Comparison of the effects of retinoic acid and nerve growth factor on PC12 cell proliferation, differentiation, and gene expression.

Retinoic acid (RA) produced a dose-dependent inhibition of PC12 cell growth and the appearance of cell clusters without neurite extension. RA-induced cell clumping was similar to that caused by dexamethasone (Dx). Nerve growth factor (NGF) induced neurite extension, and the combination of RA plus NGF produced a maximal decrease in cell proliferation with a mixed morphology in which part of the cell population had neurites and part formed clumps. Transcriptional effects of RA were demonstrated by the increase in the activity of reporter constructs that contain an RA response element. RA also regulated expression of endogenous genes in PC12 cells. The retinoid produced a two- to threefold increase in level of p75LNGFR mRNA (the low-affinity NGF receptor), without altering expression of the trk protoon-cogene (the high-affinity NGF receptor carrying tyrosine kinase activity). RA also caused a transient increase in level of tyrosine hydroxylase (TH) mRNA (twofold after 16 h), which returned to basal levels and then decreased relative to basal levels at 48 h. The effect of NGF on the expression of these genes was identical to that produced by RA. However, incubation with Dx did not induce p75LNGFR mRNA and produced a strong and sustained increase of TH mRNA level (three- to fivefold after 48 h). These results show that, despite the common morphological changes produced by RA and glucocorticoids in PC12 cells, the biochemical changes caused by RA are similar to those produced by NGF. Therefore, RA could initiate a biochemical program of neuronal differentiation in PC12 cells, although a fully differentiated phenotype with neurite extension is not obtained.