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  • Author: F. J. Ballard x
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F. Z. Upton, G. L. Francis, M. Ross, J. C. Wallace and F. J. Ballard

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.

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F. Z. Upton, L. Szabo, J. C. Wallace and F. J. Ballard

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.

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P. McKinnon, M. Ross, J. R. E. Wells, F. J. Ballard and G. L. Francis

ABSTRACT

Recombinant human insulin-like growth factor-I (hIGF-I) and a biologically potent variant lacking the N-terminal tripeptide (des(1–3)IGF-I) were produced from transfected Chinese hamster ovary cells. The constructs encoding the signal peptide, sequence of the mature peptide and a C-terminal extension peptide were expressed under the control of a Rous sarcoma virus promoter. Successfully transfected clones secreting correctly processed recombinant hIGF-I or des(1–3)IGF-I were selected by their secretion of IGF-I-like activity into the culture medium. The recombinant peptides were purified to homogeneity as assessed by high-performance liquid chromatography and N-terminal sequence analysis. The purified recombinant peptides exhibited biological potencies equivalent to authentic IGF-I and des(1–3)IGF-I respectively.

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Z Upton, G L Francis, K Kita, J C Wallace and F J Ballard

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.

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R. King, J. R. E. Wells, P. Krieg, M. Snoswell, J. Brazier, C. J. Bagley, J. C. Wallace, F. J. Ballard, M. Ross and G. L. Francis

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.

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G. L. Francis, M. Ross, F. J. Ballard, S. J. Milner, C. Senn, K. A. McNeil, J. C. Wallace, R. King and J. R. E. Wells

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.