Richard K. Assoian

Transforming growth factor type beta (TGF-beta), when injected subcutaneously in newborn mice, causes formation of granulation tissue (induction of angiogenesis and activation of fibroblasts to produce collagen) at the site of injection. These effects occur within 2-3 days at dose levels than 1 microgram. Parallel in vitro studies show that TGF-beta causes marked increase of either proline or leucine incorporation into collagen in either an NRK rat fibroblast cell line or early passage human dermal fibroblasts. Epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) do not cause these same in vivo and in vitro effects; in both rat and human fibroblast cultures, EGF antagonizes the effects of TGF-beta on collagen formation. We have obtained further data to support a role for TGF-beta as an intrinsic mediator of collagen formation: conditioned media obtained from activated human tonsillar T lymphocytes contain greatly elevated levels of TGF-beta compared to media obtained from unactivated lymphocytes. These activated media markedly stimulate proline incorporation into collagen in NRK cells; this effect is blocked by a specific antibody to TGF-beta. The data are all compatible with the hypothesis that TGF-beta is an important mediator of tissue repair.

Acidic ethanol extracts of human platelets induced non-neoplastic normal rat kidney fibroblasts to undergo anchorage-independent growth. Less than 100 ng/ml of the crude extract elicits 50% of the maximal biological response when assayed in the presence of epidermal growth factor (2.5 ng/ml). In the absence of epidermal growth factor, the potency of the extract decreased 1,000-fold. These results show that platelets contain a type beta transforming growth factor. The specific activity of the platelet extract is 100-fold greater than that of other non-neoplastic tissues. The growth factor was purified to homogeneity by sequential gel filtration on Bio-Gel P-60 columns, first in the absence and then in the presence of urea. As determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, this transforming growth factor-beta is a protein of 25,000 daltons. It is composed of two 12,500-dalton subunits held together by disulfide bonds. These results, as well as its amino acid composition and its lack of strong mitogenic activity, show that this protein is distinct from platelet-derived growth factor. When completely purified, transforming growth factor-beta elicits 50% of its maximal biological response at concentrations less than 5 x 10(-12) M.

Transforming growth factor-beta (TGF-beta) is a multifunctional peptide that controls proliferation, differentiation, and other functions in many cell types. Many cells synthesize TGF-beta and essentially all of them have specific receptors for this peptide. TGF-beta regulates the actions of many other peptide growth factors and determines a positive or negative direction of their effects. Its marked ability to enhance formation of connective tissue in vivo suggests several therapeutic applications.

A specific radioimmunoassay for type beta transforming growth factor (TGF-beta) was developed and used to show that human platelets treated with thrombin release TGF-beta as a consequence of degranulation. The thrombin concentrations required to induce release of TGF-beta parallel those concentrations that release the alpha-granule marker, beta-thromboglobulin. Related studies showed that TGF-beta acts on early passage, explant cultures of bovine aortic smooth muscle cells by inhibiting the effect of mitogens on proliferation of subconfluent cell monolayers yet synergizing with mitogens to stimulate growth of the same cells when cultured in soft agar. The results show that primary cultures of bovine aortic smooth muscle cells and established normal rat kidney cells behave similarly with regard to TGF-beta action. Moreover, the data suggest that platelet-mediated proliferation of aortic smooth muscle cells in vivo may not result solely from the stimulatory effect of platelet-derived growth factor (PDGF), but rather from an interaction of platelet factors which has the intrinsic ability to limit as well as stimulate mitosis.