Degradation of somatomedins by the thioredoxin system

Thioredoxin, an antioxidant redox protein, in ovarian follicles of women undergoing in vitro fertilization

Oxidative stress has a bidirectional role in the development and maturation of zygotes and embryos. Reduction-oxidation reactions and regulatory proteins, such as thioredoxin (TRX) and thioredoxin reductase (TRXR), are intimately involved in the regulation of oxidative stress. The aim of this study was to determine the levels of TRX mRNA and protein in ovarian follicles collected from women undergoing in vitro fertilization (IVF) and to assess these levels relative to follicle size, presence of oocytes, and responsiveness to superovulation. Follicular fluid (FF) and/or granulosa cells (GCs) from large and small follicles were collected at the time of ovum pick-up from 42 IVF patients enrolled in this study. We divided the patients into normal and poor responders (NR and PR, respectively) based on the serum estradiol levels on the day of human chorionic gonadotropin (hCG) administration. We also compared the TRX concentration in FF (FF-TRX) between oocyte-containing follicles (Oc+) and empty follicles (Oc-). The transcript levels of TRX, but not TRXR, were significantly higher in GCs derived from follicles collected from NR than PR, as determined by semi-quantitative RT-PCR analysis. In NR, the FF-TRX was significantly higher in Oc+ follicles than in Oc- follicles and also in large Oc+ follicles than in large Oc- follicles. Unlike NR, PR exhibited no positive association with elevated FF-TRX and presence of oocytes. Based on its collective anti-oxidative, cytoprotective, and cytokine-like properties of TRX, TRX is likely to be involved in the optimal growth and maturation of ovarian follicles and responsiveness to hyperstimulation.

Disulfide exchange folding of insulin-like growth factor I

The disulfide exchange folding properties of insulin-like growth factor I (IGF-I) have been analyzed in a redox buffer containing reduced (10 mM) and oxidized (1 mM) glutathione. Under these conditions, the 3 disulfide bridges of the 70 amino acid peptide were not quantitatively formed. Instead, five major forms of IGF-I were detected, and these components were concluded to be in equilibrium as their relative amounts were similar starting from either reduced, native, or a mismatched variant of IGF-I containing two non-native disulfides. The different components in the mixtures were trapped by thiol alkylation using vinylpyridine and subsequently isolated by reverse-phase HPLC. The purified variants were further characterized using plasma desorption mass spectrometry and peptide mapping. Two of the five different forms were identified as native and mismatched IGF-I. One form was a variant with only one disulfide bond, and the other two major components had two disulfides formed. In a separate experiment, early refolding intermediates were trapped by pyridylethylation after only 90 s of refolding in the glutathione buffer, starting from reduced IGF-I. The intermediates were identical to the components observed at equilibrium, but at different relative concentrations. On the basis of the disulfide bond patterns of the different components in the equilibrium mixtures, we conclude that the disulfide between cysteines-47 and -52 in IGF-I is an unfavorable high-energy bond that may exist in the native molecule in a strained configuration.

Development and hormonal control of thioredoxin and the thioredoxin-reductase system in the rat liver during the perinatal period

The development and hormonal regulation of thioredoxin and of the thioredoxin-reductase system were investigated during the perinatal period in rat liver. An immunological procedure was developed in order to quantify thioredoxin in fetal and neonatal hepatocytes. Both immunoreactive thioredoxin and thioredoxin-reductase activity appeared on day 16.5 of pregnancy. The level of immunoreactive thioredoxin increased during the late fetal period, and its level was the same 24 h after birth. Moreover, its development was not subjected to hormonal regulation by corticosteroids and glucagon. In contrast, thioredoxin-reductase activity increased 3 times during the late fetal period and presented a marked increase 24 h after birth. In the absence of glucocorticoids there was no increase in the level of thioredoxin reductase, while administration of hydrocortisone acetate and glucagon to fetuses prematurely evoked its activity. This study suggests that if thioredoxin acts physiologically, this activity is related to the state of reduction of the molecule rather than to the total concentration in the liver.

Effects of a single cleavage in insulin-like growth factors I and II on binding to receptors, carrier proteins and antibodies

Two somatomedin-like peptides were extracted from Cohn fraction IV of human plasma and brought to homogeneity: one focused at pH 7.8 and the other at pH less than 5.6. Each consisted of two peptide chains interlinked by disulphide bonds. The basic peptide was identical to insulin-like growth factor I (IGF-I) and had a single cleavage in the C-domain before Arg37 [IGF-I(Arg36cl)]. The acid peptide showed identity with IGF-II, with a cleavage in the B-domain before Arg30 [IGF-II(Ser29cl)]. The effects of these cleavages on the characteristics of binding to type I and type II receptor sites, to binding proteins and to antibodies was studied. Binding of IGF-I(Arg36cl) to antibodies directed against the B-domain or against the AD-domain of IGF-I was the same as IGF-I binding. Thus the cleavage does not influence these antigenic sites. In contrast, binding of IGF-I(Arg36cl) to the type I receptor on human and bovine placental cell membranes was markedly decreased compared with IGF-I binding. Binding to the insulin receptor on human placental cell membranes was slightly diminished, whereas the interaction with specific type II receptors on bovine placental cell membranes was unaffected. There was only a minor influence of the cleavage on the region involved in binding to binding proteins. The cleavage in IGF-II(Ser29cl) diminished binding to antibodies directed against the C-domain of IGF-II, compared with binding of IGF-II itself. Binding to receptors (type I and type II) was changed less profoundly. With 125I-labelled IGF-II(Ser29cl), less insulin was needed in order to obtain 50% displacement of the tracer compared with displacement of 125I-labelled IGF-II. The cleaved form of IGF-II probably has a greater affinity towards the common receptor population than does native IGF-II. Binding to binding proteins was not affected by the cleavage in IGF-II.

Changes in the distribution of insulin-like growth factor I, thioredoxin, thioredoxin reductase and ribonucleotide reductase during the development of the retina

The postnatal development of the rat retina offers good opportunities to follow migration and differentiation of neuroectodermal cells. In this study we have analyzed, by immunohistochemical methods, the distribution of several protein antigens and the trophic peptide insulin-like growth factor I (IGF-I; somatomedin-C). The latter contains disulfide bonds, which in vitro are reduced by the thioredoxin system, i.e. thioredoxin, thioredoxin reductase and NADPH. Ribonucleotide reductase provides growing cells with deoxyribonucleotides, necessary for DNA synthesis, and thioredoxin is an in vitro hydrogen donor. By immunofluorescence IGF-I immunoreactivity was observed throughout the Müller neuroglial cells in the developing retina, but only to a very small extent in the mature retina. Nerve cells showed transient expression of IGF-I during their development. The IGF-I immunoreactivity is likely to be due to local synthesis, since we could demonstrate retinal IGF-I mRNA. Treatment with the transport-blocking agent colchicine caused the Müller glial cells and the retinal pigment epithelium cells to become IGF-I immunoreactive. Thioredoxin and thioredoxin reductase immunoreactivities are confined to neurons and photoreceptor cells in the developing retina. Ribonucleotide reductase subunit M1 immunoreactivity was only observed during the first postnatal week in proliferating neuroectodermal cells. In conclusion, IGF-I and ribonucleotide reductase subunit M1 immunoreactivities are transiently expressed by neuroectodermal cells in the developing rat retina. In contrast, the proteins of the thioredoxin system are demonstrable also in the mature retina.

Rapid axoplasmic transport of insulin-like growth factor I in the sciatic nerve of adult rats

Somatomedin C (Sm-C; insulin-like growth factor I; IGF-I) is a polypeptide (Mr 7649), often dependent on growth hormone (GH), with trophic effects on several different tissues. Monospecific IGF-I antisera were used to investigate its localization in the sciatic nerve and corresponding nerve cells, as well as its possible axoplasmic transport in the adult rat. IGF-I-like immunoreactivity was demonstrated in anterior horn motor nerve cells in the spinal cord and in spinal- and autonomic ganglion nerve cells. Faint IGF-I immunoreactivity was under normal conditions observed in axons of the sciatic nerve and in the Schwann cells. Using crush technique, accumulation of IGF-I immunoreactivity was seen in dilated axons within 2 h, both proximal and distal to the crush. However, only a small fraction of the anterogradely transported IGF-I immunoreactive material could be demonstrated to be transported in retrograde direction. Colchicine injected proximal to a crush prevented accumulation of IGF-I immunoreactivity proximal to the crush, but not distal to it. IGF-I-immunoreactive material is synthesized in the cell bodies of peripheral sensory and motor nerve cells. It is transported at rapid rates in the axoplasm of the sciatic nerve of adult rats both in anterograde and retrograde directions. We propose that axonally transported IGF-I may be released and exert trophic influence on innervated cells, tissues and organs.

The influence of purified somatomedins and insulin on foetal rat brain DNA synthesis in vitro

The two main forms of somatomedins, termed insulin-like growth factor I and II (IGF-I and IGF-II), have been isolated from human plasma. Pure IGF-I or IGF-II was incubated with foetal rat brain cells and found to be equipotent in stimulating the [3H]thymidine incorporation into DNA, despite the fact that IGF-I not is present during foetal life.