Heterogeneity of latent transforming growth factor-beta isolated from bone matrix proteins

Transforming growth factor-beta (TGF beta) is a family of 25-kDa peptides that appear to be important regulators of cell proliferation, differentiation, and differentiated function in many tissues. TGF beta is present in platelets and serum and is released by cultured cells as several distinct large mol wt complexes of TGF beta with other proteins. These complexes are biologically inactive and are generically called latent TGF beta (L-TGF beta). Large quantities of TGF beta are present in bone matrix. This study was undertaken to determine whether the TGF beta in bone matrix was present as a free 25-kDa peptide or a large mol wt L-TGF beta complex. TGF beta activity was determined by inhibition of [3H]methylthymidine incorporation in mink lung epithelial cells. The specificity of inhibition was determined by treating fractions with a polyclonal rabbit antiporcine TGF beta-blocking antibody before assay. Latency was examined by assaying untreated and acid-treated fractions for TGF beta activity. Acid treatment of EDTA extracts of the bovine bone matrix proteins increased TGF beta activity from a mean of 0.8 pg/microgram protein to 56 pg/micrograms. Under native conditions L-TGF beta eluted from S400 between the 600-400 kDa mol wt standards. No activity eluted in the fractions with authentic 25-kDa TGF beta. Eighty-five percent of the L-TGF beta bound to lentil lectin, and this separated into four discrete L-TGF beta peaks (I-IV) at 0.22, 0.25, 0.35, and 0.42 M NaCl with Mono-Q anion exchange chromatography. Mono-Q pools II and III were reseparated by molecular size on Superose-12 under native and dissociative conditions. Under native conditions TGF beta activity was latent and eluted in the large mol wt fractions. No 25-kDa TGF beta was present. With dissociating conditions (4 M GuHCl) all TGF beta activity eluted in the small mol wt fractions identical to the elution position of authentic 25-kDa TGF beta. The active fractions from the dissociative separation of Mono-Q pool III were separated by C4 reverse phase HPLC. There were three discrete peaks of TGF beta activity corresponding to TGF beta 1, TGF beta 2, and an unidentified form of TGF. Maximum activation of L-TGF beta in each Mono-Q peak occurred at pH 3-3.5. There was partial activation at pH 4.5, but no additional activation at pH 1.5.(ABSTRACT TRUNCATED AT 400 WORDS)