ABSTRACT
ATF4 is a pro-oncogenic transcription factor whose translation is activated by eIF2 phosphorylation through delayed re-initiation involving two uORFs in the mRNA leader. However, in yeast, the effect of eIF2 phosphorylation can be mimicked by eIF5 overexpression, which turns eIF5 into translational inhibitor, thereby promoting translation of GCN4, the yeast ATF4 equivalent. Furthermore, regulatory protein termed eIF5-mimic protein (5MP) can bind eIF2 and inhibit general translation. Here, we show that 5MP1 overexpression in human cells leads to strong formation of 5MP1:eIF2 complex, nearly comparable to that of eIF5:eIF2 complex produced by eIF5 overexpression. Overexpression of eIF5, 5MP1 and 5MP2, the second human paralog, promotes ATF4 expression in certain types of human cells including fibrosarcoma. 5MP overexpression also induces ATF4 expression in Drosophila The knockdown of 5MP1 in fibrosarcoma attenuates ATF4 expression and its tumor formation on nude mice. Since 5MP2 is overproduced in salivary mucoepidermoid carcinoma, we propose that overexpression of eIF5 and 5MP induces translation of ATF4 and potentially other genes with uORFs in their mRNA leaders through delayed re-initiation, thereby enhancing the survival of normal and cancer cells under stress conditions.
Subject(s)
Activating Transcription Factor 4/metabolism , DNA-Binding Proteins/metabolism , Eukaryotic Initiation Factor-2/metabolism , Eukaryotic Initiation Factor-5/metabolism , Peptide Chain Initiation, Translational , Animals , Carcinogenesis/pathology , Cell Line, Tumor , Drosophila melanogaster/metabolism , Eukaryotic Initiation Factor-3 , Fibrosarcoma/pathology , Gene Knockdown Techniques , HEK293 Cells , HeLa Cells , Humans , Male , Mass Spectrometry , Mice, NudeABSTRACT
Three hand-reared, 50-53 day-old, red-legged seriema (Cariama cristata) chicks were evaluated for acute lameness and reluctance to ambulate. Two of the 3 chicks presented with angular limb deformities of the proximal tarsometatarsi and external rotation of the legs. Radiographs demonstrated decreased opacity of the long bone of the legs, with poorly delineated cortices and deviation of the proximal tarsometarsi. Serum concentrations of 25-hydroxycholecalciferol revealed all 3 chicks were deficient in vitamin D(3) at presentation. The chicks were administered injectable vitamin D(3) (cholecalciferol), oral vitamin D(3), and an ultraviolet B (UV-B) light was placed in their enclosure. Elastic, therapeutic taping was used to correct angular limb deformities present in 2 of the 3 chicks. Taping was continued until the angular limb deformities were corrected and lameness resolved. Hypovitaminosis D is a common cause of metabolic bone disease in captive avian species. Cholecalciferol administration, UV-B light supplementation, and elastic, therapeutic taping were effective treatments for osteodystrophy and secondary angular limb deformities due to hypovitaminosis D. This multifaceted treatment may be useful in other long-legged juvenile birds with similar clinical signs.