COMPARISON OF THE EFFECTS OF RETINOIC ACID AND NERVE GROWTH-FACTOR ONPC12 CELL-PROLIFERATION, DIFFERENTIATION, AND GENE-EXPRESSION

Retinoic acid (RA) produced a dose-dependent inhibition of PC12 cell g rowth and the appearance of cell clusters without neurite extension. R A-induced cell clumping was similar to that caused by dexamethasone (D x). Nerve growth factor (NGF) induced neurite extension, and the combi nation of RA plus NGF produced a maximal decrease in cell proliferatio n with a mixed morphology in which part of the cell population had neu rites and part formed clumps. Transcriptional effects of RA were demon strated by the increase in the activity of reporter constructs that co ntain an RA response element. RA also regulated expression of endogeno us genes in PC12 cells. The retinoid produced a two- to threefold incr ease in level of p75(LNGFR) mRNA (the low-affinity NGF receptor), with out altering expression of the trk protooncogene (the high-affinity NG F receptor carrying tyrosine kinase activity). RA also caused a transi ent 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 gen es was identical to that produced by RA. However, incubation with Dr d id not induce p75(LNGFR) mRNA and produced a strong and sustained incr ease of TH mRNA level (three- to fivefold after 48 h). These results s how that, despite the common morphological changes produced by RA and glucocorticoids in PC12 cells, the biochemical changes caused by RA ar e similar to those produced by NGF. Therefore, RA could initiate a bio chemical program of neuronal differentiation in PC12 cells, although a fully differentiated phenotype with neurite extension is not obtained .