Modulation of TGF-β activity by latent TGF-β-binding protein 1 in human malignant glioma cells

High biological activity of the transforming growth factor (TGF)-β-Smad pathway characterizes the malignant phenotype of malignant gliomas and confers poor prognosis to glioma patients. Accordingly, TGF-β has become a novel target for the experimental treatment of these tumors. TGF-β is processed by furin-like proteases (FLP) and secreted from cells in a latent complex with its processed propeptide, the latency-associated peptide (LAP). Latent TGF-β-binding protein 1 (LTBP-1) covalently binds to this small latent TGF-β complex (SLC) and regulates its function, presumably via interaction with the extracellular matrix (ECM). We report here that the levels of LTBP-1 protein in vivo increase with the grade of malignancy in gliomas. LTBP-1 is associated with the ECM as well as secreted into the medium in cultured malignant glioma cells. The release of LTBP-1 into the medium is decreased by the inhibition of FLP activity. Gene-transfer mediated overexpression of LTBP-1 in glioma cell lines results in an increase in TGF-β activity. Accordingly, Smad2 phosphorylation as an intracellular marker of TGF-β activity is enhanced. Conversely, LTBP-1 gene silencing reduces TGF-β activity and Smad2 phosphorylation without affecting TGF-δ protein levels. Collectively, we identify LTBP-1 as an important modulator of TGF-β activation in glioma cells, which may contribute to the malignant phenotype of these tumors.