Regulation of chondrocytic gene expression by biomechanical signals.

Cartilage is a mechanosensitive tissue, which means that it can perceive and respond to biomechanical signals. Despite the known importance of biomechanical signals in the etiopathogenesis of arthritic diseases and their effectiveness in joint restoration, little is understood about their actions at the cellular level. Recent molecular approaches have revealed that specific biomechanical stimuli and cell interactions generate intracellular signals that are powerful inducers or suppressors of proinflammatory and reparative genes in chondrocytes. Biomechanical signals are perceived by cartilage in magnitude-, frequency-, and time-dependent manners. Static and dynamic biomechanical forces of high magnitudes induce proinflammatory genes and inhibit matrix synthesis. Contrarily, dynamic biomechanical signals of low/physiologic magnitudes are potent antiinflammatory signals that inhibit interleukin-1beta (IL-1beta)-induced proinflammatory gene transcription and abrogate IL-1beta/tumor necrosis factor-alpha-induced inhibition of matrix synthesis. Recent studies have identified nuclear factor-kB (NF-kB) transcription factors as key regulators of biomechanical signal-mediated proinflammatory and antiinflammatory actions. These signals intercept multiple steps in the NF-kappaB signaling cascade to regulate cytokine gene expression. Taken together, these findings provide insight into how biomechanical signals regulate inflammatory and reparative gene transcription, underscoring their potential in enhancing the ability of chondrocytes to curb inflammation in diseased joints.

Relationships between masticatory rhythmicity, body mass and cephalometrically-determined aesthetic and functional variables during development in humans.

OBJECTIVE: We studied the relationship between chewing rhythmicity, craniomandibular morphology, and age in humans. DESIGN: Sixty subjects (10M:10F/group×three age groups, viz., 4-8, 10-14, and 17-21 years) participated. Subjects chewed gum for 2min while jaw movements in the frontal plane were videorecorded. Mean and variation in mean chewing cycle duration (TC) were quantified using maximum opening to maximum opening as cycle boundaries. Five "aesthetic" cephalometric variables (e.g., ANB) and seven "functional" variables (e.g., jaw length) were quantified from subjects' lateral cephalographs. Simple linear regression models and several multivariate analyses were used in comparisons. RESULTS: Mean TC increased and variation in TC decreased significantly with age. Body mass correlated with age, height, TC, all seven "functional" variables and only two "aesthetic" variables. Mean TC was correlated significantly with jaw length, distance from condylion to first molar point, distance from gonion to zygomatic arch, and distance from hyoid to menton. CONCLUSIONS: TC appeared to adapt with age. Although TC scaled most significantly with age, it is more likely that TC is mechanistically linked to jaw length or size. The decrease in TC variation with age suggests improved efficiency. TC did not scale with "aesthetic" variables, suggesting that these do not impact chewing rate; however, clinical procedures that impact jaw length may. The negative allometric scaling of TC with "functional" variables may reflect the pedomorphic jaw and face of humans. Further human studies will provide insights into the nature of scaling and adaptation of rhythmic chewing during development.

Mechanical signals control SOX-9, VEGF, and c-Myc expression and cell proliferation during inflammation via integrin-linked kinase, B-Raf, and ERK1/2-dependent signaling in articular chondrocytes.

INTRODUCTION: The importance of mechanical signals in normal and inflamed cartilage is well established. Chondrocytes respond to changes in the levels of proinflammatory cytokines and mechanical signals during inflammation. Cytokines like interleukin (IL)-1beta suppress homeostatic mechanisms and inhibit cartilage repair and cell proliferation. However, matrix synthesis and chondrocyte (AC) proliferation are upregulated by the physiological levels of mechanical forces. In this study, we investigated intracellular mechanisms underlying reparative actions of mechanical signals during inflammation. METHODS: ACs isolated from articular cartilage were exposed to low/physiologic levels of dynamic strain in the presence of IL-1beta. The cell extracts were probed for differential activation/inhibition of the extracellular signal-regulated kinase 1/2 (ERK1/2) signaling cascade. The regulation of gene transcription was examined by real-time polymerase chain reaction. RESULTS: Mechanoactivation, but not IL-1beta treatment, of ACs initiated integrin-linked kinase activation. Mechanical signals induced activation and subsequent C-Raf-mediated activation of MAP kinases (MEK1/2). However, IL-1beta activated B-Raf kinase activity. Dynamic strain did not induce B-Raf activation but instead inhibited IL-1beta-induced B-Raf activation. Both mechanical signals and IL-1beta induced ERK1/2 phosphorylation but discrete gene expression. ERK1/2 activation by mechanical forces induced SRY-related protein-9 (SOX-9), vascular endothelial cell growth factor (VEGF), and c-Myc mRNA expression and AC proliferation. However, IL-1beta did not induce SOX-9, VEGF, and c-Myc gene expression and inhibited AC cell proliferation. More importantly, SOX-9, VEGF, and Myc gene transcription and AC proliferation induced by mechanical signals were sustained in the presence of IL-1beta. CONCLUSIONS: The findings suggest that mechanical signals may sustain their effects in proinflammatory environments by regulating key molecules in the MAP kinase signaling cascade. Furthermore, the findings point to the potential of mechanosignaling in cartilage repair during inflammation.

Biomechanical signals suppress TAK1 activation to inhibit NF-kappaB transcriptional activation in fibrochondrocytes.

Exercise/joint mobilization is therapeutic for inflammatory joint diseases like rheumatoid and osteoarthritis, but the mechanisms underlying its actions remain poorly understood. We report that biomechanical signals at low/physiological magnitudes are potent inhibitors of inflammation induced by diverse proinflammatory activators like IL-1beta, TNF-alpha, and lipopolysaccharides, in fibrochondrocytes. These signals exert their anti-inflammatory effects by inhibiting phosphorylation of TAK1, a critical point where signals generated by IL-1beta, TNF-alpha, and LPS converge to initiate NF-kappaB signaling cascade and proinflammatory gene induction. Additionally, biomechanical signals inhibit multiple steps in the IL-1beta-induced proinflammatory cascade downstream of IkappaB kinase activation to regulate IkappaBalpha and IkappaBbeta degradation and synthesis, and promote IkappaBalpha shuttling to export nuclear NF-kappaB and terminate its transcriptional activity. The findings demonstrate that biomechanical forces are but another important signal that uses NF-kappaB pathway to regulate inflammation by switching the molecular activation of discrete molecules involved in proinflammatory gene transcription.

Biomechanical signals inhibit IKK activity to attenuate NF-kappaB transcription activity in inflamed chondrocytes.

OBJECTIVE: While the effects of biomechanical signals in the form of joint movement and exercise are known to be beneficial to inflamed joints, limited information is available regarding the intracellular mechanisms of their actions. This study was undertaken to examine the intracellular mechanisms by which biomechanical signals suppress proinflammatory gene induction by the interleukin-1-beta (IL-1beta)-induced NF-kappaB signaling cascade in articular chondrocytes. METHODS: Primary rat articular chondrocytes were exposed to biomechanical signals in the form of cyclic tensile strain, and the effects on the NF-kappaB signaling cascade were examined by Western blot analysis, real-time polymerase chain reaction, and immunofluorescence. RESULTS: Cyclic tensile strain rapidly inhibited the IL-1beta-induced nuclear translocation of NF-kappaB, but not its IL-1beta-induced phosphorylation at serine 276 and serine 536, which are necessary for its transactivation and transcriptional efficacy, respectively. Examination of upstream events revealed that cyclic tensile strain also inhibited the cytoplasmic protein degradation of IkappaBbeta and IkappaBalpha, as well as repressed their gene transcription. Additionally, cyclic tensile strain induced a rapid nuclear translocation of IkappaBalpha to potentially prevent NF-kappaB binding to DNA. Furthermore, the inhibition of IL-1beta-induced degradation of IkappaB by cyclic tensile strain was mediated by down-regulation of IkappaB kinase activity. CONCLUSION: These results indicate that the signals generated by cyclic tensile strain act at multiple sites within the NF-kappaB signaling cascade to inhibit IL-1beta-induced proinflammatory gene induction. Taken together, these findings provide insight into how biomechanical signals regulate and reduce inflammation, and underscore their potential in enhancing the ability of chondrocytes to curb inflammation in diseased joints.

Dynamic biophysical strain modulates proinflammatory gene induction in meniscal fibrochondrocytes.

Fibrochondrocytes of meniscus adapt to changes in their biomechanical environment by mechanisms that are yet to be elucidated. In this study, the mechanoresponsiveness of fibrochondrocytes under normal and inflammatory conditions was investigated. Fibrochondrocytes from rat meniscus were exposed to dynamic tensile forces (DTF) at various magnitudes and frequencies. The mechanoresponsiveness was assessed by examining the expression of inducible nitric oxide synthase (iNOS), tumor necrosis factor-alpha (TNF-alpha), and matrix metalloproteinase-13 mRNA expression. The mRNA and protein analyses revealed that DTF at magnitudes of 5% to 20% did not induce proinflammatory gene expression. IL-1beta induced a rapid increase in the iNOS mRNA. DTF strongly repressed IL-1beta-dependent iNOS induction in a magnitude-dependent manner. Exposure to 15% DTF resulted in >90% suppression of IL-1beta-induced mRNA within 4 h and this suppression was sustained for the ensuing 20 h. The mechanosensitivity of fibrochondrocytes was also frequency dependent and maximal suppression of iNOS mRNA expression was observed at rapid frequencies of DTF compared with lower frequencies. Like iNOS, DTF also inhibited IL-1beta-induced expression of proinflammatory mediators involved in joint inflammation. The examination of temporal effects of DTF revealed that 4- or 8-h exposure of DTF was sufficient for its sustained anti-inflammatory effects during the next 20 or 16 h, respectively. Our findings indicate that mechanical signals act as potent anti-inflammatory signals, where their magnitude and frequency are critical determinants of their actions. Furthermore, mechanical signals continue attenuating proinflammatory gene transcription for prolonged periods of time after their removal.