Characterization of the binding defect in insulin-like growth factor binding protein-3 from pregnancy serum

Mecasermin rinfabate: rhIGF-I/rhIGFBP-3 complex: iPLEX

BACKGROUND

Mecasermin rinfabate (iPLEX), comprising rhIGF-I complexed to rhIGFBP-3, was developed in an attempt to prolong the half-life of IGF-I and potentially reduce side effects. It is administered as a once-daily subcutaneous injection. Treatment with rhIGF-I has been explored in a number of growth and endocrine disorders.

OBJECTIVE

To review the published literature regarding the pharmacokinetics, safety profile and clinical efficacy of Mecasermin rinfabate.

METHODS

A comprehensive search via the NCBI PubMed portal was performed using the search terms rhIGF-I/rhIGFBP-3 complex, iPLEX and Somatokine.

RESULTS

The effects of Mecasermin rinfabate have been explored in a number of clinical situations including diabetes, severe insulin resistance, osteopaenia, burns and growth hormone insensitivity syndrome, with outcomes similar to those of rhIGF-I alone.

CONCLUSIONS

The biological effects of Mecasermin rinfabate are largely similar to those previously reported with rhIGF-I. There are little published data pertaining to pharmacokinetic properties in human subjects, and the side effect profile appears similar to that of rhIGF-I alone.

Free insulin-like growth factors -- measurements and relationships to growth hormone secretion and glucose homeostasis

IGF-I is a multipotent growth factor with important actions on normal tissue growth and regeneration. In addition, IGF-I has been suggested to have beneficial effects on glucose homeostasis due to its glucose lowering and insulin sensitizing actions. However, not all effects of IGF-I are considered to be favorable; thus, epidemiological studies suggest that IGF-I is also involved in the development of common cancers, atherosclerosis and type 2 diabetes. The biological actions of IGF-I are modulated by at least six IGF-binding proteins, which bind approximately 99% of the circulating IGF-I pool. So far, most in vivo studies have used serum or plasma total (extractable IGF-I) as an estimate of the bioactivity of IGF-I in vivo. However, within the last decade, validated assays for measurement of free IGF-I have been described. This review aims to discuss the current assays for free IGF-I and their advances in relation to the traditional measurement of total IGF-I. The literature overview will focus on the role of circulating free versus total IGF-I in the feedback regulation of GH release, and the possible involvement of the circulating IGF-system in glucose homeostasis.

A purified bovine serum albumin preparation contains an insulin-like growth factor (IGF) binding protein-3 fragment that forms ternary complexes selectively with IGF-II and the acid-labile subunit

Among the six insulin-like growth factor binding proteins (IGFBP), only IGFBP-3 and IGFBP-5 form ternary complexes with IGFs and the acid-labile sunbunit (ALS). In a commercial, highly-purified BSA preparation, ternary complex formation was detected using radio-labeled IGF-II and human serum-derived ALS, with precipitation by ALS antiserum. In contrast, no complexes with radio-labeled IGF-I were detected under the same conditions. Size-fractionation of the BSA on Superose-12 showed the peak of ternary complex forming activity at approximately 30 kDa. To purify the active factor, a solution of the BSA was pumped onto a [Gly(1)]IGF-II affinity column, and eluted fractions were lyophilized and applied to a C18 HPLC column. The eluted fractions showing ternary complex forming activity maintained a preference for IGF-II in forming ternary complexes and a slight preference in forming binary complexes with IGF-II rather than IGF-I. By silver staining after non-reducing SDS-PAGE, the peak activity in the HPLC-eluted fractions appeared as 30 kDa and 21-24 kDa bands. Amino-terminal sequencing of this peak activity revealed bovine IGFBP-3. These results demonstrate that amino-terminal proteolyzed bovine IGFBP-3 is present in a highly purified BSA preparation. In contrast to intact human IGFBP-3 and IGFBP-5, this form of bovine IGFBP-3 forms ternary complexes preferentially with IGF-II rather than IGF-I.

Serum levels of total and free IGF-I and IGFBP-3 are increased and maintained in long-term training

The goals of this study were to determine whether the long-term training regimens experienced by competitive collegiate swimmers would result in altered levels of total and free serum insulin-like growth factor I (IGF-I) as well as IGF-binding proteins (BP) IGFBP-1 and -3. Two male (Teams 1M and 2M) and one female (Team 2F) teams were studied at the start of training, after 2 mo of training, after 4 mo (2-4 mo had the highest volume of training), after 5 mo (near the end of tapering; only for Team 1M), and several days after training was over. For Team 1M, total IGF-I concentrations were increased by 76% after 4 mo and were subsequently maintained at this level. Total IGF-I responses were more variable for Teams 2F and 2M. Free IGF-I levels were increased nearly twofold for all teams at 2 mo and were maintained or increased further with subsequent training. Only the levels of free IGF-I for Team 1M returned to pretraining values after training had ended. Training had little effect on IGFBP-1 levels. For all teams, serum IGFBP-3 was elevated by 4 mo of training (for Team 2F it was increased at 2 mo) by 30-97% and remained at these higher levels thereafter. The ratio of total IGF-I to IGFBP-3 was not increased by training in any group. These data indicate that serum levels of total and free IGF-I and total IGFBP-3 can be increased with intense training and maintained with reduced training (tapering). The findings show that changes in free IGF-I levels are not accounted for by alterations in the total IGF-I/IGFBP-3 complex or in IGFBP-3 levels and indicate that there are other important determinants of free IGF-I.

N-Linked glycosylation and sialylation of the acid-labile subunit. Role in complex formation with insulin-like growth factor (IGF)-binding protein-3 and the IGFs

Over 75% of the circulating insulin-like growth factors (IGF-I and -II) are bound in 140-kDa ternary complexes with IGF-binding protein-3 (IGFBP-3) and the 84-86-kDa acid-labile subunit (ALS), a glycoprotein containing 20 kDa of carbohydrate. The ternary complexes regulate IGF availability to the tissues. Since interactions of glycoproteins can be influenced by their glycan moieties, this study aimed to determine the role of ALS glycosylation in ternary complex formation. Complete deglycosylation abolished the ability of ALS to associate with IGFBP-3. To examine this further, seven recombinant ALS mutants each lacking one of the seven glycan attachment sites were expressed in CHO cells. All the mutants bound IGFBP-3, demonstrating that this interaction is not dependent on any single glycan chain. Enzymatic desialylation of ALS caused a shift in isoelectric point from 4.5 toward 7, demonstrating a substantial contribution of anionic charge by sialic acid. Ionic interactions are known to be involved in the association between ALS and IGFBP-3. Desialylation reduced the affinity of ALS for IGFBP-3. IGF complexes by 50-80%. Since serum protein glycosylation is often modified in disease states, the dependence of IGF ternary complex formation on the glycosylation state of ALS suggests a novel mechanism for regulation of IGF bioavailability.

The insulin-like growth factor (IGF) binding site of bovine insulin-like growth factor binding protein-2 (bIGFBP-2) probed by iodination

The insulin-like growth factor (IGF) binding site of bovine insulin-like growth factor binding protein 2 (bIGFBP-2) has been probed by chemical iodination. Tyrosyl residues of bIGFBP-2 were reacted by chloramine T-mediated iodination. The modification patterns of free bIGFBP-2 and bIGFBP-2 associated with insulin-like growth factor II (IGF-II) were compared by tryptic mapping using electrospray mass spectrometry and N-terminal sequencing. The presence of bound IGF-II resulted in protection of tyrosine at position 60 from iodination measured by the relative loss of tyrosine specific fluorescence and the incorporation of the radioisotope 125I. In addition, the pattern of iodine incorporation of bIGFBP-2 was not different whether IGF-I or IGF-II was the protective ligand. bIGFBP-2, when iodinated alone sustained a 8-fold loss of binding affinity for IGF-I and a 4-fold loss in binding affinity for IGF-II. In contrast, bIGFBP-2 iodinated while complexed with either IGF-I or IGF-II retained the same binding affinity for IGF-I or IGF-II as non-iodinated bIGFBP-2. We conclude that tyrosine 60 lies either in a region of bIGFBP-2 which directly interacts with both IGF-I and IGF-II or lies in a region of bIGFBP-2 which undergoes a conformational change that is important for IGF binding. Furthermore, iodination of tyrosine residues at positions 71, 98, 213, 226, and 269 has no detectable impact on binding of bIGFBP-2 to the IGFs.

Insulin-like growth factor I (IGF-I) and IGF binding proteins in seminal plasma before and after vasectomy in normal men

OBJECTIVE

To examine the levels and origins of insulin-like growth factor I (IGF-I) and insulin-like growth factor binding proteins (IGFBPs) in the human male genital tract.

DESIGN

Examining seminal plasma before and 3 months after vasectomy.

SUBJECTS

Fifteen men who were candidates for vasectomy were included in the study.

MAIN OUTCOME MEASURES

Seminal plasma and serum levels of IGF-I, IGFBP-1, and IGFBP-3 were determined by commercially available assays, furthermore, samples were subjected to Western ligand blotting.

RESULTS

Seminal plasma concentrations of IGF-I were significantly lower after vasectomy: 18.0 +/- 2.4 micrograms/L (before) and 12.5 +/- 1.2 micrograms/L (after). When the total ejaculate content of IGF-I was calculated, the figures were reduced by 50% after vasectomy: 45.64 +/- 7.8 ng (before) and 23.45 +/- 3.8 ng (after). The patterns observed for seminal plasma IGFBP-3 concentrations were 844.9 +/- 59 micrograms/L (before) and 816.5 +/- 65 micrograms/L (after). When the total ejaculate IGFBP-3 content was calculated there was a 36% reduction after vasectomy: 2,300 +/- 251 ng (before) and 1,474 +/- 217 ng (after).

CONCLUSIONS

A considerable amount of seminal plasma IGF-I and IGFBP-3 may be of testicular origin. Although the physiological significance of IGF-I and IGFBPs in the male reproductive system still remains uncertain, the demonstration of their presence in the testes add support to a functional role in the regulation of gonadal function.

Hematologic and growth-related effects of frequent prenatal ultrasound exposure in the long-tailed macaque (Macaca fascicularis)

Prior investigations have shown that reduced birth weights and transient neutropenias result from frequent exposure of monkey fetuses to ultrasound. To further explore these findings, 26 animals were studied (16 exposed, 10 controls; "triple mode"; ATL Ultramark 9 with HDI; ISPTAd approximately 645 to 714 mW/cm2). Exposures were performed daily for 5 days each week from gestational days (GD) 21 to 35 (5 min), three times weekly from GD 36 to 60 (5 min), then weekly from GD 61 to 153 +/- 1 (10 min). Fetal blood samples (FBS) were collected for complete blood counts (CBCs), hematopoietic progenitor assay, circulating insulin-like growth factors (IGF-I, IGF-II) and binding proteins (IGFBP-3) (GD 120, 140, 153 +/- 1). Animals were delivered by Cesarean section at term (GD 153 +/- 1), and body weights, morphometrics, CBCs, and bone marrow aspirates assessed at delivery and postnatally for 3 months. Fetal neutropenias were noted in exposed animals in addition to reduced circulating progenitors (colony forming unit-granulocyte-macrophage [CFU-GM]). Growth of CFU-GM from bone marrow was exuberant at term, whereas circulating levels were diminished comparable to prenatal samples. Exposed animals were smaller at birth; marked reductions in IGFBP-3 were noted prenatally. These data suggest that frequent prenatal ultrasound exposure can transiently alter the neutrophil lineage, although these findings may be the result of enhanced margination and organ sequestration. Data also suggest that transient, altered growth patterns may be due to perturbations of the IGF axis.