Identification of a Novel AU-Rich Element in the 3′ Untranslated Region of Epidermal Growth Factor Receptor mRNA That Is the Target for Regulated RNA-Binding Proteins

Abstract

The epidermal growth factor receptor (EGF-R) plays an important role in the growth and progression of estrogen receptor-negative human breast cancers. EGF binds with high affinity to the EGF-R and activates a variety of second messenger pathways that affect cellular proliferation. However, the underlying mechanisms involved in the regulation of EGF-R expression in breast cancer cells are yet to be described. Here we show that the EGF-induced upregulation of EGF-R mRNA in two human breast cancer cell lines that overexpress EGF-R (MDA-MB-468 and BT-20) is accompanied by stabilization (>2-fold) of EGF-R mRNA. Transient transfections using a luciferase reporter identified a novel EGF-regulated ∼260-nucleotide (nt)cis-acting element in the 3′ untranslated region (3′-UTR) of EGF-R mRNA. This cis element contains two distinct AU-rich sequences (∼75 nt), EGF-R1A with two AUUUA pentamers and EGF-R2A with two AUUUUUA extended pentamers. Each independently regulated the mRNA stability of the heterologous reporter. Analysis of mutants of the EGF-R2A AU-rich sequence demonstrated a role for the 3′ extended pentamer in regulating basal turnover. RNA gel shift analysis identified cytoplasmic proteins (∼55 to 80 kDa) from breast cancer cells that bound specifically to the EGF-R1A and EGF-R2A cis-acting elements and whose binding activity was rapidly downregulated by EGF and phorbol esters. RNA gel shift analysis of EGF-R2A mutants identified a role for the 3′ extended AU pentamer, but not the 5′ extended pentamer, in binding proteins. These EGF-R mRNA-binding proteins were present in multiple human breast and prostate cancer cell lines. In summary, these data demonstrate a central role for mRNA stabilization in the control of EGF-R gene expression in breast cancer cells. EGF-R mRNA contains a novel complex AU-rich 260-nt cis-acting destabilizing element in the 3′-UTR that is bound by specific and EGF-regulatedtrans-acting factors. Furthermore, the 3′ extended AU pentamer of EGF-R2A plays a central role in regulating EGF-R mRNA stability and the binding of specific RNA-binding proteins. These findings suggest that regulated RNA-protein interactions involving this novel cis-acting element will be a major determinant of EGF-R mRNA stability.

ACKNOWLEDGMENTS

We thank Ana Zubiaga for providing the c-fos ARE plasmid, Roger Davis for the pBluescript EGF-R plasmid, Henry Furneaux for the HuR antibody, Gary Brewer for the AUF1 antibody, Robert Medcalf for the 9SWT plasmid, John Daly and Janelle Staton for technical assistance, and the Medical Illustrations Department at Royal Perth Hospital for the figures.

This work was supported by grants from the National Health and Medical Research Council of Australia, Royal Australasian College of Physicians, Cancer Foundation of Western Australia, and Royal Perth Hospital Medical Research Foundation to P.J.L.