PubMed · 8753602
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H Mikami. 1995. [Alpha-blocker].. https://pubmed.ncbi.nlm.nih.gov/8753602/
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GAP-43 is a membrane phosphoprotein that is important for the development and plasticity of neural connections. In undifferentiated PC12 pheochromocytoma cells, GAP-43 mRNA degrades rapidly ( t = 5 h), but becomes stable when cells are treated with nerve growth factor. To identify trans- acting factors that may influence mRNA stability, we combined column chromatography and gel mobility shift assays to isolate GAP-43 mRNA binding proteins from neonatal bovine brain tissue. This resulted in the isolation of two proteins that bind specifically and competitively to a pyrimidine-rich sequence in the 3'-untranslated region of GAP-43 mRNA. Partial amino acid sequencing revealed that one of the RNA binding proteins coincides with FBP (far upstream element binding protein), previously characterized as a protein that resembles hnRNP K and which binds to a single-stranded, pyrimidine-rich DNA sequence upstream of the c -myc gene to activate its expression. The other binding protein shares sequence homology with PTB, a polypyrimidine tract binding protein implicated in RNA splicing and regulation of translation initiation. The two proteins bind to a 26 nt pyrimidine-rich sequence lying 300 nt downstream of the end of the coding region, in an area shown by others to confer instability on a reporter mRNA in transient transfection assays. We therefore propose that FBP and the PTB-like protein may compete for binding at the same site to influence the stability of GAP-43 mRNA.
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Conditioned medium from PC12 cells incubated with retinoic acid (RA) increases [3H]thymidine incorporation in normal rat kidney (NRK) fibroblasts and 3D9 epithelial cells. The medium also causes anchorage-independent growth of NRK cells, which is strongly potentiated either in the presence of EGF or after activation of latent forms of transforming growth factors (TGFs) by acidification. These results suggest that RA regulates the release of more than one growth factor by PC12 cells. Conditioned media from control or NGF-treated PC12 cells causes growth of NRK cells in soft agar only after acidification. An increase in expression of the TGF-beta1 gene is coincident with NGF-induced neuronal differentiation of PC12 cells. In addition, RA also causes a dose- and time-dependent increase in content of TGF-beta1 transcripts. This increase is, at least in part, secondary to transcriptional activation. Sequences responsible for the effect of RA and NGF are located in the 5'-flanking region of the TGF-beta1 gene. The TFG-beta1 gene has two promoters and in transient transfection assays RA and NGF significantly enhance the activity of constructs containing the second promoter. High-affinity TGF-beta1 receptors were undetectable in PC12 cells both before and after NGF or RA treatment. RA and NGF decrease PC12 cell proliferation and a neutralizing anti-TGF-beta1 antibody does not reverse this inhibition. In summary, an increase in expression and secretion of TGF-beta1 accompanies RA and NGF-induced PC12 cell growth arrest, but TGF-beta1 does not play an autocrine role in this inhibition.