difference in types and concentrations of AR agonists and antagonists, AR subtypes interacting with them, and subsequent downstream signaling pathways.
In line with our results showing the suppression of IL-1β-induced catabolic responses by NE in monolayer and explant culture of human chondrocyte, several reports have demonstrated that catecholamines, including NE and dopamine, a precursor of NE, had anti-inflammatory effects in a variety of cells and in vivo models. NE reversed cartilage catabolism and inflammatory responses stimulated by IL-1β [9]. Adoptive transfer of tyrosine hydroxylase-positive neuronal cells generated from mesenchymal stem cells which exhibit a typical catecholaminergic phenotype led to markedly reduced severity of collagen induced arthritis in mice [21]. Dopamine prevented cartilage degradation in a DMM-induced OA mouse model and reduced MMPs expression and elevated Col II expression in IL-1β-treated chondrocytes via NF-κB and JAK2/STAT3 signaling pathway [22]. In lipopolysaccharide-stimulated microglia cells, dopamine suppressed nitric oxide production [23].
Bone remodeling is also under sympathetic control, such that β-blocker treatment enhanced bone mass in wild type and ovariectomized mice [11]. Clenbuterol, a β2-AR agonist, suppressed longitudinal growth of bones in young rats together with muscular hypertrophy [24]. On the contrary, another study showed that clenbuterol relieved sciatic nerve injury-induced loss of bone mineralization [25]. Regulation of the SNS by leptin via β2-AR decreases osteoblast number and increases osteoclast differentiation leading to reduction of bone mass in mice [26, 27]. β2-AR-deficient mice exhibited greater bone mass in response to mechanical loading compared to wild type, but it was not shown in β1-knockout and β1,2-AR double knockout mice. In addition, administration of isoproterenol a non-selective β-AR agonist to wildtype mice increased bone resorption, indicating the possibility that β1- and β2-AR could play opposite role in bone metabolism [28].
It is also noted that the SNS is closely related with regulation of inflammatory responses, e.g. proinflammatory cytokine production. In a study of postmenopausal women with hypertension, central blockade of the SNS with moxonidine reduced serum TNF-α level [29]. TNF production is regulated through stimulation of α-AR or β-AR by catecholamines such as NE [30, 31].
NF-κB pathway is an inducible transcription factor involved in inflammation and cellular differentiation, and in ECM homeostasis [32]. In particular, the activation of NF-κB leads to increase in catabolic gene expression such as MMPs, ADAMTS5, and proinflammatory mediators, including cyclooxygenase-2 and inducible nitric oxide synthase. We found that regulation of β-AR activity with NE and four antagonists of AR had opposite influence on activation of JNK and ERK MAPK and NF-κB compared to IL-1β-treated chondrocytes, subsequently leading to alteration in expression of anabolic and catabolic factors. Thus, the SNS modulated pro-catabolic and anti-anabolic responses induced by IL-1β through regulation of JNK and ERK MAPK and NF-κB in chondrocytes. In addition, it is likely that β-ARs induce different biological responses through activating diverse downstream signaling molecules, including adenylate cyclase, depending on types, concentration, and treatment time of agonists and antagonists.