The 5'-untranslated region of the IGF-I receptor gene modulates reporter gene expression by both pre- and post-transcriptional mechanisms

The human IGF1R IRES likely operates through a Shine-Dalgarno-like interaction with the G961 loop (E-site) of the 18S rRNA and is kinetically modulated by a naturally polymorphic polyU loop

IGF1R is a proto-oncogene with potent mitogenic and antiapoptotic activities, and its expression must be tightly regulated to maintain normal cellular and tissue homeostasis. We previously demonstrated that translation of the human IGF1R mRNA is controlled by an internal ribosome entry site (IRES), and delimited the core functional IRES to a 90-nucleotide segment of the 5'-untranslated region positioned immediately upstream of the initiation codon. Here we have analyzed the sequence elements that contribute to the function of the core IRES. The Stem2/Loop2 sequence of the IRES exhibits near-perfect Watson-Crick complementarity to the G961 loop (helix 23b) of the 18S rRNA, which is positioned within the E-site on the platform of the 40S ribosomal subunit. Mutations that disrupt this complementarity have a negative impact on regulatory protein binding and dramatically decrease IRES activity, suggesting that the IGF1R IRES may recruit the 40S ribosome by a eukaryotic equivalent of the Shine-Dalgarno (mRNA-rRNA base-pairing) interaction. The homopolymeric Loop3 sequence of the IRES modulates accessibility and limits the rate of translation initiation mediated through the IRES. Two functionally distinct allelic forms of the Loop3 poly(U)-tract are prevalent in the human population, and it is conceivable that germ-line or somatic variations in this sequence could predispose individuals to development of malignancy, or provide a selectable growth advantage for tumor cells.

The human insulin receptor mRNA contains a functional internal ribosome entry segment

Regulation of mRNA translation is an important mechanism determining the level of expression of proteins in eukaryotic cells. Translation is most commonly initiated by cap-dependent scanning, but many eukaryotic mRNAs contain internal ribosome entry segments (IRESs), providing an alternative means of initiation capable of independent regulation. Here, we show by using dicistronic luciferase reporter vectors that the 5′-UTR of the mRNA encoding human insulin receptor (hIR) contains a functional IRES. RNAi-mediated knockdown showed that the protein PTB was required for maximum IRES activity. Electrophoretic mobility shift assays confirmed that PTB1, PTB2 and nPTB, but not unr or PTB4, bound to hIR mRNA, and deletion mapping implicated a CCU motif 448 nt upstream of the initiator AUG in PTB binding. The IR-IRES was functional in a number of cell lines, and most active in cells of neuronal origin, as assessed by luciferase reporter assays. The IRES was more active in confluent than sub-confluent cells, but activity did not change during differentiation of 3T3-L1 fibroblasts to adipocytes. IRES activity was stimulated by insulin in sub-confluent cells. The IRES may function to maintain expression of IR protein in tissues such as the brain where mRNA translation by cap-dependent scanning is less effective.

Caveolin-1 up-regulates IGF-I receptor gene transcription in breast cancer cells via Sp1- and p53-dependent pathways

The insulin-like growth factor (IGF) system plays an important role in the biology of breast cancer. Most of the biological actions of IGF-I and IGF-II are mediated by the IGF-I receptor (IGF-IR), a membrane-bound heterotetramer with potent antiapoptotic and cell survival activities. Caveolin-1 (Cav-1) is one of the main components of caveolae, and it has been shown to interact with multiple signaling molecules. In view of the important roles of IGF-IR and Cav-1 in oncogenically transformed mammary gland cells, in the present study we addressed the potential regulation of IGF-IR gene expression by Cav-1. The results obtained showed that MCF7/Cav-1 cells, expressing the Cav-1 gene in a stable manner, contain significantly higher levels of IGF-IR protein and mRNA than native MCF7 cells. These elevated levels of expression are mediated at the level of transcription, as shown by the results of experiments showing that the activity of the proximal IGF-IR promoter was higher in Cav-1-expressing MCF7 cells than in untransfected MCF7 cells. Furthermore, in subcellular localization studies, intensive IGF-IR staining in membrane ruffles and projections in MCF7/Cav-1 cells were noted, in contrast to typical membrane staining in MCF7 cells. In addition, we demonstrated that transcriptional activation of the IGF-IR gene by Cav-1 requires an intact p53 signaling pathway, since Cav-1 was unable to elevate IGF-IR levels in p53-null cells. Finally, the effect of Cav-1 was associated with an elevation in the levels of Sp1, a zinc-finger protein with important roles in IGF-IR gene transactivation. In summary, we identified the IGF-IR gene as a downstream target for Cav-1 action in breast cancer cells.

The ELAV RNA-stability factor HuR binds the 5'-untranslated region of the human IGF-IR transcript and differentially represses cap-dependent and IRES-mediated translation

The type I insulin-like growth factor receptor (IGF-IR) is an integral component in the control of cell proliferation, differentiation and apoptosis. The IGF-IR mRNA contains an extraordinarily long (1038 nt) 5'-untranslated region (5'-UTR), and we have characterized a diverse series of proteins interacting with this RNA sequence which may provide for intricate regulation of IGF-IR gene expression at the translational level. Here, we report the purification and identification of one of these IGF-IR 5'-UTR-binding proteins as HuR, using a novel RNA crosslinking/RNase elution strategy. Because HuR has been predominantly characterized as a 3'-UTR-binding protein, enhancing mRNA stability and generally increasing gene expression, we sought to determine whether HuR might serve a different function in the context of its binding the IGF-IR 5'-UTR. We found that HuR consistently repressed translation initiation through the IGF-IR 5'-UTR. The inhibition of translation by HuR was concentration dependent, and could be reversed in trans by addition of a fragment of the IGF-IR 5'-UTR containing the HuR binding sites as a specific competitor, or abrogated by deletion of the third RNA recognition motif of HuR. We determined that HuR repressed translation initiation through the IGF-IR 5'-UTR in cells as well, and that siRNA knockdown of HuR markedly increased IGF-IR protein levels. Interestingly, we also found that HuR potently inhibited IGF-IR translation mediated through internal ribosome entry. Kinetic assays were performed to investigate the mechanism of translation repression by HuR and the dynamic interplay between HuR and the translation apparatus. We found that HuR, occupying a cap-distal position, significantly delayed translation initiation mediated by cap-dependent scanning, but was eventually displaced from its binding site, directly or indirectly, as a consequence of ribosomal scanning. However, HuR perpetually blocked the activity of the IGF-IR IRES, apparently arresting the IRES-associated translation pre-initiation complex in an inactive state. This function of HuR as a 5'-UTR-binding protein and dual-purpose translation repressor may be critical for the precise regulation of IGF-IR expression essential to normal cellular homeostasis.

Evidence for differential ribonucleoprotein complex assembly in vitro on the 5'-untranslated region of the human IGF-IR transcript

The type I insulin-like growth factor receptor (IGF-IR) plays a key role in the control of cellular proliferation and survival. The human IGF-IR transcript is characterized by an unusually long 1038 nucleotide 5'-untranslated region (5'-UTR). We hypothesized that the contribution of this complex 5'-untranslated RNA sequence to the post-transcriptional regulation of IGF-IR expression would involve a dynamic interplay between RNA structure and specific RNA-binding proteins. Here we have detected and characterized a diverse series of regulatory proteins binding the IGF-IR 5'-UTR under disparate conditions. One pair of proteins ( approximately 42/38 kDa) binds readily to the intact 5'-UTR, which is predicted to adopt a highly base-paired, highly favorable (dG=-498 kcal/mol) three-domain structure. Another protein(s) (p20*) specifically induces formation of a novel RNA structure from within the initial 209 nucleotides of the nascent IGF-IR transcript, but fails to UV crosslink to this RNA sequence. A third group of proteins recognizes and binds the IGF-IR 5'-UTR under highly stringent conditions, but only after higher-ordered RNA structure has been disrupted. Our in vitro results indicate that the IGF-IR 5'-UTR may exist in at least three distinct states, and we propose that interconversion between these states might take place in vivo and differentially alter IGF-IR transcript utilization.

An in vitro system for expression analysis of mutations of the beta-globin gene: validation and application to two mutations in the 5' UTR

We describe the setting up of an in vitro expression system for the analysis of mutations of the beta-globin gene. The system is based on the stable transfection of a normal or mutated beta-globin gene into mouse erythroleukaemia (MEL) cells. The expression construct contains an Agamma gene as an internal control and both globin genes are under the control of the HS2 element of the beta LCR. The system enables analysis of transcription, RNA processing and transport, as well as mRNA stability. With non-mutant genes, high-level expression of both beta and Agamma genes is seen and both mRNAs are stable. The system was validated by comparing the expression of the beta654 thalassaemia splicing mutation in MEL cells with its well-characterized expression in vivo. The level of the initial transcript, the proportion of abnormally spliced mRNA and its instability during erythroid cell maturation were all faithfully reproduced. The system was used to examine the mechanism by which two mutations in the beta-globin 5' untranslated region (5' UTR) result in beta thalassaemia. Surprisingly, the mechanism appeared to differ in the two cases, with the C-G substitution at position +33 affecting transcription, whereas the -T deletion at position +10 resulted in a translational defect. The stably transfected MEL cells, with an internal control and an endogenous enhancer, appear to be a valid and realistic experimental model, superior to transient expression studies. This system should find wide application in the analysis of the effects and mechanisms of gene inactivation in mutations affecting the beta-globin as well as other genes.

Thyroid hormone controls the expression of insulin-like growth factor I receptor gene at different levels in lung and heart of developing and adult rats

Thyroid hormone exerts profound effects on the insulin-like growth factors (IGFs)/IGF factor I receptor (IGF-IR) system through its action on the production of IGF-I peptide and IGF-binding proteins. Most of these actions are mediated by the direct control of pituitary GH gene by thyroid hormone. In this work, we have analyzed the possible effect of hypothyroidism on the expression of IGF-IR gene, both in adult and developing animals. Our results show that in the lung and heart, thyroid hormone exerts a negative effect on IGF-IR gene expression in the adult animals and during perinatal life (from day 15 onwards). This negative effect is exerted at different levels. In the heart, this regulation occurs at a pretranslational level, indicated by the fact that parallel changes in the number of membrane IGF-I receptors and IGF-IR transcripts were observed, whereas in lung, no effect of thyroid hormone was noted in the amount of IGF-IR transcripts, suggesting a translational or posttranslational control. GH does not seem to mediate T3 effects on this gene. In contrast, retinoic acid increases the expression of IGF-IR gene at a transcriptional or posttranscriptional level in adult lung and heart. Because the IGFE/ IGF-IR system is depressed in hypothyroid animals, the specific increase in the number of IGF-IRs in the lung and heart of these animals could represent a mechanism to ameliorate the negative effects of hypothyroidism on these important organs.

The regulation of IGF-I receptor gene expression

The insulin-like growth factor I (IGF-I) receptor mediates most of the biological effects of IGF-I and -II. Despite its structural similarity to the insulin receptor, the IGF-I receptor is mainly involved in the transduction of growth and differentiation types of signals. The IGF-I receptor gene is constitutively expressed by most cells in the organism as well as in culture, consistent with the role of the IGFs as survival factors. In addition, the expression of the IGF-I receptor gene is modulated by a number of physiological and pathological factors, including developmental stage, nutritional status, hormones, growth disorders and malignancy. The regulatory region of the IGF-I receptor gene has been characterized and shown to display a high level of basal promoter activity. Transcription factor Sp1 is a strong activator of IGF-I receptor gene expression, whereas tumor suppressor WT1 represses its activity. The biological implications of these findings in both normal development and disease are described in this review.