Expression, promoter usage and parental imprinting status of insulin-like growth factor II (IGF2) in human hepatoblastoma: uncoupling of IGF2 and H19 imprinting

We have studied the promoter utilization and parental imprinting status of human IGF2 in three genetically informative hepatoblastomas from patients ranging in age from 9 months to 3 years. In all three cases, there is a downregulation of promoter P1 in the tumor tissues while the P2 and P3 promoters are upregulated compared to the normal liver. One of three patients displayed loss of imprinting (LOI) of IGF2 in the tumor tissue. We also investigated the expression of the H19 gene in all three cases and the methylation pattern in H19 from the patient with LOI of IGF2. The expression of H19 was greatly reduced in all tumors. Monoallelic H19 expression however, was retained even in the case which showed LOI of IGF2. Unlike the situation in Wilms' tumor, no differences in the methylation pattern between the normal liver and tumor tissues were observed in the H19 promoter or 3' region, using HpII analysis. We show here, that in contrast to the situation in Wilms' tumor, H19 expression is not a prerequisite for maintaining a monoallelic IGF2 expression.

Monoallelic expression of IGF2 at the human fetal/maternal boundary

IGF2 is expressed in both placental and decidual tissues, enabling an analysis of the parental imprinting over the fetomaternal boundary. Evidence is provided that IGF2 is monoallelically expressed in both placenta and pregnant, as well as nonpregnant, endometrium. These observations suggest that the maternally derived IGF2 allele is inactivated during germline transmission. Comparison of promoter usage in decidua and placental samples shows that the P3 promoter appears to regulated independently of the others. These observations are discussed with respect to current models of IGF2 imprinting and the hypothesized conflict of parental reproductive interests which bears on the phenomenon of parental imprinting.

Promoter-specific IGF2 imprinting status and its plasticity during human liver development

IGF2 has been shown to be expressed preferentially from the paternally derived allele, although the maternal allele can be found active during both prenatal and postnatal development as well as in neoplastic tumours in humans. We addressed here whether or not the biallelic expression patterns that can be seen during postnatal human liver development reflected a coordinated change in the activities of the four promoters of human IGF2. We show here that the P2, P3 and P4 promoters, but not the P1 promoter, display monoallelic activity in embryonic, neonatal and younger infant liver specimens. The P2, P3 and P4 promoters can, however, be found active either monoallelically or biallelically or even monoallelically on opposite parental alleles in older infant and adult liver specimens. In contrast, H19, which is closely linked to IGF2, is monoallelically expressed in all postnatal liver samples analysed. We conclude that the functional imprinting status of IGF2 during postnatal liver development appears to be promoter/enhancer-specific and either partly or completely independent of H19.

Overlapping patterns of IGF2 and H19 expression during human development: biallelic IGF2 expression correlates with a lack of H19 expression

The spatial patterns of IGF2 and H19 gene expression are strikingly similar during parts of human embryonic/fetal and early postnatal development. Notable exceptions were found with the ciliary anlage of the embryonic retina and the choroid plexus/leptomeninges, where transcripts from the IGF2 but not the H19 locus could be detected. Moreover, in contrast to the other tissue samples examined, the choroid plexus/leptomeninges expressed both parental IGF2 alleles. Whilst RNase protection analysis revealed a weak activity of the P1 promoter in the choroid plexus/leptomeninges, the P2, P3 and P4 promoters were all active wherever IGF2 was expressed. We discuss these observations with respect to a hypothesized coordinated control of the reciprocally imprinted and closely linked IGF2 and H19 loci.

Parental imprinting and the IGF2 gene

The phenomenon of parental imprinting has become increasingly important in disciplines such as evolution, genetics, molecular biology, embryology and pathology. Principally, parental imprinting refers to a parent-of-origin dependent expression of a subset of autosomal loci, independent of the sex of the offspring. Today, at least seven such loci have been identified, including the human IGF2 gene. It appears that the set of imprinted genes is not always identical between the species, although the importance of maintaining this kind of gene regulation is evolutionarily conserved. It is particularly interesting from the clinical point of view that a number of human diseases, such as the Beckwith-Wiedemann and Prader-Willi/Angelman syndromes, appear to involve unbalanced parental contributions of imprinted loci. We show here that the four different human IGF2 promoters are expressed mono- and/or biallelically in complex patterns in postnatal liver specimens.

Insulin-like growth factor II in the mink (Mustela vison): determination of a cDNA nucleotide sequence and developmental regulation of its expression

Mink cDNA for insulin-like growth factor II (IGF-II) has been isolated by cDNA synthesis from bulk mRNA and subsequent PCR-screening. Positive clones were sequenced and analyzed. Analysis of a cDNA revealed that, compared with human, the mature mink IGF-II peptide contains 68 instead of 67 amino acids with a serine insertion at residue 40. With this exception, the homology between the human and mink mature peptide is 100% and is 94% between rat and mink. Comparing IGF-II transcripts in fetal and adult mink liver, fetal tissue contains three transcripts of 5.8, 4.5, and 4.0 kb. In contrast, adult liver expresses low levels of a 4.6-kb transcript.

IGF2 is parentally imprinted during human embryogenesis and in the Beckwith-Wiedemann syndrome

The phenomenon of parental imprinting involves the preferential expression of one parental allele of a subset of chromosomal genes and has so far only been documented in the mouse. We show here, by exploiting sequence polymorphisms in exon nine of the human insulin-like growth factor 2 (IGF2) gene, that only the paternally-inherited allele is active in embryonic and extra-embryonic cells from first trimester pregnancies. In addition, only the paternal allele is expressed in tissues from a patient who suffered from Beckwith-Wiedemann syndrome. Thus the parental imprinting of IGF2 appears to be evolutionarily conserved from mouse to man and has implications for the generation of the Beckwith-Wiedemann syndrome.

Heparin binding growth factors and the control of teratoma cell proliferation

The role of basic fibroblast growth factor (bFGF) in the control of teratoma cell proliferation was examined. It was found that bFGF stimulates proliferation at low concentrations but induces cell migration at higher doses. These effects could be efficiently counteracted by addition of protamine sulphate. Moreover the bFGF gene is actively transcribed in primary human testicular embryonal carcinomas but was not expressed in any other embryonic tumour examined. The biological implications of these findings will be discussed.

Promoter elements and transcriptional regulation of the acetylcholinesterase gene

The 5' region of the acetylcholinesterase gene from the electric ray Torpedo californica has been cloned and its cap site identified. The 5' untranslated region is divided into two exons where a small exon extending between bp -22 to -60 is alternatively spliced. Cap sites are defined at two positions, bp -138 and -143. Twenty-one base pairs 5' of the -143 cap site a repeating TATA sequence is found. Further upstream in the gene consensus sequences for Sp1, AP1, and AP2 factors are evident. The promoter region of the acetylcholinesterase gene enhances transcription of a luciferase reporter gene transfected into C2 myoblasts. However, increased transcription was not evident after C2 myoblasts were induced to form myotubes. Cotransfection of this construct with c-Jun (AP1) and AP2 expression vectors shows marked increases of transcription rates in HepG2 and C2 cells. Protein kinase A elicited regulation of expression is also evident in quail fibroblasts. In gel retardation experiments both recombinant c-Jun (AP1) and AP2 proteins bind to the appropriate Torpedo sequences. Cellular extracts from the Torpedo electric organ exhibit AP2 binding activity. Thus, although all facets of specific regulation expected upon differentiation of mammalian muscle cells were not evident, the 5'-flanking region from the Torpedo AChE gene contains consensus sequences and functional promoter elements typical of mammalian nerve and muscle systems.

Isolation of an insulin-like growth factor II cDNA from guinea pig liver: expression and developmental regulation

Insulin-like growth factor II (IGF-II) cDNA was isolated from adult guinea pig liver by polymerase chain reaction (PCR) screening. A cDNA sequence was obtained corresponding to part of the preproIGF-II, including the signal peptide, the mature IGF-II and 37 amino acids of the acid carboxy-terminal E-domain. Amino acid sequence prediction, based on the cDNA clone, showed that mature guinea pig IGF-II has a high homology with both human and rat IGF-II, 100 and 94% identity, respectively. Levels of IGF-II mRNA in guinea pigs of different ages were analyzed by solution hybridization/RNase protection assay using part of the isolated IGF-II cDNA as a probe. There is a marked developmental regulation of IGF-II after birth. IGF-II mRNA levels were high in fetal livers, and decreased 15- to 30-fold in adults. As in man, but in contrast to rats, adult guinea pigs have significant levels of IGF-II mRNA in the liver. In fetal guinea pigs, the expression of IGF-II mRNA was 5-, 2- and 70-fold lower in kidney, skeletal muscle and brain cortex, respectively, than in liver. IGF-II mRNA levels in kidney and skeletal muscle of fetal guinea pigs were 5- and 4-fold higher, respectively, compared with adults. Similar sizes of IGF-II mRNA transcripts could be observed on Northern blots in newborn rats and in fetal guinea pigs. Our conclusions are that the mature IGF-II peptide in the guinea pig is 100% identical to the mature peptide in the human.(ABSTRACT TRUNCATED AT 250 WORDS)

Differentiation associated modulation of K-FGF expression in a human teratocarcinoma cell line and in primary germ cell tumours

The human teratocarcinoma cell line Tera 2 can be induced to differentiate in vitro after exposure to retinoic acid. We show in this paper that whereas the K-FGF oncogene is expressed in undifferentiated cells, addition of retinoic acid rapidly (less than 60 min) downregulates the expression of this gene. However, when cells are cultured in RA for an extended period of time (greater than 15 days) K-FGF transcripts reappear. We also report that K-FGF is expressed in approximately one-third of primary human germ cell tumours but not in the corresponding normal testicular tissue.

Molecular cloning of mouse acetylcholinesterase: tissue distribution of alternatively spliced mRNA species

We have isolated cDNA clones encoding acetylcholinesterase from mouse muscle and brain. The polymerase chain reaction was used to amplify cDNA clones from C2 myotubes encoding the entire open reading frame and large segments of the 5' and 3' untranslated regions. The muscle cDNA clones were used to isolate clones from a brain library encoding the same mRNA species. The mouse clones encode a catalytic subunit containing a C-terminal sequence similar to that of the hydrophilic species of Torpedo. The mouse acetylcholinesterase sequence shares approximately 88% and 61% amino acid identity with bovine and Torpedo acetylcholinesterases, respectively, but only 52% identity with mouse butyrylcholinesterase, the sequence of which we have also deduced by molecular cloning. Northern blot and RNAase protection analyses indicate that the cDNA clones were derived from the acetylcholinesterase transcript that predominates in most expressing tissues. In contrast, erythroid cells are enriched in an mRNA species whose sequence diverges from that of the cDNA in the region encoding the C-terminus of the enzyme.

Single gene encodes glycophospholipid-anchored and asymmetric acetylcholinesterase forms: alternative coding exons contain inverted repeat sequences

Polymorphic forms of acetylcholinesterase are tethered extracellularly either as dimers membrane-anchored by a glycophospholipid or as catalytic subunits disulfidelinked to a collagen tail that associates with the basal lamina. Genomic clones of acetylcholinesterase from T. californica revealed that individual enzyme forms are encoded within a single gene that yields multiple mRNAs. Each enzyme form is encoded in three exons: the first two exons, bases -22 to 1502 and 1503 to 1669, encode sequence common to both forms, while alternative third exons encode a hydrophobic C-terminal region, to which a glycophospholipid is added upon processing, and a nonprocessed C-terminus, yielding a catalytic subunit that disulfide-links with a collagen-like structural unit. The 3' untranslated region of each alternative exon contains tandem repeat sequences that are inverted with respect to the other exon. This may either dictate alternative exon usage by formation of cis stem-loops or affect the abundance of translatable mRNA by trans-hybridization between the alternative spliced mRNA species.

The alpha-saturation and terminal events in dolichol biosynthesis

Incubations of 10,000 X g supernatant from rat liver with [3H]mevalonate were performed and the labeling of polyprenols was studied. It was demonstrated that factors like pH, substrate concentration, and presence of detergent not only greatly influence the total incorporation but also the relative distribution of radioactivity among the isoprenologues. The synthesis was shown to be extremely sensitive to Triton X-100. Substrate concentrations of 1 and 100 microM mostly gave polyprenols with 18 and 20 isoprenes, respectively. At a given substrate concentration, pH 6.5 resulted in shorter polyprenols than did pH 7.5. Ozonolytic fragmentation demonstrated that in the initial phase of incubation, polyprenols are elongated by 1 isoprene residue and saturated to give dolichols. No substantial dephosphorylation of polyprenyl phosphates to the free alcohol occurred. The production of dolichol in vitro was shown to utilize NADH for the saturation event. This seemed to occur concomitantly with the synthesis. alpha-Saturation of polyprenyl-P could not be achieved with the procedures employed. It is proposed that the synthesis of dolichol and dolichyl-P do not share the same terminal steps; saturation and terminal isoprene condensation occur in cooperation; and substrate concentration and pH influence the terminal enzyme(s) and the nature of the final product in the polyprenol biosynthesis.

Metabolic labeling of dolichol and dolichyl phosphate in isolate hepatocytes

Isolated hepatocytes from rat liver were incubated with [3H]mevalonate, and the labeling of polyprenols in the microsomal fraction was followed. After a 1-min incubation the alpha-unsaturated forms of polyprenyl-P2, -P, and polyprenol were mainly labeled and at this time point only 2, 8, and 17%, respectively, of the label was associated with the saturated forms. In the case of the free alcohol 2 h of incubation was required before all the labeling was recovered in the saturated form. After 1 min polyprenols and polyprenyl-P with 20 and 21 isoprene residues demonstrated much higher specific labeling than the shorter compounds, but after 5 min these differences were greatly reduced. In experiments utilizing short incubation times and chasing no evidence has been obtained that the phosphorylated form is a precursor of the free alcohol or vice versa, that the free alcohol is a precursor of the phosphorylated form. In human liver about 1% of the dolichol is present in the alpha-unsaturated form. These experiments suggest that: 1) the alpha-unsaturated form is the precursor of the alpha-saturated free alcohol, 2) dolichol does not necessarily arise directly from dephosphorylation of the phosphorylated form, and 3) the free alcohol is for the most part not phosphorylated under in vivo conditions in rat liver.