GROWTH-PLATE CHONDROCYTES STORE LATENT TRANSFORMING-GROWTH-FACTOR (TGF)-BETA-1 IN THEIR MATRIX THROUGH LATENT TGF-BETA-1 BINDING PROTEIN-1

Osteoblasts produce a 100 kDa soluble form of latent transforming grow th factor beta (TGF-beta) as well as a 290 kDa form containing latent TCF-beta binding protein-1 (LTBP1), which targets the latent complex t o the matrix for storage. The nature of the soluble and stored forms o f latent TGF-beta in chondrocytes, however, is not known. In the prese nt study, resting zone and growth zone chondrocytes from rat costochon dral cartilage were cultured to fourth passage and then examined for t he presence of mRNA coding for LTBP1 protein. In addition, the matrix and media were examined for LTBP1 protein and latent TGF-beta. Norther n blots, RT-PCR, and in situ hybridization showed that growth zone cel ls expressed higher levels of LTBP1 mRNA in vitro than resting zone ce lls. Immunohistochemical staining for LTBP1 revealed fine fibrillar st ructures around the cells and in the cell matrix. When the extracellul ar matrix of these cultures was digested with plasmin, LTBP1 was relea sed, as determined by immunoprecipitation. Both active and latent TGF- beta 1 were found in these digests by TGF-beta 1 ELISA and Western blo tting. Immunoprecipitation demonstrated that the cells also secrete LT BP1 which is not associated with latent TGF-beta, in addition to LTBP1 that is associated with the 100 kDa latent TGF-beta complex. These st udies show for the first time that latent TGF-beta is present-in the m atrix of costochondral chondrocytes and that LTBP1 is responsible for storage of this complex in the matrix. The data suggest that chondrocy tes are able to regulate both the temporal and spatial activation of l atent TGF-beta, even at sites distant from the cell, in a relatively a vascular environment. Cell. Physiol. 177:343-354, 1998. (C) 1998 Wiley -Liss, Inc.