The Regional Sensitivity of Chondrocyte Gene Expression to Coactive Mechanical Load and Exogenous TNF-α Stimuli

Mechanical loading and elevated levels of proinflammatory cytokines have been implicated in the risk for premature OA following traumatic knee injuries such as rupture of the anterior cruciate ligament (ACL) or meniscal injury [1–3]. Changes in the in vivo functional joint motion [4–6] alone or in combination with changes in joint biochemistry [7–12] after injury is one potential kinematic pathway to the initiation of post traumatic OA [13,14]. Evidence for the support of a kinematic pathway has been based on evidence of regional topological variations in the properties of knee articular cartilage structure that are thought to arise in response to the variation in local mechanical environment [15]. It has been shown that chondrocyte populations in different regions of the tibial plateau (covered by meniscus versus not covered by meniscus) respond differently to equivalent tissue level mechanical loading [16]. Thus alteration in the in vivo functional joint motion may result in subtle shifts in the load distribution at the knee altering the local mechanical environment and exposing chondrocytes to loading patterns they are unaccustomed to thereby contributing to degenerative changes. While changes in the in vivo function of the joint may alone contribute to degenerative changes in articular cartilage of the knee, changes in joint biochemistry are also likely contributing factors. In the healthy knee, synovial fluid concentrations of proinflammatory cytokines are small [7,8,11] with moderate levels for the protective cytokine interleukin-1 receptor antagonist (IL-1ra) [8]. Following ACL injury the most consistent acute cytokine responses observed are increased levels of IL-6, IL-8, and TNF-α [7,8,10]. Although these levels decrease within weeks of the injury, they are still elevated relative to healthy subjects [7–9,11] and are similar to synovial fluid concentrations observed in patients with OA [10]. One cytokine of interest, TNF-α has the ability to induce chondrocyte-mediated cartilage degradation by elevating production of degradative enzymes [17] and inhibiting synthesis [18–20] and expression [20] of structural proteins, leading to increased matrix degradation [21–23]. In vitro studies [24,25] have demonstrated that the specific effects of certain cytokines may be modulated through mechanical loading. Moderate levels of dynamic compression appear to inhibit some of the catabolic effects of interleukin-1 on articular chondrocytes [24,25]. However, proteoglycan (PG) loss in cartilage explants subjected to injurious compression and cultured in the presence of TNF-α was greater than that produced by either injurious compression or TNF-α treatment alone [26]. The coactive effects of TNF-α and in vitro dynamic compression on chondrocyte gene expression are not well understood, particularly for cartilage from distinct regions of the tibial plateau. Thus, the purpose of this study was to evaluate the regional response of chondrocytes to the coaction of load and biological stimuli by testing the following hypotheses: (1) There are significant differences in the gene expression of chondrocytes to a combined application of dynamic compression with exogenous proinflammatory cytokine (Load + TNF-α) relative to the gene expression in response to isolated Load or TNF-α stimuli. (2) Gene expression in response to Load, TNF-α and Load + TNF-α stimuli will be different in the central and peripheral regions of tibial cartilage.