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Posttranscriptional regulation of glucocorticoid-regulated functions
Abstract:
Relying heavily on studies of TAT regulation in cultured rat hepatoma cell lines, we have attempted in this brief review to discuss possible mechanisms for posttranscriptional regulation of glucocorticoid-sensitive enzymes and to chronicle the evidence for and against posttranscriptional mechanisms for specific enzyme induction by glucocorticoids. Initially, mechanisms were considered that would reconcile results showing sensitivity of both induction and deinduction of TAT to inhibitors of RNA synthesis with studies demonstrating first that glucocorticoids regulate the rates of specific enzyme synthesis and, then, that glucocorticoids regulate levels of enzyme-specific mRNA. Such reconciliation proved unnecessary when it was demonstrated that inhibitors of RNA synthesis such as actinomycin D were not specific for RNA synthesis, but also had effects on mRNA turnover and protein metabolism. The bulk of evidence to date establishes that glucocorticoids promote the production of enzyme-specific mRNA for the proteins whose synthesis is regulated by thses steroids. Nevertheless, there is still very little direct evidence that steroids can modulate rates of specific gene transcription. The glucocorticoid stimulation of mouse mammary tumor virus RNA production in cultured cell lines is the only example to date where such a mechanism is supported by RNA-DNA hybridization studies. Posttranscriptional actions of steroids on the turnover, processing, or extranuclear transport of specific mRNA precursors remain potential steps at which glucocorticoids might function. The rapid turnover of some glucocorticoid-regulated enzymes and their mRNAs not only ensures a rapid response to steroid addition or withdrawal, but also subjects these proteins to relatively large fluctuations upon alterations in overall protein or mRNA metabolism. Thus many of the inductions and repressions of hepatic TAT and TO by mediators other than the glucocorticoids may be attributable entirely to nonspecific mechanisms.
Insights
Glucocorticoids primarily increase enzyme production by boosting messenger RNA levels, not by altering gene transcription rates. Post-transcriptional regulation of enzyme synthesis remains a possibility.
Area of Science:
- Molecular Biology
- Endocrinology
Background:
- Glucocorticoids regulate enzyme synthesis, particularly tyrosine aminotransferase (TAT).
- Early studies suggested TAT regulation by glucocorticoids involved both induction and deinduction sensitive to RNA synthesis inhibitors.
Purpose of the Study:
- To review evidence for and against post-transcriptional regulation of glucocorticoid-sensitive enzymes.
- To discuss potential mechanisms of post-transcriptional regulation by glucocorticoids.
Main Methods:
- Review of existing studies on TAT regulation in rat hepatoma cell lines.
- Analysis of data regarding RNA synthesis inhibitors and their effects on mRNA turnover and protein metabolism.
Main Results:
- Evidence strongly supports glucocorticoids increasing enzyme-specific mRNA production.
- Direct evidence for steroid modulation of gene transcription rates is limited, with mouse mammary tumor virus RNA production being a notable exception.
- Post-transcriptional mechanisms involving mRNA turnover, processing, or transport are potential regulatory points.
Conclusions:
- Glucocorticoids predominantly enhance enzyme synthesis via increased mRNA levels.
- While direct transcriptional regulation is not well-established, post-transcriptional mechanisms remain plausible.
- Rapid turnover of regulated enzymes and mRNAs contributes to swift cellular responses to steroid fluctuations.