Increased Cell‐Cell Contact Stimulates the Transcription Rate of the Tyrosine Hydroxylase Gene in Rat Pheochromocytoma PC 18 Cells

Abstract

Cell aggregation is one of several environmental cues that influence the expression of neurotransmitter phenotype during development. The expression of the catecholaminergic phenotype is increased in rat pheochromocytoma cells cultured at high density. In the present study we have investigated whether this cell density‐mediated effect on the catecholaminergic phenotype is due to the stimulation of the tyrosine hydroxylase gene. When rat pheochromocytoma PC 18 cells are cultured at high density (2 ± 105 cells/cm2), tyrosine hydroxylase enzymatic activity and tyrosine hydroxylase protein increase two‐to threefold over that observed in cells cultured at low density (1 ± 104 cells/cm2). This increase in tyrosine hydroxylase protein observed in high‐density cultures is fully accounted for by a preceding increase in tyrosine hydroxylase mRNA levels. The relative transcription rate of the tyrosine hydroxylase gene, measured using a nuclear run‐on assay, is two‐to threefold greater in PC 18 cells cultured at high density than in cells cultured at low density. Using flow cytometry, we have determined that in high‐density cultures, there are approximately twice as many cells in the G0‐G1 phases of the cell cycle compared with the number of G0‐G1 cells observed in low‐density cultures. However, when G0‐G1 cells are isolated by cellular elutriation, tyrosine hydroxylase gene transcription rate remains two‐to threefold greater in G0‐G1 cells from high‐density cultures than in G0‐G1 cells from low‐density cultures. These results indicate that increased cell‐cell contact stimulates the transcription rate of the tyrosine hydroxylase gene, resulting in the subsequent increased expression of tyrosine hydroxylase mRNA and protein.