Glucocorticoid-mediated destabilization of cyclin D3 mRNA involves RNA-protein interactions in the 3'-untranslated region of the mRNA.

Glucocorticoids regulate the expression of the G(1) progression factor, cyclin D3. Cyclin D3 messenger RNA (CcnD3 mRNA) stability decreases rapidly when murine T lymphoma cells are treated with the synthetic glucocorticoid dexamethasone. Basal stability of CcnD3 mRNA is regulated by sequences within the 3'-untranslated region (3'-UTR). RNA-protein interactions occurring within the CcnD3 3'-UTR have been analyzed by RNA electrophoretic mobility shift assay. Three sites of RNA-protein interaction have been mapped using this approach. These elements include three pyrimidine-rich domains of 25, 26, and 37 nucleotides. When the cyclin D3 3'-UTR was stably overexpressed, the endogenous CcnD3 mRNA was no longer regulated by dexamethasone. Likewise, overexpression of a 215-nucleotide transgene that contains the 26- and 37-nucleotide elements blocks glucocorticoid inhibition of CcnD3 mRNA expression. These observations suggest that the 215-nucleotide 3'-UTR element may act as a molecular decoy, competing for proteins that bind to the endogenous transcript and thereby attenuating glucocorticoid responsiveness. UV-cross-linking experiments showed that two proteins of approximate molecular weight 37,000 and 52,000 bind to this 3'-UTR element.