Distribution of Bone Morphogenetic Proteins and BMP receptors in Osteochondromas and Modulation of Transforming Growth Factor-beta1 Actions by Heparan Sulfate and Chondroitin Sulfate in Normal Articular Cartilage

Bone morphogenetic proteins (BMPs) are morphogens involved in the regulation of chondrocyte proliferation and differentiation during endochondral bone development. BMPs bind heparin and heparan sulfate (HS). Osteochondromas are dysplastic cartilaginous bony outgrowths arising from the surface of endochondral bones that resemble a displaced growth plate. The expression of BMPs and their cognate receptors (BMPRs) in osteochondromas has not been investigated. We determined the immunohistochemical localization and distribution of BMP-2/4, -6 and -7; BMP receptors BMPR-1A, BMPR-1B and BMPR-2; signal transducing proteins phosphorylated Smad1/5/8; and BMP antagonist noggin in the cartilaginous cap of solitary (SO) and multiple (MO) human osteochondromas and compared these with the bovine growth plate and articular cartilage.;The distribution and localization patterns for BMP-6, BMP-7, BMPR-1A and BMPR-2 were similar between the cartilaginous cap and the growth plate. BMP-2/4 and BMPR-1B were present throughout the growth plate. However, in the cartilaginous cap of osteochondromas, BMP-2/4 was mainly detected in proliferating chondrocytes whereas BMPR-1B was found in hypertrophic chondrocytes of SO and proliferating chondrocytes of MO. Phosphorylated Smad1/5/8 were detected in hypertrophic chondrocytes of the cartilaginous cap compared to their presence in resting and proliferating chondrocytes in the growth plate. Noggin, a specific BMP antagonist, was observed in resting chondrocytes and, to a lesser extent, in clustered proliferating chondrocytes in SO, similar to the growth plate. On the other hand, noggin was observed in proliferating chondrocytes in MO. Since BMPs can stimulate proliferation and hypertrophic differentiation of chondrocytes, these findings suggests that there is an imbalance of BMP-2/4, its receptor and noggin interactions that may lead to abnormal regulation of chondrocyte proliferation and differentiation in the cartilaginous cap of human osteochondromas.;HS and chondroitin sulfate (CS) are known to bind and modulate growth factor and BMP activity. Transforming growth factor (TGF)-beta is involved in the regulation of superficial zone protein (SZP) in articular cartilage. SZP, also known as lubricin and proteoglycan4 (PRG4), plays an important role in the boundary lubrication of articular cartilage. The role of HS and CS in the actions of TGF-beta1 on SZP secretion has not been investigated. Therefore, we evaluated the role of exogenous and endogenous HS and CS during TGF-beta1 stimulation of SZP in superficial zone articular chondrocytes. We utilized primary monolayer superficial zone articular chondrocyte cultures from bovine and treated them with various concentrations of TGF-beta1, in the presence or absence of HS, heparin and CS. The cell surface HS and CS were removed by pretreatment with heparinase I (HEP-I) and/or chondroitinase-ABC (C-ABC) prior to TGF-beta1 stimulation. Accumulation of SZP protein in the culture medium in response to stimulation with TGF-beta1 and various exogenous glycosaminoglycans (GAGs) was quantified.;We show that TGF-beta1 and exogenous HS enhanced SZP accumulation of superficial zone chondrocytes both in the presence and absence of endogenous HS and CS. At the dose of 1 ng/ml of TGF-beta1, the presence of exogenous heparin reduced SZP accumulation and this inhibitory effect with exogenous heparin was magnified following the digestion of endogenous HS and CS. Digestion of endogenous HS and CS significantly decreased SZP accumulation stimulated by TGF-beta1. Enzymatic digestion of CS on the surface of superficial zone chondrocytes enhanced the ability of TGF-beta1 to stimulate SZP accumulation in the presence of exogenous CS. Together; the findings suggest HS and CS at the surface of superficial zone articular chondrocytes influence the response of TGF-beta1 and exogenous GAGs to stimulate SZP accumulation. Cell surface HS and CS modulate superficial zone chondrocytes response to TGF-beta1 and exogenous HS.