OA is a debilitating disease of the joints characterized by the irreversible erosion of articular cartilage. OA has multiple risk factors including joint dysplasia, genetic and developmental joint abnormalities, ageing and joint injuries 
. In younger people without genetic/developmental abnormalities, mechanical factors due to trauma are primarily implicated in the initiation and progression of OA lesions 
. The adult articular chondrocytes, although quiescent in normal cartilage, are able to respond to mechanical forces. Excessive mechanical loading of cartilage producing hydrostatic stress, tensile strain and fluid flow 
, adversely affects chondrocyte function and precipitates OA. The objective of our study was to identify the similarities in the gene expression profiles of shear-activated and OA chondrocytes. Using the cDNA microarray technology, we found that 42 of the 131 differentially regulated genes in sheared chondrocytes have been reported previously in OA chondrocytes, and are related to ECM/matrix degradation, cell growth/differentiation, inflammation and cell survival/apoptosis. It is likely that the 15 histone- and cell cycle- related genes, found to be differentially regulated in sheared chondrocytes, are also involved in OA, since distinct histone 
and cell cycle 
related genes were recently reported in microarray studies of OA chondrocytes. In addition, the gene expression patterns of other well-established markers of OA such as COX-2 
, L-PGDS, IL-1β, COL2A1 and AGC 
, are similar to those detected in sheared chondrocytes. Taken together, at least 60 genes display akin regulation in both sheared and OA chondrocytes.
As shown in , there were a few genes whose regulation patterns were opposite in shear-activated relative to OA chondrocytes. These differences could be attributed to several reasons such as the distinct etiologies underlying OA, the stage of OA, and the inherent variability of gene expression levels in chondrocytes isolated from different donors. Although high variability might be expected for the disease samples due to different etiology and/or stage of OA, Aigner and coworkers 
reported a comparable high variability among normal donors. This high variability might also explain why their microarray analysis of OA chondrocytes revealed the downregulation of an array of genes involved in cytokine signaling including IL-1β, IL-8 and leukemia inhibitory factor 
, whereas a recent study showed upregulation of these same genes in OA 
. Controversy exists among others about whether COL2A1 expression is increased or suppressed in OA cartilage. Aigner and colleagues have suggested that the expression of COL2A1 is suppressed in the upper zones of early OA cartilage, but increased in late-stage OA cartilage relative to normal controls 
. However, upregulation of collagen genes applies predominantly to those chondrocytes found in the middle and deep zones of OA cartilage, whereas the anabolic phenotype is less obvious in the upper regions 
We have demonstrated the critical role of COX-2 in the regulation of shear-induced IL-6 and apoptosis in human chondrocytes 
. Using cDNA microarrays, we identified genes that were either positively or negatively regulated by COX-2 in shear-activated chondrocytes. The former genes are related to inflammation, matrix degradation and apoptosis. A positive association in the expression levels of COX-2 and caveolin-1 
or EPH receptor A2 
is supported by findings of other studies employing different cell types. Caveolin-1 and -2 co-localize and form a hetero-oligomeric complex in vivo 
. Moreover, integrin alpha 2 (ITGA2) is associated with caveolin-1 in tumor cells 
. Interestingly, our data suggest that EPH receptor A2, caveolins-1 and -2 and ITGA2 are under the control of COX-2 in sheared chondrocytes. Caveolin-1 
and FAS 
, also positively regulated by COX-2, have been reported to be up-regulated in OA cartilage. In view of our recent observations suggesting that p53 phosphorylation is regulated by COX-2 in sheared chondrocytes 
, it is not surprising that apoptosis enhancing nuclease (AEN) is also under COX-2 control.
Two major classes of genes were identified to be negatively modulated by COX-2 in shear-activated T/C-28a2 chondrocytes: histone and cell-cycle-related genes. We and others have shown that COX-2 overexpression induces cell cycle arrest in diverse cells including chondrocytes, NIH 3T3 fibroblasts, human embryonic kidney 293 cells 
. Here, we provide evidence for the first time suggesting that overexpression of COX-2 also negatively regulates histone gene expression in sheared chondrocytes. Downregulation of histone gene expression has been detected after DNA damage induced by ionizing radiation in different cells such as human fibroblasts and osteosarcoma 
. Endogenous degradation of histones was also observed in K562 human leukemic cells after oxidative challenge 
. The precise role of histones in OA has yet to be defined. Two histone family genes, H2AFO and H3F3B, were shown to be differentially down-regulated in OA chondrocytes relative to healthy control samples, which is in general agreement with our observations in sheared chondrocytes. Moreover, injection of histone H1 into collagen-induced arthritis (CIA) mice dramatically suppressed CIA 
. Prior work has shown that transcriptional downregulation of histone occurs in parallel with the inhibition of DNA synthesis by p53 
. We recently demonstrated that PGD2
and/or its metabolite 15d-PGJ2
mediate chondrocyte apoptosis via PKA-dependent regulation of p53 phosrphorylation 
. Indeed, L-PGDS knockdown reverses the shear-mediated histone transcriptional downregulation.
ROS play an important role in the pathogenesis of OA 
. Excessive levels of ROS generated by abnormal chondrocyte metabolism tip the balance of anabolic and catabolic events, resulting in oxidative stress and loss of homeostasis. We and others have shown that elevated mechanical stress, including shear stress, releases ROS from chondrocytes 
, and that antioxidants repress stress-induced chondrocyte death 
. L-PGDS knockdown inhibits shear-induced ROS formation, suggesting the involvement of PGD2
and/or its metabolite 15d-PGJ2
in this process.
In summary, we have demonstrated that prolonged application of high fluid shear to T/C-28a2 chondrocytes recapitulates the earmarks of OA, thereby providing further support to the link between exposure of chondrocytes/cartilage to high mechanical loading and the development of OA. Fluid shear is a well-defined biophysical stimulus for in vitro studies of mechanotransduction of articular chondrocytes. Delineating the responses of chondrocytes to high fluid shear may help us understand how OA develops. These studies may also lead to identification of ideal hydrodynamic environments for culturing artificial cartilage in bioreactors.