Background: Perlecan is a heparan sulfate proteoglycan that has a core protein of 466 kDa and is composed of five modules, four of which share a high degree of homology with a variety of biologically important molecules including the low-density lipoprotein receptor, laminin, and the neural cell adhesion molecule. Previously, this specialized proteoglycan, characteristic of the basement membrane, was not identified as a constitutive extracellular component of human synovium.
Experimental design: Using human synovium and cultured human synovial cells, we examined perlecan gene expression in human synovium, a tissue lacking a classic basement membrane. The modulation of gene expression of the perlecan core protein by transforming growth factor-beta (TGF-beta) and basic fibroblast growth factor (FGF) was examined in cultures of normal human synovial cells, and by metabolic labeling using radioactive sodium sulfate, the question of whether this specialized protein is produced by synovial cells as a proteoglycan was addressed.
Results: Cultures of normal human synovial cells were shown to contain the large 14.5 kb perlecan mRNA and produced substantial amounts of perlecan core protein as shown by immunohistochemistry employing specific human perlecan Ab. Immunohistochemical detection showed intense staining in the intimal and subintimal layers of human synovial membrane. Moreover, the perlecan core protein was shown to be up-regulated by TGF-beta and down-regulated by basic FGF. In addition, a sizable portion (approximately 25%) of the synovial cell-produced proteoglycan was shown to contain heparan sulfate, providing evidence that synovial cell perlecan is produced as a proteoglycan.
Conclusions: The presence of perlecan in human synovium and the modulation of the biosynthesis of its core protein by TGF-beta and FGF suggest that, in addition to its structural role, this unique heparan sulfate proteoglycan may be involved in normal synovial membrane function and in the pathogenesis of arthritis.