RETINOIC ACID INDUCES SECRETION OF TRANSFORMING GROWTH-FACTORS BY PC12 PHEOCHROMOCYTOMA CELLS

Conditioned medium from PC12 cells incubated with retinoic acid (RA) i ncreases [H-3]thymidine incorporation in normal rat kidney (NRK) fibro blasts and 3D9 epithelial cells. The medium also causes anchorage-inde pendent growth of NRK cells, which is strongly potentiated either in t he 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. C onditioned media from control or NGF-treated PC12 cells causes growth of NRK cells in soft agar only after acidification. An increase in exp ression of the TGF-beta 1 gene is coincident with NGF-induced neuronal differentiation of PC12 cells. In addition, RA also causes a dose- an d time-dependent increase in content of TGF-beta 1 transcripts. This i ncrease 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-beta 1 gene. The TFG-beta 1 gene has tw o promoters and in transient transfection assays RA and NGF significan tly enhance the activity of constructs containing the second promoter, High-affinity TGF-beta 1 receptors were undetectable in PC12 cells bo th before and after NGF or RA treatment. RA and NGF decrease PC12 cell proliferation and a neutralizing anti-TGF-beta 1 antibody does not re verse this inhibition. In summary, an increase in expression and secre tion of TGF-beta 1 accompanies RA and NGF-induced PC12 cell growth arr est, but TGF-beta 1 does not play an autocrine role in this inhibition .