Fluid-induced shear stress stimulates chondrocyte proliferation partially mediated via TGF-beta1

There is growing evidence that a hydrodynamic environment is beneficial for growing cartilage tissue-engineered constructs; however, the mechanisms by which fluid shear provides for a better construct are not well understood. In this study, we investigated one possible mechanism by which constructs grow faster under fluid shear: fluid shear upregulates chondrocyte proliferation. Further, we investigated if this effect is mediated by TGF-beta1, a known mediator of fluid shear effects in other cell types and a mitogen for chondrocytes. To test the hypotheses, primary bovine articular chondrocytes were cultured in monolayers (approximately 40,000 cells/cm(2)) to 80-85% confluency. After 24 h of growth arrest, cells were exposed to 3.5 Pa fluid shear stress for 96 h. Total DNA was compared between flow and static culture slides. Total TGF-beta1 was quantified in flow-conditioned media (CM) and static culture-CM. Mitogenic capacity of the CM, with or without anti-TGF-beta1 or anti-TbetaRII (TGF beta receptor type II) antibodies, was also assessed. Results show that fluid shear significantly up-regulates chondrocyte proliferation (p < 0.02). Further, total TGF-beta1 in the flow-CM was more than 3.5-fold higher (p < 0.03) and its mitogenicity significantly higher (p < 0.007) as compared to static culture-CM. Adding excess anti-TGF-beta1 or anti-TbetaRII antibodies partially, but significantly depressed mitogenicity (approximately 20% decrease) of the flow-CM. These results show that fluid shear stress upregulates chondrocyte proliferation and that this effect is partially mediated by TGF-beta1.