Reduced levels of transforming growth factor-beta type I receptor in human gastric carcinomas

The expressions of transforming growth factor beta (TGF-beta) and its receptor and TGF-beta inhibitory element (TIE)-binding protein were examined on human gastric carcinomas by Northern blot hybridization, immunohistochemistry, affinity labeling and gel retardation analysis. TGF-beta mRNA was expressed in tumor and normal tissues at various levels. Immunohistochemically, TGF-beta expression was confirmed to be present within tumor cells. Out of the 17 human gastric carcinoma tissues, 14 (82%) showed a reduction in the level of type I receptor (65 kDa) for TGF-beta when compared to corresponding normal mucosas. Interestingly, in seven of the 14 tumors the level of TIE-binding protein in the tumor tissue was lower than that in normal mucosa. Human gastric carcinoma cell line TMK-1, whose growth was inhibited by TGF-beta, had only type I receptor for TGF-beta and showed a high level of TIE-binding protein. Conversely, MKN-1, a TGF-beta-resistant cell line, exhibited an extremely low level of TGF-beta receptor and had no TIE-binding protein. These results overall indicate that although human gastric carcinoma cells produced TGF-beta, they showed a reduction in TGF-beta type I receptor and a low level of TIE-binding protein, resulting in escape from growth inhibition by TGF-beta.

Purification of transforming growth factor-beta 1 binding proteins from porcine uterus membranes

We have identified several transforming growth factor-beta 1 (TGF-beta 1) binding proteins in solubilized and glycoprotein-enriched porcine uterus membrane fractions by affinity cross-linking and in-gel ligand binding using 125I-labeled TGF-beta 1. By a ligand affinity chromatography using a column of immobilized recombinant TGF-beta 1, four components of apparent molecular weights 160,000, 80,000, 50,000, and 40,000 under reducing conditions were eluted at a pH of 3.5; the 160-,80-, and 40-kDa components were demonstrated to bind TGF-beta 1 specifically by the 125I-TGF-beta 1 binding assays. Further purification was performed by gel chromatography using a Superose 12 column eluted in 70% formic acid. The 40-kDa component was purified to an apparently homogenous form, whereas the 160-kDa component eluted in a broad peak overlapping the peak of the 80-kDa component. It remains to be elucidated whether these TGF-beta 1 binding proteins are related to cell surface receptors for TGF-beta s.

A role of the latent TGF-beta 1-binding protein in the assembly and secretion of TGF-beta 1

Transforming growth factor-beta 1 (TGF-beta 1) is synthesized as latent complexes with high molecular weights. The large latent complex of TGF-beta 1 in platelets is composed of three components, i.e. the mature TGF-beta 1, which is non-covalently associated with a disulphide-bonded complex of the N-terminal remnant of the TGF-beta 1 precursor (TGF-beta 1-latency associated peptide) and the latent TGF-beta 1 binding protein (LTBP). The TGF-beta 1-latency associated peptide is sufficient for the latency of TGF-beta 1, whereas the functions of LTBP remain to be elucidated. In a human erythroleukemia cell line, HEL, the production of the latent form of TGF-beta 1 was induced more than 100-fold by phorbol 12-myristate 13-acetate. Analysis by Northern blotting revealed that both the TGF-beta 1 precursor and LTBP were induced in a coordinated fashion. Analysis by immunoprecipitation using antibodies against LTBP and the TGF-beta 1 precursor dimer revealed that LTBP has a molecular size of 205 kd under reducing conditions in this cell type, i.e. similar to that from cells transfected with cDNA for LTBP, but larger than the platelet form (125-160 kd). Limited tryptic digestion of LTBP in HEL cells and analysis by SDS-PAGE showed protein bands of similar sizes to those of platelet LTBP, suggesting that the difference in molecular sizes of LTBP involves cell-specific processing. The biosynthesis of the latent TGF-beta 1 was studied by pulse-chase analysis. LTBP became covalently associated with the TGF-beta 1 precursor within 15 min after synthesis in this cell line. Secretion of the large latent TGF-beta 1 complex was observed as early as 30 min after the synthesis of LTBP; at the same time, a free form of LTBP not bound to the TGF-beta 1 precursor was seen. In contrast, the TGF-beta 1 precursor remained inside the cells in an unprocessed form for a longer time period and the TGF-beta 1 precursor dimer without LTBP was secreted only very slowly. Furthermore, the results of partial tryptic digestion of this molecule suggested that it contained improper disulphide bonding. These results suggest that LTBP plays a critical role in the assembly and secretion of the latent TGF-beta 1.

Molecular cloning of the large subunit of transforming growth factor type beta masking protein and expression of the mRNA in various rat tissues

Masking protein (MP), which neutralizes the activity of transforming growth factor type beta 1 (TGF-beta 1), is composed of a dimeric N-terminal part of a TGF-beta 1 precursor of Mr 39,000 and an unknown large subunit of Mr 105,000-120,000. The deduced primary structure of the MP large subunit was elucidated by determining the nucleotide sequence of its cDNA. The cDNA encodes a prepro-precursor of 1712 amino acid residues with a calculated Mr of 186,596. The mature large subunit seems to be derived proteolytically from a prepro-precursor and the calculated Mr is 91,606. The precursor has seven N-linked glycosylation sites and an unusual structure containing 18 epidermal growth factor-like domains and four cysteine-rich internal repeats. The large subunit mRNA is synthesized in parallel with the expression of TGF-beta 1 mRNA in various rat tissues.

TGF-beta 1 binding protein: a component of the large latent complex of TGF-beta 1 with multiple repeat sequences

TGF-beta occurs in a latent complex of high Mr. We report the cDNA cloning and an initial structural and functional characterization of a component of the large latent TGF-beta 1 complex, denoted TGF-beta 1 binding protein (TGF-beta 1-BP). Most of the sequence of fibroblast TGF-beta 1-BP is made up of cysteine-rich repeats of two different kinds; there are 16 EGF-like repeats and three repeats with a distant resemblance to EGF, but of a distinct type hitherto not found in any other protein. beta-hydroxylated asparagine residues were identified in two of the EGF-like repeats. TGF-beta 1-BP purified from human platelets is considerably smaller than the fibroblast form (125-160 kd vs. 170-190 kd), suggesting that there is alternative splicing of the TGF-beta 1-BP gene or that TGF-beta 1-BP undergoes cell-specific proteolysis. TGF-beta 1-BP was found not to bind and inactive TGF-beta 1; its role in the latent complex is discussed.

One of two subunits of masking protein in latent TGF-beta is a part of pro-TGF-beta

A high molecular mass latent form of transforming growth factor type-beta (TGF-beta) was purified to homogeneity from rat platelets by a seven-step procedure involving group-specific affinity chromatographies on Red-Toyopearl and zinc chelating-Sepharose. The purified latent TGF-beta was a complex of TGF-beta (25 kDa) and the binding protein previously named masking protein (approximately 400 kDa) [(1986) Biochem. Biophys. Res. Commun. 141, 176-184]. Analysis of the peptide structure by gel electrophoresis showed that the masking protein consisted of two subunits of 39 kDa and 105-120 kDa linked by disulfide bonds. N-terminal amino-acid sequencing of the 39 kDa subunit indicated that this subunit was identical to the N-terminal part of the TGF-beta precursor.

Partial purification and characterization of masking protein for beta-type transforming growth factor from rat platelets

beta-Transforming growth factor (TGF-beta) is stored in platelets and secreted as a high molecular weight latent form associated with a carrier protein of about 440 KD. This carrier protein could be separated from TGF-beta in 1 N acetic acid and could again mask the activity of TGF-beta under neutral conditions. Therefore, it was named the masking protein of TGF-beta. The masking protein was separated from TGF-beta by gel filtration on a Sephacryl S-300 column or by anion-exchanger FPLC on a Mono Q column in the presence of 6 M urea. Partially purified masking protein from rat platelets neutralized the activity of TGF-beta dose-dependently and was effective at 0.3 microgram/ml. This masking protein could also mask the activity of human TGF-beta, suggesting that it was not species specific. The masking protein was a heat- and acid-stable protein, but was inactivated by treatment with dithiothreitol. The Physiological role of the masking protein in the mechanisms of wound healing and liver regeneration is discussed.