Comprehensive Physiology Wiley Online Library

Insulin‐Like Growth Factors: Gene Structure and Regulation

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Insulin‐Like Growth Factor I
1.1 Insulin‐Like Growth Factor I cDNAs and Encoded Precursors
1.2 Multiple Insulin‐Like Growth Factor I mRNAs and Precursors in Mammals
1.3 Multiple Size Classes of Insulin‐Like Growth Factor I mRNAs
1.4 Insulin‐Like Growth Factor I Gene Structure
1.5 Functional Significance of Multiple Mammalian Insulin‐Like Growth Factor I mRNAs and Precursors
1.6 Insulin‐Like Growth Factor I mRNA Regulation
1.7 Structure And Regulation of the Insulin‐Like Growth Factor I Promoters
2 Insulin‐Like Growth Factor II
2.1 Insulin‐Like Growth Factor II cDNAs and Encoded Precursors
2.2 Multiple Insulin‐Like Growth Factor II mRNAs
2.3 Insulin‐Like Growth Factor II Gene Structure
2.4 Insulin‐Like Growth Factor II mRNA Regulation
2.5 Transcriptional Control of Insulin‐Like Growth Factor II Synthesis
2.6 Post‐Transcriptional Control of Insulin‐Like Growth Factor II Synthesis
3 Manipulation of the Expression of Insulinlike Growth Factors I and II
3.1 Gain of Function Insulin‐Like Growth Factor Transgenics
3.2 Loss of Function Insulin‐Like Growth Factor Transgenics
3.3 Chimeric Genes and Insulin‐Like Growth Factor Gene Regulation
4 Future Directions
Figure 1. Figure 1.

Comparison of amino‐acid sequences of human proinsulin, insulin‐like growth factors I and II, (IGF‐I and IGF‐II). Amino acids, in single letter code, are aligned to show regions of homology. Regions of homology are boxed. Peptide domains are identified by the letter above each group.

Figure 2. Figure 2.

Schematic of structure of isolated mammalian insulin‐like growth factor I (IGF‐I) cDNAs to illustrate mRNA and precursor heterogeneity. Open bar shows identical coding sequence in all isolated IGF‐I cDNAs; shaded bar indicates distinct 5′ sequences in class 1 or class 2 IGF‐I cDNAs that specify variable leader peptide and/or 5′UT and distinct 3′ sequences on Ea and Eb type cDNAs that encode different E domains and 3′UT.

Figure 3. Figure 3.

Evolutionary conservation of insulin‐like growth factor I (IGF‐I) precursor sequences. a: Comparison of amino‐acid sequences of mature IGF‐I peptides in several species. Dashes indicate identical amino acids. b: Comparisons of precursor peptide sequences (class 1 or class 2 leader peptides and Ea or Eb domains) predicted by isolated IGF‐I cDNAs. Cow and sheep class 1 and class 2 leader peptides are prefixed with an asterisk, because a glutamine (Q) residue has been omitted to maximize alignment with leader peptides from other species. Asterisks above class 1 and class 2 leader peptides indicate methionine residues common to both coding sequences. Note that the first 16 amino acids shown for the COOH terminal Ea and Eb domains are identical, followed by the distinct precursor peptides. In the salmon Ea domain, 27 amino acids were deleted to maximize homology with the Ea domains of other species.

Figure 4. Figure 4.

Autoradiograms of RNase protection and Northern hybridization assays to illustrate tissue‐specific expression of insulin‐like growth factor I mRNA subtypes. A: RNAse protection shows expression of class 1 and class 2 IGF‐I mRNAs in rat liver and low or barely detectable expression in nonhepatic tissues such as brain and ileum. B: Northern hybridization with probes specific for class 1, class 2, Ea, or Eb type on RNA from adult rat liver illustrates the size heterogeneity of each IGF‐I mRNA sub‐type.

Figure 5. Figure 5.

Structure of human and rat insulin‐like growth factor I (IGF‐I) genes. Schematic to show human (top) and rat (bottom) IGF‐I gene structure. Solid boxes indicate the exons common to all characterized human or rat IGF‐I mRNAs. Open boxes indicate exons that are alternately spliced. Alternate splicing of exons 1, 2, 5, and 6 that leads to four potential mRNA coding sequences for human or rat IGF‐I precursors is indicated.

Figure 6. Figure 6.

Schematic of the transcription starts sites and characterized gene regulatory regions in human and rat insulin‐like growth factor I (IGF‐I) genes. a: Top panel shows a schematic of exons 1–3, introns 1 and 2, and 5′ flanking sequence in human and rat IGF‐I genes. Major (tall arrows) and minor (short arrows) transcription start sites are shown together with translation initiation AUG/Met codons that would be included in derived mRNAs as a result of use of alternate transcription start sites. Open arrow indicates a proximal transcription start site mapped only in exon 1 of the rat IGF‐I gene that would exclude AUG/Met codon −48 from the derived mRNA 3. A DNase 1 hypersensitive site in intron 2, HS7, that is associated with growth hormone (GH) induction of the IGF‐I gene is also shown in the line underneath intron 2 265,266. // indicates that size of introns 1 and 2 is not to scale relative to the size of the exons. Bottom panel shows expanded view of exon 1 and 5′ flanking DNA to indicate the major regions implicated as cis‐acting regulatory sequences and binding sites for transcription factors. Scale above the sequence is in nucleotides. The first base of the most downstream major transcription start site in human and rat IGF‐I genes is designated +1. Upstream and downstream regions are indicated by negative and positive numbers, respectively. The location of this major transcription start site relative to other transcription start sites is indicated by the tall and short arrows above the scale, as also indicated in the top panel. The transcribed portion of exon 1 is indicated by the wide bar and the nontranscribed flanking region by the narrow bar. Binding of transcription factors HNF‐3β, HNF‐1α, and C/EBPα to the indicated regions and functional transcriptional activation have been documented only for the human IGF‐I gene 190,191,192. Open triangles (numbered 1–4) are regions that confer liver‐specific transcriptional activation of rat promoter 1 in an in vitro transcription system 199. FP1 indicates region of rat exon 1 defined as a major transcription regulatory sequence and site of protein binding in C6 glioma cells 166. HS3A, HS3B, and HS3C indicate the location of 3 DNase I footprints in rat IGF‐I gene implicated as sites of DNA: protein interaction during GH activation of the IGF‐I gene in rat liver 265. Labeled line beneath indicates location of two DNase I hypersensitive sites implicated in developmental induction of hepatic IGF‐I gene expression in the rat 140. The overlap in gene regulatory elements in human and rat IGF‐I genes defined by multiple different approaches and in multiple systems indicates a growing consensus about molecular mechanisms of transcriptional activation of promoter 1. B: Summary of sequences in human and rat IGF‐I exon 1 and 5′ flanking DNA corresponding to the predicted locations of gene regulatory elements and sites of transcription factor binding. The location of each sequence is indicated in nucleotides upstream (‐) or downstream (+) of the downstream major transcription site in human or rat exons 1 designated +1 in Figure 6A. In the original reports describing these sites, not all authors use this site as +1 and so their numerical designation may differ from that used here. The transcription factors indicated in parentheses correspond to factors with consensus binding sites that show some degree of homology with the putative gene regulatory elements.

Figure 7. Figure 7.

Comparison of amino acid sequences of insulin‐like growth factor II (IGF‐II) precursors from several species. Dashes indicate identical amino acids compared with human precursor.

Figure 8. Figure 8.

Comparison of the rat and human insulin‐like growth factor II (IGF‐II) genes. Solid bars indicate coding exons for IGF‐II precursor. Open bars indicate exons that specify 5′ or 3′ UTs. Alternate splicing of human exons 1–6 that leads to human IGF‐II mRNAs that differ in 5′ UT and size is indicated. The 1.8 kb mRNA fragment is comprised entirely of exon 9 sequence. In the human gene, hP1–hP4 indicate four promoters that control expression of different IGF‐II mRNAs. In the rat, promoters rP1, rP2, and rP3 are counterparts of hP2, hP3, and hP4, respectively. ins indicates the insulin gene that lies close to the IGF‐II gene in human and rat.

Figure 9. Figure 9.

A schematic of human insulin‐like growth factor II (IGF‐II) promoter 3 and putative gene regulatory regions. Binding sites for EGR and WT1 transcription factors 68,69,257,258,281 are shown together with important sites of DNA: protein interaction mapped by DNase footprinting, gel shift, or in vitro trancription (PE3‐1‐PE3‐4) 257,258,275.

Figure 10. Figure 10.

“Enhancer competition” model of insulin‐like growth factor II (IGF‐II) imprinting. Top: Schematic of IGF‐II and H19 locus in wild‐type mouse showing two distal enhancers (•) downstream of H19. On the paternal allele, shading indicates methylation of 5′ portion of H19 gene. Large arrows indicate transcription of IGF‐II gene from the paternal allele and H19 from the maternal allele as indicated in figure on right. Middle: Schematic of the consequences of deletion of enhancers on maternal allele as reported in 157. No effect on IGF‐II expression from paternal allele but decreased expression of H19 from maternal allele as indicated. Bottom: Schematic of consequences of deletion of enhancers on paternal allele as reported in 157. IGF‐II expression from paternal allele is decreased whereas expression of H19 from maternal allele is unaffected.



Figure 1.

Comparison of amino‐acid sequences of human proinsulin, insulin‐like growth factors I and II, (IGF‐I and IGF‐II). Amino acids, in single letter code, are aligned to show regions of homology. Regions of homology are boxed. Peptide domains are identified by the letter above each group.



Figure 2.

Schematic of structure of isolated mammalian insulin‐like growth factor I (IGF‐I) cDNAs to illustrate mRNA and precursor heterogeneity. Open bar shows identical coding sequence in all isolated IGF‐I cDNAs; shaded bar indicates distinct 5′ sequences in class 1 or class 2 IGF‐I cDNAs that specify variable leader peptide and/or 5′UT and distinct 3′ sequences on Ea and Eb type cDNAs that encode different E domains and 3′UT.



Figure 3.

Evolutionary conservation of insulin‐like growth factor I (IGF‐I) precursor sequences. a: Comparison of amino‐acid sequences of mature IGF‐I peptides in several species. Dashes indicate identical amino acids. b: Comparisons of precursor peptide sequences (class 1 or class 2 leader peptides and Ea or Eb domains) predicted by isolated IGF‐I cDNAs. Cow and sheep class 1 and class 2 leader peptides are prefixed with an asterisk, because a glutamine (Q) residue has been omitted to maximize alignment with leader peptides from other species. Asterisks above class 1 and class 2 leader peptides indicate methionine residues common to both coding sequences. Note that the first 16 amino acids shown for the COOH terminal Ea and Eb domains are identical, followed by the distinct precursor peptides. In the salmon Ea domain, 27 amino acids were deleted to maximize homology with the Ea domains of other species.



Figure 4.

Autoradiograms of RNase protection and Northern hybridization assays to illustrate tissue‐specific expression of insulin‐like growth factor I mRNA subtypes. A: RNAse protection shows expression of class 1 and class 2 IGF‐I mRNAs in rat liver and low or barely detectable expression in nonhepatic tissues such as brain and ileum. B: Northern hybridization with probes specific for class 1, class 2, Ea, or Eb type on RNA from adult rat liver illustrates the size heterogeneity of each IGF‐I mRNA sub‐type.



Figure 5.

Structure of human and rat insulin‐like growth factor I (IGF‐I) genes. Schematic to show human (top) and rat (bottom) IGF‐I gene structure. Solid boxes indicate the exons common to all characterized human or rat IGF‐I mRNAs. Open boxes indicate exons that are alternately spliced. Alternate splicing of exons 1, 2, 5, and 6 that leads to four potential mRNA coding sequences for human or rat IGF‐I precursors is indicated.



Figure 6.

Schematic of the transcription starts sites and characterized gene regulatory regions in human and rat insulin‐like growth factor I (IGF‐I) genes. a: Top panel shows a schematic of exons 1–3, introns 1 and 2, and 5′ flanking sequence in human and rat IGF‐I genes. Major (tall arrows) and minor (short arrows) transcription start sites are shown together with translation initiation AUG/Met codons that would be included in derived mRNAs as a result of use of alternate transcription start sites. Open arrow indicates a proximal transcription start site mapped only in exon 1 of the rat IGF‐I gene that would exclude AUG/Met codon −48 from the derived mRNA 3. A DNase 1 hypersensitive site in intron 2, HS7, that is associated with growth hormone (GH) induction of the IGF‐I gene is also shown in the line underneath intron 2 265,266. // indicates that size of introns 1 and 2 is not to scale relative to the size of the exons. Bottom panel shows expanded view of exon 1 and 5′ flanking DNA to indicate the major regions implicated as cis‐acting regulatory sequences and binding sites for transcription factors. Scale above the sequence is in nucleotides. The first base of the most downstream major transcription start site in human and rat IGF‐I genes is designated +1. Upstream and downstream regions are indicated by negative and positive numbers, respectively. The location of this major transcription start site relative to other transcription start sites is indicated by the tall and short arrows above the scale, as also indicated in the top panel. The transcribed portion of exon 1 is indicated by the wide bar and the nontranscribed flanking region by the narrow bar. Binding of transcription factors HNF‐3β, HNF‐1α, and C/EBPα to the indicated regions and functional transcriptional activation have been documented only for the human IGF‐I gene 190,191,192. Open triangles (numbered 1–4) are regions that confer liver‐specific transcriptional activation of rat promoter 1 in an in vitro transcription system 199. FP1 indicates region of rat exon 1 defined as a major transcription regulatory sequence and site of protein binding in C6 glioma cells 166. HS3A, HS3B, and HS3C indicate the location of 3 DNase I footprints in rat IGF‐I gene implicated as sites of DNA: protein interaction during GH activation of the IGF‐I gene in rat liver 265. Labeled line beneath indicates location of two DNase I hypersensitive sites implicated in developmental induction of hepatic IGF‐I gene expression in the rat 140. The overlap in gene regulatory elements in human and rat IGF‐I genes defined by multiple different approaches and in multiple systems indicates a growing consensus about molecular mechanisms of transcriptional activation of promoter 1. B: Summary of sequences in human and rat IGF‐I exon 1 and 5′ flanking DNA corresponding to the predicted locations of gene regulatory elements and sites of transcription factor binding. The location of each sequence is indicated in nucleotides upstream (‐) or downstream (+) of the downstream major transcription site in human or rat exons 1 designated +1 in Figure 6A. In the original reports describing these sites, not all authors use this site as +1 and so their numerical designation may differ from that used here. The transcription factors indicated in parentheses correspond to factors with consensus binding sites that show some degree of homology with the putative gene regulatory elements.



Figure 7.

Comparison of amino acid sequences of insulin‐like growth factor II (IGF‐II) precursors from several species. Dashes indicate identical amino acids compared with human precursor.



Figure 8.

Comparison of the rat and human insulin‐like growth factor II (IGF‐II) genes. Solid bars indicate coding exons for IGF‐II precursor. Open bars indicate exons that specify 5′ or 3′ UTs. Alternate splicing of human exons 1–6 that leads to human IGF‐II mRNAs that differ in 5′ UT and size is indicated. The 1.8 kb mRNA fragment is comprised entirely of exon 9 sequence. In the human gene, hP1–hP4 indicate four promoters that control expression of different IGF‐II mRNAs. In the rat, promoters rP1, rP2, and rP3 are counterparts of hP2, hP3, and hP4, respectively. ins indicates the insulin gene that lies close to the IGF‐II gene in human and rat.



Figure 9.

A schematic of human insulin‐like growth factor II (IGF‐II) promoter 3 and putative gene regulatory regions. Binding sites for EGR and WT1 transcription factors 68,69,257,258,281 are shown together with important sites of DNA: protein interaction mapped by DNase footprinting, gel shift, or in vitro trancription (PE3‐1‐PE3‐4) 257,258,275.



Figure 10.

“Enhancer competition” model of insulin‐like growth factor II (IGF‐II) imprinting. Top: Schematic of IGF‐II and H19 locus in wild‐type mouse showing two distal enhancers (•) downstream of H19. On the paternal allele, shading indicates methylation of 5′ portion of H19 gene. Large arrows indicate transcription of IGF‐II gene from the paternal allele and H19 from the maternal allele as indicated in figure on right. Middle: Schematic of the consequences of deletion of enhancers on maternal allele as reported in 157. No effect on IGF‐II expression from paternal allele but decreased expression of H19 from maternal allele as indicated. Bottom: Schematic of consequences of deletion of enhancers on paternal allele as reported in 157. IGF‐II expression from paternal allele is decreased whereas expression of H19 from maternal allele is unaffected.

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Pauline Kay Lund. Insulin‐Like Growth Factors: Gene Structure and Regulation. Compr Physiol 2011, Supplement 24: Handbook of Physiology, The Endocrine System, Hormonal Control of Growth: 537-571. First published in print 1999. doi: 10.1002/cphy.cp070518