LATENT FORMS OF TRANSFORMING GROWTH-FACTOR-BETA (TGF-BETA) DERIVED FROM BONE CULTURES - IDENTIFICATION OF A NATURALLY-OCCURRING 100-KDA COMPLEX WITH SIMILARITY TO RECOMBINANT LATENT TGF-BETA

Transforming growth factor-beta (TGF-beta) is produced by most tissues, including bone, as a complex that is biologically inert. Release of TGF-beta homodimer from this latent complex is necessary for TGF-beta to exert effects on target cells. Thus, the nature of the latent complex and the mechanisms responsible for TGF-beta release are the key to understanding TGF-beta actions. We have found that murine calvarial bone cultures secrete multiple latent forms of TGF-beta. Using analytical chromatography and Western blot analysis, we have compared bone latent TGF-beta with the previously characterized latent complex present in platelets and with simian TGF-beta precursor, which is stably expressed in a latent form by Chinese hamster ovarian (CHO) cells. A major component of the bone material appears to be a latent complex of 100 kDa, consisting of mature TGF-beta (25-kDa homodimer) noncovalently associated with the remainder of the TGF-beta precursor proregion (75-kDa homodimer). Like the recombinant TGF-beta precursor, it elutes from a Mono-Q fast pressure liquid chromatography anion exchange column at 0.2 M NaCl and shows a very similar banding pattern on Western blots. Thus, this bone complex closely resembles recombinant TGF-beta precursor expressed in a latent form by CHO cells and differs from the naturally occurring platelet complex, which has an additional 135-kDa binding protein that is bound through disulfide bonds to the precursor proregion. Western blot analysis also indicates that, like CHO cells, which express recombinant TGF-beta precursor, but unlike other cell types, the bone cultures secrete detectable amounts of uncleaved TGF-beta precursor. The bone calvarial culture is the first example of a naturally occurring system that expresses the 100-kDa latent TGF-beta complex.