Differential expression of Myc1 and Myc2 isoforms in cells transformed by eIF4E: evidence for internal ribosome repositioning in the human c-myc 5 ' UTR
Ps. Carter et al., Differential expression of Myc1 and Myc2 isoforms in cells transformed by eIF4E: evidence for internal ribosome repositioning in the human c-myc 5 ' UTR, ONCOGENE, 18(30), 1999, pp. 4326-4335
eIF4E is essential for translation initiation, but its overexpression cause
s malignant transformation. Recent work demonstrated that eIF4E/F participa
tes in exposing and locating alternate translation start codons during scan
ning. Translation initiation of several important protooncogenes and growth
-regulators, such as Myc and FGF-2, can start at CUG start codon(s) upstrea
m of the normal open reading frame (ORF), The resulting amino-terminal exte
nsion alters the properties of these proteins and their intracellular distr
ibution, In cells overexpressing eIF4E, c-myc is overexpressed and particul
arly the larger, CUG-initiated form (Myc1), Recent reports suggest that syn
thesis of Myc2, the normally expressed AUG-initiated form, is mediated by a
n IRES. To determine,what role eIF4E might play in c-myc expression, the c-
myc 5' untranslated region (UTR) was fused in-frame to CAT reporters, and s
everal more derivative constructs were made. In vitro translation experimen
ts (with and without eIF4E/F); expression in CHO cells transformed with eIF
4E; and deletion/mutation analysis demonstrated that Myc1 is translated by
a scanning mechanism, while Myc2 is translated by Internal Ribosome Reposit
ioning. Moreover, the existence of a true IRES in the 5'UTR was contradicte
d by its failure to direct translation of a circular transcript, in contras
t to hsp70, The c-myc 5'UTR also failed to engage in translation in the abs
ence of functional eIF4F, after cleavage of the eIF4G component with CVB4 p
rotease-2A, The Internal Repositioning Element (IRPE) in c-myc 5'UTR was de
limited to nucleotides (nt) 394-440 from the P1 transcription start site.