Developmental pattern of a serum binding protein for multiplication stimulating activity in the rat

Soluble insulin-like growth factor II/mannose 6-phosphate receptor carries multiple high molecular weight forms of insulin-like growth factor II in fetal bovine serum

We have characterized a soluble form of the insulin-like growth factor II/mannose 6-phosphate receptor (sIGF-II/MPR) and bound ligands from bovine serum. Fetal serum contained 2-8 mg/liter sIGF-II/MPR. Affinity-purified receptor isolated by adsorption to phosphomannan-agarose and elution with mannose 6-phosphate contained nearly stoichiometric amounts of bound 7.5-kDa IGF-II. In addition, at least 12 distinct 12-20-kDa proteins immunologically related to IGF-II also copurified with receptor. Receptor was separated from its associated ligands by acidification and gel filtration chromatography. Sequence analysis revealed that the 12-20-kDa proteins have the same amino termini as mature 7.5-kDa IGF-II. Protease and glycosidase treatments revealed that the different high molecular weight IGF-II species contain an identical COOH-terminal extension that is differentially glycosylated with O-linked sugars. Radiolabeled tracer experiments demonstrated that the sIGF-II/MPR carries approximately 1/4 of the IGF-II in fetal bovine serum. These results support a significant role for sIGF-II/MPR in the transport of circulating IGF-II isoforms during development.

Characterisation of the rat acid-labile subunit gene

The acid-labile subunit (ALS) of the ternary insulin-like growth factor binding protein (IGFBP) complex has a central role in regulating the bioavailability of circulating IGF. We have shown that gene expression of ALS in vivo and in vitro is regulated by a variety of factors, including growth hormone (GH). Our aim was to isolate and characterise the ALS gene as a step in defining the mechanism of its regulation. Southern analysis of rat genomic DNA suggests that the ALS gene exists as a single copy in the rat genome. In order to isolate this gene we screened 5 x 10(5) clones and selected fragments of two genomic clones were sequenced. Comparison of this sequence with the cDNA identified two exons and a single approximately 1.1 kb intron. Primer extension experiments suggest two major transcription initiation sites at -539 and -396 nts relative to the translational initiation codon, although there are no consensus TATA-boxes in this region. Analysis of 2.3 kb of 5' flanking sequence identified two LF-A1 sites which may confer the liver-specific expression of the ALS gene. In addition, there are several elements that may be involved in regulation by growth hormone and cytokines.

Insulin-like growth factor-II is a substrate for dipeptidylpeptidase I (cathepsin C). Biological properties of the product

We observed that the lysosomal enzyme, dipeptidylaminopeptidase I (DAP-I) caused the release of trichloroacetic-acid-soluble radioactivity from rat 125I-insulin-like growth factor-II (IGF-II). This activity could be blocked by dipeptide inhibitors of DAP-I, and was enhanced by chloride. Treatment of unlabeled rat IGF-II with DAP-I converted approximately 50% of the IGF-II to a species with a slightly shorter elution time on reverse-phase HPLC, whereas treatment of human IGF-II caused complete conversion to the species with the shorter elution time. Rat IGF-II purified from the rat BRL 3A cell line is a mixture of two molecules beginning with Ala-Tyr-Arg-Pro-Ser- and Tyr-Arg-Pro-Ser- [Marquardt, H., Todaro, G. J., Henderson, L. E. & Oroszlan, S. (1981) J. Biol. Chem. 256, 6859-6865] while human IGF-II begins with Ala-Tyr-Arg-Pro-Ser-. Determination of the N-terminal amino acid sequence of human IGF-II before and after digestion with DAP-I showed that DAP-I cleaved Ala-Tyr, terminating at Arg-Pro-; the rat IGF-II species beginning with Tyr-Arg-Pro-Ser- was resistant to digestion. In order to compare DAP-I-treated IGF-II with native IGF-II for binding to IGF receptors and IGF-binding proteins and in a bioassay, rat and human IGF-II were treated with DAP-I and the digested and undigested species were isolated by reverse-phase HPLC. The IGF-II/mannose 6-phosphate receptor was purified from rat placental membranes, the IGF-I receptor was solubilized from human placental membranes and IGF-binding proteins were partially purified from adult and three-day-old rat sera by sequential gel filtration on Sephadex G-200 (pH 8.0) and Sephadex G-50 (acid pH). The dose/response curves of the two IGF-II species were indistinguishable in radioreceptor assays utilizing the IGF-II/mannose 6-phosphate receptor and the IGF-I receptor and in IGF competitive binding assays utilizing partially purified IGF-binding proteins. The DAP-I-digested and native IGF-II species were also equipotent in stimulating [3H]thymidine incorporation into DNA in the human osteosarcoma cell line, MG-63. We conclude that DAP-I cleaves an N-terminal dipeptide from IGF-II and that this does not result in a change in the biological activity of the molecule.

The impact of maternal serum on development of enolase activity in fetal rat brain cell culture

The effect of gestational age on serum-mediated changes in enolase activity was tested in a fetal rat brain cell culture. After 6 days exposure to graded concentrations (10 and 20%) of nonpregnant female rat sera, enolase activity in brain cell cultures increased from 2.83 +/- 0.03 to 3.74 +/- 0.19 mumol/min/mg protein, P less than 0.01. By contrast, similar concentrations of 20-day maternal serum progressively decreased enzyme activity from 1.52 +/- 0.14 to 1.19 +/- 0.08 mumol/min/mg protein. The inhibitory effect was apparent at 14 days gestation and became progressively greater during late gestation to reach a maximum at 20 days. Combining equal concentrations of 20-day pregnant with either nonpregnant or adult male serum neutralized the inhibitory activity. When serum from 20-day pregnant rats was partitioned by a dialysis membrane with a 50,000 MW pore size, inhibitory activity could be similarly neutralized by male or nonpregnant female serum. When 20-day maternal serum was passed successively through filters with a greater than 300,000, 100,000, and 50,000 MW exclusion, the inhibitory activity was apparent in all fractions excluded by a molecular weight of 50,000. No inhibition was apparent in fractions that were not excluded by 50,000 MW pore size. Inhibition of enolase activity was greatest in the fraction with MW greater than 300,000. Binding of IGF II could also be demonstrated in this fraction. Binding of IGF II was evident in the fraction greater than 100,000 MW but could not be demonstrated in fractions with a lower molecular weight. The presence of mRNA for IGF II in 20-day fetal rat brain cell cultures was evident when total cellular RNA was analyzed by an RNAase protection assay. It is proposed that a high-molecular-weight component of maternal serum in late gestation can bind endogenously generated IGF II. Such binding, by depleting the necessary growth factors, could inhibit in vitro growth and development of enolase activity.

Isolation of a biologically active fragment from the carboxy terminus of the fetal rat binding protein for insulin-like growth factors

We have purified a 14 kDa fragment of the 30 kDa binding protein for insulin-like growth factors (IGFs) from BRL-3A cell conditioned medium. The fragment binds IGF-I and IGF-II with similar specificity to the 30 kDa binding protein, but with lower affinity. It corresponds to the carboxy terminus of the native binding protein (residues 148-270), and is thought to arise by proteolysis. We infer that this region of the native binding protein contains, at least in part, the IGF binding domain.

Isolation and NH2-terminal amino acid sequences of rat serum carrier proteins for insulin-like growth factors

Three N-glycosylated carrier proteins (CP) for insulin-like growth factors (apparent molecular weights 30-32, 42 and 45 kDa) were isolated from adult rat serum. They share the same amino terminus (up to amino acid 31) and are constituents of the growth hormone-dependent native 150-200 kDa IGF carrier complex. Residues 12-31 display 60 and 50% sequence homology, respectively, to residues 2-21 of fetal rat and to residues 4-22 of a human amniotic fluid IGF carrier protein. No homology exists with the type I or II IGF receptors. Adult rat serum also contains a fourth IGF CP (24 kDa) whose 9 NH2-terminal amino acids are identical to those of the fetal form. Our findings suggest that the three N-glycosylated components originate from the same IGF carrier protein (adult form) and that the 24 kDa protein is a separate (fetal) species.

Type II insulin-like growth factor receptor is present in rat serum

We previously identified in fetal rat serum a component capable of specifically binding radiolabeled insulin-like growth factor type II (IGF-II) that is considerably larger than both the fetal (40 kDa) and the adult (150 kDa) carrier proteins. We now present immunologic and affinity crosslinking data to show that this binding species is the type II IGF receptor. Rat serum was gel-filtered on a Sephadex G-200 column (0.05 M NH4HCO3, pH 8), and 125I-labeled IGF-II (125I-IGF-II) binding was measured in individual column fractions. 125I-IGF-II binding activity was found in the void volume of the column in addition to the carrier protein regions. Competitive binding studies using 125I-IGF-II and binding activity from the Sephadex G-200 void volume showed the characteristics of the type II receptor: IGF-II was more potent than IGF-I, and insulin did not compete. Moreover, a specific anti-type II IGF receptor antibody (no. 3637) completely blocked 125I-IGF-II binding. 125I-IGF-I did not bind to the void volume pool, demonstrating the absence of the type I IGF receptor in rat serum. Affinity crosslinking of 125I-IGF-II to the Sephadex G-200 void volume material demonstrated a specific band at 210 kDa without reduction and at 240 kDa after reduction of disulfide bonds. The serum type II IGF receptor size was confirmed by immunoblotting the void volume material with antiserum 3637, which revealed a band slightly smaller (approximately 10 kDa) than the type II IGF receptor from rat placental membranes. Immunoquantitation by immunoblotting using pure type II IGF receptor from rat placental membranes as standard showed a developmental pattern. In fetal rat serum (19-days gestation) and in sera from 3-, 10-, and 20-day-old rats, the concentrations of receptor protein were similar (1-5 micrograms/ml). The level of the type II IGF receptor in serum declined dramatically between age 20 and 40 days, but the receptor was still measurable at age 12 mo. We conclude that the type II IGF receptor is present in rat serum and is developmentally regulated.

Insulin-like growth factor II receptor as a multifunctional binding protein

The primary structure of human insulin-like growth factor II receptor, predicted from the complementary DNA sequence, reveals a transmembrane receptor molecule with a large extracellular domain made up of fifteen repeat sequences and a small region homologous to the collagen-binding domain of fibronectin. The structural and biochemical features of the IGF-II receptor appear identical to those of the cation-independent mannose-6-phosphate receptor.

The influence of maternal protein deprivation on the developmental pattern of serum immunoreactive insulin-like growth factor 1 (IGF-1) levels

The influence of maternal protein deprivation on body and brain growth and on serum immunoreactive insulin-like growth factor I (IGF-I) levels was examined in rats during development. The offspring of protein-deprived mothers were significantly growth-retarded postnatally and showed an altered developmental pattern of serum immunoreactive IGF-I levels. In contrast to control animals, between days 5 and 20 postnatally, a significant elevation in serum immunoreactive IGF-I levels which peaked at day 15 was found. However, in adult growth-retarded rats a significant reduction in serum immunoreactive IGF-I levels was found. These findings suggested the preweaning period to be critical for the development of the mature somatomedin system.

Hybrid molecules containing the B-domain of insulin-like growth factor I are recognized by carrier proteins of the growth factor

The insulin-like growth factors (IGFs) are polypeptides in plasma that are chemically related to insulin and have mitogenic and insulin-like activity. Unlike insulin, the IGFs circulate in plasma bound to specific high molecular weight carrier proteins that regulate their delivery to target tissues. To define the sites on the IGFs that allow them to be recognized by carrier proteins, we constructed hybrid molecules containing different portions of the insulin, IGF-I, and IGF-II molecules. The presence of the B domain of IGF-I, but not the D domain of IGF-II, enables these insulin-IGF hybrid molecules to be recognized by acid-stripped IGF carrier proteins from rat serum and other sources. By contrast, neither the BIGF-I nor DIGF-II domain is sufficient to enable binding to type II IGF receptors, despite the fact that type II receptors, like the carrier protein, specifically bind IGF-I and IGF-II but do not interact with insulin. By differentiating those sites on the IGF molecule required for binding to IGF carrier protein and receptors, the insulin-IGF hybrid molecules should help delineate the role of the carrier protein in presenting biologically active IGF to target tissues.