Superficial zone protein (lubricin) in the different tissue compartments of the knee joint: Modulation by transforming growth factor beta 1 and interleukin-1 beta

Superficial-zone protein (SZP), also known as lubricin, is a key mediator of boundary lubrication and plays an important role in the functional integrity of the diarthrodial joint. The aim of this investigation was to examine the role of transforming growth factor beta (TGF-β) and interleukin-1 beta (IL-1β) on the expression of SZP in various compartments of the bovine knee joint: the superficial zone of articular cartilage, synovium, meniscus, and anterior and posterior cruciate ligaments. The effects of TGF-β1 and IL-1β on SZP expression were examined in explants and cells from the different tissue compartments. TGF-β1 up-regulated the expression of SZP in cultured explants, but IL-1β down-regulated it. Quantitative analysis of secreted proteins in the medium of the cells demonstrated significant stimulation by TGF-β1 and inhibition by IL1-β of the accumulation of SZP protein in all four tissues. Real-time polymerase chain reaction analysis revealed that TGF-β1 significantly up-regulated SZP expression and that IL-1β down-regulated it. These results revealed the modulation of SZP expression in various compartments of the knee joint by TGF-β1 and IL-1β. In addition, SZP was found to be immunolocalized at the surface layer of cells in histological sections of all four tissue compartments. Collectively, results of the current study on regulation of SZP expression by TGF-β and IL-1 help provide new insights, into tissue engineering strategies to repair and regenerate the different tissue compartments in the articular joint with optimal lubrication.