The role of IGFBP-3 in the regulation of IGFBP-4 proteolysis

Reduced pericellular sensitivity to IGF-I in fibroblasts from girls with Turner syndrome: a mechanism to impair clinical responses to GH

Girls with Turner syndrome (TS) are treated with supraphysiological doses of growth hormone (GH) to improve final height; however in some girls, the growth response can be poor. This may reflect aberrations in GH and/or IGF-I actions at the cellular level, and thus this study compared the response of skin fibroblasts from normal children (n = 5) and girls with TS (n = 8) to GH, IGF-I, or a combination, by assessing the IGF binding protein (IGFBP) profile of conditioned medium harvested over 7 d. The two cell types had a comparable IGFBP profile; IGFBP-3 and IGFBP-4 were the most abundant species. TS fibroblasts produced more IGFBP-3 (d 7, 51.4 ± 45 ng/mL versus 20 ± 22 ng/mL; p < 0.05) than control cells; levels of IGFBP-4 were similar (21 ± 12 ng/mL versus 30 ± 21 ng/mL). GH did not influence IGFBP production. IGF-I treatment did not affect IGFBP-4 levels but enhanced the production of IGFBP-3 by both cell types (p < 0.05). However, the response of TS fibroblasts to IGF-I was approximately half that observed in normal cells (p < 0.05). Altered IGF-I activity, because of reduced bioavailability and/or reduced sensitivity, could contribute to the need for high GH doses in TS and for the poor response to GH in some girls with TS.

Role of insulin-like growth factor binding protein-3 (IGFBP-3) in the differentiation of primary human adult skeletal myoblasts

Although muscle satellite cells were identified almost 40 years ago, little is known about the induction of their proliferation and differentiation in response to physiological/pathological stimuli or to growth factors/cytokines. In order to investigate the role of the insulin-like growth factor (IGF)/IGF binding protein (IGFBP) system in adult human myoblast differentiation we have developed a primary human skeletal muscle cell model. We show that under low serum media (LSM) differentiating conditions, the cells secrete IGF binding proteins-2, -3, -4 and -5. Intact IGFBP-5 was detected at days 1 and 2 but by day 7 in LSM it was removed by proteolysis. IGFBP-4 levels were also decreased at day 7 in the presence of IGF-I, potentially by proteolysis. In contrast, we observed that IGFBP-3 initially decreased on transfer of cells into LSM but then increased with myotube formation. Treatment with 20 ng/ml tumour necrosis factor-alpha (TNFalpha), which inhibits myoblast differentiation, blocked IGFBP-3 production and secretion whereas 30 ng/ml IGF-I, which stimulates myoblast differentiation, increased IGFBP-3 secretion. The TNFalpha-induced decrease in IGFBP-3 production and inhibition of differentiation could not be rescued by addition of IGF-I. LongR(3)IGF-I, which does not bind to the IGFBPs, had a similar effect on differentiation and IGFBP-3 secretion as IGF-I, both with and without TNFalpha, confirming that increased IGFBP-3 is not purely due to increased stability conferred by binding to IGF-I. Furthermore reduction of IGFBP-3 secretion using antisense oligonucleotides led to an inhibition of differentiation. Taken together these data indicate that IGFBP-3 supports myoblast differentiation.

Insulin-like growth factor (IGF)-binding protein-4 proteolytic degradation in bovine, equine, and porcine preovulatory follicles: regulation by IGFs and heparin-binding domain-containing peptides

We recently showed that insulin-like growth factor-binding protein-4 (IGFBP-4) proteolytic degradation in ovine preovulatory ovarian follicles is IGF-dependent and regulated by the heparin-binding domain (HBD) from IGFBP-3 and from connective tissue growth factor (CTGF), heparan/heparin-interacting protein (HIP), and vitronectin. The present study investigated regulation of IGFBP-4 proteolytic degradation in porcine, bovine, and equine ovarian preovulatory follicles. Follicular fluid from such preovulatory follicles contains proteolytic activity, degrading exogenous IGFBP-4. An excess of IGF-I enhanced IGFBP-4 degradation. In contrast, IGFBP-2 or -3 or monoclonal antibodies against IGF-I or -II dose-dependently inhibited IGFBP-4 degradation, and IGF-I or -II reversed this inhibition in a dose-dependent manner. Heparin-binding peptides derived from the C-terminal domain of IGFBP-3 or -5 inhibited IGFBP-4 degradation. Other heparin-binding peptides derived from CTGF, HIP, and vitronectin also inhibited IGFBP-4 degradation, except in porcine follicles. Finally, IGFBP-3 that was mutated in its HBD was less effective at inhibiting IGFBP-4 degradation. Thus, in bovine, porcine, and equine preovulatory follicles, IGFBP-4 proteolytic degradation both depends on IGFs and is inhibited by peptides containing HBD. Overall, these results suggest that during terminal development of follicles to the preovulatory stage in domestic animal species, the increase in IGF bioavailability might enhance IGFBP-4 degradation. In contrast, in atretic follicles, the decrease in IGF bioavailability, resulting partly from the increase in IGFBP-2 (sow, heifer, mare) and IGFBP-5 (heifer) expression would participate in the decrease of IGFBP-4 degradation. In bovine atretic follicles, IGFBP-5 would also strengthen the inhibition of IGFBP-4 degradation by direct interaction of its HBD with the protease. The involvement of other HBD-containing proteins in the modulation of intrafollicular proteases degrading IGFBP-4 remains to be investigated.

Role of insulin-like growth factors and their binding proteins in growth control and carcinogenesis

Interest in the role of the insulin-like growth factor (IGF) axis in growth control and carcinogenesis has recently been increased by the finding of elevated serum insulin-like growth factor I (IGF-I) levels in association with three of the most prevalent cancers in the United States: prostate cancer, colorectal cancer, and lung cancer. IGFs serve as endocrine, autocrine, and paracrine stimulators of mitogenesis, survival, and cellular transformation. These actions are mediated through the type 1 IGF-receptor (IGF-1R), a tyrosine kinase that resembles the insulin receptor. The availability of free IGF for interaction with the IGF-1R is modulated by the insulin-like growth factor-binding proteins (IGFBPs). IGFBPs, especially IGFBP-3, also have IGF-independent effects on cell growth. IGF-independent growth inhibition by IGFBP-3 is believed to occur through IGFBP-3-specific cell surface association proteins or receptors and involves nuclear translocation. IGFBP-3-mediated apoptosis is controlled by numerous cell cycle regulators in both normal and disease processes. IGFBP activity is also regulated by IGFBP proteases, which affect the relative affinities of IGFBPs, IGFs and IGF-1R. Perturbations in each level of the IGF axis have been implicated in cancer formation and progression in various cell types.

Role of insulin-like growth factors and their binding proteins in growth control and carcinogenesis

Interest in the role of the insulin-like growth factor (IGF) axis in growth control and carcinogenesis has recently been increased by the finding of elevated serum insulin-like growth factor I (IGF-I) levels in association with three of the most prevalent cancers in the United States: prostate cancer, colorectal cancer, and lung cancer. IGFs serve as endocrine, autocrine, and paracrine stimulators of mitogenesis, survival, and cellular transformation. These actions are mediated through the type 1 IGF-receptor (IGF-1R), a tyrosine kinase that resembles the insulin receptor. The availability of free IGF for interaction with the IGF-1R is modulated by the insulin-like growth factor-binding proteins (IGFBPs). IGFBPs, especially IGFBP-3, also have IGF-independent effects on cell growth. IGF-independent growth inhibition by IGFBP-3 is believed to occur through IGFBP-3-specific cell surface association proteins or receptors and involves nuclear translocation. IGFBP-3-mediated apoptosis is controlled by numerous cell cycle regulators in both normal and disease processes. IGFBP activity is also regulated by IGFBP proteases, which affect the relative affinities of IGFBPs, IGFs and IGF-1R. Perturbations in each level of the IGF axis have been implicated in cancer formation and progression in various cell types.

Novel aspects of the insulin-like growth factor binding proteins

The insulin-like growth factors (IGFs), IGF binding proteins (IGFBPs), and IGFBP proteases regulate somatic growth and cellular proliferation both in vivo and in vitro. IGFs are potent mitogens whose actions are determined by the availability of free IGFs to interact with IGF receptors. IGFBPs comprise a family of six proteins that bind IGFs with high affinity and specificity and thereby regulate IGF-dependent actions. IGFBPs have also recently emerged as IGF-independent regulators of cell growth. Several IGFBP association proteins have been discovered recently which can affect IGFBP action. Cleavage of IGFBPs by specific proteases modulates levels of free IGFs and IGFBPs and thereby their actions. IGFBP-related proteins (IGFBP-rPs) are an emerging group of proteins which bind IGFs with low affinity and also play important roles in cell growth and differentiation. The IGFBPs appear to have emerging roles in the mechanisms underlying human cancer. The GH-IGF-IGFBP axis is complex and powerful. Future research on its physiology promises exciting insights into cell biology as well as advancements in the treatment of a wide range of disease states including cancer, diabetes, vascular disease, asthma, and growth disorders.

The role of cell surface attachment and proteolysis in the insulin-like growth factor (IGF)-independent effects of IGF-binding protein-3 on apoptosis in breast epithelial cells

We have recently reported that insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) can significantly increase ceramide-induced apoptosis in an Hs578T breast carcinoma cell line in an IGF-independent manner. It was observed in that study that IGFBP-3 added to the cultures was proteolytically modified, generating a specific pattern of fragmentation. We have also previously reported that almost all of the IGFBP-3 outside the circulation in extravascular fluids is in a fragmented form, apparently due to the activity of a cation-dependent serine protease. The aim of this study was to investigate the role of proteolysis in the IGFBP-3 enhancement of C2-induced apoptosis. In this study we confirmed that preincubation of Hs578T cells with IGFBP-3 enhances the apoptotic effect of the ceramide analog C2. The presence of IGF-I completely inhibited the enhancement effect, apparently by inhibiting cell surface association and proteolytic modification. The presence of a serine protease inhibitor [4-(2-aminoethyl)benesulfonyl fluoride] completely inhibited the enhancement effect of IGFBP-3, and Western immunoblotting of conditioned medium and cell surface-associated IGFBP-3 revealed that proteolytic fragmentation of the IGFBP-3 was reduced. In addition, fragments from the incubation of IGFBP-3 with plasmin were able to enhance the susceptibility of Hs578T cells to C2. The effect of these fragments could, however, also be reduced by 4-(2-aminoethyl)benesulfonyl fluoride despite the fact that IGFBP-3 was already fragmented. This suggests additional roles for serine proteases in the IGFBP-3 effect on C2-induced apoptosis in addition to the cleavage of the binding protein.

Cellular actions of insulin-like growth factor binding proteins

The insulin-like growth factors (IGFs), insulin-like growth factor binding proteins (IGFBPs), and the IGFBP proteases are involved in the regulation of somatic growth and cellular proliferation both in vivo and in vitro. IGFs are potent mitogenic agents whose actions are determined by the availability of free IGFs to interact with the IGF receptors. IGFBPs comprise a family of proteins that bind IGFs with high affinity and specificity and thereby regulate IGF-dependent actions. IGFBPs have recently emerged as IGF-independent regulators of cell growth. Various IGFBP association proteins as well as cleavage of IGFBPs by specific proteases modulate levels of free IGFs and IGFBPs. The ubiquity and complexity of the IGF axis promise exciting discoveries and applications for the future.

Recombinant insulin-like growth factor-I (IGF-I) production in Super-CHO results in the expression of IGF-I receptor and IGF binding protein 3

Previously, we described the genetic construction Super- CHO, a cell line capable of autocrine growth under fully defined protein-free conditions. Super-CHO cells constitutively express insulin growth factor-I (IGF-I) and transferrin in sufficient amounts to support long-term, stable growth without the addition of exogenous growth factors, thus making it an ideal host for the production of recombinant biopharmaceuticals. although IGF-I has been successfully expressed in Chinese Hamster Ovary (CHO) cells, the long term effects of recombinant IGF-I expression have not been explored. In particular, the expression of the endogenous IGF-I receptor in response to IGF-I production has not been reported. We report here the transcriptional induction of the type I IGF receptor gene in Super-CHO. In addition, we examined the conditioned medium for the presence of IGF-I binding proteins. Ligand blot analysis reveals the presence of IGF binding proteins present in the medium conditioned by Super-CHO cells as well as CHO cells incubated in the presence of IGF-I. Furthermore, immunoaffinity reveals that Super-CHO expresses IGF binding protein-3 in response to IGF-I production. These results suggest the autocrine growth of Super-CHO involves a complex interaction of cell type specific factors which regulate its utility of IGF-I.

Expression of insulin-like growth factor binding proteins in bronchoalveolar lavage fluid of patients with pulmonary sarcoidosis

Pulmonary sarcoidosis involves development of parenchymal granulomata that usually resolve spontaneously; however, it remains unclear what pathogenic mechanisms are responsible for the progression to local or diffuse fibrosis with irreversible lung remodeling that occurs in 20% of patients. Alveolar macrophages have a pivotal role in sarcoidosis, releasing mediators including insulin-like growth factor (IGF)-1, a potent profibrogenic molecule. IGF-1 bioavailability in the lung is dependent on at least six high-affinity IGF-binding proteins (IGFBP), which mainly inhibit IGF-1 action. We have investigated their presence in patients with established stage III sarcoidosis to determine whether IGF-1 and IGFBP contribute to the fibrogenic process in these patients and as such contribute to the (clinical) progression of the disease. The fibroblast mitogenic potential of bronchoalveolar lavage fluid (BALF) was more than 3-fold higher (P < 0.005) in sarcoid patients. Sarcoid BALF-induced activity could be inhibited (P < 0.0005) by neutralizing antibodies to IGF-1. We established the IGFBP profile of BALF with Western ligand analysis and quantified expression of IGFBP-3 by immunoblotting. IGFBP-2 and IGFBP-4 predominate in normal and sarcoid BALF, but IGFBP-3 occurs only as a modified, smaller, 29-kD form, expression of which was raised (P < 0.003) in sarcoid patients. Gene expression of IGF-1 and IGFBP-3 was demonstrated by reverse transcription-polymerase chain reaction in BAL cells. Thus, local production of pro-fibrogenic IGF-1 may be subject to substantial post-translational regulation by associated IGFBP and IGFBP proteases that may contribute to enhanced fibrogenesis in sarcoidosis patients with evidence of progression or (development) of fibrosis.

Peak oxygen uptake, muscle volume, and the growth hormone-insulin-like growth factor-I axis in adolescent males

PURPOSE

The growth effects of exercise appear to be mediated in part by central neuroendocrine control reflected in circulating levels of growth hormone (GH), insulin-like growth factor-I (IGF-I), and their binding proteins (BP). In previous studies positive correlations between peak VO2 and circulating IGF-I have been demonstrated. The relationship between peak oxygen uptake and these potential regulating factors has not been examined in adolescent males where patterns of GH pulsatility and levels of IGF-I are rapidly changing.

METHODS

Forty-three healthy adolescent males (age 16 +/- 0.7 yr, 70% at Tanner V) performed cycle ergometry to determine p oxygen uptake (peak VO2), and magnetic resonance images to determine the thigh muscle volume. Baseline blood samples were collected for GHBP, the extracellular portion of the GH tissue receptor (by ligand mediated immunofunctional assay), IGF-I (by RIA), and IGFBPs 1-5 (by RIA). Mean GH was determined from samples obtained every 20 min overnight.

RESULTS

Peak VO2/kg was positively correlated with mean overnight GH levels (r = 0.41, P < 0.005). Both peak VO2/kg and thigh muscle volume/kg were negatively correlated with GHBP (r = -0.33, P < 0.02) and IGFBP-4 (r = -0.52, P < 0.005). There were no correlations between peak VO2/kg and IGF-I or IGFBPs 1-3, and 5.

CONCLUSIONS

GH pulsatility is increased adolescent males who have higher peak VO2, but this did not translate into increases in IGF-I. We speculate that in the fitter males, lower GHBP levels may reduce hepatic sensitivity to GH. Thus, circulating IGF-I was unchanged despite higher mean GH in subjects with higher peak VO2. IGFBP-4 which is known to inhibit IGF-I was negatively correlated with peak VO2 leading, possibly, to increased IGF-I bioactivity. Fitness (as assessed by muscle mass and peak VO2) does modulate the GH-IGF-I axis, but not solely through circulating IGF-I; both GHBP and IGFBPs play important roles.

Proteolysis of insulin-like growth factor-binding protein-3 by human skin keratinocytes in culture in comparison to that in skin interstitial fluid: the role and regulation of components of the plasmin system

Proteolysis of insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) is an important determinant of IGF action on cells. We have investigated this in a human skin keratinocyte cell line HaCaT. Although these cells did not normally produce an active IGFBP-3 protease, addition of plasminogen resulted in a dose-dependent proteolysis of endogenous and exogenous IGFBP-3, producing fragments similar to those cleaved by skin interstitial fluid, but different from those generated by plasmin. Protease inhibitor profiles suggested the enzyme in the conditioned medium to be a calcium-dependent serine protease. Exogenous IGFBP-3 either inhibited or slightly stimulated IGF-I-induced cell proliferation when it was coincubated or preincubated with the cells, respectively. Both effects were attenuated in the presence of plasminogen. Preincubation of cells with IGF-I or long R3 IGF-I divergently changed plasminogen activator inhibitor-1 and -2 secretion, but only IGF-I blocked IGFBP-3 proteolysis. Such inhibition was also observed in a cell-free protease assay. IGF-I, however, had no effect on plasmin-induced IGFBP-3 degradation. Together, these data indicate that an IGFBP-3 protease similar to that in skin interstitial fluid is generated in plasminogen-treated HaCaT cells, and it attenuates the effects of IGFBP-3 on IGF action. IGF-I, probably by coupling with IGFBP-3, can protect it from the action of this protease.

Local disruption of the insulin-like growth factor system in the arthritic joint

OBJECTIVE

To identify differences in levels of insulin-like growth factor (IGF) and IGF binding proteins (IG-FBPs) between 30 patients with arthritis (14 with rheumatoid arthritis [RA], 16 with osteoarthritis [OA]) and 11 normal control subjects. IGF and IGFBP levels were correlated to the disease activity marker C-reactive protein (CRP) to determine whether they were disease related. We also examined the degree of proteolytic modification of the IGFBPs.

METHODS

Radioimmunoassays were used for measuring IGF and IGFBP-3 levels; CRP was measured by enzyme-linked immunosorbent assay. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by Western blotting, chemiluminescence, and autoradiography were used for visualizing binding proteins.

RESULTS

There was a significant increase in synovial fluid levels of both IGF-1 and IGFBP-3 in both RA and OA. This resulted in an elevated IGFBP-3 to IGF molar ratio of 1.49 in the OA group and 1.47 in the RA group, compared with 0.86 in the normal control group (P = 0.0002 for both). A significantly lower degree of IGFBP-3 proteolysis was also seen in the synovial fluids from the patients compared with the controls. There were significant correlations between the CRP level and levels of IGF-1, IGF-2, and IGFBP-3 in the RA patients (r = 0.62-0.898, P = 0.04-0.0007).

CONCLUSION

There was significant local disruption of the IGF system in patients with arthritis. This may result in a lower amount of IGF that is able to bind to IGF receptors in the arthritic joint. Levels of IGF-1 IGF-2, and IGFBP-3 all correlated with the CRP level in patients with RA, which indicates the possibility that the IGF system is involved in the disease process.