Structure of the rat insulin-like growth factor II transcriptional unit: heterogeneous transcripts are generated from two promoters by use of multiple polyadenylation sites and differential ribonucleic acid splicing

The H19 endodermal enhancer is required for Igf2 activation and tumor formation in experimental liver carcinogenesis

The expression of the linked but reciprocally imprinted Igf2 and H19 genes is activated in adult liver in the course of tumor development. By in situ hybridization analysis we have shown that both the Igf2 and H19 RNAs are expressed in the majority of the neoplastic nodules, and that hepatocellular carcinomas are developed in an experimental model of liver carcinogenesis. H19 is also highly activated in smaller and less distinct hyperplastic regions. The few neoplastic areas showing Igf2 but no H19 RNA display loss of the maternally inherited allele at the Igf2/H19 locus. These data are compatible with the existence of a common activation mechanism of these two genes during liver carcinogenesis and with a stronger H19 induction in the pre-neoplastic lesions. By using mice carrying a deletion of the H19 endodermal enhancer, we show that this regulatory element is necessary for the activation of the Igf2 and H19 genes upon induction of liver carcinogenesis. Furthermore, multiple sites of the H19 endodermal enhancer region become hypersensitive to DNase I when the carcinogenesis process is induced. Lastly, liver tumors developed in mice paternally inheriting the H19 enhancer deletion are found to have marked growth delays, increased frequency of apoptotic nuclei, and lack of Igf2 mRNA expression, thus indicating that this regulatory element plays a major role in the progression of liver carcinogenesis, since it is required for the activation of the anti-apoptotic Igf2 gene.

Relaxation of insulin-like growth factor-2 imprinting in rat cultured cells

The parental-specific expression of the insulin-like growth factor-2 (Igf-2) and H19 genes was studied in rat fibroblast cells derived from a 3 day-old first-generation hybrid animal obtained by crossing Fisher and Wistar strains (F x W cells). Results showed that the reciprocal imprinting of the Igf-2 and H19 genes was conserved in the rat tissues and in the derived F x W cells when cultured with frequent transfer. Igf-2 and H19 gene expression was coordinately up-regulated upon reaching confluence, but Igf-2 RNA levels were further increased in a time-dependent manner and the repressed state of the maternal Igf-2 allele was progressively relaxed in cultures held in the confluent state and in the presence of low serum for more than 3 days. The active expression and relaxed imprinting status of the Igf-2 gene persisted over cell generations when the growth-constraining conditions were released by trypsinization and dilution. On the contrary, the imprinting of the H19 gene appeared to be unaffected by changes in growth conditions and its expression was down-regulated when the confluent cells were passaged. Methylation of the H19 promoter and Igf-2 coding regions was increased in the F x W cells extensively held under confluence and in the derived 'post-confluent' cultures. The heritable changes in the expression, and imprinting status of the Igf-2 and H19 genes observed in the F x W cells closely resembles events described in human embryonal cancers and cancer-predisposing syndromes. The occurrence of imprinting relaxation under strong growth-inhibitory conditions supports the hypothesis that it is an epigenetic change.

Early responses to dynamic strain change and prostaglandins in bone-derived cells in culture

Mechanical loading of bone explants stimulates prostaglandin E2 (PGE2) and prostacyclin (PGI2) release and increases glucose 6-phosphate dehydrogenase (G6PD) activity. This response is blocked by indomethacin and imitated by exogenous PGs. In the experiments reported here, primary cultures of rat long bone-derived osteoblast-like cells were exposed to a dynamic strain and exogenous PGs in the culture dish. Strain (3400 mu epsilon, 600 cycles, 1 Hz) caused an immediate release of PGI2 into the culture medium but had no effect on PGE2. Strain also caused an increase in G6PD activity per cell and an increase in the smallest transcript of insulin-like growth factor II (IGF-II) (IGF-II T3) but had no effect on the expression of transforming growth factor-beta1 (TGF-beta1). Indomethacin inhibited strain-induced release of PGI2 and suppressed strain-induced stimulation of IGF-II T3 transcript. PGI2 (1 microM) increased G6PD activity and mRNA levels of all three transcripts of IGF-II but had no effect on the mRNA levels of IGF-I or TGF-beta1. PGE2 (1 microM) stimulated G6PD activity and caused a marked increase in IGF-I and the largest transcript of IGF-II (IGF-II T1) but had no effect on the IGF-II transcripts T2 and T3 or on TGF-beta1 mRNA levels. These findings show similarities in response between osteoblast-like cells strained in monolayer culture and bone cells in loaded bone explants in situ. They provide support for a role for IGF-II and PGI2 in the early strain-related response of osteoblasts in loading-related bone modeling/remodeling.

Constitutive insulin-like growth factor-II expression interferes with the enterocyte-like differentiation of CaCo-2 cells

In this study we have examined the role of insulin-like growth factor-II (IGF-II) in the differentiation of the CaCo-2 human colon carcinoma cell line. We have shown previously that IGF-II is an autocrine growth factor for CaCo-2 cells. IGF-II expression is high in proliferating, undifferentiated CaCo-2 cells and markedly decreases when cells become confluent and start to differentiate. To evaluate whether differentiation of CaCo-2 cells depends on an IGF-II related pathway, we treated cells with a blocking antibody to the IGF-I receptor that mediates most IGF-II biological effects. Treatment of preconfluent CaCo-2 cells with this antibody decreased by 40% autonomous cell proliferation and induced differentiation as shown by an increase in sucrase isomaltase activity and apolipoprotein A-I (apoA-I) mRNA levels. To examine the significance of autocrine IGF-II production in CaCo-2 cell differentiation, we generated stable CaCo-2 cell lines that constitutively express rat IGF-II under the control of a Rous sarcoma virus promoter. Sustained expression of IGF-II resulted in: (a) increased proliferative rate; (b) high IGF-I receptor number, even after reaching confluence; (c) increased capability of anchorage-independent growth; (d) inhibition of the expression of apoA-I and SI mRNAs. Analysis of several independent IGF-II-transfected clones showed an inverse correlation between IGF-II mRNA levels and expression of the differentiation markers, the cells expressing the higher levels of the transfected IGF-II being the less differentiated ones. Our data suggest that perturbation of IGF-II-mediated cell proliferation interferes with the enterocyte-like differentiation pathway of CaCo-2 cells.

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.

Expression of the gene encoding an insulin-related peptide in Locusta (Insecta, Orthoptera). Evidence for alternative promoter usage

A peptide with significant sequence similarity to mammalian insulins, Locusta insulin-related peptide (LIRP) has recently been isolated from neurohaemal lobes of Locusta migratoria and the corresponding cDNA has been cloned. We report here the cloning of the LIRP gene and we show that this gene is present as a single copy/haploid genome. The organization of this gene is similar to that of mammalian insulin genes. There are at least two LIRP transcripts, LIRP T1 and LIRP T2, differing in their 5' untranslated regions and in the 5' end of the coding region resulting in an additional ATG present in LIRP T2. The two transcripts are differentially expressed in Locusta; whereas LIRP T1 is only expressed in neurosecretory cells of the brain, LIRP T2 is present at low levels in all tissues. Our data indicate that the LIRP gene has at least two promoters, the alternative usage of which could account for a differential regulation of expression of LIRP in the neurosecretory cells and in various peripheral tissues.

Characterization of the linked ovine insulin and insulin-like growth factor-II genes

The ovine insulin-like growth factor-II (oIGF-II) gene is comprised of 9 exons that span approximately 25 kb. Approximately 750 nucleotides upstream of oIGF-II exon 1 are the three exons of the ovine insulin gene that are transcribed in the same direction as oIGF-II. The genomic organization and expression of the oIGF-II gene is similar to that of the human IGF-II gene. Four putative promoters direct the transcription of six 5' noncoding exons (1, 3, 4, 5, 6, and 7), which are alternatively spliced to the shared exons 8, 9, and 10. An ovine exon comparable to human exon 2 has not been identified. Multiple transcription initiation sites were identified for exons 1 and 6 by primer extension analysis. Using a reverse transcriptase polymerase chain reaction (RT-PCR) assay, exon 1 and 3 transcripts were shown to be expressed in adult but not fetal liver. In addition, a novel transcript, which contained exon 1 spliced directly to exon 8, was detected in adult liver. Exon 4 transcripts were not detected in either fetal or adult liver, whereas exon 6 and 7 transcripts were detected in both fetal and adult liver. Exon 5 transcripts were also expressed in both fetal and adult liver, which is in contrast to the tumor cell-specific expression of human exon 5. Like the human and rodent genes, the regulation of expression of the oIGF-II gene is under complex control.

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.

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.

Regulation of insulin-like-growth-factor-II gene expression in rat liver cells

The rat insulin-like-growth-factor-(IGF)-II gene is expressed at high levels during embryonic and fetal life and at low levels in adult animals. To study the regulation of IGF-II gene expression, we analyzed the synthesis and localization of the IGF-II transcripts in cultured rat liver cells either expressing (BRL3A cells) or not expressing (BRL30E and FAO cells) the IGF-II mRNA. The IGF-II gene is transcribed at a similar rate in expressing and non-expressing cells, whereas its nuclear and cytoplasmic RNA levels are diversely distributed in the cells. IGF-II RNA is more abundant in the cytoplasmic than in the nuclear RNA fraction of BRL3A cells and is present in the nucleus but not in the cytoplasm of the FAO cells. However, both precursor and mature IGF-II nuclear RNA levels are reduced in FAO cells. Our data indicate that the IGF-II gene expression is regulated by mechanisms affecting the subcellular distribution and the abundance of the transcripts.

Nucleotide sequence of the central coding region of bovine erythrotropin/insulin-like growth factor II cDNA from fetal intestine and northern analysis of the major IGF II transcripts at the time of hepatic erythropoiesis

1. cDNA for insulin-like growth factor II (IGF II) was synthesized from poly A(+)-RNA from fetal bovine intestine and amplified. 2. The sequence corresponding to amino acids 6-62 was identical to a published ovine IGF II sequence cDNA with the exception of a single nucleotide change (G to A). 3. Northern blot analysis of intestine, liver, kidney and spleen from bovine fetuses showed multiple IGF II RNA species which are more similar to the human than to the rodent mRNAs. 4. Under the hybridization conditions used, synthesis of antisense strands, as described in embryonic chicken IGF II transcripts, was undetectable.

The liver-specific promoter of the human insulin-like growth factor II gene is activated by CCAAT/enhancer binding protein (C/EBP)

The human gene coding for insulin-like growth factor II (IGF-II) contains four promoters (P1-P4), that are subjected to tissue-specific and development-dependent regulation. Expression of promoter P1 is detected only in adult liver tissue, whereas promoter P3 is the major IGF-II promoter in fetal liver and is further expressed in other fetal tissues and in adult non-hepatic tissues. C/EBP is a tissue- and development-specific transcription factor that is expressed predominantly in adult liver, adipose tissue and lung. The effect of C/EBP on the expression of constructs containing IGF-II promoter P1 or P3 linked to the luciferase gene was investigated in cotransfection assays using Hep3B cells. We found that promoter P1 can be activated by C/EBP, whereas this transcription factor has no effect on the expression of promoter P3. By gel retardation and DNasel footprinting it was demonstrated that C/EBP can bind to a region of P1 located between 82 and 109 basepairs upstream of the cap site. Furthermore, we showed that deletion of this C/EBP binding region strongly reduces the ability of C/EBP to stimulate transcription from P1. These results indicate that C/EBP is a major component in the specific activation of the human IGF-II promoter P1 in adult liver.

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.

Specific endonucleolytic cleavage of IGF-II mRNAs

Expression of the human insulin-like growth factor II (IGF-II) gene gives rise to a family of mRNAs which are expressed in a tissue- and developmental-specific manner. Recently, an IGF-II RNA species of 1.8 kb was detected which consists of the 3' terminal untranslated region of the last IGF-II exon. We have investigated the mechanism of formation of this RNA employing IGF-II minigenes. Analysis of the expression of these genes has revealed that the 1.8-kb RNA does not result from transcription activation, but arises by specific cleavage of IGF-II transcripts.

Developmental and tissue-specific regulation of ovine insulin-like growth factor II (IGF-II) mRNA expression

Insulin-like growth factor II (IGF-II) is believed to be involved in the development of the fetus. Northern and dot-blot analysis of RNA isolated from different sheep tissues at various stages of development were undertaken, revealing that the ovine IGF-II gene is expressed as a multitranscript family (6.0, 5.1, 5.0, 4.7, 3.8, 2.9, 2.3, 1.9, 1.6, 1.3 kb). Evidence that the ovine IGF-II gene may be regulated in a developmental, tissue-specific, co-ordinate or independent manner is presented. The developmental profile of IGF-II gene expression correlates with plasma levels (Mesiano et al. (1989) Endocrinology 124, 1485-1491), and suggests that the rapid fall in plasma concentration at term can be attributed to regulation at the transcriptional level. With the exception of the kidney, IGF-II expression was down-regulated at birth in all tissues examined. As in man but not rat, an adult liver-specific transcript was detected and attributed to different 5' untranslated regions in the fetal and adult IGF-II mRNAs. The finding of IGF-II transcripts in all tissues examined supports evidence from other species of autocrine/paracrine roles for IGF-II in the development of the fetus.

The presence and active transcription of three independent leader exons in the mouse insulin-like growth factor II gene

The presence of multiple leader exons is one of the common features in the insulin-like growth factor II (IGFII) genes. Among them the 5' most exon sequence, rE1, was so far reported to be present only in the rat genome. We have found a rE1-homologous sequence in the mouse genome (mE1) and have isolated it by genomic cloning. The mE1 sequence was located in the 5' region of the IGFII gene and was considered to take an integral part in the mouse IGFII gene construction, just like in the rat gene. Overall homology between mE1 and rE1 regions was approx. 95%. The mE1 was actively transcribed in the newborn tissues and generated approx. 3.8 kb RNA species. Since the other two leader exon sequences were also active, producing 4.6 kb and 3.6 kb RNA species, respectively, transcription units of the mouse IGFII gene were, thus, composed of three leader exon systems.

Structure, evolution, expression and regulation of insulin-like growth factors I and II

Insulin-like growth factors (IGF) I and II are chemically-related single-chain peptides with diverse actions on cellular growth and metabolism. This review will focus on recent information pertinent to the biochemical and molecular biological aspects of these peptides. Three areas will be examined: The structure of the two IGF molecules and their precursors will be analyzed; the complicated anatomy of the IGF genes and their mRNAs will be described; and the multiple ways in which the expression of IGF-I and IGF-II can be regulated will be discussed. Gaps in our understanding of these peptides will be highlighted in the context of opportunities for further investigation in this field.

Evolution of insulin-like growth factor II: characterization of the mouse IGF-II gene and identification of two pseudo-exons

We have cloned the mouse insulin-like growth factor II (IGF-II) gene as a series of overlapping cosmid and lambda recombinants and have characterized its six exons. The gene extends over approximately 12 kb of mouse chromosome 7 and is located 18 kb 3' to the insulin 2 gene and in the same transcriptional polarity. Exons 1-3 encode distinct 5' untranslated regions and are transcribed by three different promoters, P1, P2, and P3, into three IGF-II mRNAs sharing common coding and 3' untranslated sequences. Promoters P2 and P3 each contain a TATA box and appear to direct transcription from single initiation sites. By contrast, exon 1 has three major transcriptional start sites distributed over 556 nucleotides, and P1 lacks a TATA region and other typical transcriptional control sequences. Exons 4-6 code for the 180-amino-acid IGF-II precursor, and exon 6 also contains a 3,045-nucleotide 3' untranslated region which ends at a single polyadenylation site. In addition to six functional IGF-II exons, we identified two 5' "pseudo-exons," which appear to be evolutionarily retained remnants of an alternative promoter-exon cassette that is active in human IGF-II. Loss of the homolog of this promoter, which directs "adult-specific" expression of the IGF-II gene in some human tissues, may explain the disappearance of this growth factor from most murine tissues in the early postnatal period.

Functional analysis of multiple promoters of the rat insulin-like factor II gene

We have characterized the multiple promoters of the rat insulin-like growth factor II (rIGFII) gene by in vivo transient expression assay using a series of deletion mutant templates. Among the four promoters (P1, P2, P3 and P6), two (P2 and P3) showed relatively strong promoter activities compared with the other two. One of the four promoters, P2, was further characterized by gel band-shift and footprinting analysis using HeLa cell nuclear extract, showing two retarded bands and at least one protected sequence stretch. The results indicated that P2 has a very simple structure like P3, and consists of no more than 141 base-pairs (bp) including a TATA box and two GC core hexanucleotides. Promoter strength shown by in vivo transient expression in different cell types failed to explain the differential employment of P2 and P3 in these cells, suggesting the involvement of other regulatory mechanisms that might operate only in the native state.

Multiple polyadenylation sites in a large 3'-most exon of the rat insulin-like growth factor II gene

The rat insulin-like growth factor II (rIGFII) gene produces, in addition to three major mRNA species 3.6 kilobases (kb), 4.6 kb and 3.8 kb in length which represent transcripts from three independent leader-exons, multiple smaller-sized products that distribute broadly in the 1-3 kb region on Northern blots. Structural constituents of these RNAs were analyzed by hybridization with region-specific probes prepared from the entire rIGFII genome. Most of these shorter RNAs contained both 5'-untranslated and coding regions, but only parts of the 3'-untranslated region. At least nine protected sites were mapped within a single 3'-most exon E6 by S1 nuclease analysis. Some but not all of these sites were associated with the upstream polyadenylation signal, AATAAA, or its variants. Since none of the shorter subspecies contained intronic sequences, aberration in splicing is not involved in their generation. Thus, the main parts of submature materials are a collection of discrete species of RNAs, most, if not all, of which are produced by alternative polyadenylation site selection.

Molecular aspects of insulin-like growth factors, their binding proteins and receptors

As we have tried to illustrate in the preceding brief review of some of the current research on the molecular biology of the IGF system, the physiological function of these important and pluripotent molecules will undoubtedly prove to be extraordinarily complex. This prediction is based upon the extensive heterogeneity of the IGF-I and IGF-II ligands themselves, the multiplicity of BPs which may influence IGF action either positively or negatively at numerous levels, and the ability of these hormones/growth factors (and possibly their BPs) to interact with disparate receptor moieties, both singly and in concert, in order to elicit their various effects.