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ABSTRACT

This study describes the biosynthesis of a human epidermal growth factor fusion protein, Long EGF, that has a 53 amino acid extension peptide derived from the 46 N-terminal amino acids of porcine GH. The approach allowed the production of Long EGF at high efficiency due to the expression of the fusion protein in high yield as inclusion bodies in Escherichia coli. Long EGF had a slightly lower potency compared with native EGF in a range of assays, including binding to anti-EGF antibodies or the EGF receptor, stimulation of Balb/3T3 fibroblast and rat intestinal epithelial cell growth, as well as counteracting the inhibition of mink lung epithelial cell proliferation by transforming growth factor-β1.

Degradation of Long EGF and native EGF was compared in gastrointestinal flushings as an indication of whether the EGF domain of the fusion protein would be protected from proteolytic cleavage and be useful as a trophic agent in the gut. Incubation with flushings from the stomach or jejunum of rats caused rapid cleavage of the extension peptide, releasing native EGF. A C-terminal truncation of Arg⁵³ in the stomach and a removal of the C-terminal pentapeptide (⁴⁹Trp-Trp-Glu-Leu-Arg⁵³) in the small bowel was demonstrated by N-terminal sequencing and mass spectrometry. The degradation patterns were reflected by changes in migration of products on SDS-PAGE and in subsequent binding activities to the EGF receptor and anti-EGF antibodies. The data show that a human EGF fusion protein can be produced efficiently in a bacterial expression system and that it retains biological activity in vitro. Although the extension peptide was rapidly cleaved from Long EGF in both stomach and small bowel producing similar biological activity to native EGF, it could not prevent subsequent degradation of the EGF domain. Other strategies are being investigated to develop an effective oral form of EGF that resists digestion by proteases in the gastrointestinal tract.

ABSTRACT

Recombinant chicken insulin-like growth factor-I (cIGF-I) has been produced in Escherichia coli after first modifying a plasmid that coded for a human IGF-I (hIGF-I) fusion protein, in order to introduce codons for the eight amino acid substitutions. The cIGF-I fusion protein, deposited in bacterial inclusion bodies, was dissolved under reducing conditions, desalted, subjected to anion-exchange chromatography to remove proteinases, refolded and partially purified by reverse-phase high-performance liquid chromatography. The fusion protein was cleaved with hydroxylamine after which cIGF-I was purified to homogeneity by three additional chromatographic steps. Recombinant cIGF-I was equipotent with hIGF-I in cell culture bioassays of protein synthesis and breakdown using rat L6 myoblasts and chick embryo fibroblasts. Binding of radiolabelled cIGF-I and hIGF-I was also equivalent in the two cell lines, as was their binding in ligand blots of chicken, sheep and human plasma. The cross-reactivity of cIGF-I in a polyclonal hIGF-I radioimmunoassay was 60% of that observed with hIGF-I. The availability of recombinant cIGF-I will facilitate investigations into the role of IGF-I in chicken growth and development.

ABSTRACT

The primary structure of the insulin-like growth factor-binding protein (IGFBP) produced by the bovine kidney cell line, MDBK, has been deduced from the cDNA clone. The MDBK binding protein precursor consists of a hydrophobic pre-peptide of at least 26 amino acids and a mature protein of 284 amino acids. The predicted protein sequence shares extensive sequence similarity with both the rat (82%) and human (89%) IGFBP-2s, so that the MDBK binding protein is clearly the bovine counterpart of IGFBP-2. The protein has limited similarity with classes 1 (31%) and 3 (31%) human IGFBPs, except that all 18 cysteine residues are conserved. Other features deduced from the bovine IGFBP-2 cDNA include: an abundance of leucine in the pre-peptide, an Arg-Gly-Asp sequence, absence of N-linked glycosylation sites, and an imperfect polyadenylation signal as well as an ATTTA motif in the 3′ non-coding DNA. Western blotting indicated that this binding protein is widely distributed in bovine fluids as well as in media conditioned by bovine cell lines. Proteins immunologically related to bovine IGFBP-2 were detected not only in sheep, but also in chickens, indicating that this IGFBP is not exclusively mammalian.

ABSTRACT

Recombinant chicken (c)IGF-II has been produced in Escherichia coli after first modifying a plasmid that coded for a human (h)IGF-II fusion protein. The cIGF-II fusion protein, deposited in bacterial inclusion bodies, was dissolved under reducing conditions, desalted, subjected to anion-exchange chromatography and refolded. Recombinant cIGF-II was then released from the fusion protein using a genetically engineered serine protease and purified to homogeneity by reverse-phase HPLC. In vitro analysis of recombinant cIGF-II revealed differences between cIGF-II and its human counterpart. Recombinant cIGF-II was less potent than hIGF-II in stimulating protein synthesis in rat myoblasts. This appeared to be due to a decreased affinity for the type-1 IGF receptor. The human and chicken peptides were similar, however, in studies assessing binding to the type-2 IGF receptor and to IGF-binding proteins. Moreover, recombinant cIGF-II and hIGF-II were equipotent in both biological and receptor binding studies in chick embryo fibroblasts, suggesting that there may be a difference between mammalian and avian type-1 IGF receptors.

Understanding of the principal pathways of steroid hormone biosynthesis was established over two decades ago through advances in steroid radioisotopic and chromatographic techniques. When the enzymes of individual pathways could be examined in more detail, the dissection of the complex pattern of enzyme activities began. At many points, separate pathways employ precisely the same enzyme for equivalent catalytic steps, e.g. for 21-hydroxylase, 11 β-hydroxylase, aromatase and several dehydrogenases (Orth et al. 1992). A further economy was found for 17α-hydroxylase and 17,20-lyase activities, which co-purify with the same P450c17 polypeptide. This enzyme was later cloned and expressed in tissue culture cells, revealing that, contrary to the enzyme in rat, human and cattle, 17α-hydroxylase cannot convert 17α-hydroxyprogesterone to androstenedione (Bradshaw et al. 1987, Fevold et al. 1989). Further complexity emerged with the existence of multiple tissue-specific forms of 5α-reductase (Wilson et al. 1993), and 3β-, 11β- and 17β-hydroxysteroid dehydrogenases, most of which

ABSTRACT

The development of an efficient expression system for insulin-like growth factor-I (IGF-I) in Escherichia coli as a fusion protein is described. The fusion protein consists of an N-terminal extension made up of the first 46 amino acids of methionyl porcine GH ([Met¹]-pGH) followed by the dipeptide Val-Asn. The latter two residues provide a unique hydroxylamine-sensitive link between [Met¹]-pGH(1-46) and the N-terminal Gly of IGF-I. Downstream processing of the fusion proteins involved isolation of inclusion bodies, cleavage at the Asn-Gly bond, refolding of the reduced IGF-I peptide and purification to homogeneity. This expression system was also used to produce two variants of IGF-I in which Glu³ was substituted by either Gly or Arg to give [Gly³]-IGF-I and [Arg³]-IGF-I respectively. Production of milligram quantities of IGF-I peptide was readily achieved. The purity of the IGF-I, [Gly³]-IGF-I and [Arg³]-IGF-I was established by high-performance liquid chromatography and N-terminal sequence analysis. [Gly³]-IGF-I and [Arg³]-IGF-I were more potent than IGF-I in biological assays measuring stimulation of protein synthesis and DNA synthesis or inhibition of protein breakdown in rat L6 myoblasts. Both analogues bound very poorly to bovine IGF-binding protein-2 and slightly less well than IGF-I to the type-1 receptor on rat L6 myoblasts. We conclude that reduced binding to IGF-binding proteins rather than increased receptor binding is the likely explanation for the greater biological potency of the analogues compared with IGF-I.

ABSTRACT

Three different molecular mass forms of IGF-binding proteins (IGFBPs) were purified from ovine plasma by IGF-I affinity chromatography and reverse-phase HPLC: a 46 kDa doublet and 29 kDa and 24 kDa forms. Amino-terminal sequence analysis confirmed that these proteins were ovine (o)IGFBP-3 (46 kDa) and two molecular size variants of oIGFBP-4. oIGFBP-3 and the 29 kDa form of oIGFBP-4 were shown to be N-glycosylated. Isoelectric points were determined to be at approximately pH 6 for oIGFBP-3 and at pH 7 and pH 7·5 for the 29 and 24 kDa forms of oIGFBP-4 respectively. The two different molecular mass variants of oIGFBP-4 had similar IGF-binding properties. Compared with human IGFBP-3 and oIGFBP-3, the two variants of oIGFBP-4 exhibited lower relative binding to amino-terminally modified IGF-I analogues in a competitive IGF-binding assay. The full protein sequence of oIGFBP-4, as deduced from the cDNA sequence, showed a high degree of identity with rat (90%), human (96%) and bovine (98%) IGFBP-4. The cDNA sequence also showed homology over regions of the 3′ non-coding sequence, particularly in comparison with bovine IGFBP-4 (96%). Northern analysis of mRNA for oIGFBP-4 indicated a 26 kb major transcript and two minor transcripts of approximately 21 and 1·8 kb. oIGFBP-4 mRNA transcripts were detected in adult ewe liver>kidney>lung>>heart and also in several fetal tissues, thus suggesting tissue-specific and developmental regulation. The availability of purified oIGFBP-4 and oIGFBP-3 as well as DNA probes for oIGFBP-4 will enable further study of the properties and functions of these proteins, as well as the establishment of specific assays for these IGFBPs.

ABSTRACT

An efficient expression system in Escherichia coli for several biologically active insulin-like growth factor-I (IGF-I) fusion peptide analogues is described. These novel IGF-I fusion protein analogues have properties that make them very useful reagents in the investigation of IGF-I action. The analogues comprise an IGF-I sequence and the first 11 amino acids of methionyl porcine growth hormone (pGH) and include [Met¹]-pGH(1–11)-Val-Asn-IGF-I, which contains the authentic IGF-I sequence, and two analogues, [Met¹]-pGH(1–11)-Val-Asn-[Gly³]-IGF-I and [Met¹]-pGH(1–11)-Val-Asn-[Arg³]-IGF-I, where Glu3 in the human IGF-I sequence has been replaced by Gly or Arg respectively. The three peptides are referred to as Long IGF-I, Long [Gly³]-IGF-I or Long [Arg³]-IGF-I depending on the IGF-I sequence present. Production of the purified fusion peptides was aided by folding the reduced and denatured fusion peptide sequence under conditions that gave very high yields of biologically active product. Introduction of a hydrophobic N-terminal extension peptide appears to facilitate the correct folding of the IGF-I analogues compared with that obtained previously when folding normal-length IGFs. The biological activities of the IGF-I fusion peptides were compared with authentic IGF-I and the truncated analogue, des(1–3)IGF-I. In L6 rat myoblasts, all the analogues were more potent than authentic IGF-I in their abilities to stimulate protein and DNA synthesis and inhibit protein breakdown. In H35 hepatoma cells, where the IGFs act through the insulin receptor, the Long IGF-I analogues maintained a similar potency relative to IGF-I as was observed in the L6 myoblasts. The order of biological potency in cell lines secreting IGF-binding proteins (IGFBPs) into the medium was Long [Arg³]IGF-I-des(1–3)IGF-I>Long [Gly³]-IGF-I>Long IGF-I>IGF-I. In chicken embryo fibroblasts, a cell line that does not secrete detectable IGFBPs into the medium, Long [Arg³]-IGF-I, was less potent than IGF-I. Investigation of receptor and IGFBP association by these analogues reinforced our previous findings that N-terminal analogues of IGF-I show increased biological potency due to changes in the degree of their IGFBP interactions.

ABSTRACT

A 5-year-old XY pseudohermaphrodite was found to have a defect of steroid biosynthesis consistent with a partial deficiency of the enzyme 3β-hydroxysteroid dehydrogenase (3β-HSD). Circulating concentrations of Δ⁵ steroids and Δ⁵ urinary steroid metabolites were elevated and remained elevated after orchidectomy. There was no evidence of salt loss, plasma renin being within normal limits, and no detectable glucocorticoid abnormality. The coding sequences of the genes for 3β-HSD types I and II were amplified by PCR and screened for mutations by denaturing gradient gel electrophoresis (DGGE) and manual and automatic DNA sequencing. A mutation in the gene for 3β-HSD type II was observed at codon 173 (CTA→CGA), leading in the affected patient to a homozygous substitution in which the leucine at residue 173 was altered to an arginine (L173R). The propositus's 2-year-old XX sister was also homozygous for L173R and showed the biochemical characteristics of partial 3β-HSD deficiency without clinical symptoms or signs. The mutation segregated as an autosomal recessive. Three related heterozygous adult females showed evidence of a small over-production of Δ⁵ steroids and steroid metabolites and a variable reduction in ovarian function. Concentrations of Δ⁵ steroids and steroid metabolites in the heterozygous father of the propositus were within the normal range.

These data are discussed in relation to the endocrine causes of pseudohermaphroditism and hirsutism. Evidence for tight linkage between the genes for 3β-HSD types I and II was obtained using a microsatellite polymorphism in the third intron of the gene for 3β-HSD type II and synonymous and non-synonymous mutations and polymorphisms in the gene for 3β-HSD type I. The latter polymorphisms were located 88 bp apart at the 3′ end of the type I coding sequence and could be physically resolved as haplotypes using DGGE. The application of DGGE to the analysis of mutations in members of a multigene family is discussed.