Specific endonucleolytic cleavage of IGF-II mRNAs

Identification and expression analysis of cDNA encoding insulin-like growth factor 2 in horses

Insulin-like growth factor 2 (IGF2) is responsible for a broad range of physiological processes during fetal development and adulthood, but genomic analyses of IGF2 containing the 5ʹ- and 3ʹ-untranslated regions (UTRs) in equines have been limited. In this study, we characterized the IGF2 mRNA containing the UTRs, and determined its expression pattern in the fetal tissues of horses. The complete equine IGF2 mRNA sequence harboring another exon approximately 2.8 kb upstream from the canonical transcription start site was identified as a new transcript variant. As this upstream exon did not contain the start codon, the amino acid sequence was identical to the canonical variant. Analysis of the deduced amino acid sequence revealed that the protein possessed two major domains, IlGF and IGF2_C, and analysis of IGF2 sequence polymorphism in fetal tissues of Hokkaido native horse and Thoroughbreds revealed a single nucleotide polymorphism (T to C transition) at position 398 in Thoroughbreds, which caused an amino acid substitution at position 133 in the IGF2 sequence. Furthermore, the expression pattern of the IGF2 mRNA in the fetal tissues of horses was determined for the first time, and was found to be consistent with those of other species. Taken together, these results suggested that the transcriptional and translational products of the IGF2 gene have conserved functions in the fetal development of mammals, including horses.

The IGF axis in HPV associated cancers

Human papillomaviruses (HPV) infect and replicate in stratified epithelium at cutaneous and mucosal surfaces. The proliferation and maintenance of keratinocytes, the cells which make up this epithelium, are controlled by a number of growth factor receptors such as the keratinocyte growth factor receptor (KGFR, also called fibroblast growth factor receptor 2b (FGFR2b)), the epithelial growth factor receptor (EGFR) and the insulin-like growth factor receptors 1 and 2 (IGF1R and IGF2R). In this review, we will delineate the mutation, gene transcription, translation and processing of the IGF axis within HPV associated cancers. The IGFs are key for developmental and postnatal growth of almost all tissues; we explore whether this crucial axis has been hijacked by HPV.

RNA quadruplexes

Opposed to DNA quadruplex sequences, RNA quadruplexes are still less well characterized. On the other hand, RNA quadruplexes are found to be at least as stable as their DNA counterparts. They show the same dependence on metal ions but seem to be much more restricted with respect to the adopted conformations. Other than DNA, which is mostly found to be double-stranded inside cells, RNAs are produced during transcription without its complementary sequence. The absence of a second strand that is able to hybridize and form a duplex makes the folding of RNA quadruplexes a likely event of intramolecular structure formation. Consequently, the formation of RNA quadruplexes in cellular RNAs has recently been suggested and the study of their influence and potential roles in cellular processes has just started. Here we give an overview of the RNA quadruplex field, summarizing issues such as structures, stabilities, and anticipated roles of these interesting four-stranded, guanosine-rich sequences.

Mechanisms of endonuclease-mediated mRNA decay

Endonuclease cleavage was one of the first identified mechanisms of mRNA decay but until recently it was thought to play a minor role to the better-known processes of deadenylation, decapping, and exonuclease-catalyzed decay. Most of the early examples of endonuclease decay came from studies of a particular mRNA whose turnover changed in response to hormone, cytokine, developmental, or nutritional stimuli. Only a few of these examples of endonuclease-mediated mRNA decay progressed to the point where the enzyme responsible for the initiating event was identified and studied in detail. The discovery of microRNAs and RISC-catalyzed endonuclease cleavage followed by the identification of PIN (pilT N-terminal) domains that impart endonuclease activity to a number of the proteins involved in mRNA decay has led to a resurgence of interest in endonuclease-mediated mRNA decay. PIN domains show no substrate selectivity and their involvement in a number of decay pathways highlights a recurring theme that the context in which an endonuclease function is a primary factor in determining whether any given mRNA will be targeted for decay by this or the default exonuclease-mediated decay processes.

Quantitated transcript haplotypes (QTH) of AGTR1, reduced abundance of mRNA haplotypes containing 1166C (rs5186:A>C), and relevance to metabolic syndrome traits

The angiotensin II type 1 receptor (AGTR1) is the main target through which angiotensin II influences cardiovascular tone, cell growth, and fluid and electrolyte balance. AGTR1 polymorphism has been reported to associate with hypertension, myocardial infarction (MI), and metabolic traits. Here we describe a novel approach to quantitation of transcript haplotypes (QTH) of AGTR1. To determine relative allelic expression from haplotypes, within-individual-between-allele ratiometric analyses in placental cDNA were developed for the transcribed SNPs rs5182:C>T (encoding p.L191) and rs5186:A>C (3'-noncoding "A1166C"). Additionally, between-individual comparisons were made using TaqMan assays applied to both homozygous and heterozygous genotypes and haplotypes. In conjunction, linkage disequilibrium (LD) and genomic haplotype associations with metabolic syndrome were examined. There was no significant difference of mRNA level for alleles of rs5182:C>T, but allele and mRNA haplotypes carrying 1166C exhibited reduced abundance. The effect was much greater in CC homozygotes than in heterozygotes. The promoter region was confirmed to be in a separate haplotype block from the AGTR1 3' region containing rs5182:C>T and rs5186:A>C. Metabolic syndrome trait associations were strongest for the 3' block generally and for the C allele of rs5186:A>C specifically. All effects were much more prominent in homozygotes, possibly reflecting interallelic interaction through feedback loops of mRNA regulation. Differential abundance of AGTR1 mRNA haplotypes may mediate clinical phenotypic observations of the AGTR1 genotype.

Messenger RNA turnover and its regulation in herpesviral infection

The ability to regulate cellular gene expression is a key aspect of the lifecycles of a diverse array of viruses. In fact, viral infection often results in a global shutoff of host cellular gene expression; such inhibition serves not only to ensure maximal viral gene expression without competition from the host for essential machinery and substrates but also aids in evasion of immune responses detrimental to successful viral replication and dissemination. Within the herpesvirus family, host shutoff is a prominent feature of both the alpha- and gamma-herpesviruses. Intriguingly, while both classes of herpesviruses block cellular gene expression by inducing decay of messenger RNAs, the viral factors responsible for this phenotype as well as the mechanisms by which it is achieved are quite distinct. However, data suggest that the host shutoff functions of alpha- and gamma-herpesviruses are likely achieved both through the activity of virally encoded nucleases as well as via modulation of cellular RNA degradation pathways. This review highlights the processes governing normal cellular messenger RNA decay and then details the mechanisms by which herpesviruses promote accelerated RNA turnover. Parallels between the viral and cellular degradation systems as well as the known interactions between viral host shutoff factors and the cellular RNA turnover machinery are highlighted.

A systematic analysis of disease-associated variants in the 3' regulatory regions of human protein-coding genes II: the importance of mRNA secondary structure in assessing the functionality of 3' UTR variants

In an attempt both to catalogue 3' regulatory region (3' RR)-mediated disease and to improve our understanding of the structure and function of the 3' RR, we have performed a systematic analysis of disease-associated variants in the 3' RRs of human protein-coding genes. We have previously analysed the variants that have occurred in two specific domains/motifs of the 3' untranslated region (3' UTR) as well as in the 3' flanking region. Here we have focused upon 83 known variants within the upstream sequence (USS; between the translational termination codon and the upstream core polyadenylation signal sequence) of the 3' UTR. To place these variants in their proper context, we first performed a comprehensive survey of known cis-regulatory elements within the USS and the mechanisms by which they effect post-transcriptional gene regulation. Although this survey supports the view that RNA regulatory elements function within the context of specific secondary structures, there are no general rules governing how secondary structure might exert its influence. We have therefore addressed this question by systematically evaluating both functional and non-functional (based upon in vitro reporter gene and/or electrophoretic mobility shift assay data) USS variant-containing sequences against known cis-regulatory motifs within the context of predicted RNA secondary structures. This has allowed us not only to establish a reliable and objective means to perform secondary structure prediction but also to identify consistent patterns of secondary structural change that could potentiate the discrimination of functional USS variants from their non-functional counterparts. The resulting rules were then used to infer potential functionality in the case of some of the remaining functionally uncharacterized USS variants, from their predicted secondary structures. This not only led us to identify further patterns of secondary structural change but also several potential novel cis-regulatory motifs within the 3' UTRs studied.

Control of fetal growth and neonatal survival by the RasGAP-associated endoribonuclease G3BP

The regulation of mRNA stability plays a major role in the control of gene expression during cell proliferation, differentiation, and development. Here, we show that inactivation of the RasGAP-associated endoribonuclease (G3BP)-encoding gene leads to embryonic lethality and growth retardation. G3BP-/- mice that survived to term exhibited increased apoptotic cell death in the central nervous system and neonatal lethality. Both in mouse embryonic fibroblasts and during development, the absence of G3BP altered the expression of essential growth factors, among which imprinted gene products and growth arrest-specific mRNAs were outstanding. The results demonstrate that G3BP is essential for proper embryonic growth and development by mediating the coordinate expression of multiple imprinted growth-regulatory transcripts.

mRNA degradation machines in eukaryotic cells

The steady-state levels of mRNAs depend upon their combined rates of synthesis and processing, transport from the nucleus to cytoplasm, and decay in the cytoplasm. In eukaryotic cells, the degradation of mRNA is an essential determinant in the regulation of gene expression, and it can be modulated in response to developmental, environmental, and metabolic signals. This level of regulation is particularly important for proteins that are active for a brief period, such as growth factors, transcription factors, and proteins that control cell cycle progression. The mechanisms by which mRNAs are degraded and the sequence elements within the mRNAs that affect their stability are the subject of this review. We will summarize the current state of knowledge regarding cis-acting elements in mRNA and trans-acting factors that contribute to mRNA regulation decay. We will then consider the mechanisms by which specific signaling proteins seem to contribute to a dynamic organization of the mRNA degradation machinery in response to physiological stimuli.

Kinetics and regulation of site-specific endonucleolytic cleavage of human IGF-II mRNAs

Human insulin-like growth factor II (IGF-II) mRNA can be cleaved at a specific site in its 4 kb long 3'-UTR. This yields a stable 3' cleavage product of 1.8 kb consisting of a 3'-UTR and a poly(A) tail and an unstable 5' cleavage product containing the IGF-II coding region. After cleavage, the 5' cleavage product is targeted to rapid degradation and consequently is no longer involved in IGF-II protein synthesis. Cleavage is therefore thought to provide an additional way to control IGF-II gene expression. In this paper the kinetics and the efficiency of cleavage of IGF-II mRNAs are examined. The cleavage efficiency of IGF-II mRNAs carrying four different leaders (L1-L4) is enhanced in the highly structured leaders L1 and L3. Additionally, under standard cell culture conditions cleavage is a slow process that only plays a limited role in destabilisation and translation of the IGF-II mRNAs. However, in human Hep3B cells and CaCo2 cells which express IGF-II endogenously, cleavage is upregulated 3-5-fold at high cell densities. Regulated endonucleolytic cleavage of IGF-II mRNAs is restricted to cells in which IGF-II expression is related to specific cell processes.

Characterization of nuclear RNases that cleave hepatitis B virus RNA near the La protein binding site

Hepatitis B virus (HBV) RNA is downregulated by inflammatory cytokines induced in the liver by adoptively transferred HBV-specific cytotoxic T lymphocytes (CTLs) and during murine cytomegalovirus (MCMV) infections of the livers of HBV transgenic mice. The disappearance of HBV RNA is tightly associated with the cytokine-induced proteolytic cleavage of a previously defined HBV RNA-binding protein known as La autoantigen. La binds to a predicted stem-loop structure at the 5' end of the posttranscriptional regulatory element of HBV RNA between nucleotides 1243 and 1333. In the present study, we searched for nuclear RNase activities that might be involved in HBV RNA decay. Nuclear extracts derived from control livers and CTL-injected and MCMV-infected livers were analyzed for the ability to cleave HBV RNA. Endonucleolytic activity that cleaved HBV RNA at positions 1269 to 1270 and 1271 to 1272, immediately 5' of the stem-loop bound by the La protein (positions 1272 to 1293), was detected. Furthermore, we provide evidence that the cytokine-dependent downregulation of HBV RNA following MCMV infection is temporally associated with the upregulation of the endonucleolytic activity herein described. Collectively, these results suggest a model in which the steady-state HBV RNA content is controlled by the stabilizing influence of La and the destabilizing influence of nuclear RNase activities.

Extensive tissue-specific variation of allelic methylation in the Igf2 gene during mouse fetal development: relation to expression and imprinting

The imprinted Igf2 gene is active only on the paternal allele in most tissues. Its imprinting involves a cis-acting imprinting-control region (ICR) located upstream of the neighboring and maternally expressed H19 gene. It is thought that differential methylation of the parental alleles at the ICR is crucial for parental imprinting of both genes. Differentially methylated regions (DMRs) have also been identified within the Igf2 gene and their differential methylation is thought to be established during early development. To gain further insight into the function of these DMRs, we performed a quantitative analysis of their allelic methylation levels in different tissues during fetal development and the postnatal period in the mouse. Surprisingly, we found that the methylation levels of Igf2 DMRs vary extensively during fetal development, mostly on the expressed paternal allele. In particular, in skeletal muscle, differential allelic methylation in both DMR 1 and DMR 2 occurs only after birth, whereas correct paternal monoallelic expression is always observed, including in the embryonic stages. This suggests that differential methylation in the DMR 1 and DMR 2 of the Igf2 gene is dispensable for its imprinting in skeletal muscle. Furthermore, progressive methylation of the Igf2 paternal allele appears to be correlated with concomitant postnatal down-regulation and silencing of the gene. We discuss possible relations between Igf2 allelic methylation and expression during fetal development.

Distinct RNA structural domains cooperate to maintain a specific cleavage site in the 3'-UTR of IGF-II mRNAs

The insulin-like growth factor II mRNAs are targets for site-specific endonucleolytic cleavage in the 3'-UTR, which results in a very stable 3' cleavage product of 1.8 kb, consisting of 3'-UTR sequences and a poly(A) tail. The 5' cleavage product contains the coding region and is rapidly degraded. Thus, cleavage is thought to provide an additional way to control IGF-II protein synthesis. We had established that cleavage requires two widely separated sequence elements (I and II) in the 3'-UTR that form a stable duplex of 83 nucleotides. The cleavage-site itself is located in an internal loop preceded by two stable stem-loop structures. Furthermore, in a study which was based on RNA folding algorithms, we have shown that there are specific sequence and structural requirements for the cleavage reaction. Here, the functions of the different structural domains in cleavage were assessed by deletion/mutational analyses, and biochemical structure probing assays were performed to characterize better the RNA structures formed and to verify the computer folding predictions. The data suggest that the stem-loop domain contributes to maintain a highly specific c leavage-site by preventing the formation of alternative structures in the cleavage-site domain. Involvement of the nucleotides in the cleavage-site loop itself in non-Watson-Crick interactions may be important for providing a specific recognition surface for an endoribonuclease activity.

Characterization of mRNA endonucleases

Endonucleases are key effectors of mRNA degradation, particularly for mRNAs whose turnover rates are regulated by extracellular stimuli. The rapid clearance of mRNA degradation products in vivo and the need to selectively identify mRNA endonucleases in the presence of many other cellular ribonucleases make the study of these enzymes particularly challenging. We have successfully purified and cloned one such enzyme, termed polysomal RNase 1, or PMR-1. Presented here are protocols either developed in our laboratory or adapted from the work of others that we have used successfully in characterizing PMR-1. We first describe methods to determine whether a particular mRNA is degraded in vivo through an endonuclease-initiated mechanism, and then present approaches for developing an in vitro mRNA degradation system. Next we describe experiments one should perform to optimize reaction conditions, determine cofactor requirements for an endonuclease, map in vitro cleavage sites, and characterize endonucleolytic cleavage products. Finally we describe kinetic parameters one should evaluate in characterizing the enzymology of mRNA endonucleases, with particular concern focused on the relative selectivity of these enzymes for cleavage at preferred sites within target mRNAs.

Growth-condition-dependent regulation of insulin-like growth factor II mRNA stability

Insulin-like growth factor II (IGF-II) is synthesized in many tissues, but the main site of production is the liver. In this paper we show that IGF-II mRNA levels are dependent on the growth conditions of the cells. In Hep3B cells, serum deprivation leads to a marked increase in IGF-II mRNA levels. Serum stimulation of starved Hep3B cells induces a decrease in the amount of IGF-II mRNA, which is not caused by a change in promoter activity. IGF-II mRNAs are subject to endonucleolytic cleavage, a process that requires two widely separated elements in the 3' untranslated region of the mRNA. Specific regions of these elements can form a stable stem structure which is involved in the formation of RNA-protein complexes. By employing electrophoretic mobility shift assays, two complexes have been identified in cytoplasmic extracts of Hep3B cells. The formation of these complexes is related to the growth conditions of the cells and is correlated with the regulation of IGF-II mRNA levels. Our data suggest that, depending on whether serum is present or absent, a transition from one complex to the other occurs. A decrease in the IGF-II mRNA level is also observed when IGF-I or IGF-II is added to serum-deprived Hep3B cells, possibly providing a feedback mechanism for IGF-II production. The serum-induced degradation of IGF-II mRNAs does not require de novo protein synthesis, and is abolished by rapamycin, an inhibitor of p70 S6 kinase.

The cis-acting elements involved in endonucleolytic cleavage of the 3' UTR of human IGF-II mRNAs bind a 50 kDa protein

Site-specific cleavage of human insulin-like growth factor II mRNAs requires two cis-acting elements, I and II, that are both located in the 3' untranslated region and separated by almost 2 kb. These elements can interact and form a stable RNA-RNA stem structure. In this study we have initiated the investigation of transacting factors involved in the cleavage of IGF-II mRNAs. The products of the cleavage reaction accumulate in the cytoplasm, suggesting that cleavage occurs in this cellular compartment. By electrophoretic mobility shift assays, we have identified a cytoplasmic protein with an apparent molecular weight of 48-50 kDa, IGF-II cleavage unit binding protein (ICU-BP), that binds to the stem structure formed by interaction of parts of the cis-acting elements I and II. The binding is resistant to high K+ concentrations and is dependent on Mg2+. In addition, ICU-BP binding is dependent on the cell density and correlates inversely with the IGM-II mRNA levels. In vivo cross-linking data show that this protein is associated with IGF-II mRNAs in vivo.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

Long-range RNA interaction of two sequence elements required for endonucleolytic cleavage of human insulin-like growth factor II mRNAs

Human insulin-like growth factor II (IGF-II) mRNAs are subject to site-specific endonucleolytic cleavage in the 3' untranslated region, leading to an unstable 5' cleavage product containing the IGF-II coding region and a very stable 3' cleavage product of 1.8 kb. This endonucleolytic cleavage is most probably the first and rate-limiting step in degradation of IGF-II mRNAs. Two sequence elements within the 3' untranslated region are required for cleavage: element I, located approximately 2 kb upstream of the cleavage site, and element II, encompassing the cleavage site itself. We have identified a stable double-stranded RNA stem structure (delta G = -100 kcal/mol [418.4 kJ/mol]) that can be formed between element I and a region downstream of the cleavage site in element II. This structure is conserved among human, rat, and mouse mRNAs. Detailed analysis of the requirements for cleavage shows that the relative position of the elements is not essential for cleavage. Furthermore, the distance between the coding region and the cleavage site does not affect the cleavage reaction. Mutational analysis of the long-range RNA-RNA interaction shows that not only the double-stranded character but also the sequence of the stable RNA stem is important for cleavage.

A guanosine quadruplex and two stable hairpins flank a major cleavage site in insulin-like growth factor II mRNA

Insulin-like growth factor II (IGF-II) mRNAs are cleaved by an endonucleolytic event in a conserved part of their 3' untranslated region that is predicted to exhibit a complex higher-order RNA structure. In the present study, we have examined the putative secondary structures of in vitro transcripts from the conserved part of human and rat mRNAs by enzymatic and chemical probing. The results show that the cleavage site is situated between two highly structured domains. The upstream domain consists of two large hairpins, whereas the downstream domain is guanosine-rich. The guanosine-rich domain adopts a compact unimolecular conformation in Na+ or K+ but not in Li+, and it completely arrests reverse transcription in K+ but only partially in Na+, indicating the presence of an intramolecular guanosine quadruplex. The flanking higher-order structures may ensure that the cleavage site is not sequestered in stable RNA structures, thus allowing interactions with RNA or proteins at posttranscriptional stages of IGF-II expression.

A novel synapse-associated noncoding RNA

Synaptic nuclei of innervated muscle transcribe acetylcholine receptor (AChR) genes at a much higher level than extrasynaptic nuclei. To isolate candidate synaptic regulatory molecules responsible for the unique transcriptional potential of synaptic nuclei, we have taken a subtractive hybridization approach. Here, we report the cloning and characterization of a novel synapse-associated RNA, 7H4. 7H4 is expressed selectively in the endplate zone of skeletal muscle and is upregulated during early postnatal development and after denervation. Interestingly, the 7H4 gene has no introns, and yet two different-size RNAs with identical polyadenylated 3' ends are generated. Most intriguingly, the nucleotide sequence does not contain any significant open reading frames, suggesting that 7H4 may function as a noncoding RNA.

A novel synapse-associated noncoding RNA

Synaptic nuclei of innervated muscle transcribe acetylcholine receptor (AChR) genes at a much higher level than extrasynaptic nuclei. To isolate candidate synaptic regulatory molecules responsible for the unique transcriptional potential of synaptic nuclei, we have taken a subtractive hybridization approach. Here, we report the cloning and characterization of a novel synapse-associated RNA, 7H4. 7H4 is expressed selectively in the endplate zone of skeletal muscle and is upregulated during early postnatal development and after denervation. Interestingly, the 7H4 gene has no introns, and yet two different-size RNAs with identical polyadenylated 3' ends are generated. Most intriguingly, the nucleotide sequence does not contain any significant open reading frames, suggesting that 7H4 may function as a noncoding RNA.

Discriminating translation of insulin-like growth factor-II (IGF-II) during mouse embryogenesis

The problem is to discover which of the promoters of the insulin-like growth factor-II gene stimulate the transcription of mRNA which is translated into protein. Three alternative leader exons are attached to the coding sequences in RNA transcribed from this gene in other systems, and it is mainly the paternal allele which is expressed in mouse development. Transcripts bearing each of the three leader exons were found in the RNA from the chorio-allantoic placenta, visceral yolk sac, and embryo, starting at 9.5 days. A varying proportion of one abundant transcript was disengaged from the polysomes at different days of development. This transcript was prefixed by the longest of the three alternative untranslated 5' leader exons (exon 2), and it was consistently associated with polysomes in the choroid plexus and leptomeninges of the brain. Many exon 2 transcripts were abbreviated by endonucleolytic cleavage and lacked a poly(A) tail. In contrast, the transcripts with the shortest leader (exon 3) were mainly displayed on polysomes at all the stages of development which were examined. During mouse development, the production of IGF-II protein must be partly controlled by the mechanisms which regulate translation.

Transcriptional and post-transcriptional regulation of the human IGF-II gene expression

The human insulin-like growth factor II (IGF-II) gene consists of nine exons and has four promoters (P1-4). The promoters exhibit a tissue-specific and developmental stage-dependent expression pattern. In fetal liver promoters P2-4 are expressed, but after birth these promoters are shut off and another promoter, P1, is activated. We have investigated some properties of the human promoters P1 and P3 and identified a number of sequence elements, that are recognized by transcription factors. Promoter P1 is stimulated by the liver-enriched transcription factors C/EBP and LAP, whereas in the proximal region of P3 we have identified several elements that are recognized by transcription factors, including krox20/egr2 and krox24/erg1. Besides transcriptional regulation of expression also regulation at the post-transcriptional level occurs. We have found that the IGF-II mRNAs are subjected to site-specific endonucleolytic cleavage yielding a labile 5' specific fragment and a stable polyadenylated 3' specific cleavage product of 1.8 kb. Two widely separated sequence elements within the last exon were identified that are able to interact and yield a double-stranded stem structure. It is likely that this structure is essential for post-transcriptional cleavage of IGF-II mRNAs.

Differential expression of the human, mouse and rat IGF-II genes

The IGF-II gene is a complex transcription unit. Multiple transcripts are synthesized as a result of alternate promoter usage and the splicing of unique 5' untranslated regions to common coding exons. In order to characterize the mechanisms of IGF-II gene regulation we performed comparative studies to define essential features of IGF-II expression in human, rat and mouse. Homologous promoter regions of the human, mouse and rat IGF-II genes were fused to the luciferase reporter gene and expression was measured in various cell lines that have an endogenously active or inactive IGF-II gene expression pattern, respectively. The transient promoter activity of the human, mouse and rat IGF-II constructs was further compared with the endogenous activity of the IGF-II gene in various tissues and cell lines of human, mouse and rat origin. The results indicate that in transient expression assays employing heterologous systems (e.g., mouse promoter in human cells), most IGF-II promoter constructs are active, albeit at low levels. Maximal promoter activity is only observed, however, in homologous systems (e.g., human promoter constructs tested in human cells). This suggests that each promoter, despite the strong sequence conservation of the homologous human, rat and mouse promoters, is adapted to the levels of the transcription factors present in its natural environment. Finally, IGF-II gene expression is not only regulated at the level of transcription but also depends on mRNA stability. We show that human, rat and also mouse IGF-II mRNAs are subjected to specific endonucleolytic cleavage, suggesting that specific cleavage of IGF-II mRNAs must be of general physiological importance.

Insulin-like growth factors

The purpose of this review has been to emphasize, in general terms, the major aspects of the structure, expression, and regulation of the IGF-I and IGF-II genes. The complex organization of these genes provides ample opportunities for control of gene expression at multiple levels. It is important to realize that regulation at one level can influence regulation at a different level. While such regulatory interactions are characteristic of both the IGF-I and IGF-II genes, they are particularly evident in the case of IGF-I gene expression. For example, the choice of transcription start site influences the length and the sequence of the 5'-UTR, which can influence mRNA translatability and prepeptide sequence, which may influence the amounts of protein produced and, potentially, the intracellular processing and secretion of the final gene product, the mature hormone. Another example is provided by the alternative splicing of E-peptide-encoding exons, which determines the primary structure of the prohormone, which could influence its processing, stability, or function. Thus, this complex gene organization may reflect the need to carefully control, through a multilevel process, the synthesis, processing, and secretion of these important regulatory peptides.

Regulation of insulin-like growth factor-IL expression and its role in autocrine growth of human neuroblastoma cells

Insulin-like growth factor-II (IGF-II) is highly expressed in fetal tissues and may act as an autocrine growth factor during early embryogenesis. The SH-SY5Y human neuroblastoma cell line also expresses IGF-II and its receptors and responds to exogenous IGF-II with increased DNA synthesis, cell division, and neuritic outgrowth. For this study, we tested the hypothesis that IGF-II mediates autocrine growth of SH-SY5Y cells in serum-free media. SH-SY5Y cells plated at high densities proliferated in serum-free media, whereas sparsely plated cells did not. IGF-II mRNA levels increased within 24 hours of serum deprivation and were associated with increased immunoreactive IGF-II protein. Exogenous addition of IGF-II increased 3H-TdR incorporation and cell number in a dose- and time-dependent fashion. By nuclear labelling experiments using 5-Bromo-2' deoxyuridine (BrdU), we detected a twofold higher percentage of S phase nuclei after a 24-hour incubation in IGF-II. Treatment of SH-SY5Y cells with anti-IGF-II antibodies in serum-free media inhibited cell proliferation, and this inhibition was partially overcome by the addition of increasing concentrations of IGF-II. Collectively, our results indicate that IGF-II mediates an autocrine growth mechanism in SH-SY5Y cells that is associated with increased IGF-II expression.

Tumor necrosis factor-alpha and interferon-gamma inhibit insulin-like growth factor II gene expression in human fetal adrenal cell cultures

Insulin-like growth factor-II (IGF-II) gene expression is induced by adrenocorticotropic hormone (ACTH) in human fetal adrenals (HFA), which suggests an important role for IGF-II in HFA growth and differentiation. Many cytokines have different regulatory actions in the endocrine glands. In the present study we have investigated the effects of two cytokines, tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma), on the regulation of IGF-II gene expression in cultured HFA cells. Both TNF-alpha and IFN-gamma inhibited basal and ACTH-induced accumulation of IGF-II mRNA dose-dependently. Cell viability was not altered by treatment with TNF-alpha or IFN-gamma. In addition, the combination of TNF-alpha and IFN-gamma decreased ACTH-induced IGF-II mRNAs more potently than each cytokine alone. Our results suggest that TNF-alpha and IFN-gamma may be involved in the regulation of HFA growth and differentiation via local IGF-II production.

Site-specific cleavage of IGF-II mRNAs requires sequence elements from two distinct regions of the IGF-II gene

The human insulin-like growth factor II (IGF-II) gene constitutes a complex transcriptional unit that contains nine exons and four promoters. Expression of the IGF-II gene yields a family of mRNAs that all encode prepro-IGF-II. In addition, a stable 1.8 kb RNA is formed that is derived from the 3' untranslated region of exon 9. Recently, we have shown that this RNA species arises by site-specific endonucleolytic cleavage of IGF-II mRNAs and not by transcription from a separate promoter. In the present study we establish that two widely separated sequence elements of approximately 300 nucleotides, both located within exon 9, are required for this cleavage reaction. The first element encompasses about 200 nucleotides upstream and 100 nucleotides downstream of the cleavage site, while the second element is located within a region of 330 nucleotides about 2 kb upstream of the cleavage site. Interestingly, site-specific cleavage also occurred when a fragment from exon 9 of the IGF-II gene containing these two elements was inserted into the 3' untranslated part of the beta-globin gene. Apparently, the expressed hybrid beta-globin-IGF-II mRNA contains all the regulatory elements to confer site-specific endonucleolytic cleavage.

The molecular and cellular biology of insulin-like growth factor II

Insulin-like growth factor II (IGF-II) is a 67 amino acid polypeptide that belongs to the family of insulin-like peptides. The IGF-II gene is coupled to the insulin gene and paternally imprinted. Multiple IGF-II mRNAs with identical coding regions and 3' untranslated regions (UTRs) but different 5' UTRs are generated from 3 promoters. The transcripts are translationally discriminated and inactivated by a specific endonucleolytic cleavage in their 3' UTR. These features may be important in the control of IGF-II production. IGF-II functions in an auto- and paracrine manner and binds to two types of receptors. The IGF-I receptor that is a tyrosine kinase and closely related with the insulin receptor and the IGF-II/mannose 6-phosphate (IGF-II/Man 6-P) receptor that is identical with the cation-independent mannose 6-phosphate receptor. The mitogenic and metabolic actions of IGF-II are propagated by the IGF-I receptor. In contrast, the IGF-II/Man 6-P receptor, that target lysosomal enzymes from the Golgi apparatus or the plasma membrane to the lysosomes, mediates the rapid internalization and degradation of IGF-II. IGF-II is expressed at high levels during foetal life and it is a major growth factor for the foetus in rodents. The developmental profiles and tissue distribution of the IGF-I and the maternally imprinted IGF-II/Man 6-P receptors both parallel that of IGF-II. In this scenario IGF-II promotes the growth of the embryo through the IGF-I receptor, whereas the IGF-II/Man 6-P receptor balance the activity by controlling the extracellular level of IGF-II.