Physical and biological characterization of a growth-inhibitory activity purified from the neuroepithelioma cell line A673

Epithelial- and haematopoietic-cell growth-inhibitory activities have been identified in the conditioned medium of the human peripheral neuroepithelioma cell line A673. An A673-cell-derived growth-inhibitory activity was previously fractionated into two distinct components which inhibited the proliferation of human carcinoma and leukaemia cells in culture. One inhibitory activity was shown to comprise interleukin-1α (IL-1α). Here, we have purified to homogeneity a distinct activity which inhibited the growth of the epithelial cells in vitro. Using a combination of protein-sequence analysis and mass spectrometry, we demonstrated that biological activity can be assigned to a dimeric protein with a molecular mass of 25576 (±4) Da and an N-terminal sequence identical with that of transforming growth factor-β1 (TGF-β1). Further characterization of the growth inhibitor with TGF-β-isoform-specific antibodies showed that > 90 % of the bioactivity consists of TGF-β1 and not TGF-β2 or TGF-β3. Although A673 cells were growth-inhibited by exogenous TGF-β1, we showed that TGF-β1 in A673-cell-conditioned media was present in the latent, biologically inactive, form which did not act as an autocrine growth modulator of A673 cells in vitro.