Post-transcriptional regulation of tyrosine hydroxylase expression in adrenal medulla and brain

Abstract

It is well established that long-term stress leads to induction of tyrosine hydroxylase (TH) mRNA and TH protein in adrenal medulla and brain. This induction is usually associated with stimulation of the TH gene transcription rate. However, a number of studies have reported major discrepancies between the stress-induced changes in TH gene transcription, TH mRNA, and TH protein. These discrepancies suggest that post-transcriptional mechanisms also play an important role in regulating TH expression in response to stress and other stimuli. In this report, we summarize some of our findings and literature reports that demonstrate these discrepancies in adrenal medulla, locus ceruleus, and midbrain dopamine neurons. We then describe our recent work investigating the molecular mechanisms that mediate this post-transcriptional regulation in adrenal medulla and midbrain. Our results suggest that trans-acting factors binding to the polypyrimidine-rich region of the 3' untranslated region of TH mRNA play a role in these post-transcriptional mechanisms. A hypothetical cellular model describing this post-transcriptional regulation is proposed.

Figure 1. TH mRNA is induced by dexamethasone, but not by stimulatory receptor agonists in midbrain slices

Organotypic midbrain slice cultures were incubated in the presence of 100 uM glutamate, 20 uM nicotine, 50 uM muscarine or 100 uM NMDA for 6 hr or by 1 uM dexamethasone for 24 hr. TH mRNA levels were measured using semiquantitative RT-PCR. The data represent the means ± SE from 3 cultures for each of the agonists in the bar graph and 5 cultures for the dexamethasone experiment.

Figure 2. 8-CPT-cAMP elicits induction of TH protein, but not TH mRNA in midbrain slice cultures

Organotypic midbrain slice cultures were incubated in the presence of 1 mM 8-CPT-cAMP for the periods of time designated in the figure. TH protein was measured using western analysis (see representative western blot above bar graph) and TH mRNA was measured using semiquantitative RT-PCR (see representative autoradiogram above bar graph). The bar graph presents the changes in TH mRNA and TH protein expressed as fold-increases over control values, which were 1.0 ± 0.1 for TH mRNA/28S and 12 ± 2 density units/ug protein for TH protein. The autoradiograms showing the RT-PCR results were overexposed for display purposes; quantification of the density values was made using a phosphorimager within the linear range. The data represent the means ± SE from 3-4 cultures.

Figure 3

(A) Diagram of TH mRNA, showing the sequences comprising the putative polypyrimidine-rich region of TH mRNA in different species. (B) Hypothetical mechanism by which cAMP, stress and other stimuli regulate TH mRNA translation and degradation via the induction and consequent increased binding of PCBP and possibly other unknown factors to the polypyrimidine-rich region of the 3′UTR.