Insulin-like growth factor binding protein (IGFBP) proteolysis: occurrence, identification, role and regulation

The Stanniocalcin-PAPP-A-IGFBP-IGF Axis

The pappalysin metalloproteinases, PAPP-A and PAPP-A2, have emerged as highly specific proteolytic enzymes involved in the regulation of insulin-like growth factor (IGF) signaling. The only known pappalysin substrates are a subset of the IGF binding proteins (IGFBPs), which bind IGF-I or IGF-II with high affinity to antagonize receptor binding. Thus, by cleaving IGFBPs, the pappalysins have the potential to increase IGF bioactivity and hence promote IGF signaling. This is relevant both in systemic and local IGF regulation, in normal and several pathophysiological conditions. Stanniocalcin-1 and -2 were recently found to be potent pappalysin inhibitors, thus comprising the missing components of a complete proteolytic system, the stanniocalcin-PAPP-A-IGFBP-IGF axis. Here, we provide the biological context necessary for understanding the properties of this molecular network, and we review biochemical data, animal experiments, clinical data, and genetic data supporting the physiological operation of this branch as an important part of the IGF system. However, although in vivo data clearly illustrate its power, it is a challenge to understand its subtle operation, for example, multiple equilibria and inhibitory kinetics may determine how, where, and when the IGF receptor is stimulated. In addition, literally all of the regulatory proteins have suspected or known activities that are not directly related to IGF signaling. How such activities may integrate with IGF signaling is also important to address in the future.

Insulin-like growth factors: Ligands, binding proteins, and receptors

BACKGROUND

The insulin-like growth factor family of ligands (IGF-I, IGF-II, and insulin), receptors (IGF-IR, M6P/IGF-IIR, and insulin receptor [IR]), and IGF-binding proteins (IGFBP-1-6) play critical roles in normal human physiology and disease states.

SCOPE OF REVIEW

Insulin and insulin receptors are the focus of other chapters in this series and will therefore not be discussed further. Here we review the basic components of the IGF system, their role in normal physiology and in critical pathology's. While this review concentrates on the role of IGFs in human physiology, animal models have been essential in providing understanding of the IGF system, and its regulation, and are briefly described.

MAJOR CONCLUSIONS

IGF-I has effects via the circulation and locally within tissues to regulate cellular growth, differentiation, and survival, thereby controlling overall body growth. IGF-II levels are highest prenatally when it has important effects on growth. In adults, IGF-II plays important tissue-specific roles, including the maintenance of stem cell populations. Although the IGF-IR is closely related to the IR it has distinct physiological roles both on the cell surface and in the nucleus. The M6P/IGF-IIR, in contrast, is distinct and acts as a scavenger by mediating internalization and degradation of IGF-II. The IGFBPs bind IGF-I and IGF-II in the circulation to prolong their half-lives and modulate tissue access, thereby controlling IGF function. IGFBPs also have IGF ligand-independent cell effects.

The role of insulin-like growth factors in the development of prostate cancer

INTRODUCTION

Preclinical, clinical, and population studies have provided robust evidence for an important role for the insulin-like growth factor (IGF) system in the development of prostate cancer.

AREAS COVERED

An overview of the IGF system is provided. The evidence implicating the IGF system in the development of prostate cancer is summarized. The compelling evidence culminated in a number of clinical trials of agents targeting the system; the reasons for the failure of these trials are discussed.

EXPERT OPINION

Clinical trials of agents targeting the IGF system in prostate cancer were terminated due to limited objective clinical responses and are unlikely to be resumed unless a convincing predictive biomarker is identified that would enable the selection of likely responders. The aging population and increased screening will lead to greater diagnosis of prostate cancer. Although the vast majority will be indolent disease, the epidemics of obesity and diabetes will increase the proportion that progress to clinical disease. The increased population of worried men will result in more trials aimed to reduce the risk of disease progression; actual clinical endpoints will be challenging and the IGFs remain the best intermediate biomarkers to indicate a response that could alter the course of disease.

Systemic Metabolism, Its Regulators, and Cancer: Past Mistakes and Future Potential

There has been a resurgence of interest in cancer metabolism; primarily in the resetting of metabolism within malignant cells. Metabolism within cells has always been a tightly regulated process; initially in protozoans due to metabolic enzymes, and the intracellular signaling pathways that regulate these, being directly sensitive to the availability of nutrients. With the evolution of metazoans many of these controls had been overlaid by extra-cellular regulators that ensured coordinated regulation of metabolism within the community of cells that comprised the organism. Central to these systemic regulators is the insulin/insulin-like growth factor (IGF) system that throughout evolution has integrated the control of tissue growth with metabolic status. Oncological interest in the main systemic metabolic regulators greatly subsided when pharmaceutical strategies designed to treat cancers failed in the clinic. During the same period, however the explosion of new information from genetics has revealed the complexity and heterogeneity of advanced cancers and helped explain the problems of managing cancer when it reaches such a stage. Evidence has also accumulated implying that the setting of the internal environment determines whether cancers progress to advanced disease and metabolic status is clearly an important component of this local ecology. We are in the midst of an epidemic of metabolic disorders and there is considerable research into strategies for controlling metabolism. Integrating these new streams of information suggests new possibilities for cancer prevention; both primary and secondary.

Pregnancy-associated plasma protein A and mortality in haemodialysis

BACKGROUND

Elevated pregnancy-associated plasma protein A (PAPP-A) levels are associated with increased risk of death in ischaemic heart disease as well as in haemodialysis patients. Previous research indicates that the prognostic value of PAPP-A may be stronger in patients with concomitant diabetes mellitus or signs of inflammation. We studied the association between PAPP-A and outcomes in prevalent haemodialysis patients and hypothesized that diabetes mellitus and inflammation status act as effect modifiers.

MATERIALS AND METHODS

Circulating PAPP-A levels were quantified using ELISA. Cox proportional hazards and quantile regression models were used for associations between PAPP-A and mortality. PAPP-A levels were log-transformed for Normality.

RESULTS

During 60-month follow-up, 37 (40%) of the 92 participants died. Higher PAPP-A was associated with increased risk of mortality in unadjusted analysis (HR per SD = 1.4, 95% CI = 1-1.9, P = .03) and when adjusted for confounders and cardiovascular risk factors (HR = 1.8, 95% CI = 1.18-2.73, P = .006). An interaction between PAPP-A levels and diabetes mellitus on mortality was found (HR for the multiplicative interaction term = 2.74 95% CI = 1.02-7.37, P = .05). In a quantile regression adjusted for age and sex, one SD increase in PAPP-A was associated with 22 months shorter estimated time until 25% of the patients died (95% CI -35 to -9.1 months).

CONCLUSIONS

Increased PAPP-A levels are associated with higher all-cause mortality in prevalent haemodialysis patients with concomitant diabetes mellitus.

Relationships of insulin-like growth factor-1, its binding proteins, and cardiometabolic risk in hypertensive perimenopausal women

PURPOSE

During the transition from premenopause to postmenopause, many women experience weight gain and central fat deposition; therefore, we hypothesized that circulating growth factors can play a role in the pathogenesis of hypertension, metabolic syndrome, and subclinical organ damage in perimenopausal women.

BASIC PROCEDURES

The study included 192 women aged 40 to 60years; 152 had newly diagnosed essential hypertension that had never been treated, and 40 were normotensive age-matched controls. For all subjects, 24-h ambulatory blood pressure monitoring (ABPM), echocardiographic examination with assessment of left ventricular mass (LVM) and systolic and diastolic functions (GE Vivid 7.0, General Electric Vingmed Ultrasound, Horten, Norway), carotid ultrasound with measurement of intima-media thickness, and carotid-femoral pulse wave velocity (PWV) measurement (SphygmoCor, AtCor Medical, Sydney, Australia) were performed. Serum levels of insulin-like growth factor 1 (IGF-1), insulin-like growth factor-binding protein 2 (IGFBP-2), and insulin-like growth factor-binding protein 3 (IGFBP-3) were measured using an immunochemical assay.

MAIN FINDINGS

Hypertensive women had significantly lower IGFBP-2 levels than did normotensive controls (162.9±83.7 vs. 273.1±103.0μg/L, p<0.001); the groups did not differ regarding IGF and IGFBP-3 concentrations. After adjusting the covariates, multivariate analysis showed that IGFBP2 was significantly negatively correlated with 24-h systolic blood pressure (β=-0.31, p=0.02). The adjusted odds ratio for hypertension per standard deviation decrease in IGFBP-2 was 3.43 (95% confidence interval [CI] 1.65-7.13). IGFBP-2 showed a negative correlation with the number of metabolic syndrome components. Independent of body composition, IGFBP-2 was significantly related to left ventricular relative wall thickness and the ratio of mitral inflow velocities as parameter of diastolic function.

PRINCIPAL CONCLUSIONS

In perimenopausal women, decreased IGFBP-2 levels may play a role in blood pressure regulation and the development of subclinical left ventricular diastolic dysfunction. Whether IGFBP-2 is a marker or a mediator of cardiovascular disease in this population merits further investigation.

Maternally Administered Cyclic Glycine-Proline Increases Insulin-Like Growth Factor-1 Bioavailability and Novelty Recognition in Developing Offspring

Cyclic glycine-proline (cGP), a metabolite of IGF-1, is an endogenous neuropeptide that improves memory in adult rats. The presence and concentrations of endogenous cGP, and its association with IGF-1 and IGF binding protein-3 (IGFBP-3) in rat milk and plasma, were evaluated during postnatal development. Maternal-infantile transfer of cGP during lactation and its efficacy on the memory of developing offspring were also investigated. Dams were gavaged with either cGP (3 mg/kg) or saline daily from postnatal days 8-22. Concentrations of cGP were measured in dams' milk, and concentrations of cGP, IGF-1, and IGFBP-3 were measured in the plasma of dams, pups, and young adults. The recognition memory, locomotor function, and anxiety-like behavior of offspring were evaluated using behavioral tests. Endogenous cGP was detected in rat milk, and its concentration was higher during peak lactation compared with late lactation. Comparisons within control groups showed low endogenous IGF-1 and IGFBP-3 and high endogenous cGP concentrations in the plasma of male pups. The reduced IGFBP-3 and increased cGP may be a response to increase the bioavailability of IGF-1 during infancy. Exogenous cGP showed oral bioavailability and effective maternal-infantile transfer through milk. Maternally transferred cGP also led to improved recognition memory in the developing offspring, possibly through increased IGF-1 bioavailability, with no effect on locomotor activity and anxiety-like behavior. These results show that cGP is an essential endogenous peptide during early postnatal development as it improves the bioavailability of IGF-1 during infancy. Furthermore, maternal cGP supplementation offers an effective and natural route of administration for improving memory in the developing offspring.

The therapeutic potential of insulin-like growth factor-1 in central nervous system disorders

Central nervous system (CNS) development is a finely tuned process that relies on multiple factors and intricate pathways to ensure proper neuronal differentiation, maturation, and connectivity. Disruption of this process can cause significant impairments in CNS functioning and lead to debilitating disorders that impact motor and language skills, behavior, and cognitive functioning. Recent studies focused on understanding the underlying cellular mechanisms of neurodevelopmental disorders have identified a crucial role for insulin-like growth factor-1 (IGF-1) in normal CNS development. Work in model systems has demonstrated rescue of pathophysiological and behavioral abnormalities when IGF-1 is administered, and several clinical studies have shown promise of efficacy in disorders of the CNS, including autism spectrum disorder (ASD). In this review, we explore the molecular pathways and downstream effects of IGF-1 and summarize the results of completed and ongoing pre-clinical and clinical trials using IGF-1 as a pharmacologic intervention in various CNS disorders. This aim of this review is to provide evidence for the potential of IGF-1 as a treatment for neurodevelopmental disorders and ASD.

Insulin-like growth factor binding-protein-3 (IGFBP-3)

The IGF system comprises two IGFs (IGF-1, IGF-2), two IGF-receptors (IGF-R1, IGF-R2), and six IGF binding proteins (IGFBPs) with a high affinity for IGFs. The IGFBPs, of which IGFBP-3 is the most abundant in postnatal blood, link with IGFs and prevent them from being degraded; they also facilitate IGF transport through body compartments. The interaction between IGFs and their specific receptors is partly regulated by structural modifications inherent to the IGFBPs. IGFBPs also have IGF-independent biological effects. Since serum IGFBP-3 is GH-dependent and correlates quantitatively with GH secretion, its measurement is useful in tests of abnormal GH secretion. Particularly during childhood, IGFBP-3 values play an important role in ascertaining alterations in GH secretion and action (i.e., primary IGF deficiency states). A new role for IGFBP-3 and other IGFBPs with natural or altered structures is likely to be established through current studies investigating their application in promoting apoptotic processes in malignancies.

Free dissociable IGF-I: Association with changes in IGFBP-3 proteolysis and insulin sensitivity after surgery

BACKGROUND

Patients receiving a carbohydrate drink (CHO) before major abdominal surgery display improved insulin sensitivity postoperatively and increased proteolysis of IGFBP-3 (IGFBP-3-PA) compared to patients undergoing similar surgery after overnight fasting.

AIMS

We hypothesized that serum IGFBP-3-PA increases bioavailability of circulating IGF-I and preserves insulin sensitivity in patients given CHO.

DESIGN

Matched control study.

METHODS

At Karolinska University Hospital, patients given CHO before major elective abdominal surgery (CHO,n = 8) were compared to patients undergoing similar surgical procedures after overnight fasting (FAST,n = 10). Results from two different techniques for determination of free-dissociable IGF-I (fdIGF-I) were compared with changes in IGFBP-3-PA and insulin sensitivity.

RESULTS

Postoperatively, CHO displayed 18% improvement in insulin sensitivity (hyperinsulinemic clamp) and increased IGFBP-3-PA vs. FAST. As determined by IRMA, fdIGF-I increased by 48 ± 25% in CHO while fdIGF-I decreased by 13 ± 18% in FAST (p < 0.01 vs. CHO, when corrected for duration of surgery). However, fdIGF-I determined by ultra-filtration decreased similarly in both groups (-22 ± 8% vs. -25 ± 8%, p = 0.8) and IGFBP-1 increased similarly in both groups. Patients with less insulin resistance after surgery demonstrated larger increases in fdIGF-I by IRMA (r = 0.58, p < 0.05). Fifty-three % of the variability of the changes in fdIGF-I by IRMA could be explained by changes in IGFBP-3-PA and total IGF-I levels (p < 0.05), while IGFBP-1 did not contribute significantly.

CONCLUSION

During conditions when serum IGF-I bioavailability is regulated by IGFBP-3 proteolysis, measurements of fdIGF-I by IRMA is of physiological relevance as it correlates with the associated changes in insulin sensitivity.

IGFBP-3 inhibits cytokine-induced insulin resistance and early manifestations of atherosclerosis

Metabolic syndrome is associated with visceral obesity, insulin resistance and an increased risk of cardiovascular diseases. Visceral fat tissue primarily consists of adipocytes that secrete cytokines leading to a state of systemic inflammation in obese conditions. One of the IGF-independent functions of IGFBP-3 is its role as an anti-inflammatory molecule. Our study in obese adolescents show a decrease in total IGFBP-3 levels and increase in proteolyzed IGFBP-3 in circulation when compared to their normal counterparts and establishes a positive correlation between IGFBP-3 proteolysis and adiposity parameters as well as insulin resistance. In human adipocytes, we show that IGFBP-3 inhibits TNF-α-induced NF-κB activity in an IGF-independent manner, thereby restoring the deregulated insulin signaling and negating TNF-α-induced inhibition of glucose uptake. IGFBP-3 further inhibits TNF-α, CRP and high glucose-induced NF-κB activity in human aortic endothelial cells (HAECs) and subsequently suppresses monocyte adhesion to HAEC through the IGFBP-3 receptor. In conclusion, these findings suggest that reduced levels of IGFBP-3 in circulation and reduced expression of IGFBP-3 in macrophages in obesity may result in suppression of its anti-inflammatory functions and therefore IGFBP-3 may present itself as a therapeutic for obesity-induced insulin resistance and for events occurring in the early stages of atherosclerosis.

Insulin-like growth factor physiology: what we have learned from human studies

Although very similar to insulin and its receptor; the modus operandi of the insulin-like growth factors (IGFs) within the body is very different from that of the traditional peptide hormone. The IGF-binding proteins bind the IGFs with greater affinity than the cell surface receptors, enabling them to tightly control tissue activity. In addition to their role in fetal and childhood growth, IGFs play an important role in metabolic regulation. This article describes the basic underlying human physiology of IGFs, how this differs from that of experimental models, and why some information can only be learned from human clinical studies.

Insulin-like growth factor-binding protein-2 and -3 in gingival crevicular fluid

BACKGROUND AND OBJECTIVE

Insulin-like growth factor-binding proteins (IGFBPs) are crucial regulators of insulin-like growth factor (IGF). They enhance or inhibit IGF functions, but also exhibit IGF-independent effects. In a previous study, we detected, qualitatively, IGFBP-2 and -3 in gingival crevicular fluid using a cytokine antibody array. Here we extended these results using an ELISA to determine the concentrations of IGFBP-2 and -3 in gingival crevicular fluid. In addition, we explored whether the expression of IGFBP-2 and IGFBP-3 correlates with periodontal disease severity.

MATERIAL AND METHODS

Gingival crevicular fluid samples from 92 sites of 12 patients affected with periodontal disease and from 100 sites of 19 healthy volunteers, were collected, divided into two groups and analyzed by ELISA for IGFBP-2 and -3 expression. The potential correlation among probing depth, gingival index and the concentrations of IGFBP-2 and -3 was analyzed.

RESULTS

Positive correlations were observed between the concentration of IGFBP-2 and probing depth and gingival index, but not for IGFBP-3. The IGFBP-2 concentrations at bleeding on probing-positive sites and at sites with a probing depth of ≥ 4 mm were higher than at bleeding on probing-negative sites and at sites with a probing depth of ≤ 3 mm.

CONCLUSION

These results indicate that IGFBP-2 is a potential novel marker for periodontal disease progression. As IGFBP-2 modulates bone metabolism and cell migration, IGFBP-2 in the gingival crevicular fluid may reflect periodontal disease activity.

Insulin-like growth factor binding protein: a possible marker for the metabolic syndrome?

Insulin-like growth factor (IGF) binding proteins (IGFBPs) moved on to contain both IGF high- and low-affinity binders, exerting mitogenic and metabolic actions through its complex interplay between IGF/insulin and its IGF/insulin-independent manner. Progress on the metabolic-related function of IGFBPs has been rapid in recent years. A wealth of studies in 3T3-L1 adipocytes and the transgenic mice models demonstrated that IGFBPs played important roles in the pathogenesis of obesity and insulin resistance. Studies conducted in humans demonstrated the close relation between IGFBPs and the components of the metabolic syndrome. Abnormal expression of IGFBP was detected in various states of the metabolic disorders, suggesting that it could be used as a convenient and sensitive marker of insulin resistance, identification of insulin-resistant individuals at high cardiovascular risk, and may be an earlier marker of the metabolic syndrome. These exciting findings bring us new insight into the elucidation of the metabolic syndrome, which may have important clinical implications.

Targeting insulin and insulin-like growth factor pathways in epithelial ovarian cancer

Ovarian cancer is the most lethal of all gynecological malignancies, due in part to the diagnosis at an advanced stage caused by the lack of specific signs and symptoms and the absence of reliable tests for screening and early detection. Most patients will respond initially to treatment but about 70% of them will suffer a recurrence. Therefore, new therapeutic modalities are urgently needed to overcome chemoresistance observed in ovarian cancer patients. Evidence accumulates suggesting that the insulin/insulin growth factor (IGF) pathways could act as a good therapeutic target in several cancers, including ovarian cancer. In this paper, we will focus on the role of insulin/IGF in ovarian cancer tumorigenesis and treatment.

Separation of the molecular forms of the insulin-like growth factor (IGF)-Binding proteins by affinity chromatography

Association of IGFBP-1, IGFBP-2 and IGFBP-3 with other proteins in human serum and placental cell membranes was investigated using affinity chromatography matrix with immobilized antibodies. Circulating IGFBP-1 was found to be predominantly bound to alpha(2)-macroglobulin and not in the binary complex with its ligand, IGFBP-2 complexes and/or polymers were detected, which was not acknowledged before, and IGFBP-3 molecular forms were differentiated into those that form binary/ternary complexes and those that form stable associations with other serum proteins. As for placental membranes, both IGFBP-1 dimers and high molecular mass IGFBP-1 associations, most probably with alpha(2)-macroglobulin, were recognized and resolved.

Fresh-frozen plasma as a source of exogenous insulin-like growth factor-I in the extremely preterm infant

CONTEXT

Preterm birth is followed by a decrease in circulatory levels of IGF-I and IGF binding protein (IGFBP)-3, proteins with important neurogenic and angiogenic properties.

OBJECTIVE

Our objective was to evaluate the effects of iv administration of fresh-frozen plasma (FFP) from adult donors on circulatory levels of IGF-I and IGFBP-3 in extremely preterm infants.

DESIGN, SETTING, AND PATIENTS

A prospective cohort study was performed in 20 extremely preterm infants [mean (SD) gestational age 25.3 (1.3) wk] with clinical requirement of FFP during the first postnatal week. Sampling was performed before initiation of transfusion, directly after, and at 6, 12, 24, and 48 h after completed FFP transfusion.

MAIN OUTCOME MEASURES

Concentrations of IGF-I and IGFBP-3 before and after transfusion of FFP were determined.

RESULTS

FFP with a mean (SD) volume of 11 ml/kg (3.1) was administered at a median postnatal age of 2 d (range 1-7). Mean (SD) IGF-I and IGFBP-3 concentrations in administered FFP were 130 (39) and 2840 microg/liter (615), respectively. Immediately after FFP transfusion, mean (SD) concentrations of IGF-I increased by 133% from 11 (6.4) to 25 microg/liter (9.3) (P < 0.001) and IGFBP-3 by 61% from 815 (451) to 1311 microg/liter (508) (P < 0.001). Concentrations of IGF-I and IGFBP-3 remained higher at 6 (P < 0.001, P = 0.009) and 12 h (P = 0.017, P = 0.018), respectively, as compared with concentrations before FFP transfusion. Typical half-life of administrated IGF-I was 3.4 h for a 1-kg infant.

CONCLUSION

Transfusion of FFP to extremely preterm infants during the first postnatal week elevates levels of IGF-I and IGFBP-3.

Reactivation of the insulin-like growth factor-II signaling pathway in human hepatocellular carcinoma

Constitutive activation of the insulin-like growth factor (IGF)-signaling axis is frequently observed in human hepatocellular carcinoma (HCC). Especially the overexpression of the fetal growth factor IGF-II, IGF-Ireceptor (IGF-IR), and cytoplasmic downstream effectors such as insulin-receptor substrates (IRS) contribute to proliferation, anti-apoptosis, and invasive behavior. This review focuses on the relevant alterations in this signaling pathway and independent in vivo models that support the central role IGF-II signaling during HCC development and progression. Since this pathway has become the center of interest as a target for potential anti-cancer therapy in many types of malignancies, various experimental strategies have been developed, including neutralizing antibodies and selective receptor kinase inhibitors, with respect to the specific and efficient reduction of oncogenic IGF-II/IGF-IR-signaling.

Insulin-like growth factor system regulates oligodendroglial cell behavior: therapeutic potential in CNS

Amongst the many soluble extracellular factors stimulating intracellular signal transduction pathways and driving cellular processes such as proliferation, differentiation and survival, insulin-like growth factors (IGFs) stand out as indispensable factors for proper oligodendrocyte differentiation and accompanying myelin production. Owing to its potent myelinogenic capacity and its neuroprotective properties, IGFs hold therapeutic potential in demyelinating and neurodengenerative diseases. However, the IGF system is comprised of a complex molecular network involving regulatory binding proteins, proteases, cell surface and extracellular matrix components which orchestrate IGF-specific functions. Thus, the complexity by which these factors are tightly regulated makes a simplistic therapeutic approach towards treating demyelinating conditions unfeasible. In the present review, we address these issues and consider current therapeutic prospects of oligodendrocyte-targeted IGF-based therapies.

Insulin-like growth factor (IGF) binding protein-4 is both a positive and negative regulator of IGF activity in vivo

IGFs are required for normal prenatal and postnatal growth. Although actions of IGFs can be modulated by a family of IGF-binding proteins (IGFBPs) in vitro, these studies have identified a complicated pattern of stimulatory and inhibitory IGFBP effects, so that understanding relevant aspects of IGFBP action in vivo has been limited. Here we have produced a null mutation of one specific IGFBP, IGFBP-4, which is coexpressed with IGF-II early in development. Surprisingly, mutation of IGFBP-4, believed from in vitro studies to be exclusively inhibitory, leads to a prenatal growth deficit that is apparent from the time that the IGF-II growth deficit first arises, which strongly suggests that IGFBP-4 is required for optimal IGF-II-promoted growth during fetal development. Mice encoding a mutant IGFBP-4 protease (pregnancy-associated plasma protein-A), which facilitates IGF-II release from an inactive IGF-II/IGFBP-4 complex in vitro, are even smaller than IGFBP-4 mutant mice. However, the more modest IGFBP-4 growth deficit is completely restored in double IGFBP-4/pregnancy-associated plasma protein-A-deficient mice. Taken together these results indicate not only that IGFBP-4 functions as a local reservoir to optimize IGF-II actions needed for normal embryogenesis, but also establish that IGFBP-4 proteolysis is required to activate most, if not all, IGF-II mediated growth-promoting activity.

Activity of ulilysin, an archaeal PAPP-A-related gelatinase and IGFBP protease

Human growth and development are conditioned by insulin-like growth factors (IGFs), which have also implications in pathology. Most IGF molecules are sequestered by IGF-binding proteins (IGFBPs) so that exertion of IGF activity requires disturbance of these complexes. This is achieved by proteolysis mediated by IGFBP proteases, among which the best characterised is human PAPP-A, the first member of the pappalysin family of metzincins. We have previously identified and studied the only archaeal homologue found to date, Methanosarcina acetivorans ulilysin. This is a proteolytically functional enzyme encompassing a pappalysin catalytic domain and a pro-domain involved in maintenance of latency of the zymogen, proulilysin. Once activated, the protein hydrolyses IGFBP-2 to -6 and insulin chain beta in vitro. We report here that ulilysin is also active against several other substrates, viz (azo)casein, azoalbumin, and extracellular matrix components. Ulilysin has gelatinolytic but not collagenolytic activity. Moreover, the proteolysis-resistant skeletal proteins actin and elastin are also cleaved, as is fibrinogen, but not plasmin and alpha1-antitrypsin from the blood coagulation cascade. Ulilysin develops optimal activity at pH 7.5 and strictly requires peptide bonds preceding an arginine residue, as determined by means of a novel fluorescence resonance energy transfer assay, thus pointing to biotechnological applications as an enzyme complementary to trypsin.

Alteration of the insulin-like growth factor axis during in vitro differentiation of the human osteosarcoma cell line HOS 58

The insulin-like growth factors I and II (IGF-I, IGF-II), their receptors, and high affinity binding proteins (IGFBPs) represent a family of cellular modulators that play essential roles in the development and differentiation of cells and tissues including the skeleton. Recently, the human osteosarcoma cell line HOS 58 cells were used as an in vitro model of osteoblast differentiation characterized by (i) a rapid proliferation rate in low-density cells that decreased continuously with time of culture and (ii) an increasing secretion of matrix proteins during their in vitro differentiation. In the present paper, HOS 58 cells with low cell density at early time points of the in vitro differentiation (i) displayed a low expression of IGF-I and -II; (ii) synthesized low levels of IGFBP-2, -3, -4, and -5, but (iii) showed high expression levels of both the type I and II IGF receptors. During the in vitro differentiation of HOS 58 cells, IGF-I and -II expressions increased continuously in parallel with an upregulation of IGFBP-2, -3, -4, and -5 whereas the IGF-I receptor and IGF-II/M6P receptor mRNA were downregulated. In conclusion, the high proliferative activity in low cell density HOS 58 cells was associated with high mRNA levels of the IGF-IR, but low concentrations of IGFBP-2. The rate of proliferation of HOS 58 cells continuously decreased during cultivation in parallel with a decline in IGF-IR expression, but increase of mitoinhibitory IGFBP-2. These data are indicative for a role of the IGF axis during the in vitro differentiation of HOS 58 cells.

Calpain proteolysis of insulin-like growth factor binding protein (IGFBP) -2 and -3, but not of IGFBP-1

Calpains are cytoplasmic Ca(2+)-regulated cysteine proteases that may regulate insulin-like growth factor (IGF)-independent actions of insulin-like growth factor binding proteins (IGFBPs) through IGFBP proteolysis. In this study, [(125)I]-labeled IGFBP-2 and -3, but not IGFBP-1, were proteolyzed by Ca(2+)-activated m-calpain in vitro. Degradation of higher concentrations of the recombinant proteins IGFBP-2 and -3 by m-calpain was dose-dependent, but was terminated within 20 min by autolysis. By subjecting proteolytic fragments to N-terminal amino acid sequence analysis, the primary cleavage sites in IGFBP-2 and -3 were localized to the non-conserved central linker regions. Using the biosensor technique, in vitro binding of m-calpain to IGFBP-3 was demonstrated to be a Ca(2+)-dependent reaction with a rapid on/off rate.

Insulin-like growth factor binding proteins: regulation in chronic active plaques in multiple sclerosis and functional analysis of glial cells

Studies in experimental allergic encephalomyelitis, an animal model of multiple sclerosis (MS), suggest that astrocyte-secreted insulin-like growth factor binding protein-2 (IGFBP-2) helps target IGF-1 to IGF-1 receptor-expressing oligodendrocytes and promote remyelination. We examined the presence of IGFBPs 1-6 in astrocytes in normal post-mortem human brain tissue and lesions of MS by means of immunohistochemistry. Under normal conditions all six IGFBPs were detected. Compared to controls, hypertrophic astrocytes at the borders of chronic active MS lesions displayed increased immunoreactivity for IGFBP-2 and IGFBP-4. In vitro studies were performed to analyse the effects of IGFBPs on cellular proliferation of neonatal rat glial cells. Treatment of astrocytes with IGF-1 and -2 enhanced proliferation whereas IGFBP-2 and -4 inhibited cellular growth. Interestingly, combined treatment with IGFBP-2 and IGF-1 potentiated effects on cellular proliferation whereas combined treatment with IGFBP-2 and IGF-2 inhibited growth. Unlike IGFBP-2, IGFBP-4 inhibited proliferation in combined treatment with IGF-1. In contrast, combined treatment with IGFBP-2 and IGF-1 resulted in decreased cell survival of oligodendrocyte precursor cells. Our results suggest that the up-regulation of IGFBP-2 in reactive astrocytes in MS lesions may primarily serve to enhance the IGF-1-mediated mitogenic stimulus for astrocytes rather than supporting oligodendrocyte survival.

The insulin-like growth factor system in multiple sclerosis

Multiple sclerosis (MS) is a chronic disorder of the central nervous system characterized by inflammation, demyelination, and axonal degeneration. Present therapeutic strategies for MS reduce inflammation and its destructive consequences, but are not effective in the progressive phase of the disease. There is a need for neuroprotective and restorative therapies in MS. Insulin-like growth factor-1 (IGF-1) is of considerable interest because it is not only a potent neuroprotective trophic factor but also a survival factor for cells of the oligodendrocyte lineage and possesses a potent myelinogenic capacity. However, the IGF system is complex and includes not only IGF-1 and IGF-2 and their receptors but also modulating IGF-binding proteins (IGFBPs), of which six have been identified. This chapter provides an overview of the role of the IGF system in the pathophysiology of MS, relevant findings in preclinical models, and discusses the possible use of IGF-1 as a therapeutic agent for MS.

Substrate specificity of a metalloprotease of the pappalysin family revealed by an inhibitor and a product complex

Human pappalysin-1 is a multi-domain metalloprotease engaged in the homeostasis of insulin-like growth factors and the founding member of the pappalysin family within the metzincin clan of metalloproteases. We have recently identified an archaeal relative, ulilysin, encompassing only the protease domain. It is a 262-residue active protease with a novel 3D structure with two subdomains separated by an active-site cleft. Despite negligible overall sequence similarity, noticeable similarity is found with other metzincin prototypes, adamalysins/ADAMs and matrix metalloproteinases. Ulilysin has been crystallised in a product complex with an arginine-valine dipeptide occupying the active-site S(1') and S(2') positions and in a complex with the broad-spectrum hydroxamic acid-based metalloprotease inhibitor, batimastat. This molecule inhibits mature ulilysin with an IC(50) value of 61 microM under the conditions assayed. The binding of batimastat to ulilysin evokes binding to vertebrate matrix metalloproteases but is much weaker. These data give insight into substrate specificity and mechanism of action and inhibition of the novel pappalysin family.

Identification of membrane-bound serine proteinase matriptase as processing enzyme of insulin-like growth factor binding protein-related protein-1 (IGFBP-rP1/angiomodulin/mac25)

Insulin-like growth factor (IGF) binding protein-related protein-1 (IGFBP-rP1) modulates cellular adhesion and growth in an IGF/insulin-dependent or independent manner. It also shows tumor-suppressive activity in vivo. We recently found that a single-chain IGFB-rP1 is proteolytically cleaved to a two-chain form by a trypsin-like, endogenous serine proteinase, changing its biological activities. In this study, we attempted to identify the IGFBP-rP1-processing enzyme. Of nine human cell lines tested, seven cell lines secreted IGFBP-rP1 at high levels, and two of them, ovarian clear cell adenocarcinoma (OVISE) and gastric carcinoma (MKN-45), highly produced the cleaved IGFBP-rP1. Serine proteinase inhibitors effectively blocked the IGFBP-rP1 cleavage in the OVISE cell culture. The conditioned medium of OVISE cells did not cleave purified IGFBP-rP1, but their membrane fraction had an IGFBP-rP1-cleaving activity. The membrane fraction contained an 80-kDa gelatinolytic enzyme, which was identified as the membrane-type serine proteinase matriptase (MT-SP1) by immunoblotting. When the membrane fraction was separated by SDS/PAGE, the IGFBP-rP1-cleaving activity comigrated with matriptase. A soluble form of matriptase purified in an inhibitor-free form efficiently cleaved IGFBP-rP1 at the same site as that found in a naturally cleaved IGFBP-rP1. Furthermore, small interfering RNAs for matriptase efficiently blocked both the matriptase expression and the cleavage of IGBP-rP1 in OVISE cells. These results demonstrate that IGFBP-rP1 is processed to the two-chain form by matriptase on the cell surface.

The role of insulin-like growth factor binding proteins

Insulin-like growth factors (IGFs) are fundamental cell regulators with an evolutionary conserved role synchronising tissue growth, development and function according to metabolic conditions. Although structurally very similar to insulin, the IGFs act in a very different way as cell regulators. Whereas insulin is stored in a specific gland and released when needed, the IGFs are stored outside of cells with soluble binding proteins. A very complex system of six IGF binding proteins, each of which exists in various modified states and interacts with other proteins, provides a sophisticated system for conferring specificity to provide a finely tuned system for local regulation at the tissue level.

Insulin-like growth factor binding proteins in development

IGFBPs regulate growth and development by regulating IGF transport to tissues and IGF bioavailability to IGF receptors at cell membrane level. IGFBP excess leads predominantly to inhibition of IGF action and growth retardation with impaired organogenesis. Absence of human and also mouse ALS leads to decreased IGF-I levels in circulation and causes mild growth retardation. Although IGFBP KO mice demonstrate relatively minor phenotypes, the possibility of compensatory mechanisms that mask the phenotypic manifestation of lack of individual binding proteins needs to be further investigated. Recent studies of hepatic regeneration in IGFBP-1 KO mice and also with mutant IGFBP-3 Tg mice provide some limited support for the existence of IGF-independent mechanism of action in vivo.

The insulin-like growth factor system: IGFs, IGF-binding proteins and IGFBP-proteases

Insulin-like growth factors (IGF-I/-II) are not only the endocrine mediators of growth hormone-induced metabolic and anabolic actions but also polypeptides that act in a paracrine and autocrine manner to regulate cell growth, differentiation, apoptosis and transformation. The IGF system is a complex network comprised of two growth factors (IGF-I and -II), cell surface receptors (IGF-IR and -IIR), six specific high affinity binding proteins (IGFBP-I to IGFBP-6), IGFBP proteases as well as several other IGFBP-interacting molecules, which regulate and propagate IGF actions in several tissues. Besides their broad-spectrum physiological and pathophysiological functions, recent evidence suggests even a link between IGFs and different malignancies.

Diversity of human insulin-like growth factor (IGF) binding protein-2 fragments in plasma: primary structure, IGF-binding properties, and disulfide bonding pattern

The insulin-like growth factor binding proteins (IGFBPs) play a major role in the regulation of the effects and the bioavailability of the insulin-like growth factors (IGFs). IGFs are released from IGFBP-IGF complexes by proteolysis of IGFBPs generating fragments with reduced ligand-binding properties. To identify naturally occurring fragments of IGFBP-2, a peptide library generated from human hemofiltrate was immunologically screened. Purification of immunoreactive IGFBP-2 fragments was performed by consecutive chromatographic steps. A total of 18 different IGFBP-2 fragments was isolated and characterized. The peptides exhibited different N-terminal amino acid residues that were located in the variable midregion of IGFBP-2. Four major cleavage sites were determined to be between Tyr103 and Gly104, Leu152 and Ala153, Arg156 and Glu157, and Gln165 and Met166. The resulting fragments were further processed by amino and/or carboxy peptidases and comprised 37-185 amino acid residues. Ligand blotting, solution binding assays, and BIAcore analyses revealed that all tested fragments retained low IGF-binding capacity. The most abundant fragment IGFBP-2 (167-279) showed 10% of IGF-II binding compared to recombinant human (rh)IGFBP-2. Furthermore, the disulfide bonding pattern of the C-terminal domain of rhIGFBP-2 was defined, indicating linkages between cysteine residues 191-225, 236-247, and 249-270. This study provides the most comprehensive molecular characterization of human IGFBP-2 fragments formed in vivo, exhibiting both residual IGF-binding capacities and the integrin-binding sequence.

Interaction of Insulin-like Growth Factor II (IGF-II) with Multiple Plasma Proteins

In the circulation, most of the insulin-like growth factors (IGFs), IGF-binding proteins (IGFBPs), and IGFBP proteases are bound in high molecular mass complexes of > or =150 kDa. To investigate molecular interactions between proteins involved in IGF.IGFBP complexes, Cohn fraction IV of human plasma was subjected to IGF-II affinity chromatography followed by reversed-phase high pressure liquid chromatography and analysis of bound proteins. Mass spectrometry and Western blotting revealed the presence of IGFBP-3, IGFBP-5, transferrin, plasminogen, prekallikrein, antithrombin III, and the soluble IGF-II/mannose 6-phosphate receptor in the eluate. Furthermore, an IGFBP-3 protease cleaving also IGFBP-2 but not IGFBP-4 was co-purified from the IGF-II column. Inhibitor studies and IGFBP-3 zymography have demonstrated that the 92-kDa IGFBP-3 protease belongs to the class of serine-dependent proteases. IGF-II ligand blotting and surface plasmon resonance spectrometry have been used to identify plasminogen as a novel high affinity IGF-II-binding protein capable of binding to IGFBP-3 with 50-fold higher affinity than transferrin. In combination with transferrin, the overall binding constant of plasminogen/transferrin for IGF-II was reduced 7-fold. Size exclusion chromatography of the IGF-II matrix eluate revealed that transferrin, plasminogen, and the IGFBP-3 protease are present in different high molecular mass complexes of > or =440 kDa. The present data indicate that IGFs, low and high affinity IGFBPs, several IGFBP-associated proteins, and IGFBP proteases can interact, which may result in the formation of binary, ternary, and higher molecular weight complexes capable of modulating IGF binding properties and the stability of IGFBPs.

Pregnancy-associated plasma protein-A proteolytic activity in rat vertebral cell cultures: Stimulation by dexamethasone-a potential mechanism for glucocorticoid regulation of osteoprogenitor proliferation and differentiation

Glucocorticoids (GCs) at physiological concentrations stimulate osteoprogenitor proliferation and differentiation in rat bone cell populations, and this is mediated in part by an increased response to insulin-like growth factors (IGFs). Since IGF binding proteins (IGFBPs) modulate IGF actions, we evaluated whether the increased IGF responsiveness might be associated with decreased inhibitory IGFBP-4 peptide levels. Rat vertebral cells were cultured for up to 20 days with or without dexamethasone (Dex). Cell layer proteins were extracted at day 6, 8, 14, and 20, conditioned media (CM) collected at day 8, 14, and 20, and total RNA isolated at day 14 and 20 of culture. Western blotting showed that cell layer IGFBP-4 levels were lower, while IGFBP-4 protease activity in CM was higher, in Dex-treated cultures. Addition of pregnancy-associated plasma protein-A (PAPP-A) antibody to CM abrogated IGFBP-4 proteolysis. PAPP-A mRNA levels were the same in control and Dex-treated cultures as evaluated by RT-PCR. Our data demonstrate that activity of the IGFBP-4 protease, PAPP-A, in rat bone cell cultures is increased by Dex via post-transcriptional mechanisms. Since IGFBP-4 mRNA levels in Dex-treated cultures were the same as in controls at day 8, slightly lower than in controls at day 14, and higher than in controls at day 20 as shown previously, the decreased IGFBP-4 peptide levels in Dex-treated cultures likely result from increased IGFBP-4 proteolysis by the elevated PAPP-A enzymatic activity. Our findings underscore a novel mechanism whereby GCs increase IGF responses in rat bone cells via PAPP-A-induced IGFBP-4 proteolysis.

IGF-binding protein-3 fragments in plasma of a child with acute renal failure

The insulin-like growth factors (IGF) -I and -II promote cellular growth and differentiation of various organs. Their growth-stimulating effects are modulated by a family of six IGF-binding proteins (IGFBPs). Altered patterns of intact and fragmented IGFBPs have been reported in serum and urine of children with chronic renal failure (CRF), and it has been suggested that this may contribute to the growth failure observed in these patients. In the present study, a rapid and comprehensive method is presented to analyze IGFBPs and IGFBP fragments in the plasma of a child with acute renal failure (ARF) who had undergone plasmapheresis. The plasma IGF-I and IGFBP-3 levels were drastically reduced. Plasmapheresis filtrate (3 l) was fractionated by cation-exchange chromatography and reversed-phase high-performance liquid chromatography. The fractions obtained were tested by ligand and immunoblotting. In addition to IGFBP-1 and -4 fragments, the majority of IGF-binding polypeptides were IGFBP-3 immunoreactive. N-terminal sequence analysis of a 17-kDa polypeptide revealed the isolation of a C-terminal fragment of IGFBP-3 starting with Lys 160. The IGF-II-binding polypeptide pattern in the ARF plasma resembles the pattern in hemofiltrate from CRF patients, suggesting that similar or identical proteases are involved in IGFBP-3 fragmentation and common mechanisms may lead to the accumulation of the fragments in both diseases.

An evaluation of the role of insulin-like growth factors (IGF) and of type-I IGF receptor signalling in hepatocarcinogenesis and in the resistance of hepatocarcinoma cells against drug-induced apoptosis

Strong evidence emphasizes the role of the insulin-like growth factor (IGF) system and of type-I IGF receptor (IGF-IR) signalling in tumourigenesis. In this connection: (i) changes in the expression pattern of components of the IGF system (autocrine/paracrine expression of IGF-I and -II, overexpression of IGF-IR, decreased expression of IGF-binding proteins (IGFBPs) and of type-II IGF receptor/cation-independent mannose-6-phosphate receptor (IGF-II/M6PR) and (ii) increased serum concentrations of proteases that cleave the IGFBPs (e.g., cathepsin D) were observed in patients with hepatocellular carcinomas (HCC), in human hepatoma cell lines and in their conditioned culture medium, as well as in rodent models of hepatocarcinogenesis. Accordingly, studies carried out with animal models do suggest that the IGF system and IGF-IR signalling may play a role in hepatocarcinogenesis and in deregulated proliferation and apoptosis of HCC cells. Finally the instrumental role of Raf/MEK/ERK, one of the signalling cascades stimulated by IGF-IR, in anthracycline-induced apoptosis of HepG2 and Huh-7 human hepatoma cell lines emphasizes that care must be taken when designing combinations of antitumoural molecules for antineoplastic treatment. This review addresses the putative roles of the IGF system in primary HCC, with a special focus on the underlying molecular mechanisms. In a second part it emphasizes the putative interference of IGF-IR signalling with chemotherapeutic drug-induced apoptosis in human hepatoma cells.

Matrix Metalloprotease-3 and -9 Proteolyze Insulin-Like Growth Factor-Binding Protein-11

Growth in utero depends on adequate development and function of the fetal/maternal interface. During pregnancy, the insulin-like growth factors (IGFs), which are known to be critically involved in placental development, are controlled by a binding protein-IGFBP-1-produced by maternal decidualized endometrium. We have previously found that decidua also produces a protease that cleaves IGFBP-1; because proteolysis of IGFBP-1 may represent a mechanism for increasing IGF bioavailability, the present study aimed to identify the protease and its regulators to understand the control of IGF activity at the maternal/fetal interface. Immunochemical methods were used to show that decidualized endometrial cells from first-trimester pregnancy produced matrix metalloprotease (MMP)-3; incubation of IGFBP-1 with either this enzyme or MMP-9, which is produced by the trophoblast, produced a series of fragments that were unable to bind IGF-I. Western immunoblot analysis and immunocytochemistry demonstrated that decidual cells also produce tissue inhibitor of metalloproteinase (TIMP)-1, TIMP-2, and alpha(2)-macroglobulin, and all three inhibitors attenuated the proteolysis of IGFBP-1 by MMPs. The N-terminal sequence analysis of the fragments revealed that the enzymes cleave IGFBP-1 at (145)Lys/Lys(146), resulting in a small (9-kDa) C-terminal peptide of IGFBP-1. These findings suggest cleavage of IGFBP-1 as a novel mechanism in the control of placental development by matrix metalloproteases.

Proteolytic Degradation of Insulin-Like Growth Factor Binding Proteins by Ovarian Follicles: A Control Mechanism for Selection of Dominant Follicles1

This review summarizes evidence for the role of proteolytic enzymes that degrade and inactivate insulin-like growth factor binding proteins (IGFBP) during follicular development in mammals. In some species (e.g., bovine), evidence indicates that decreases in IGFBP-4 and -5 levels in estrogen-dominant preovulatory follicles are likely due, in part, to increased protease activity, whereas lower levels of IGFBP-2 are not due to increased proteolysis. Increased IGFBP-4 and -5 protease along with lower amounts of IGFBP-4 binding activity and greater amounts of free IGF-I are some of the earliest developmental changes documented in bovine growing antral follicles. This protease activity has recently been ascribed to serine metalloprotease(s), including pregnancy-associated plasma protein-A (PAPP-A), which was first detected in human follicular fluid nearly 20 yr ago. Other recent studies verified the presence of PAPP-A mRNA in granulosa cells of humans, monkeys, cattle, mice, and pigs. Increases in the amount of PAPP-A mRNA in granulosa cells during follicular development occurs in some but not all species, indicating that other proteases or protease inhibitors may be involved in IGFBP degradation. Whether the hormonal control of PAPP-A production/activity by the ovary differs between monotocous and polytocous animals will require further study. These protease-induced decreases in IGFBP-4 and -5 likely cause increased levels of bioavailable (or free) IGFs that stimulate steroidogenesis and mitogenesis in developing dominant follicles, which ultimately prepare the follicle(s) and oocyte(s) for successful ovulation and fertilization.

Epidermal homeostasis: the role of the growth hormone and insulin-like growth factor systems

GH and IGF-I and -II were first identified by their endocrine activity. Specifically, IGF-I was found to mediate the linear growth-promoting actions of GH. It is now evident that these two growth factor systems also exert widespread activity throughout the body and that their actions are not always interconnected. The literature highlights the importance of the GH and IGF systems in normal skin homeostasis, including dermal/epidermal cross-talk. GH activity, sometimes mediated via IGF-I, is primarily evident in the dermis, particularly affecting collagen synthesis. In contrast, IGF action is an important feature of the dermal and epidermal compartments, predominantly enhancing cell proliferation, survival, and migration. The locally expressed IGF binding proteins play significant and complex roles, primarily via modulation of IGF actions. Disturbances in GH and IGF signaling pathways are implicated in the pathophysiology of several skin perturbations, particularly those exhibiting epidermal hyperplasia (e.g., psoriasis, carcinomas). Additionally, many studies emphasize the potential use of both growth factors in the treatment of skin wounds; for example, burn patients. This overview concerns the role and mechanisms of action of the GH and IGF systems in skin and maintenance of epidermal integrity in both health and disease.

Proteolytic processing of IGFBP-related protein-1 (TAF/angiomodulin/mac25) modulates its biological activity

Insulin-like growth factor (IGF) binding protein-related protein-1 (IGFBP-rP1) was previously identified as tumor-derived adhesion factor (TAF) secreted from human bladder carcinoma cells. It exhibits growth-stimulatory activity in synergy with insulin or IGFs. In the present study, we found that IGFBP-rP1 was proteolytically cleaved to a two-chain form. The cleavage sequence suggested that a trypsin-like serine proteinase may be responsible for the processing. The cleavage of IGFBP-rP1 led to an almost complete loss of both insulin/IGF-1-binding activity and insulin/IGF-1-dependent growth-stimulatory activity. On the other hand, the cell attachment activity of IGFBP-rP1 was markedly increased by the proteolytic processing. Syndecan-1 was thought to be a cell surface receptor for both intact and cleaved IGFBP-rP1 forms. Although the proteolytic cleavage of IGFBP-rP1 decreased its heparin-binding activity, the cleaved form could bind syndecan-1 efficiently. Thus the proteolytic processing of IGFBP-rP1 seems to modulate its insulin/IGF-dependent and -independent biological functions.

Insulin-like growth factor-binding protein 6 inhibits survival and differentiation of rat oligodendrocyte precursor cells

Insulin-like growth factor 1 (IGF-1) is a growth and survival factor for oligodendrocyte lineage cells and promotes myelination. We demonstrate that IGF-binding protein 6 (IGFBP-6) is expressed and localized to the Golgi complex in rat oligodendrocyte precursor (O2A) cells. IGFBP-6 mRNA showed a developmentally regulated expression pattern, displaying a transient decrease during early development, and enhanced levels upon cell maturation. IGFBP-6 mRNA expression could be reduced by addition of basic fibroblast growth factor and progesterone while estrogen increased IGFBP-6 mRNA. IGF-1, platelet-derived growth factor, and insulin had no effect. When added exogenously, IGFBP-6 reduced O2A cell survival in the absence of IGF-1 and inhibited IGF-1-stimulated survival in a partially IGF-1-dependent and partially IGF-1-independent fashion. In addition, IGFBP-6 reduced the IGF-stimulated expression of two myelin proteins, CNPase and MAG. Taken together, the data show that IGFBP-6 is a new negative effector of oligodendrocyte survival and differentiation.

The role of the IGF system in cancer: from basic to clinical studies and clinical applications

Insulin-like growth factors (IGFs) are important mediators of growth, development, and survival, are synthesized by almost any tissue in the body, and their action is modulated by a complex network of molecules, including binding proteins, proteases and receptors, which all comprise the IGF system. Evidence from in vitro and animal studies suggests that overexpression of IGFs by cancer cells and/or the nearby stroma as well as the type IGF-I receptor by the cancer cells may play a significant role in establishing a transformed phenotype in an increasing number of malignancies. More specifically, IGFs may promote cell cycle progression and inhibition of apoptosis either by directly associating with other growth factors or indirectly by interacting with other molecular systems which have an established role in carcinogenesis and cancer promotion, such as the steroid hormones and integrins. In addition, a growing number of epidemiologic studies suggest that increased serum levels of IGFs and/or altered levels of their binding proteins are associated with increased risk for developing several malignancies. These data indicate that IGF dysregulation should now be considered as an important independent factor for cancer risk, and a potential target for novel antineoplastic therapies and/or preventative strategies in high-risk groups.

Treatment of metastatic osteosarcoma with the somatostatin analog OncoLar: significant reduction of insulin-like growth factor-1 serum levels

BACKGROUND

Insulin-like growth factor-1 (IGF-1) has been implicated in the growth and/or metastasis of osteosarcoma (OS) and chondrosarcoma based on in vitro and experimental animal studies.

STUDY PURPOSE

To determine the degree of growth hormone (GH), IGF-1 axis blockade, toxicities, and antitumor effect of OncoLar (ONC) (Novartis, East Hanover, NJ, U.S.A.) in OS.

DESIGN/METHODS

A phase 1 study with ONC enrolled 21 OS patients (median age 19 y) in four cohorts: ONC 60 mg or 90 mg intramuscularly every 4 weeks with/without tamoxifen (TAM) 20 mg oral daily.

RESULTS

There were no dose-limiting toxicities. Nineteen percent of patients had grade III drug-related toxicities including: 62% of patients showed progressive disease after two courses (8 wk). Nineteen percent received four courses. No clinical responses were observed. At weeks two and eight of therapy, IGF-1 serum levels dropped 46% ( < 0.0001, n = 21) and 53% ( = 0.003, n = 10). The difference of the area under the curve (AUC) minus baseline AUC (DeltaAUC) for arginine-stimulated GH serum levels at week two was lower than baseline ( < 0.01). At weeks two and eight, GH peak values were lower than baseline ( < 0.0001 and = 0.002, respectively).

CONCLUSIONS

A long-acting somatostatin analog was able to lower IGF-1 levels of OS patients. IGF-BP-3 and GH were only transiently reduced. Although ONC was well tolerated, no sustained clinical responses were observed. The pathophysiology of serum versus tissue concentrations of IGF-1 as well as the interplay of IGFs, IGF-binding proteins, and other growth factors and cytokines in osteosarcoma warrants further investigation. A better understanding of these processes should lead to a more effective exploitation of these pathways for the targeted therapy of OS.

Insulin-like growth factor binding proteins-1 and -2 differentially inhibit rat oligodendrocyte precursor cell survival and differentiation in vitro

Insulin-like growth factor-1 (IGF-1) is a growth and survival factor for oligodendrocyte lineage cells and induces myelination. Its actions are modulated by IGF binding proteins (IGFBPs) that are present in the extracellular fluids or on the cell surface. Additionally, IGFBPs are also known to exert actions that are independent of IGF-1. We studied whether IGF-binding proteins (IGFBPs)-1 and -2 modulate rat oligodendrocyte precursor (O2A) cell survival and differentiation in vitro both in the absence and presence of exogenously added IGF-1. The data reveal that IGFBP-1 and -2 reduced O2A cell survival in the absence and presence of exogenously added IGF-1. The effects of IGFBP-1 on cell survival in the presence of exogenously added IGF-1 were IGF-1-dependent, whereas IGFBP-2 displayed both IGF-1-dependent and IGF-1-independent effects. Furthermore, IGFBP-1 and -2 inhibited O2A cell differentiation in the presence of IGF-1 as reflected by decreased expression levels of two myelin proteins, CNPase (2',3'-cyclic nucleotide 3'-phosphohydrolase) and MAG (myelin associated glycoprotein). Analysis of medium samples revealed that O2A cells do not secrete proteases that degrade these IGFBPs. Taken together the data show that IGFBP-1 and -2 are negative effectors of oligodendrocyte survival and differentiation. Accordingly, the role of IGFBPs should be explicitly taken into account when investigating IGF-1 effects on oligodendrocytes, especially in the context of therapeutic purposes.

A non-IGF binding mutant of IGFBP-3 modulates cell function in breast epithelial cells

We demonstrated previously that IGFBP-3 alone had no effect on cell death, but dramatically modulated apoptosis in Hs578T IGF non-responsive cells. We investigated whether a non-IGF binding mutant of IGFBP-3 retained its intrinsic actions in this cell line, prior to investigating its actions in IGF-responsive cells (MCF-7 and MCF-10A). In the Hs578T cells, the ceramide analogue, C2-induced apoptosis, non-glycosylated, glycosylated or mutant IGFBP-3 alone had no effect but on co-incubation with C2, all forms of IGFBP-3 markedly accentuated triggered apoptosis. In MCF-7 cells, IGFBP-3 was unable to modulate C2-induced death. In the MCF-10A cells, IGFBP-3 acted as a potent survival factor. IGFBP-3 also affected cell growth in the MCF-10A cells (inhibiting at low doses but increasing growth at higher concentrations). These actions of IGFBP-3 in the MCF-10A cells were independent of IGF-1. IGFBP-3 has differential IGF-independent effects on cell death and growth in normal breast and breast cancer cells.

Isolation and characterization of circulating fragments of the insulin-like growth factor binding protein-3

Proteolysis of insulin-like growth factor binding protein-3 (IGFBP-3), the major carrier of IGFs in the circulation, is an essential mechanism to regulate IGF bioavailability. To analyze naturally occurring IGFBP-3 fragments a peptide library established from human hemofiltrate was screened. Three IGFBP-3 fragments were detected with apparent molecular masses of 34, 16, and 11 kDa. Mass spectrometric and sequence analysis identified the 16 and 11 kDa peptides as glycosylated and non-glycosylated N-terminal fragments spanning residues Gly1-Ala98 of IGFBP-3. Both the circulating forms and those secreted from IGFBP-3(1-98) overexpressing cells bound IGF. Additionally, two smaller fragments (IGFBP-3(139-157) and IGFBP-3(139-159)) were identified in the hemofiltrate. The data indicate that proteolysis of circulating IGFBP-3 occurs in the variable domain at residues alanine 98, phenylalanine 138, glutamine 157, and tyrosine 159.

Decreased intracellular degradation of insulin-like growth factor binding protein-3 in cathepsin L-deficient fibroblasts

Proteolysis of insulin-like growth factor binding proteins (IGFBPs) is the major mechanism of releasing IGFs from their IGFBP complexes. Analysis of fibroblasts deficient for the lysosomal cysteine protease cathepsin L (CTSL) revealed an accumulation of IGFBP-3 in the medium which was due neither to alterations in IGFBP-3 mRNA expression nor to extracellular IGFBP-3 protease activity. Incubation of CTSL-deficient fibroblasts with radiolabeled IGFBP-3 followed by subcellular fractionation indicates that both intact and fragmented IGFBP-3 accumulate transiently in endosomal and lysosomal fractions of CTSL-deficient cells. This suggests the involvement of CTSL in the intracellular degradation of IGFBP-3 representing a new mechanism to regulate the extracellular concentration of IGFBP-3.

IGFBP-3 binding to endothelial cells inhibits plasmin and thrombin proteolysis

Insulin-like growth factor-binding protein (IGFBP)-3 contains a highly basic COOH-terminal heparin-binding region, the P3 region, which is thought to be important in the binding of IGFBP-3 to endothelial cells. IGFBP-3 and IGFBP-4, and their chimeras IGFBP-3(4) and IGFBP-4(3), were treated with plasmin and with thrombin, proteases known to cleave IGFBP-3. IGFBP-3 was highly susceptible to plasmin, whereas IGFBP-4 was less so. Substitution of the P3 region for the P4 region in IGFBP-4 (IGFBP-4(3)) increased the ability of the protease to digest IGFBP-4(3); substitution of the P4 region for the P3 region in IGFBP-3 (IGFBP-3(4)) decreased the digestion of IGFBP-3(4). When 125I-labeled IGFBP-3 or 125I-IGFBP-4(3) was first bound to vascular endothelial cells, subsequent proteolysis by either plasmin or thrombin was substantially inhibited. Proteolysis of 125I-IGFBP-3(4) was not inhibited in the presence of endothelial cells. The P3 peptide was cleaved by plasmin but not by thrombin. We conclude that the P3 region is central to proteolysis of IGFBP-3 by plasmin and thrombin, processes which were inhibited by association of IGFBP-3 with endothelial cells.

MDCK cells secrete neutral proteases cleaving insulin-like growth factor-binding protein-2 to -6

Proteolysis of insulin-like growth factor-binding proteins (IGFBPs) may be an important mechanism to regulate IGF availability and IGF-independent functions of IGFBPs. We analyzed the secretion of IGFBP proteases in Madin-Darby canine kidney (MDCK) cells. The results showed that several specific proteases were secreted, cleaving IGFBP-2 to -6 at neutral pH. The proteolytic activity against IGFBP-6 differed at least from IGFBP-5 protease activity in its sensitivity both to IGF-II and to the hydroxamic acid-based disintegrin metalloprotease inhibitor, as well as serine protease inhibitors. During partial purification steps, the serine protease inhibitor-sensitive fraction with IGFBP-6 protease activity was separated from fractions characterized by the presence of a 30-kDa disintegrin immunoreactive band. Whereas the IGFBP-4 and -6 proteases are predominantly secreted across the basolateral membrane, the majority of IGFBPs are sorted to the apical medium from filter-grown cells. These studies indicate that the side-specific secretion of several distinct IGFBP proteases with partially overlapping IGFBP specificities may be another level in the regulation of IGF-dependent epithelial functions.

The changes of IGF binding proteins after rhGH administration to patients totally dependent on parenteral nutrition

The objective was to study the effect of recombinant human growth hormone (rhGH) administration to patients with chronic malnutrition maintained on total parenteral nutrition (TPN) on the levels of insulin-like growth factor-I (IGF-I) and IGF binding proteins (IGFBPs) during a double-blind trial. After 1 week of TPN the patients were randomized into group I (placebo) or group II (rhGH). Samples were collected on the first day (start of the TPN) to measure basal values, the seventh day to study the effect of TPN and the 10th, 14th and 21st days to evaluate the rhGH effect. Basal laboratory evaluation, nutritional status and glucose tolerance were assessed using standard laboratory techniques. Radioimmunoassays were used to analyse IGF-I, free IGF-I (fIGF-I) and IGFBP1-3. Electrophoresis with Western ligand blotting and Western immunoblotting was applied to find the pattern of IGFBPs. TPN had no effect on the circulating IGF-I concentration and the pattern of IGFBPs present in the studied groups of patients. The rhGH administration led to significant increases of IGF-I, total IGFBP-3, glycosylated IGFBP-3 (39, 42 kDa) and the 29 kDa fragment of IGFBP-3 and the decrease of IGFBP-2 during the trial (P<0.05). The mean levels of IGFBP-1, fIGF-I and the parameters of nutritional status in group II during the trial were not significantly influenced by rhGH. However, it has been found that IGFBP-1 and fIGF-I levels were correlated with the levels of the weekly nitrogen balance of each patient in group II at the end of the trial. In spite of the significant changes of IGF-I, IGFBP-2, total IGFBP-3 and IGFBP-3 (29 kDa proteolytic fragment) after rhGH administration to patients with malnutrition, maintained on parenteral nutrition, the increase of nitrogen balance was seen only in patients who decreased their IGFBP-1 and increased bioavailable IGF-I as reflected by measurement of fIGF-I. The levels of IGFBP-1 may provide a useful marker of IGF-I bioavailability in monitoring the efficiency of the rhGH therapy in malnourished patients.