One level up: abnormal proteolytic regulation of IGF activity plays a role in human pathophysiology

The Proteinase PAPP-A has Deep Evolutionary Roots Outside of the IGF System

The animal pappalysin metalloproteinases, PAPP-A and PAPP-A2, are highly specific regulatory enzymes of the insulin-like growth factor (IGF) system. Cleavage of their only known substrates, a subset of IGF binding proteins (IGFBPs), releases bioactive IGFI and IGFII, thus promoting IGF signaling. Stanniocalcin-1 and -2 (STC1 and STC2) are potent pappalysin inhibitors, completing the STC-PAPP-A-IGFBP-IGF axis. Utilizing homology searches and phylogenetic analyses, we examined the occurrence of pappalysins in the animal kingdom and their functional conservation. This revealed the extensive presence of pappalysins across metazoans, as well as the presence of 3 pappalysins: PAPP-A, PAPP-A2, and a third group of invertebrate pappalysins, which we name invertebrate PAPP-A (invPAPP-A). We show that PAPP-A and PAPP-A2 arose by duplication during early vertebrate evolution. Despite significant evolutionary distance, the domain architecture of the metazoan pappalysins is completely conserved, and several functional domains and motifs are highly conserved across all pappalysins. However, invPAPP-A exists outside the context of IGFBPs, suggesting that the animal pappalysins may have substrates beyond the IGFBPs for PAPP-A and PAPP-A2 that remain to be discovered. Since PAPP-A is an emerging drug target, it is important to understand potential involvement in regulatory systems other than the IGF system, which might be affected upon targeting of PAPP-A.

Prepubertal Children With Obesity Have High Free IGF-1 Levels and Accelerated Growth Despite Reduced Pappalysin Levels

BACKGROUND

Prepubertal children with obesity frequently have enhanced growth, accelerated skeletal maturation, and changes in the growth hormone-insulin-like growth factor (GH-IGF) axis. However, the involvement of pappalysins (PAPP-A, PAPP-A2) and stanniocalcins (STC1, STC2) as regulators of IGF bioavailability has not been studied in obesity.

OBJECTIVE

We aimed to determine the effects of childhood obesity and weight reduction on serum levels of PAPP-A, PAPP-A2, STC1, and STC2 and their relationship with IGF bioavailability, growth, and other components of the GH-IGF system.

METHODS

Prepubertal children with severe obesity (150, 50% males/females, age: 7.72 ± 2.05 years, BMI z-score: 4.95 ± 1.70, height z-score: 1.28 ± 1.04) were studied at diagnosis and after a minimum of 0.5 BMI z-score reduction. Two hundred and six healthy age- and sex-matched children were used as controls.

RESULTS

Children with obesity had decreased serum concentrations of PAPP-A, PAPP-A2 and STC2, but increased total and free IGF-I, intact IGFBP-3, acid-labile subunit (ALS), IGF-II, and insulin levels, with no difference in the free IGF-I/total IGF-I ratio. Neither the standardized body mass index (BMI) nor height correlated with any biochemical parameter analyzed. A decrease in IGF-II, insulin, and ALS with an increase in IGFBP-2 and -5, STC2, and PAPP-A were observed after weight loss.

CONCLUSION

Increased circulating total and free IGF-I, insulin, and IGF-II may all contribute to the increased rate of prepubertal growth and bone maturation observed in children with obesity, with STC2 possibly being involved.

Regulation and function of insulin and insulin-like growth factor receptor signalling

Receptors of insulin and insulin-like growth factors (IGFs) are receptor tyrosine kinases whose signalling controls multiple aspects of animal physiology throughout life. In addition to regulating metabolism and growth, insulin-IGF receptor signalling has recently been linked to a variety of new, cell type-specific functions. In the last century, key questions have focused on how structural differences of insulin and IGFs affect receptor activation, and how insulin-IGF receptor signalling translates into pleiotropic biological functions. Technological advances such as cryo-electron microscopy have provided a detailed understanding of how native and engineered ligands activate insulin-IGF receptors. In this Review, we highlight recent structural and functional insights into the activation of insulin-IGF receptors, and summarize new agonists and antagonists developed for intervening in the activation of insulin-IGF receptor signalling. Furthermore, we discuss recently identified regulatory mechanisms beyond ligand-receptor interactions and functions of insulin-IGF receptor signalling in diseases.

Heritability and Correlation Estimates for Serum Insulin-like Growth Factor I Concentration, Weight, Weight Gain, and Height in Angus Beef Cattle in a Long-Term Divergent Selection Study for Serum Insulin-like Growth Factor I (1989 to 2017)

This study investigated the genetic parameters for serum IGF-I concentrations and growth traits in beef cattle. A divergent selection experiment for serum IGF-I concentration was initiated in 1989. One hundred spring-calving (50 high line and 50 low line) and 100 fall-calving (50 high line and 50 low line) black Angus cows with unknown IGF-I concentrations were randomly assigned to the two divergent selection lines. For the 2009 through 2017 breeding seasons, the selection criterion in the two lines was changed from serum IGF-I concentration to high vs. low maintenance energy expected progeny differences (ME EPDs). The number of records available for analysis varied from 2056 for IGF-I concentration on day 42 of the postweaning performance test to 2988 for birth weight, with the exception that only 617 records were available for IGF-I concentration at weaning. (Co)variance components were estimated for direct and maternal additive genetic effects using an animal model and multiple-trait, derivative-free, restricted maximum likelihood (MTDFREML) computer programs. Direct heritability estimates for serum IGF-I concentrations ranged from 0.34 ± 0.07 to 0.46 ± 0.07, whereas direct heritability estimates for weight traits ranged from 0.32 ± 0.05 to 0.39 ± 0.07. Maternal genetic effects on IGF-I were low, with estimates ranging from 0.02 ± 0.11 to 0.17 ± 0.05, and were similarly low for weight traits. Maternal permanent environmental effects were negligible for IGF-I but were larger for weight traits, especially for weaning weight (0.22 ± 0.04). Direct maternal correlations for postweaning IGF-I concentrations were approximately -0.90. The genetic correlations of IGF-I with weight traits were generally low, with negative correlations observed for birth weight and small positive correlations for the other weight traits. Environmental and phenotypic correlations between IGF-I concentrations and growth traits were generally small, indicating circulating IGF-I has a modest impact on growth traits in beef cattle.

Nutrition, GH/IGF-1 signaling, and cancer

Cancer is the second leading cause of death in the United States and among the most prevalent diseases globally, with an incidence expected to grow because of smoking, pollution, poor dietary habits, obesity, and the rise in the older population. Given their ability to reduce risk factors, albeit with varying efficacy, nutrition and fasting could help prevent cancer and other age-related disorders. Calorie restriction (CR), various forms of intermittent fasting (IF) or periodic fasting (PF), and fasting-mimicking diets (FMDs) have been shown to improve health span, increase lifespan, and prevent or postpone cancer in rodents. The effects of specific diets and fasting regimens on aging and cancer appear to be mediated in part by the reduction in the activity of the growth hormone (GH)/insulin-like-growth-factor-I (IGF-1) axis. Nevertheless, recent data indicate that the alternation of low and normal levels of these hormones and factors may be ideal for optimizing longevity and function. Here, we review the role of nutrition, CR, and fasting/FMD on cancer, focusing on the hypothesis that the modulation of GH, IGF-1, and insulin signaling partly mediates the effect of these dietary interventions on cancer prevention.

The metalloproteinase PAPP-A is required for IGF-dependent chondrocyte differentiation and organization

Insulin-like growth factor (IGF) signaling is required for proper growth and skeletal development in vertebrates. Consequently, its dysregulation may lead to abnormalities of growth or skeletal structures. IGF is involved in the regulation of cell proliferation and differentiation of chondrocytes. However, the availability of bioactive IGF may be controlled by antagonizing IGF binding proteins (IGFBPs) in the circulation and tissues. As the metalloproteinase PAPP-A specifically cleaves members of the IGFBP family, we hypothesized that PAPP-A activity liberates bioactive IGF in cartilage. In PAPP-A knockout mice, the femur length was reduced and the mice showed a disorganized columnar organization of growth plate chondrocytes. Similarly, zebrafish lacking pappaa showed reduced length of Meckel's cartilage and disorganized chondrocytes, reminiscent of the mouse knockout phenotype. Expression of chondrocyte differentiation markers (sox9a, ihha, and col10a1) was markedly affected in Meckel's cartilage of pappaa knockout zebrafish, indicating that differentiation of chondrocytes was compromised. Additionally, the zebrafish pappaa knockout phenotype was mimicked by pharmacological inhibition of IGF signaling, and it could be rescued by treatment with exogenous recombinant IGF-I. In conclusion, our data suggests that IGF activity in the growing cartilage, and hence IGF signaling in chondrocytes, requires the presence of PAPP-A. The absence of PAPP-A causes aberrant chondrocyte organization and compromised growth in both mice and zebrafish.

Sex-based differences in growth-related IGF1 signaling in response to PAPP-A2 deficiency: comparative effects of rhGH, rhIGF1 and rhPAPP-A2 treatments

BACKGROUND

Children with pregnancy-associated plasma protein-A2 (PAPP-A2) mutations resulting in low levels of bioactive insulin-like growth factor-1 (IGF1) and progressive postnatal growth retardation have improved growth velocity and height following recombinant human (rh)IGF1 treatment. The present study aimed to evaluate whether Pappa2 deficiency and pharmacological manipulation of GH/IGF1 system are associated with sex-specific differences in growth-related signaling pathways.

METHODS

Plasma, hypothalamus, pituitary gland and liver of Pappa2ko/ko mice of both sexes, showing reduced skeletal growth, and liver of these mice treated with rhGH, rhIGF1 and rhPAPP-A2 from postnatal day (PND) 5 to PND35 were analyzed.

RESULTS

Reduced body and femur length of Pappa2ko/ko mice was associated with increases in: (1) components of IGF1 ternary complexes (IGF1, IGFBP5/Igfbp5, Igfbp3, Igfals) in plasma, hypothalamus and/or liver; and (2) key signaling regulators (phosphorylated PI3K, AKT, mTOR, GSK3β, ERK1/2 and AMPKα) in hypothalamus, pituitary gland and/or liver, with Pappa2ko/ko females having a more prominent effect. Compared to rhGH and rhIGF1, rhPAPP-A2 specifically induced: (1) increased body and femur length, and reduced plasma total IGF1 and IGFBP5 concentrations in Pappa2ko/ko females; and (2) increased Igf1 and Igf1r levels and decreased Ghr, Igfbp3 and Igfals levels in the liver of Pappa2ko/ko females. These changes were accompanied by lower phospho-STAT5, phospho-AKT and phospho-ERK2 levels and higher phospho-AMPK levels in the liver of Pappa2ko/ko females.

CONCLUSIONS

Sex-specific differences in IGF1 system and signaling pathways are associated with Pappa2 deficiency, pointing to rhPAPP-A2 as a promising drug to alleviate postnatal growth retardation underlying low IGF1 bioavailability in a female-specific manner.

Implication of Pappalysins and Stanniocalcins in the Bioavailability of IGF-I in Children With Type 1 Diabetes Mellitus

Context

Anomalies in the growth hormone (GH)/insulin-like growth factor (IGF) axis, are common in children with type 1 diabetes mellitus (T1DM), even in those reaching a normal or near-normal final height. However, concentrations of the IGF bioavailability regulatory factors (pappalysins [PAPP-As] and stanniocalcins [STCs]) have not been reported in children with T1DM.

Objective

To determine serum concentrations of PAPP-As and STCs in children at diagnosis of T1DM and after insulin treatment and the correlation of these factors with other members of the GH/IGF axis, beta-cell insulin reserve, auxology, and nutritional status.

Methods

A single-center prospective observational study including 47 patients (59.5% male), with T1DM onset at median age of 9.2 years (interquartile range: 6.3, 11.9) was performed. Blood and anthropometric data were collected at diagnosis and after 6 and 12 months of treatment.

Results

At 6 and 12 months after T1DM diagnosis, there was improvement in the metabolic control (decrease in glycated hemoglobin [HbA1c] at 12 months -3.66 [95% CI: -4.81, -2.05], P = .001), as well as in body mass index SD and height SD (not statistically significant). STC2 increased (P < .001) and PAPP-A2 decreased (P < .001) at 6 and 12 months of treatment onset (P < .001), which was concurrent with increased total IGF-I and IGF-binding protein concentrations, with no significant modification in free IGF-I concentrations. HbA1c correlated with PAPP-A2 (r = +0.41; P < .05) and STC2 (r = -0.32; P < .05).

Conclusion

Implementation of insulin treatment after T1DM onset modifies various components of the circulating IGF system, including PAPP-A2 and STC2. How these modifications modulate linear growth remains unknown.

Elucidating the mechanism of corneal epithelial cell repair: unraveling the impact of growth factors

The repair mechanism for corneal epithelial cell injuries encompasses migration, proliferation, and differentiation of corneal epithelial cells, and extracellular matrix remodeling of the stromal structural integrity. Furthermore, it involves the consequential impact of corneal limbal stem cells (LSCs). In recent years, as our comprehension of the mediating mechanisms underlying corneal epithelial injury repair has advanced, it has become increasingly apparent that growth factors play a pivotal role in this intricate process. These growth factors actively contribute to the restoration of corneal epithelial injuries by orchestrating responses and facilitating specific interactions at targeted sites. This article systematically summarizes the role of growth factors in corneal epithelial cell injury repair by searching relevant literature in recent years, and explores the limitations of current literature search, providing a certain scientific basis for subsequent basic research and clinical applications.

Pregnancy associated plasma protein-A2 (PAPP-A2) and stanniocalcin-2 (STC2) but not PAPP-A are associated with circulating total IGF-1 in a human adult population

The pappalysins pregnancy associated plasma protein-A (PAPP-A) and -A2 (PAPP-A2) act as proteinases of insulin-like growth factor-1 (IGF-1) binding proteins, while stanniocalcin-2 (STC2) was identified as a pappalysin inhibitor. While there is some evidence from studies in children and adolescents, it is unclear whether these molecules are related to concentrations of IGF-1 and its binding proteins in adults. We investigated cross-sectionally the association of circulating PAPP-A, PAPP-A2 and STC2 with IGF-1 and its binding proteins (IGFBPs) in 394 adult pretest participants (20-69 years) of the German National Cohort Berlin North study center. Plasma PAPP-A, PAPP-A2, total and free IGF-1, IGFBP-1, IGFBP-2, IGFBP-3, IGFBP-5 and STC2 were measured by ELISAs. The associations of PAPP-A, PAPP-A2 and STC2 with IGF-1 or IGFBPs were investigated using multivariable linear regression analyses adjusting for age, sex, body mass index and pretest phase. We observed significant inverse associations of PAPP-A2 (difference in concentrations per 0.5 ng/mL higher PAPP-A2 levels) with total IGF-1 (- 4.3 ng/mL; 95% CI - 7.0; - 1.6), the IGF-1:IGFBP-3 molar ratio (- 0.34%; 95%-CI - 0.59; - 0.09), but not free IGF-1 and a positive association with IGFBP-2 (11.9 ng/mL; 95% CI 5.0; 18.8). PAPP-A was not related to total or free IGF-1, but positively associated with IGFBP-5. STC2 was inversely related to total IGF-1, IGFBP-2 and IGFBP-3 and positively to IGFBP-1. This first investigation of these associations in a general adult population supports the hypothesis that PAPP-A2 as well as STC2 play a role for IGF-1 and its binding proteins, especially for total IGF-1. The role of PAPP-A2 and STC2 for health and disease in adults warrants further investigation.

The Pregnancy-Associated Plasma Protein-A (PAPP-A) Story

Pregnancy-associated plasma protein-A (PAPP-A) was first identified in the early 1970s as a placental protein of unknown function, present at high concentrations in the circulation of pregnant women. In the mid-to-late 1990s, PAPP-A was discovered to be a metzincin metalloproteinase, expressed by many nonplacental cells, that regulates local insulin-like growth factor (IGF) activity through cleavage of high-affinity IGF binding proteins (IGFBPs), in particular IGFBP-4. With PAPP-A as a cell surface-associated enzyme, the reduced affinity of the cleavage fragments results in increased IGF available to bind and activate IGF receptors in the pericellular environment. This proteolytic regulation of IGF activity is important, since the IGFs promote proliferation, differentiation, migration, and survival in various normal and cancer cells. Thus, there has been a steady growth in investigation of PAPP-A structure and function outside of pregnancy. This review provides historical perspective on the discovery of PAPP-A and its structure and cellular function, highlights key studies of the first 50 years in PAPP-A research, and introduces new findings from recent years.

Stanniocalcin 1a regulates organismal calcium balance and survival by suppressing Trpv6 expression and inhibiting IGF signaling in zebrafish

Stanniocalcin 1 (Stc1) is well known for its role in regulating calcium uptake in fish by acting on ionocytes or NaR cells. A hallmark of NaR cells is the expression of Trpv6, a constitutively open calcium channel. Recent studies in zebrafish suggest that genetical deletion of Stc1a and Trpv6 individually both increases IGF signaling and NaR cell proliferation. While trpv6-/- fish suffered from calcium deficiency and died prematurely, stc1a-/- fish had elevated body calcium levels but also died prematurely. The relationship between Stc1a, Trpv6, and IGF signaling in regulating calcium homeostasis and organismal survival is unclear. Here we report that loss of Stc1a increases Trpv6 expression in NaR cells in an IGF signaling-dependent manner. Treatment with CdCl2, a Trpv6 inhibitor, reduced NaR cell number in stc1a -/- fish to the sibling levels. Genetic and biochemical analysis results suggest that Stc1a and Trpv6 regulate NaR cell proliferation via the same IGF pathway. Alizarin red staining detected abnormal calcium deposits in the yolk sac region and kidney stone-like structures in stc1a -/- fish. Double knockout or pharmacological inhibition of Trpv6 alleviated these phenotypes, suggesting that Stc1a inhibit epithelial Ca2+ uptake by regulating Trpv6 expression and activity. stc1a-/- mutant fish developed cardiac edema, body swelling, and died prematurely. Treatment of stc1a-/- fish with CdCl2 or double knockout of Trpv6 alleviated these phenotypes. These results provide evidence that Stc1a regulates calcium homeostasis and organismal survival by suppressing Trpv6 expression and inhibiting IGF signaling in ionocytes.

Challenges in the care of individuals with severe primary insulin-like growth factor-I deficiency (SPIGFD): an international, multi-stakeholder perspective

BACKGROUND

Severe primary insulin-like growth factor-I (IGF-I) deficiency (SPIGFD) is a rare growth disorder characterized by short stature (standard deviation score [SDS] ≤ 3.0), low circulating concentrations of IGF-I (SDS ≤ 3.0), and normal or elevated concentrations of growth hormone (GH). Laron syndrome is the best characterized form of SPIGFD, caused by a defect in the GH receptor (GHR) gene. However, awareness of SPIGFD remains low, and individuals living with SPIGFD continue to face challenges associated with diagnosis, treatment and care.

OBJECTIVE

To gather perspectives on the key challenges for individuals and families living with SPIGFD through a multi-stakeholder approach. By highlighting critical gaps in the awareness, diagnosis, and management of SPIGFD, this report aims to provide recommendations to improve care for people affected by SPIGFD globally.

METHODS

An international group of clinical experts, researchers, and patient and caregiver representatives from the SPIGFD community participated in a virtual, half-day meeting to discuss key unmet needs and opportunities to improve the care of people living with SPIGFD.

RESULTS

As a rare disorder, limited awareness and understanding of SPIGFD amongst healthcare professionals (HCPs) poses significant challenges in the diagnosis and treatment of those affected. Patients often face difficulties associated with receiving a formal diagnosis, delayed treatment initiation and limited access to appropriate therapy. This has a considerable impact on the physical health and quality of life for patients, highlighting a need for more education and clearer guidance for HCPs. Support from patient advocacy groups is valuable in helping patients and their families to find appropriate care. However, there remains a need to better understand the burden that SPIGFD has on individuals beyond height, including the impact on physical, emotional, and social wellbeing.

CONCLUSIONS

To address the challenges faced by individuals and families affected by SPIGFD, greater awareness of SPIGFD is needed within the healthcare community, and a consensus on best practice in the care of individuals affected by this condition. Continued efforts are also needed at a global level to challenge existing perceptions around SPIGFD, and identify solutions that promote equitable access to appropriate care. Medical writing support was industry-sponsored.

Time-Dependent Changes in Muscle IGF1-IGFBP5-PAPP System after Sciatic Denervation

Denervation-induced muscle atrophy is a frequent cause of skeletal muscle diseases. However, the role of the most important muscle growth factor, insulin-like growth factor (IGF-1), in this process is poorly understood. IGF-1 activity is controlled by six IGF-1 binding proteins (IGFBPs). In skeletal muscle, IGFBP-5 seems to have an important role in atrophic processes. Furthermore, pappalysins (PAPP-A) modulate muscle growth by increasing IGF-1 bioavailability through IGFBP cleavage. We aimed to study the time-dependent changes in the IGF1-IGFBP5-PAPP system and its regulators in gastrocnemius muscle after sciatic denervation. Gastrocnemius atrophy and overexpression of IGF-1 was observed from day 3 post-denervation. The proteolytic factors measured were elevated from day 1 post-denervation onwards. Expression of both IGFBP-5 and pappalysins were increased on days 1 and 3. Subsequently, on days 7 to 14 pappalysins returned to control levels while IGFBP-5 remained elevated. The ratio IGFBP-5/PAPP-A was correlated with the main proteolytic markers. All data suggest that the initial increase of pappalysins could facilitate the IGF-1 action on muscle growth, whereas their subsequent decrease could lead to further muscle wasting.

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.

The STC2-PAPP-A-IGFBP4-IGF1 axis and its associations to mortality and CVD in T2D

Objective

Physiologically, pregnancy-associated plasma protein-A (PAPP-A) serves to liberate bound IGF1 by enzymatic cleavage of IGF-binding proteins (IGFBPs), IGFBP4 in particular. Clinically, PAPP-A has been linked to cardiovascular disease (CVD). Stanniocalcin-2 (STC2) is a natural inhibitor of PAPP-A enzymatic activity, but its association with CVD is unsettled. Therefore, we examined associations between the STC2-PAPP-A-IGFBP4-IGF1 axis and all-cause mortality and CVD in patients with type 2 diabetes (T2D).

Design

We followed 1284 participants with T2D from the ADDITION trial for 5 years.

Methods

Circulating concentrations of STC2, PAPP-A, total and intact IGFBP4 and IGF1 and -2 were measured at inclusion. End-points were all-cause mortality and a composite CVD event: death from CVD, myocardial infarction, stroke, revascularisation or amputation. Survival analysis was performed by Cox proportional hazards model.

Results

During follow-up, 179 subjects presented with an event. After multivariable adjustment, higher levels of STC2, PAPP-A, as well as intact and total IGFBP4, were associated with all-cause mortality; STC2: hazard ratio (HR) = 1.84 (1.09-3.12) (95% CI); P = 0.023, PAPP-A: HR = 2.81 (1.98-3.98); P < 0.001, intact IGFBP4: HR = 1.43 (1.11-1.85); P = 0.006 and total IGFBP4: HR = 3.06 (1.91-4.91); P < 0.001. Higher PAPP-A levels were also associated with CVD events: HR = 1.74 (1.16-2.62); P = 0.008, whereas lower IGF1 levels were associated with all-cause mortality: HR = 0.51 (0.34-0.76); P = 0.001.

Conclusions

This study supports that PAPP-A promotes CVD and increases mortality. However, STC2 is also associated with mortality. Given that STC2 inhibits the enzymatic effects of PAPP-A, we speculate that STC2 either serves to counteract harmful PAPP-A actions or possesses effects independently of the PAPP-A-IGF1 axis.

Significance statement

PAPP-A has pro-atherosclerotic effects and exerts these most likely through IGF1. IGF1 is regulated by the STC2-PAPP-A-IGFBP4-IGF1 axis, where STC2, an irreversible inhibitor of PAPP-A, has been shown to reduce the development of atherosclerotic lesions in mice. We examined the association of this axis to mortality and CVD in T2D. We demonstrated an association between PAPP-A and CVD. All components of the STC2-PAPP-A-IGFBP4-IGF1 axis were associated with mortality and it is novel that STC2 was associated with mortality in T2D. Our study supports that inhibition of PAPP-A may be a new approach to reducing mortality and CVD. Whether modification of STC2 could serve as potential intervention warrants further investigation.

Integrative analyses of the mRNA expression profile reveal the involvement of STC1 in chicken folliculogenesis

Efficient ovarian follicle development, maturation, and ovulation are critical for egg production performance. Previous research has underscored the importance of messenger RNAs (mRNAs) in regulating development and folliculogenesis in chicken ovarians. However, the molecular mechanism is not fully understood, especially in the late period of the laying cycle. In the present study, ovarian tissues from 80-week-old Hy-Line Brown layers (three with high and three with low rates of egg laying) were collected for transcriptome sequencing. A total of 306 differentially expressed genes (DEGs) were identified in this study, at a false discovery rate (FDR)-corrected P-value < 0.05 and a log2|fold change| (log2|FC|) ≥1.5. Among these DEGs, stanniocalcin 1 (STC1) was mainly related to cellular processes, single-organism processes, biological regulation, metabolic processes, developmental processes, and reproductive processes. Then, we further investigated the regulation of STC1 during chicken follicle development and found that STC1 inhibited the proliferation and stimulated the apoptosis of follicular granulosa cells (GCs), and decreased the expression of progesterone (P4) and estradiol (E2). Collectively, these results suggest that STC1 plays an important role in chicken follicle development by decreasing GC proliferation and steroidogenesis and stimulating GC apoptosis. This study contributes to the understanding of the reproductive biology of laying hens in the late period of the laying cycle and further lays a foundation for the improvement of egg production in poultry breeding.

Structure of the proteolytic enzyme PAPP-A with the endogenous inhibitor stanniocalcin-2 reveals its inhibitory mechanism

The metzincin metalloproteinase PAPP-A plays a key role in the regulation of insulin-like growth factor (IGF) signaling by specific cleavage of inhibitory IGF binding proteins (IGFBPs). Using single-particle cryo-electron microscopy (cryo-EM), we here report the structure of PAPP-A in complex with its endogenous inhibitor, stanniocalcin-2 (STC2), neither of which have been reported before. The highest resolution (3.1 Å) was obtained for the STC2 subunit and the N-terminal approximately 1000 residues of the PAPP-A subunit. The 500 kDa 2:2 PAPP-A·STC2 complex is a flexible multidomain ensemble with numerous interdomain contacts. In particular, a specific disulfide bond between the subunits of STC2 and PAPP-A prevents dissociation, and interactions between STC2 and a module located in the very C-terminal end of the PAPP-A subunit prevent binding of its main substrate, IGFBP-4. While devoid of activity towards IGFBP-4, the active site cleft of the catalytic domain is accessible in the inhibited PAPP-A·STC2 complex, as shown by its ability to hydrolyze a synthetic peptide derived from IGFBP-4. Relevant to multiple human pathologies, this unusual mechanism of proteolytic inhibition may support the development of specific pharmaceutical agents, by which IGF signaling can be indirectly modulated.

Pappalysins and Stanniocalcins and Their Relationship With the Peripheral IGF Axis in Newborns and During Development

CONTEXT

Pappalysins (PAPP-A, PAPP-A2) modulate body growth by increasing insulin-like growth factor I (IGF-I) bioavailability through cleavage of insulin-like growth factor binding proteins (IGFBPs) and are inhibited by stanniocalcins (STC1, STC2). Normative data on these novel factors, as well as on free IGF-I and uncleaved fractions of IGFBPs, are not well established.

OBJECTIVE

This work aimed to determine serum concentrations of PAPP-A, PAPP-A2, STC1, and STC2 in relationship with other growth hormone (GH)-IGF axis parameters during development.

METHODS

Full-term newborns (150; gestational age: 39.30 ± 1.10 weeks), 40 preterm newborns (30.87 ± 3.35 weeks), and 1071 healthy individuals (aged 1-30 years) were included in the study and divided according to their Tanner stages (males and females): I:163 males, 154 females; II:100 males, 75 females; III:83 males, 96 females; IV: 77 males, 86 females; and V:109 males,128 females.

RESULTS

Serum concentrations of PAPP-A, PAPP-A2, STC1, STC2, IGFBP-2, total IGFBP-4, and total IGFBP-5 were elevated at birth and declined throughout childhood. In postnatal life, PAPP-A2 concentrations decreased progressively in concomitance with the free/total IGF-I ratio; however, stanniocalcin concentrations remained stable. PAPP-A2 concentrations positively correlated with the free/total IGF-I ratio (r = +0.28; P < .001) and negatively with the intact/total IGFBP-3 ratio (r = -0.23; P < .001). PAPP-A concentrations inversely correlated with intact/total IGFBP-4 ratio (r = -0.21; P < .001), with PAPP-A concentrations being lower in females at all ages. Association studies indicate the importance of stanniocalcins and pappalysins in the control of this axis in an age-specific manner.

CONCLUSION

This study provides reference values of pappalysins and stanniocalcins, which modulate IGF-I activity by changing the concentrations of cleaved and uncleaved IGFBPs.

Structure of the PAPP-ABP5 complex reveals mechanism of substrate recognition

Insulin-like growth factor (IGF) signaling is highly conserved and tightly regulated by proteases including Pregnancy-Associated Plasma Protein A (PAPP-A). PAPP-A and its paralog PAPP-A2 are metalloproteases that mediate IGF bioavailability through cleavage of IGF binding proteins (IGFBPs). Here, we present single-particle cryo-EM structures of the catalytically inactive mutant PAPP-A (E483A) in complex with a peptide from its substrate IGFBP5 (PAPP-ABP5) and also in its substrate-free form, by leveraging the power of AlphaFold to generate a high quality predicted model as a starting template. We show that PAPP-A is a flexible trans-dimer that binds IGFBP5 via a 25-amino acid anchor peptide which extends into the metalloprotease active site. This unique IGFBP5 anchor peptide that mediates the specific PAPP-A-IGFBP5 interaction is not found in other PAPP-A substrates. Additionally, we illustrate the critical role of the PAPP-A central domain as it mediates both IGFBP5 recognition and trans-dimerization. We further demonstrate that PAPP-A trans-dimer formation and distal inter-domain interactions are both required for efficient proteolysis of IGFBP4, but dispensable for IGFBP5 cleavage. Together the structural and biochemical studies reveal the mechanism of PAPP-A substrate binding and selectivity.

Increased activity of the metalloproteinase PAPP-A promotes diabetes-induced glomerular hypertrophy

BACKGROUND

Diabetic nephropathy (DN) is a serious complication of diabetes and a common cause of end stage renal failure. Insulin-like growth factor (IGF)-signaling has been implicated in DN, but is mechanistically poorly understood. Here, we assessed the activity of the metalloproteinase PAPP-A, an activator of IGF activity, and its possible interaction with the endogenous PAPP-A inhibitors stanniocalcin (STC)-1 and -2 in the mammalian kidney under normal and hyperglycemic conditions.

METHODS AND RESULTS

Immunohistochemistry demonstrated that PAPP-A, its proteolytic substrate IGF binding protein-4, STC1 and STC2 are present in the human kidney. Endogenous inhibited complexes of PAPP-A (PAPP-A:STC1 and PAPP-A:STC2) were demonstrated in media conditioned by human mesangial cells (HMCs), suggesting that PAPP-A activity is regulated by the STCs in kidney tissue. A method for the selective detection of active PAPP-A in tissue was developed and a significant increase in glomerular active PAPP-A in human diabetic kidney relative to normal was observed. In DN patients, the estimated glomerular filtration rate correlated with PAPP-A activity. In diabetic mice, glomerular growth was reduced when PAPP-A activity was antagonized by adeno-associated virus-mediated overexpression of STC2.

CONCLUSION

We propose that PAPP-A activity in renal tissue is precisely balanced by STC1 and STC2. An imbalance in this equilibrium causing increased PAPP-A enzymatic activity potentially contributes to the development of DN, and thus, therapeutic targeting of PAPP-A activity may represent a novel strategy for its treatment.

Free Insulin-like Growth Factor (IGF)-I in Children with PWS

In children with Prader−Willi syndrome (PWS), the standard growth hormone (GH) dose often results in high immunoreactive IGF-I levels. These high immunoreactive IGF-I levels lead to concern because their long-term effects are unknown. As a result, clinicians have to lower the GH dose, which worsens body composition and quality of life. As clinical features do not seem to correspond to immunoreactive IGF-I values, it is questionable whether immunoreactive IGF-I is a suitable marker for GH dosing, or whether another parameter better reflects IGF-I bioavailability and bioactivity. We, therefore, investigate serum immunoreactive IGF-I, free IGF-I and IGFBP-3 levels in 70 GH-treated children with PWS. Our study showed that, although immunoreactive IGF-I levels were high (>2 SDS) in the vast majority of prepubertal and pubertal children, free IGF-I SDS levels were <0 SDS in most and <1 SDS in all. Free IGF-I correlated with the immunoreactive IGF-I, IGFBP-3 and IGF-I/IGFBP-3 ratio. We conclude that there is a major discrepancy between immunoreactive and free IGF-I levels. While in the majority of GH-treated children with PWS, immunoreactive IGF-I levels were high, free IGF-I levels were <0 SDS in most. Our data appear to be very reassuring and suggest that free IGF-I levels should also be taken into consideration when the immunoreactive IGF-I levels are >2 SDS in GH-treated children with PWS.

Extending human healthspan and longevity: a symposium report

For many years, it was believed that the aging process was inevitable and that age-related diseases could not be prevented or reversed. The geroscience hypothesis, however, posits that aging is, in fact, malleable and, by targeting the hallmarks of biological aging, it is indeed possible to alleviate age-related diseases and dysfunction and extend longevity. This field of geroscience thus aims to prevent the development of multiple disorders with age, thereby extending healthspan, with the reduction of morbidity toward the end of life. Experts in the field have made remarkable advancements in understanding the mechanisms underlying biological aging and identified ways to target aging pathways using both novel agents and repurposed therapies. While geroscience researchers currently face significant barriers in bringing therapies through clinical development, proof-of-concept studies, as well as early-stage clinical trials, are underway to assess the feasibility of drug evaluation and lay a regulatory foundation for future FDA approvals in the future.

Pregnancy-Associated Plasma Protein (PAPP)-A2 in Physiology and Disease

The growth hormone (GH)/insulin-like growth factor (IGF) axis plays fundamental roles during development, maturation, and aging. Members of this axis, composed of various ligands, receptors, and binding proteins, are regulated in a tissue- and time-specific manner that requires precise control that is not completely understood. Some of the most recent advances in understanding the implications of this axis in human growth are derived from the identifications of new mutations in the gene encoding the pregnancy-associated plasma protein PAPP-A2 protease that liberates IGFs from their carrier proteins in a selective manner to allow binding to the IGF receptor 1. The identification of three nonrelated families with mutations in the PAPP-A2 gene has shed light on how this protease affects human physiology. This review summarizes our understanding of the implications of PAPP-A2 in growth physiology, obtained from studies in genetically modified animal models and the PAPP-A2 deficient patients known to date.

Metabolic improvement after gastric bypass correlates with changes in IGF-regulatory proteins stanniocalcin-2 and IGFBP-4

BACKGROUND

Pregnancy-associated plasma protein-A (PAPP-A) is an enzyme that increases IGF-activity through cleavage of IGF-binding proteins (IGFBPs), primarily IGFBP-4, whereby bound IGF-I becomes released as a free molecule. The enzymatic activity of PAPP-A is irreversibly suppressed by the glycoprotein stanniocalcin-2 (STC2). Pre-clinical and clinical studies suggest that the STC2 - PAPP-A - IGFBP-4 axis is important in controlling local IGF-action. STC2, PAPP-A and IGFBP-4 are expressed in adipose tissue, and as bariatric surgery markedly reduces the amount of fat, we found it relevant to study the impact of Roux-en-Y gastric bypass (RYGB) on circulating concentrations of this IGF-regulatory network.

METHODS

Analysis of fasting blood samples from 20 obese subjects, hereof 10 with preoperative type 2 diabetes, investigated before RYGB, and 1 week, 3 months and 12 months post-surgery. Members of the IGF-system were analyzed by immunoassays, bioactive IGF by cell-based IGF-I receptor activation assay. We compared changes in IGF-system components with changes in fasting plasma insulin and glucose, and HbA1c.

RESULTS

PAPP-A remained unchanged, but STC2 decreased following RYGB (p < 0.05). The PAPP-A substrate IGFBP-4 declined (p < 0.01), whereas levels of PAPP-A specific IGFBP-4 fragments increased (p < 0.05), indicating an increased PAPP-A enzymatic activity post-RYGB. Further, the reduction in intact IGFBP-4 correlated with increased levels of bioactive IGF (p < 0.05). In multivariable regression analyses, an improved glucose metabolism correlated with reductions in STC2 and IGFBP-4, and with increases in bioactive IGF and IGF-I (p < 0.05).

CONCLUSION

After 12 months, RYGB caused reduced serum concentrations of intact IGFBP-4 and STC2, whereas serum PAPP-A remained at pre-operative levels. However, concentrations of PAPP-A generated IGFBP-4 fragments increased, pointing to an overall increased PAPP-A enzymatic activity following RYGB. Notably, reductions in intact IGFBP-4 and STC2 associated with improvements in glucose metabolism. Therefore, we propose that STC2 and IGFBP-4 are involved in the metabolic improvement that follows RYGB.

Adult height and long-term outcomes after rhIGF-1 therapy in two patients with PAPP-A2 deficiency

PAPP-A2 deficiency is a novel syndrome characterized by short stature due to low IGF bioactivity, skeletal abnormalities and decreased bone mineral density (BMD). Treatment with recombinant human IGF-1 (rhIGF-1) for 1 year demonstrated to increase growth velocity and BMD, without reported adverse effects, but data regarding the long-term efficacy and safety of rhIGF-1 administration in this entity has not yet been reported. Two Spanish siblings with short stature due to a homozygous loss-of-function mutation in the PAPP-A2 gene (p.D643fs25*) were treated with rhIGF-1 twice daily for six years. Growth velocity continued to increase and both patients achieved their target height. Free IGF-1 concentrations increased notably after rhIGF-1 administration, with serum IGFBP-3, IGFBP-5 and ALS levels also being higher during treatment. BMD was progressively normalized and an increase in lean mass was also noted during treatment. No episodes of hypoglycemia or any other adverse effects were documented. An increase in the growth of kidney and spleen length was observed in one of the patients.

PAPPA2 Promote the Proliferation of Dermal Papilla Cells in Hu Sheep (Ovis aries) by Regulating IGFBP5

Hu sheep (Ovis aries) is a rare white sheep breed, with four different types of lambskin patterns that have different values. However, the genetic mechanisms underlying different types of pattern formation remains unclear. This research aimed to characterize the molecular mechanism of differentially expressed gene PAPPA2 affecting the pattern type of Hu sheep's lambskin at the cellular level. Thus, RT-qPCR, EdU and Cell Cycle detection were used to explore the effect of PAPPA2 and IGFBP5 (a protein that can be hydrolyzed by PAPPA2) on the proliferation of dermal papilla cells (DPCs) after overexpression or interference with PAPPA2 and IGFBP5. The expression level of PAPPA2 in straight DPCs was 4.79 ± 1.84 times higher than curved. Overexpression of PAPPA2 promoted the proliferation of DPCs and also increased the expression of IGFBP5. Conversely, overexpression of IGFBP5 reduced the proliferation of DPCs. However, the proliferation of DPCs was restored by co-overexpression of PAPPA2 and IGFBP5 compared with overexpression of IGFBP5 alone. Thus, PAPPA2 can affect the proliferation of DPCs through regulating IGFBP5 and then participate in lambskin pattern determination. Overall, we preliminarily clarified the critical role played by PAPPA2 during the formation of different pattern in Hu sheep lambskin.

Digital Health for Supporting Precision Medicine in Pediatric Endocrine Disorders: Opportunities for Improved Patient Care

Digitalization of healthcare delivery is rapidly fostering development of precision medicine. Multiple digital technologies, known as telehealth or eHealth tools, are guiding individualized diagnosis and treatment for patients, and can contribute significantly to the objectives of precision medicine. From a basis of "one-size-fits-all" healthcare, precision medicine provides a paradigm shift to deliver a more nuanced and personalized approach. Genomic medicine utilizing new technologies can provide precision analysis of causative mutations, with personalized understanding of mechanisms and effective therapy. Education is fundamental to the telehealth process, with artificial intelligence (AI) enhancing learning for healthcare professionals and empowering patients to contribute to their care. The Gulf Cooperation Council (GCC) region is rapidly implementing telehealth strategies at all levels and a workshop was convened to discuss aspirations of precision medicine in the context of pediatric endocrinology, including diabetes and growth disorders, with this paper based on those discussions. GCC regional investment in AI, bioinformatics and genomic medicine, is rapidly providing healthcare benefits. However, embracing precision medicine is presenting some major new design, installation and skills challenges. Genomic medicine is enabling precision and personalization of diagnosis and therapy of endocrine conditions. Digital education and communication tools in the field of endocrinology include chatbots, interactive robots and augmented reality. Obesity and diabetes are a major challenge in the GCC region and eHealth tools are increasingly being used for management of care. With regard to growth failure, digital technologies for growth hormone (GH) administration are being shown to enhance adherence and response outcomes. While technical innovations become more affordable with increasing adoption, we should be aware of sustainability, design and implementation costs, training of HCPs and prediction of overall healthcare benefits, which are essential for precision medicine to develop and for its objectives to be achieved.

Short stature with low insulin-like growth factor 1 availability due to pregnancy-associated plasma protein A2 deficiency in a Saudi family

In 2016 a new syndrome with postnatal short stature and low IGF1 bioavailability caused by biallelic loss-of-function mutations in the gene encoding the metalloproteinase pregnancy-associated plasma protein A2 (PAPP-A2) was described in two families. Here we report two siblings of a third family from Saudi Arabia with postnatal growth retardation and decreased IGF1 availability due to a new homozygous nonsense mutation (p.Glu886* in exon 7) in PAPPA2. The two affected males showed progressively severe short stature starting around 8 years of age, moderate microcephaly, decreased bone mineral density, and high circulating levels of total IGF1, IGFBP3, and the IGF acid-labile subunit (IGFALS), with decreased free IGF1 concentrations. Interestingly, circulating IGF2 and IGFBP5 were not increased. An increase in growth velocity and height was seen in the prepuberal patient in response to rhIGF1. These patients contribute to the confirmation of the clinical picture associated with PAPP-A2 deficiency and that the PAPPA2 gene should be studied in all patients with short stature with this characteristic phenotype. Hence, pediatric endocrinologists should measure circulating PAPP-A2 levels in the study of short stature as very low or undetectable levels of this protein can help to focus the diagnosis and treatment.

Circulating transcripts in maternal blood reflect a molecular signature of early-onset preeclampsia

Circulating RNA (C-RNA) is continually released into the bloodstream from tissues throughout the body, offering an opportunity to noninvasively monitor all aspects of pregnancy health from conception to birth. We asked whether C-RNA analysis could robustly detect aberrations in patients diagnosed with preeclampsia (PE), a prevalent and potentially fatal pregnancy complication. As an initial examination, we sequenced the circulating transcriptome from 40 pregnancies at the time of severe, early-onset PE diagnosis and 73 gestational age-matched controls. Differential expression analysis identified 30 transcripts with gene ontology annotations and tissue expression patterns consistent with the placental dysfunction, impaired fetal development, and maternal immune and cardiovascular system dysregulation characteristic of PE. Furthermore, machine learning identified combinations of 49 C-RNA transcripts that classified an independent cohort of patients (early-onset PE, n = 12; control, n = 12) with 85 to 89% accuracy. C-RNA may thus hold promise for improving the diagnosis and identification of at-risk pregnancies.

Calcium State-Dependent Regulation of Epithelial Cell Quiescence by Stanniocalcin 1a

The molecular mechanisms regulating cell quiescence-proliferation balance are not well defined. Using a zebrafish model, we report that Stc1a, a secreted glycoprotein, plays a key role in regulating the quiescence-proliferation balance of Ca²⁺ transporting epithelial cells (ionocytes). Zebrafish stc1a, but not the other stc genes, is expressed in a Ca²⁺ state-dependent manner. Genetic deletion of stc1a, but not stc2b, increased ionocyte proliferation, leading to elevated body Ca²⁺ levels, cardiac edema, body swelling, and premature death. The increased ionocyte proliferation was accompanied by an increase in the IGF1 receptor-mediated PI3 kinase-Akt-Tor signaling activity in ionocytes. Inhibition of the IGF1 receptor, PI3 kinase, Akt, and Tor signaling reduced ionocyte proliferation and rescued the edema and premature death in stc1a^–/– fish, suggesting that Stc1a promotes ionocyte quiescence by suppressing local IGF signaling activity. Mechanistically, Stc1 acts by inhibiting Papp-aa, a zinc metalloproteinase degrading Igfbp5a. Inhibition of Papp-aa proteinase activity restored ionocyte quiescence-proliferation balance. Genetic deletion of papp-aa or its substrate igfbp5a in the stc1a^–/– background reduced ionocyte proliferation and rescued the edema and premature death. These findings uncover a novel and Ca²⁺ state-dependent pathway regulating cell quiescence. Our findings also provide new insights into the importance of ionocyte quiescent-proliferation balance in organismal Ca²⁺ homeostasis and survival.

Growth Hormone/Insulin Growth Factor Axis in Sex Steroid Associated Disorders and Related Cancers

To date, almost all solid malignancies have implicated insulin-like growth factor (IGF) signalling as a driver of tumour growth. However, the remarkable level of crosstalk between sex hormones, the IGF-1 receptor (IGF-1R) and its ligands IGF-1 and 2 in endocrine driven cancers is incompletely understood. Similar to the sex steroids, IGF signalling is essential in normal development as well as growth and tissue homoeostasis, and undergoes a steady decline with advancing age and increasing visceral adiposity. Interestingly, IGF-1 has been found to play a compensatory role for both estrogen receptor (ER) and androgen receptor (AR) by augmenting hormonal responses in the absence of, or where low levels of ligand are present. Furthermore, experimental, and epidemiological evidence supports a role for dysregulated IGF signalling in breast and prostate cancers. Insulin-like growth factor binding protein (IGFBP) molecules can regulate the bioavailability of IGF-1 and are frequently expressed in these hormonally regulated tissues. The link between age-related disease and the role of IGF-1 in the process of ageing and longevity has gained much attention over the last few decades, spurring the development of numerous IGF targeted therapies that have, to date, failed to deliver on their therapeutic potential. This review will provide an overview of the sexually dimorphic nature of IGF signalling in humans and how this is impacted by the reduction in sex steroids in mid-life. It will also explore the latest links with metabolic syndromes, hormonal imbalances associated with ageing and targeting of IGF signalling in endocrine-related tumour growth with an emphasis on post-menopausal breast cancer and the impact of the steroidal milieu.

Development of a Sensitive Bioassay for the Analysis of IGF-Related Activation of AKT/mTOR Signaling in Biological Matrices

The bioactivity of the IGF system is not a function of isolated hormone concentrations in a given biological matrix. Instead, the biological activities of IGFs are regulated by IGFBPs, IGFBP proteases, and inhibitors of IGFBP proteases. Therefore, assays based on IGF-related bioactivity may describe functions of the complete IGF system in a given biological matrix. Of particular interest are the IGF system effects on the AKT/mTOR pathway, as a dominant system for controlling growth, metabolism, and aging. In order to improve the sensitivity of IGF-dependent bioactivity, we made use of the known short-term and enhancing effects of IGFBP2 on the intracellular PI3K pathway. As a specific readout of this pathway, and further as a marker of the mTOR pathway, we assessed the phosphorylation of AKT-Ser473. Preincubation using IGFBP2 enhanced IGF1-dependent AKT-Ser473 phosphorylation in our experimental system. The assay's specificity was demonstrated by inhibition of IGF1 receptors outside or inside the cell, using antiserum or small molecule inhibitors, which reduced AKT phosphorylation in response to exogenous IGF1 (p < 0.05). The maximal response of AKT phosphorylation was recorded 15 to 60 min after the addition of IGF1 to cell monolayers (p < 0.001). In our cellular system, insulin induced AKT phosphorylation only at supra-physiological concentrations (µM). Using this novel assay, we identified the differential biological activity of the IGF system in AKT-Ser473 phosphorylation in serum (mouse, naked mole rat, and human), in cerebrospinal fluid (human), and in colostrum or mature milk samples (dairy cow). We have developed a sensitive and robust bioassay to assess the IGF-related activation of the AKT/mTOR pathway. The assay works efficiently and does not require expensive cell culture systems. By using capillary immuno-electrophoresis, the readout of IGF-related bioactivity is substantially accelerated, requiring a minimum of hands-on time. Importantly, the assay system is useful for studying IGF-related activity in the AKT/mTOR pathway in a broad range of biological matrices.

Disorders caused by genetic defects associated with GH-dependent genes: PAPPA2 defects

Growth hormone (GH) and its mediator, insulin-like growth factor-1 (IGF-1), have long been recognized as central to human growth physiology. IGF-1 is known to complex with IGF binding proteins as well as with the acid labile subunit (ALS) in order to prolong its half-life in circulation. Factors regulating the bioavailability of IGF-1 (i.e. the balance between free and bound IGF-1) were less well understood. Recently, pregnancy-associated plasma protein-A2 (PAPP-A2) was discovered as a protease which specifically cleaves IGF-binding protein (IGFBP)-3 and -5. PAPP-A2 deficient patients present with characteristic findings including growth failure, elevated total IGF-1 and -2, IGFBPs, and ALS, but decreased percentage of free to total IGF-1. Additionally, patients with PAPP-A2 deficiency have impairments in glucose metabolism and bone mineral density (BMD). Treatment with recombinant human IGF-1 (rhIGF-1) improved height SD scores, growth velocity, body composition, and dysglycemia. Mouse models recapitulate many of the human findings of PAPP-A2 deficiency. This review summarizes the function of PAPP-A2 and its contribution to the GH-IGF axis through an examination of PAPP-A2 deficient patients and mouse models, thereby emphasizing the importance of the regulation of IGF-1 bioavailability in human growth.

Laron Syndrome Research Paves the Way for New Insights in Oncological Investigation

Laron syndrome (LS) is a rare genetic endocrinopathy that results from mutation of the growth hormone receptor (GH-R) gene and is typically associated with dwarfism and obesity. LS is the best characterized entity under the spectrum of the congenital insulin-like growth factor-1 (IGF1) deficiencies. Epidemiological analyses have shown that LS patients do not develop cancer, whereas heterozygous family members have a cancer prevalence similar to the general population. To identify genes and signaling pathways differentially represented in LS that may help delineate a biochemical and molecular basis for cancer protection, we have recently conducted a genome-wide profiling of LS patients. Studies were based on our collection of Epstein–Barr virus (EBV)-immortalized lymphoblastoid cell lines derived from LS patients, relatives and healthy controls. Bioinformatic analyses identified differences in gene expression in several pathways, including apoptosis, metabolic control, cytokine biology, Jak-STAT and PI3K-AKT signaling, etc. Genes involved in the control of cell cycle, motility, growth and oncogenic transformation are, in general, down-regulated in LS. These genetic events seem to have a major impact on the biological properties of LS cells, including proliferation, apoptosis, response to oxidative stress, etc. Furthermore, genomic analyses allowed us to identify novel IGF1 downstream target genes that have not been previously linked to the IGF1 signaling pathway. In summary, by ‘mining’ genomic data from LS patients, we were able to generate clinically-relevant information in oncology and, potentially, related disciplines.

Sex-Specific Control of Muscle Mass: Elevated IGFBP Proteolysis and Reductions of IGF-1 Levels Are Associated with Substantial Loss of Carcass Weight in Male DU6PxIGFBP-2 Transgenic Mice

In farmed animals, carcass weight represents an important economic trait. Since we had demonstrated that IGFBP-2 represents a potent inhibitor of muscle accretion in inbred mice, we wanted to quantify the inhibitory effects of IGFBP-2 under conditions of elevated protein mass in growth selected non-inbred mice (DU6P). Therefore, we crossed male DU6P mice with female IGFBP-2 transgenic mice. Male IGFBP-2 transgenic offspring (DU6P/IGFBP-2) were characterized by more than 20% reductions of carcass mass compared to male non-transgenic littermates. The carcass mass in males was also significantly lower (p < 0.001) than in transgenic female DU6P/IGFBP-2 mice, which showed a reduction of less than 10% (p < 0.05) compared to non-transgenic female DU6P/IGFBP-2 mice. Although transgene expression was elevated in the muscle of both sexes (p < 0.001), serum levels were normal in female, but significantly reduced in male transgenic DU6P/IGFBP-2 mice (p < 0.001). In this group, also IGFBP-3 and IGFBP-4 were significantly reduced in the circulation (p < 0.01). Particularly in male transgenic mice, we were able to identify proteolytic activity against recombinant IGFBP-2 included in diluted serum. IGFBP-proteolysis in males correlated with massive reductions of IGF-1 in serum samples and the presence of elevated levels of IGFBP-2 fragments. From our data, we conclude that elevated tissue expression of IGFBP-2 is an essential effector of muscle accretion and may block more than 20% of carcass mass. However, in the circulation, intact IGFBP-2 contained no reliable biomarker content. Notably, for the estimation of breeding values in meat-producing animal species, monitoring of IGFBP-2 expression in muscle appears to be supported by the present study in a model system.

Pregnancy-associated plasma proteins and Stanniocalcin-2 - Novel players controlling IGF-I physiology

IGF-I was originally discovered as a GH-dependent growth factor stimulating longitudinal growth. Currently, however, it has become evident that the biological activities of IGF-I extend well beyond those of a simple growth factor and impact such processes as insulin sensitivity, aging, cancer and cardiovascular disease. The vast majority of IGF-I is tightly bound to IGF-binding proteins (IGFBPs), which renders IGF-I unable to stimulate the IGF-I receptor (IGF-IR) in vivo. This binding means that liberation of IGF-I from the IGFBPs is an important step controlling IGF-I action. In this context, IGFBP-cleaving enzymes appear to play a key role. Enzymatic cleavage of the IGFBPs markedly lowers their ligand affinity, and as a consequence, IGF-I becomes liberated and hence available for stimulation of the IGF-IR. Two of the best-characterized IGFBP-cleaving enzymes are pregnancy-associated plasma protein-A (PAPP-A) and its paralog PAPP-A2. The two enzymes (often referred to as pappalysins) regulate the liberation of IGF-I in a highly controlled manner. PAPP-A is believed to act predominantly in tissues, serving to liberate IGF-I at the cell surface in close proximity to the IGF-IR. In keeping with this notion, mice lacking PAPP-A exhibit reduced body size, despite having normal circulating IGF-I concentrations. In contrast, human findings indicate that altered PAPP-A2 activity changes circulating IGF-I concentrations, although PAPP-A2 is also present in high concentrations in tissues. Thus, PAPP-A2 appears to impact circulating, as well as tissue, IGF-I activity. The enzymatic activity of PAPP-A and PAPP-A2 was recently discovered to be regulated by the protein Stanniocalcin-2 (STC2). By binding to the enzymatic sites of PAPP-A and PAPP-A2, STC2 inhibits their activity. To date, the majority of findings demonstrating the ability of pappalysins and STC2 to regulate IGF-I action are from preclinical studies. However, clinical studies are now beginning to emerge. In this review, we will summarize our data on STC2, PAPP-A and PAPP-A2 in humans. These results indicate that pappalysins and STC2 constitute an important IGF-I activity-regulating system that warrants further investigation.

Insulin-like growth factor-II and bioactive proteins containing a part of the E-domain of pro-insulin-like growth factor-II

Insulin-like growth factor (IGF)-II is considered to function as an important fetal growth factor, which is structurally and functionally related to IGF-I and proinsulin. At least in vitro, IGF-II actions are mediated through the IGF-I receptor and to a lesser extent the insulin receptor. After birth, the function of IGF-II is less clear although in adults the serum level of IGF-II exceeds that of IGF-I several fold. The IGF-II gene is maternally imprinted, with exception of the liver and several parts of the brain, where it is expressed from both alleles. The regulation, organization, and translation of the IGF-II gene is complex, with five different putative promotors leading to a range of noncoding and coding mRNAs. The 180-amino acid pre-pro-IGF-II translation product can be divided into five domains and include a N-terminal signal peptide of 24 amino acid residues, the 67 amino acid long mature protein, and an 89 residues extension at the COOH terminus, designated as the E-domain. After removal of the signal peptide, the processing of pro-IGF-II into mature IGF-II requires various steps including glycosylation of the E-domain followed by the action of endo-proteases. Several of these processing intermediates can be found in the human circulation. There is increasing evidence that, besides IGF-II, several incompletely processed precursor forms of the protein, and even a 34-amino acid peptide (preptin) derived from the E-domain of pro-IGF-II, exhibit distinct biological activities. This review will focus on the current insights regarding the specific roles of the latter proteins in cancer, glucose homeostasis, and bone physiology. To address this topic clearly in the right context, a concise overview of the biological and biochemical properties of IGF-II and several relevant aspects of the IGF system will be provided.

The IGF/Insulin-IGFBP Axis in Corneal Development, Wound Healing, and Disease

The insulin-like growth factor (IGF) family plays key roles in growth and development. In the cornea, IGF family members have been implicated in proliferation, differentiation, and migration, critical events that maintain a smooth refracting surface that is essential for vision. The IGF family is composed of multiple ligands, receptors, and ligand binding proteins. Expression of IGF type 1 receptor (IGF-1R), IGF type 2 receptor (IGF-2R), and insulin receptor (INSR) in the cornea has been well characterized, including the presence of the IGF-1R and INSR hybrid (Hybrid-R) in the corneal epithelium. Recent data also indicates that each of these receptors display unique intracellular localization. Thus, in addition to canonical ligand binding at the plasma membrane and the initiation of downstream signaling cascades, IGF-1R, INSR, and Hybrid-R also function to regulate mitochondrial stability and nuclear gene expression. IGF-1 and IGF-2, two of three principal ligands, are polypeptide growth factors that function in all cellular layers of the cornea. Unlike IGF-1 and IGF-2, the hormone insulin plays a unique role in the cornea, different from many other tissues in the body. In the corneal epithelium, insulin is not required for glucose uptake, due to constitutive activation of the glucose transporter, GLUT1. However, insulin is needed for the regulation of metabolism, circadian rhythm, autophagy, proliferation, and migration after wounding. There is conflicting evidence regarding expression of the six IGF-binding proteins (IGFBPs), which function primarily to sequester IGF ligands. Within the cornea, IGFBP-2 and IGFBP-3 have identified roles in tissue homeostasis. While IGFBP-3 regulates growth control and intracellular receptor localization in the corneal epithelium, both IGFBP-2 and IGFBP-3 function in corneal fibroblast differentiation and myofibroblast proliferation, key events in stromal wound healing. IGFBP-2 has also been linked to cellular overgrowth in pterygium. There is a clear role for IGF family members in regulating tissue homeostasis in the cornea. This review summarizes what is known regarding the function of IGF and related proteins in corneal development, during wound healing, and in the pathophysiology of disease. Finally, we highlight key areas of research that are in need of future study.

Pregnancy-associated plasma protein-A2 levels are increased in early-pregnancy gestational diabetes: a novel biomarker for early risk estimation

AIM

To determine whether pregnancy-associated plasma protein-A2 levels are increased in early pregnancies complicated by gestational diabetes and whether gestation age influences levels. The possible use of pregnancy-associated plasma protein-A2 as a pre-screening biomarker to reduce the need for performing oral glucose tolerance tests in pregnant women was also investigated.

METHODS

Pregnant women were diagnosed with gestational diabetes in early pregnancy after a 2-hour 75 g oral glucose tolerance test in the catchment area of Skåne University Hospital, Lund, Sweden during 2011-2015 (n = 99). Age- and BMI-matched pregnant women without diabetes were recruited at similar gestational ages from maternal healthcare centres in the same geographical area during 2014-2015 to act as controls (n = 100). Circulating pregnancy-associated plasma protein-A2 was analysed in participant serum using commercially available enzyme-linked immunosorbent assay kits.

RESULTS

Circulating pregnancy-associated plasma protein-A2 was increased in women diagnosed with gestational diabetes [13.5 (9.58-18.8) ng/ml] compared with controls [8.11 (5.74-11.3) ng/ml; P < 0.001]. Pregnancy-associated plasma protein-A2 was associated with gestational diabetes independent of age, BMI, C-peptide and adiponectin (P < 0.001). Pregnancy-associated plasma protein-A2 as a pre-screening biomarker to identify women at a decreased risk of gestational diabetes resulted in a negative predictive value of 99.7%, with a sensitivity of 96% and a specificity of 30% at a cut-off level of 6 ng/ml.

CONCLUSIONS

This is the first study to show increased pregnancy-associated plasma protein-A2 levels in gestational diabetes. Pregnancy-associated plasma protein-A2 also shows promise as a pre-screening biomarker with the potential to reduce the need for performing oral glucose tolerance tests in early pregnancy. Future prospective cohort studies in a larger group of both high- and low-risk women are, however, needed to further confirm this observation.

Reduced Fragmentation of IGFBP-2 and IGFBP-3 as a Potential Mechanism for Decreased Ratio of IGF-II to IGFBPs in Cerebrospinal Fluid in Response to Repeated Intrathecal Administration of Triamcinolone Acetonide in Patients With Multiple Sclerosis

Multiple sclerosis (MS) is a chronic autoimmune disease of the brain and spinal cord causing a wide range of symptoms such as impaired walking capability, spasticity, fatigue, and pain. The insulin-like growth factor (IGF) system has regulatory functions for the induction of inflammatory pathways in experimental encephalomyelitis. We have therefore assessed expression and regulation of the IGF system on the level of IGFs and IGFBPs in serum and cerebrospinal fluid (CSF) in the course of four repeated triamcinolone acetonide (TCA) administrations in two female and four male MS patients. Sample series of 20 treatment cycles were analyzed. IGF-I and IGF-II were quantified by ELISAs, and IGFBPs were analyzed by quantitative Western ligand (qWLB) and Western immunoblotting (WIB) in order to differentiate intact and fragmented IGFBPs. The ratios of fragmented to intact IGFBP-2 and -3 were calculated in serum and CSF. Finally, the ratios of IGF-I and IGF-II to the total IGF-binding activity, quantified by qWLB, were determined as an indicator of IGF-related bioactivity. After the fourth TCA administration, the average level of IGF-I was increased in serum (p < 0.001). The increase of IGF-I concentrations in serum resulted in an increased ratio of IGF-I to IGFBPs in the circulation. By contrast in CSF, fragmentation of IGFBP-2 and IGFBP-3 and the ratio of IGF-II to intact IGFBPs were decreased at the fourth TCA administration (p < 0.01). Furthermore, reduced fragmentation of IGFBP-3 in CSF was accompanied by increased concentrations of intact IGFBP-3 (p < 0.001). We conclude that reduced fragmentation of IGFBPs and concomitant reduction of IGF-II to IGFBP ratios indicate regulation of bioactivity of IGF-II in CSF during repeated intrathecal TCA administration in MS patients.

PAPP-A and the IGF system in atherosclerosis: what's up, what's down?

Pregnancy-associated plasma protein-A (PAPP-A) is a metalloproteinase with a well-established role in releasing bioactive insulin-like growth factor-1 (IGF-1) from IGF-binding protein-2, -4, and -5 by proteolytic processing of these. The IGF system has repeatedly been suggested to be involved in the pathology of atherosclerosis, and both PAPP-A and IGF-1 are proposed biomarkers and therapeutic targets for this disease. Several experimental approaches based on atherosclerosis mouse models have been undertaken to obtain causative and mechanistic insight to the role of these molecules in atherogenesis. However, reports seem conflicting. The literature suggests that PAPP-A is detrimental, while IGF-1 is beneficial. This raises important questions that need to be addressed. Here we summarize the various studies and discuss potential underlying explanations for this seemingly inconsistency with the objective of better understanding complexities and limitations when manipulating the IGF system in mouse models of atherosclerosis. A debate clarifying what's up and what's down is highly warranted going forward with the ultimate goal of improving atherosclerosis therapy by targeting the IGF system.

The insulin-like growth factor-I receptor stimulating activity (IRSA) in health and disease

Determination of true IGF-I bioactivity in serum and other biological fluids is still a substantial challenge. The IGF-IR Kinase Receptor Activation assay (IGF-IR KIRA assay) is a novel tool to asses IGF-IR stimulating activity (IRSA) and has opened a new era in studying the IGF system. In this paper we discuss many studies showing that measuring IRSA by the IGF-IR KIRA assay often provides fundamentally different information about the IGF system than the commonly used total IGF-I immunoassays. With the IGF-IR KIRA assay phosphorylation of tyrosine residues of the IGF-IR is used as read out to quantify IRSA in unknown (serum) samples. The IGF-IR KIRA assay gives information about net overall effects of circulating IGF-I, IGF-II, IGFBPs and IGFBP-proteases on IGF-IR activation and seems especially superior to immunoreactive total IGF-I in monitoring therapeutic interventions. Although the IRSA as measured by the IGF-IR KIRA assay probably more closely reflects true bioactive IGF-I than measurements of total IGF-I in serum, the IGF-IR KIRA assay in its current form does not give information about all the post-receptor intracellular events mediated by the IGF-IR. Interestingly, in several conditions in health and disease IRSA measured by the IGF-IR KIRA assay is considerably higher in interstitial fluid and ascites than in serum. This suggests that both the paracrine (local) and endocrine (circulating) IRSA should be measured to get a complete picture about the role of the IGF system in health and disease.

Current Insights into the Role of the Growth Hormone-Insulin-Like Growth Factor System in Short Children Born Small for Gestational Age

BACKGROUND

The reason for the insufficient catch-up growth seen in 10% of children born small for gestational age (SGA) is poorly understood. Disturbances in the growth hormone (GH) - insulin-like growth factor (IGF) axis might underlie this failure to show sufficient catch-up growth.

CONCLUSION

This review summarizes insights gained in the molecular and (epi) genetic mechanisms of the GH-IGF axis in short children born SGA. The most notable anomalies of the IGF system are the lowered IGF-I levels in both cord blood and the placenta, and the increased expression of IGF-binding proteins (IGFBP)-1 and IGFBP-2, which inhibit IGF-I, in the placenta of SGA neonates. These observations suggest a decreased bioactivity of IGF-I in utero. IGF-I levels remain reduced in SGA children with short stature, as well as IGFBP-3 and acid-labile subunit levels. Proteolysis of IGFBP-3 appears to be increased.

Role of stanniocalcin-1 in breast cancer

Breast cancer is a highly heterogeneous disease consisting of five disease subtypes with distinct histological characteristics, clinical behaviors and prognostic features. Stanniocalcin-1 (STC1) is a secreted glycoprotein hormone that has been demonstrated to regulate calcium and phosphate homeostasis. Mammalian STC1 is expressed in various tissues and is implicated in multiple physiological and pathophysiological processes. In addition, growing evidence has suggested that STC1 serves an oncogenic role in a number of different types of tumor. However, the role of STC1 in breast cancer is complex, considering that some studies have shown that it exerts an oncogenic role, whereas other studies have demonstrated the opposite. The aim of the present review article is to evaluate the currently available data on mammalian STC1 and discuss its potential roles in each subtype of breast cancer.

Circulating levels of the components of the GH/IGF-1/IGFBPs axis total and intact IGF-binding proteins (IGFBP) 3 and IGFBP 4 and total IGFBP 5, as well as PAPPA, PAPPA2 and Stanniocalcin-2 levels are not altered in response to energy deprivation and/or metreleptin administration in humans

OBJECTIVE

It remains unclear whether food deprivation induces changes in components of the GH/IGF-1/IGFBPs axis and if yes, which ones are mediated by leptin, an adipocyte secreted hormone regulating neuroendocrine response to energy deprivation in animals and humans. We aimed to investigate components of the axis that have not been studied to date, i.e. IGF-binding proteins (IGFBPs) and related proteases (total and intact IGFBP 3 and IGFBP 4, total IGFBP 5, PAPPA, PAPPA2 and Stanniocalcin-2), during acute (short-term fasting in healthy subjects) and chronic (women with hypothalamic amenorrhea [HA] due to excessive exercise) energy deprivation and whether metreleptin administration, in replacement, supraphysiologic or pharmacologic levels, may mediate any changes of circulating levels of the above molecules in healthy individuals and in women with hypothalamic amenorrhea.

METHODS

We studied: 1) 11 healthy men and women during three four day admissions i.e. a baseline admission in the fed isocaloric state and two admissions in the complete food deprivation state for 72-h with either placebo (resulting in a hypoleptinemic state) or metreleptin administration in doses designed to normalize circulating leptin levels for the duration of the study, 2) 15 healthy men and women during three 72-hour long admissions in a complete food deprivation state receiving three escalating doses of metreleptin designed to bring circulating leptin levels to physiologic, supraphysiologic, or pharmacologic levels, and 3) 18 women with HA randomized to either metreleptin treatment in replacement doses or placebo for nine months.

RESULTS

There were no significant changes in the circulating profiles of the above molecules in the fasting vs. fed state and/or with metreleptin administration during acute and chronic energy deprivation.

CONCLUSIONS

The studied components of the GH/IGF-1/IGFBPs axis are not affected by energy deprivation, leptin deficiency associated with energy deprivation, or by metreleptin administration in physiologic, supraphysiologic or pharmacologic doses.

Genome-Wide Profiling of Laron Syndrome Patients Identifies Novel Cancer Protection Pathways

Laron syndrome (LS), or primary growth hormone resistance, is a prototypical congenital insulin-like growth factor 1 (IGF1) deficiency. The recent epidemiological finding that LS patients do not develop cancer is of major scientific and clinical relevance. Epidemiological data suggest that congenital IGF1 deficiency confers protection against the development of malignancies. This ‘experiment of nature’ reflects the critical role of IGF1 in tumor biology. The present review article provides an overview of recently conducted genome-wide profiling analyses aimed at identifying mechanisms and signaling pathways that are directly responsible for the link between life-time low IGF1 levels and protection from tumor development. The review underscores the concept that ‘data mining’ an orphan disease might translate into new developments in oncology.