Retinoic acid induces secretion of transforming growth factors by PC12 pheochromocytoma cells

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-β1 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-β1 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-β1 gene. The TFG-β1 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-β1 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-β1 antibody does not reverse this inhibition. In summary, an increase in expression and secretion of TGF-β1 accompanies RA and NGF-induced PC12 cell growth arrest, but TGF-β1 does not play an autocrine role in this inhibition.