It has been shown that some types of tumour cells produce activated transforming growth factor beta-1 (TGF-beta 1). However, the mechanism for the activation of TGF-beta 1 derived from tumour cells has not been fully elucidated. The present study was undertaken to characterise an activator of latent TGF-beta 1 secreted from a human gastric cancer cen line, KATO-III. Western blot analyses using antibodies for TGF-beta 1, latency associated peptide (LAP) and latent TGF-beta 1-binding protein (LTBP) revealed that, in the cell lysate of KATO-III, TGF-beta 1 protein was expressed as a small latent complex of TGF-beta 1 and LAP. This was also confirmed by a gel chromatographic analysis of the cell lysate obtained from KATO-III. A 2.5 kb transcript of TGF-beta 1 mRNA was detected in KATO-III cells by Northern blot analysis. A gel chromatographic analysis of the conditioned medium from KATO-III cells revealed, in addition to the active form of TGF-beta 1, a factor which activated latent TGF-beta 1 from NRK-49F cells at fractions near a molecular size of 65 000. This factor was inactivated by heat (100 degrees C), acidification, trypsin and serine protease inhibitors. TGF-beta 1 activity in KATO-III cell lysate was not detected in the untreated state, but potent TGF-beta 1 activity was detected after acid treatment. These results suggest that KATO-III releases not only a latent TGF-beta 1 complex but also a type of serine protease, different from plasmin, plasminogen activator, cathepsin D, endoglycosidase F or sialidase, which activates the latent TGF-beta 1 complex as effectively as acid treatment.
