Interaction of cyclic AMP and cell-cell contact in the control of tyrosine hydroxylase RNA.

The interaction between cell-cell contact and cyclic AMP-mediated control of the rat tyrosine hydroxylase (TH) gene was investigated in subclones of the PC12 rat pheochromocytoma cell line. Increasing cell culture density and elevation of intracellular cyclic AMP levels with forskolin both cause augmentation of TH RNA levels. However, the extent of increase in TH RNA following forskolin treatment is less in cultures grown at high density than those at low density, suggesting that there may be an interaction in the mechanism by which these two treatments modulate TH RNA levels. The role of cis-acting sequences in the TH gene in the induction of TH RNA by cyclic AMP and cell density was determined by the use of plasmid constructs containing the 5'-flanking sequences of the TH gene directing the transcription of the reporter gene, chloramphenicol acetyltransferase (CAT). Using transient transfection assays in PC12 cells, we have mapped the site of cyclic AMP regulation of the TH gene to a region between -60 and -41. Stable transformants of PC12 cells which express p5'TH CAT (-773/+27) were isolated and the activity of CAT following treatment of cells with forskolin and growth at different cell densities was evaluated. CAT activity does not differ between cells grown at low or high density. Forskolin induces CAT activity 2-4 fold, but the extent of induction does not vary with changes in cell culture density. We conclude from these experiments that the intracellular mechanism by which increased cell-cell contact modulates TH RNA levels is not through interaction with the same genomic elements as those which regulate gene expression by cyclic AMP.