Secretion of human latent TGF-beta-binding protein-3 (LTBP-3) is dependent on co-expression of TGF-beta

Latent TGF-beta-binding proteins (LTBPs) were initially identified through their binding to the growth factor. Three of the four known LTBPs are able to associate covalently with the small latent forms of TGF-beta and mediate their efficient secretion. LTBPs have subsequently been found to associate with the extracellular matrix. We report here the cDNA cloning and characterization of the human LTBP-3 protein, which is the smallest LTBP. The hLTBP-3 gene consists of 28 exons, including one alternatively spliced exon. The splice variant contains an additional epidermal-growth-factor-like repeat in the C-terminus. The gene is transcribed to produce a approximately 4.6 kb mRNA, which is expressed at high levels in human heart, skeletal muscle, prostate and ovaries and in certain osteosarcoma and fibroblastic cell lines. Antibodies were generated against recombinant fragment of hLTBP-3 and used to detect the protein and its secretion from cultured COS-7 and osteosarcoma cells. Immunoblotting analysis indicated that efficient secretion of overexpressed hLTBP-3 from COS-7 cells required co-expression of TGF-beta1, which resulted in the secretion of high molecular weight complexes of approximately 240 kDa. hLTBP-3 protein was secreted from cultured osteosarcoma cells as high molecular weight complexes rather than in the free form. Similar complexes were recognized with antibodies specific to beta1*LAP. These findings indicate that human LTBP-3 has an essential role in the secretion and targeting of TGF-beta1.