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PubMed1997

Characterization of a 60-kDa cell surface-associated transforming growth factor-beta binding protein that can interfere with transforming growth factor-beta receptor binding.

We have characterized a 60-kDa transforming growth factor-beta (TGF-beta) binding protein that was originally identified on LNCaP adenocarcinoma prostate cells by affinity cross-linking of cell surface proteins by using 125I-TGF-beta 1. Binding of 125I-TGF-beta 1 to the 60-kDa protein was competed by an excess of unlabeled TGF-beta 1 but not by TGF-beta 2, TGF-beta 3, activin, or osteogenic protein-1 (OP-1), also termed bone morphogenetic protein-7 (BMP-7). In addition, no binding of 125I-TGF-beta 2 and 125I-TGF-beta 3 to the 60-kDa binding protein on LNCaP cells could be demonstrated by using affinity labeling techniques. The 60-kDa TGF-beta binding protein showed no immunoreactivity with antibodies against the known type I and type II receptors for members of the TGF-beta superfamily. Treatment of LNCaP cells with 0.25 M NaCl, 1 microgram/ml heparin, or 10% glycerol caused a release of the 60-kDa protein from the cell surface. In addition, we found that the previously described TGF-beta type IV receptor on GH3 cells, which does not form a heterometric complex with TGF-beta receptors, could be released from the cell surface by these same treatments. This suggests that the 60-kDa protein and the similarly sized TGF-beta type IV receptor are related proteins. The eluted 60-kDa LNCaP protein was shown to interfere with the binding of TGF-beta to the TGF-beta receptors. Thus, the cell surface-associated 60-kDa TGF-beta binding protein may play a role in regulating TGF-beta binding to TGF-beta receptors.

Adaptor Proteins, Signal Transducing↗

PubMed1996

Oct-4 regulates alternative platelet-derived growth factor alpha receptor gene promoter in human embryonal carcinoma cells.

Expression of the platelet-derived growth factor alpha-receptor (PDGFalphaR) gene is tightly controlled in mammalian embryogenesis. A well established model system to study human embryogenesis is the embryonal carcinoma cell line Tera2. We have shown previously that retinoic acid-differentiated Tera2 cells express two PDGFalphaR transcripts of 6.4 kilobase pairs (kb) (encoding the full-length receptor) and 3.0 kb, respectively, whereas in contrast, undifferentiated Tera2 cells express PDFGalphaR transcripts of 1.5 kb and 5.0 kb. Here we show that this switch in PDGFalphaR expression pattern during differentiation of Tera2 cells results from alternative promoter use. In undifferentiated cells, a second promoter is used, which is located in intron 12 of the PDGFalphaR gene. Functional analysis shows that this promoter contains a consensus octamer motif, which can be bound by the POU domain transcription factor Oct-4. Oct-4 is expressed in undifferentiated Tera2 cells but not in retinoic acid-induced differentiated cells. Mutation of the octamer motif decreases promoter activity, while ectopic expression of Oct-4 in differentiated Tera2 cells specifically enhances the activity of this PDGFalphaR promoter. Therefore, we suggest that an important aspect in the maintenance of the undifferentiated state of human embryonal carcinoma cells results from Oct-4 expression, which thereupon activates this PDGFalphaR promoter.

Alternative Splicing↗