Metabolic consequences of pregnancy-associated plasma protein-A deficiency in mice: exploring possible relationship to the longevity phenotype

Gene deletion of Pregnancy-associated Plasma Protein-A (PAPP-A) improves pathology and cognition in an Alzheimer's disease mouse model

Alzheimer's disease (AD) is a progressive neurodegenerative disease of age with no effective preventative or treatment approaches. Deeper understanding of the mechanisms underlying the accumulation of toxic β-amyloid oligopeptides and the formation of amyloid plaque in AD has the potential to identify new therapeutic targets. Prior research links the insulin-like growth factor (IGF) system to pathologic mechanisms underlying AD. Suppression of local IGF-I receptor (IGFIR) signaling in AD mice has been shown to reduce plaque formation in the brain and delay neurodegeneration and behavioral changes. However, direct inhibitors of IGFIR signaling are not a viable treatment option for AD due to the essentiality of the IGFIR in physiological growth and metabolism. We have previously demonstrated a more selective means to reduce local IGFIR signaling through inhibition of PAPP-A, a novel zinc metalloprotease that regulates local IGF-I bioavailability through cleavage of inhibitory IGF binding proteins. Here we tested if deletion of PAPP-A in a mouse model of AD provides protection against pathology and behavioral changes. We show that compared to AD mice, AD/PAPP-A KO mice had significantly less plaque burden, reduced astrocytic activation, decreased IGF-IR activity, and improved cognition. Human senile AD plaques showed specific immunostaining for PAPP-A. Thus, inhibition of PAPP-A expression or activity may represent a novel treatment strategy for AD.

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.

Recapitulation of anti-aging phenotypes by global, but not by muscle-specific, deletion of PAPP-A in mice

Deletion of pregnancy-associated plasma protein-A (PAPP-A), a protease that cleaves some but not all IGF1 binding proteins, postpones late-life diseases and extends lifespan in mice, but the mechanism of this effect is unknown. Here we show that PAPP-A knockout (PKO) mice display a set of changes, in multiple tissues, that are characteristic of other varieties of slow-aging mice with alterations in GH production or GH responsiveness, including Ames dwarf, Snell dwarf, and GHRKO mice. PKO mice have elevated UCP1 in brown and white adipose tissues (WAT), and a change in fat-associated macrophage subsets that leads to diminished production of inflammatory cytokines. PKO mice also show increased levels of muscle FNDC5 and its cleavage product, the myokine irisin, thought to cause changes in fat cell differentiation. PKO mice have elevated production of hepatic GPLD1 and plasma GPLD1, consistent with their elevation of hippocampal BDNF and DCX, used as indices of neurogenesis. In contrast, disruption of PAPP-A limited to muscle ("muPKO" mice) produces an unexpectedly complex set of changes, in most cases opposite in direction from those seen in PKO mice. These include declines in WAT UCP1, increases in inflammatory macrophages and cytokines in WAT, and a decline in muscle FNDC5 and plasma irisin. muPKO mice do, however, resemble global PKO mice in their elevation of hippocampal BDNF and DCX. The data for the PKO mice support the idea that these changes in fat, macrophages, liver, muscle, plasma, and brain are consistent and biologically significant features of the slow-aging phenotype in mice. The results on the muPKO mice provide a foundation for further investigation of the complex, local, and global circuits by which PAPP-A modulates signals ordinarily controlled by GH and/or IGF1.

Mice with gene alterations in the GH and IGF family

Much of our understanding of GH's action stems from animal models and the generation and characterization of genetically altered or modified mice. Manipulation of genes in the GH/IGF1 family in animals started in 1982 when the first GH transgenic mice were produced. Since then, multiple laboratories have altered mouse DNA to globally disrupt Gh, Ghr, and other genes upstream or downstream of GH or its receptor. The ability to stay current with the various genetically manipulated mouse lines within the realm of GH/IGF1 research has been daunting. As such, this review attempts to consolidate and summarize the literature related to the initial characterization of many of the known gene-manipulated mice relating to the actions of GH, PRL and IGF1. We have organized the mouse lines by modifications made to constituents of the GH/IGF1 family either upstream or downstream of GHR or to the GHR itself. Available data on the effect of altered gene expression on growth, GH/IGF1 levels, body composition, reproduction, diabetes, metabolism, cancer, and aging are summarized. For the ease of finding this information, key words are highlighted in bold throughout the main text for each mouse line and this information is summarized in Tables 1, 2, 3 and 4. Most importantly, the collective data derived from and reported for these mice have enhanced our understanding of GH action.

Plausible Links Between Metabolic Networks, Stem Cells, and Longevity

Aging is an inevitable consequence of life, and all multicellular organisms undergo a decline in tissue and organ functions as they age. Several well-known risk factors, such as obesity, diabetes, and lack of physical activity that lead to the cardiovascular system, decline and impede the function of vital organs, ultimately limit overall life span. Over recent years, aging research has experienced an unparalleled growth, particularly with the discovery and recognition of genetic pathways and biochemical processes that control to some extent the rate of aging.In this chapter, we focus on several aspects of stem cell biology and aging, beginning with major cellular hallmarks of aging, endocrine regulation of aging and its impact on stem cell compartment, and mechanisms of increased longevity. We then discuss the role of epigenetic modifications associated with aging and provide an overview on a most recent search of antiaging modalities.

Pappa2 deletion alters IGFBPs but has little effect on glucose disposal or adiposity

OBJECTIVE

Insulin-like growth factor binding proteins (IGFBPs) are involved in glucose and lipid metabolism, and their actions are modulated by proteases. The aim of this study was to examine the effects of an IGFBP-5 protease, pregnancy associated plasma protein-A2 (PAPP-A2), on glucose metabolism and susceptibility to diet-induced obesity.

DESIGN

Postnatal growth, circulating IGF-I, IGFBP-3 and IGFBP-5 levels, and glucose tolerance were measured in Pappa2 deletion mice and littermate controls on a chow diet. Males were subsequently fed a high-fat diet for 8 weeks to measure weight gain and adiposity, as well as glucose tolerance in response to a metabolic challenge.

RESULTS

Circulating IGFBP-5 levels were ~2-fold higher in mice with no functional PAPP-A2 than in littermate controls, as expected. In contrast, circulating IGFBP-3 levels were reduced by ~15-fold, and total IGF-I levels were ~60% higher in Pappa2 deletion mice. There was no effect of Pappa2 deletion on fasting blood glucose levels or glucose clearance after intraperitoneal injection of 2g glucose/kg body weight in mice on a chow diet. In males on a high-fat diet, there was no difference between genotypes in weight gain or adiposity, adjusting for differences in initial body weight, or in fasting blood glucose or insulin levels, or in glucose clearance.

CONCLUSIONS

Despite a dramatic disruption of the balance between circulating IGF-I, IGFBP-3 and -5, we found no effects of Pappa2 deletion on glucose metabolism, weight gain or adiposity on a high-fat diet.

Syntaxin 4 Overexpression Ameliorates Effects of Aging and High-Fat Diet on Glucose Control and Extends Lifespan

Indirect evidence suggests that improved insulin sensitivity may contribute to improved lifespan of mice in which aging has been slowed by mutations, drugs, or dietary means, even in stocks of mice that do not show signs of late-life diabetes. Peripheral responses to insulin can be augmented by overexpression of Syntaxin 4 (Syn4), a plasma-membrane-localized SNARE protein. We show here that Syn4 transgenic (Tg) mice with high level expression of Syn4 had a significant extension of lifespan (33% increase in median) and showed increased peripheral insulin sensitivity, even at ages where controls exhibited age-related insulin resistance. Moreover, skeletal muscle GLUT4 and islet insulin granule exocytosis processes were fully protected in Syn4 Tg mice challenged with a high-fat diet. Hence, high-level expressing Syn4 Tg mice may exert better glycemic control, which slows multiple aspects of aging and extends lifespan, even in non-diabetic mice.

Female PAPP-A knockout mice are resistant to metabolic dysfunction induced by high-fat/high-sucrose feeding at middle age

Longevity and aging are influenced by common intracellular signals of the insulin/insulin-like growth factor (IGF)-1 pathway. Abnormally high levels of bioactive IGF-1 increase the development of various cancers and may contribute to metabolic diseases such as insulin resistance. Enhanced availability of IGF-1 is promoted by cleavage of IGF binding proteins (IGFBPs) by proteases, including the pregnancy-associated plasma protein-A (PAPPA). In vitro, PAPP-A is regulated by pro-inflammatory cytokines (PICs) such as interleukin (IL)-6 and tumor necrosis factor (TNF). Mice born with deficiency of the Papp-a gene (PAPP-A knockout (KO) mice) live ~30-40 % longer than their normal littermates and have decreased bioactive IGF-1 on standard diets. Our objective was to elucidate how the effects of high-fat, high-sucrose diet (HFHS) promote obesity, induce metabolic dysfunction, and alter systemic cytokine expression in PAPP-A KO and normal mice. PAPP-A KO mice fed HFHS diet for 10 weeks were more glucose tolerant and had enhanced insulin sensitivity compared to normal mice fed HFHS diet. PAPP-A KO mice fed HFHS diet had lower levels of pro-inflammatory cytokines (IL-2, IL-6, and TNF-α) compared to normal mice fed the same diet. However, anti-inflammatory cytokine levels (IL-4 and adiponectin) were higher in PAPP-A KO mice fed HFHS diet compared to normal mice fed HFHS. Circulating PAPP-A levels were elevated in normal mice fed an HFHS diet compared to normal mice fed a standard, low-fat, low-sucrose (LFLS) diet. Indirect calorimetry showed, at 10 weeks of feeding HFHS diet, significantly increased oxygen consumption (VO2) in PAPP-A KO mice fed HFHS diet compared to normal mice fed the same diet. Furthermore, respiratory quotient (RQ) was significantly lower in PAPP-A KO mice fed HFHS diet compared to normal (N) mice fed HFHS diet indicating PAPP-A KO mice fed HFHS diet are able to rely on fat as their primary source of energy more so than normal controls. We conclude that PAPP-A KO mice are resistant to the HFHS diet induction of metabolic dysfunction associated with higher levels of anti-inflammatory cytokines and a remarkably metabolic flexible phenotype and that some of the effects of HFHS diet in normal animals may be due to increased levels of PAPP-A.

Altered metabolism and resistance to obesity in long-lived mice producing reduced levels of IGF-I

The extension of lifespan due to reduced insulin-like growth factor 1 (IGF-I) signaling in mice has been proposed to be mediated through alterations in metabolism. Previously, we showed that mice homozygous for an insertion in the Igf1 allele have reduced levels of IGF-I, are smaller, and have an extension of maximum lifespan. Here, we tested whether this specific reduction of IGF-I alters glucose metabolism both on normal rodent chow and in response to high-fat feeding. We found that female IGF-I-deficient mice were lean on a standard rodent diet but paradoxically displayed an insulin-resistant phenotype. However, these mice gained significantly less weight than normal controls when placed on a high-fat diet. In control animals, insulin response was significantly impaired by high-fat feeding, whereas IGF-I-deficient mice showed a much smaller shift in insulin response after high-fat feeding. Gluconeogenesis was also elevated in the IGF-I-deficient mice relative to controls on both normal and high-fat diet. An analysis of metabolism and respiratory quotient over 24 h indicated that the IGF-I-deficient mice preferentially utilized fatty acids as an energy source when placed on a high-fat diet. These results indicate that reduction in the circulating and tissue IGF-I levels can produce a metabolic phenotype in female mice that increases peripheral insulin resistance but renders animals resistant to the deleterious effects of high-fat feeding.

A novel neutralizing antibody targeting pregnancy-associated plasma protein-a inhibits ovarian cancer growth and ascites accumulation in patient mouse tumorgrafts

The majority of ovarian cancer patients acquire resistance to standard platinum chemotherapy and novel therapies to reduce tumor burden and ascites accumulation are needed. Pregnancy-associated plasma protein-A (PAPP-A) plays a key role in promoting insulin-like growth factor (IGF) pathway activity, which directly correlates to ovarian cancer cell transformation, growth, and invasiveness. Herein, we evaluate PAPP-A expression in tumors and ascites of women with ovarian cancer, and determine the antitumor efficacy of a neutralizing monoclonal PAPP-A antibody (mAb-PA) in ovarian cancer using primary patient ovarian tumorgrafts ("Ovatars"). PAPP-A mRNA expression in patient ovarian tumors correlated with poor outcome and was validated as a prognostic surrogate in Ovatar tumors. Following confirmation of mAb-PA bioavailability and target efficacy in vivo, the antitumor efficacy of mAb-PA in multiple Ovatar tumor models was examined and the response was found to depend on PAPP-A expression. Strikingly, the addition of mAb-PA to standard platinum chemotherapy effectively sensitized platinum-resistant Ovatar tumors. PAPP-A protein in ascites was also assessed in a large cohort of patients and very high levels were evident across the entire sample set. Therefore, we evaluated targeted PAPP-A inhibition as a novel approach to managing ovarian ascites, and found that mAb-PA inhibited the development, attenuated the progression, and induced the regression of Ovatar ascites. Together, these data indicate PAPP-A as a potential palliative and adjunct therapeutic target for women with ovarian cancer.

Specific suppression of insulin sensitivity in growth hormone receptor gene-disrupted (GHR-KO) mice attenuates phenotypic features of slow aging

In addition to their extended lifespans, slow-aging growth hormone receptor/binding protein gene-disrupted (knockout) (GHR-KO) mice are hypoinsulinemic and highly sensitive to the action of insulin. It has been proposed that this insulin sensitivity is important for their longevity and increased healthspan. We tested whether this insulin sensitivity of the GHR-KO mouse is necessary for its retarded aging by abrogating that sensitivity with a transgenic alteration that improves development and secretory function of pancreatic β-cells by expressing Igf-1 under the rat insulin promoter 1 (RIP::IGF-1). The RIP::IGF-1 transgene increased circulating insulin content in GHR-KO mice, and thusly fully normalized their insulin sensitivity, without affecting the proliferation of any non-β-cell cell types. Multiple (nonsurvivorship) longevity-associated physiological and endocrinological characteristics of these mice (namely beneficial blood glucose regulatory control, altered metabolism, and preservation of memory capabilities) were partially or completely normalized, thus supporting the causal role of insulin sensitivity for the decelerated senescence of GHR-KO mice. We conclude that a delayed onset and/or decreased pace of aging can be hormonally regulated.

Motor and memory testing of long-lived pregnancy-associated plasma protein--a knock-out mice

Mice deficient in pregnancy-associated plasma protein-A (PAPP-A), an IGF binding protein protease, have been shown to be resistant to experimentally induced atherosclerosis and diabetic nephropathy, and, in the laboratory environment, live 30-40% longer than wild-type littermates in association with delayed incidence and occurrence of age-related neoplasms and degenerative diseases.

OBJECTIVE

PAPP-A is highly expressed in the cerebellum and hippocampus of the mouse brain. Therefore, the studies presented here were aimed at determining motor behavior, learning and retention in PAPP-A knock-out (KO) mice compared to wild-type (WT) littermates with age.

DESIGN

Balance and coordination were assessed using an accelerating rotarod; learning and memory were assessed in a Stone T-maze.

RESULTS

Time on the rotarod decreased with age but there was no significant difference between PAPP-A KO and WT mice at any of the testing ages. Latency to reach the goal box and number of errors committed in the Stone T-maze did not change with age and there were no significant differences between PAPP-A KO and WT mice.

CONCLUSION

Lack of PAPP-A in mice did not impact central regulation of coordination, learning or memory.

Interaction of growth hormone receptor/binding protein gene disruption and caloric restriction for insulin sensitivity and attenuated aging

The correlation of physiological sensitivity to insulin ( vis-à-vis glycemic regulation) and longevity is extensively established, creating a justifiable gerontological interest on whether insulin sensitivity is causative, or even predictive, of some or all phenotypes of slowed senescence (including longevity). The growth hormone receptor/ binding protein gene-disrupted (GHR-KO) mouse is the most extensively investigated insulin-sensitive, attenuated aging model. It was reported that, in a manner divergent from similar mutants, GHR-KO mice fail to respond to caloric restriction (CR) by altering their insulin sensitivity. We hypothesized that maximized insulin responsiveness is what causes GHR-KO mice to exhibit a suppressed survivorship response to dietary (including caloric) restriction; and attempted to refute this hypothesis by assessing the effects of CR on GHR-KO mice for varied slow-aging-associated phenotypes. In contrast to previous reports, we found GHR-KO mice on CR to be less responsive than their ad libitum (A.L.) counterparts to the hypoglycemia-inducing effects of insulin. Further, CR had negligible effects on the metabolism or cognition of GHR-KO mice. Therefore, our data suggest that the effects of CR on the insulin sensitivity of GHR-KO mice do not concur with the effects of CR on the aging of GHR-KO mice.

Preservation of blood glucose homeostasis in slow-senescing somatotrophism-deficient mice subjected to intermittent fasting begun at middle or old age

Poor blood glucose homeostatic regulation is common, consequential, and costly for older and elderly populations, resulting in pleiotrophically adverse clinical outcomes. Somatotrophic signaling deficiency and dietary restriction have each been shown to delay the rate of senescence, resulting in salubrious phenotypes such as increased survivorship. Using two growth hormone (GH) signaling-related, slow-aging mouse mutants we tested, via longitudinal analyses, whether genetic perturbations that increase survivorship also improve blood glucose homeostatic regulation in senescing mammals. Furthermore, we institute a dietary restriction paradigm that also decelerates aging, an intermittent fasting (IF) feeding schedule, as either a short-term or a sustained intervention beginning at either middle or old age, and assess its effects on blood glucose control. We find that either of the two genetic alterations in GH signaling ameliorates fasting hyperglycemia; additionally, both longevity-inducing somatotrophic mutations improve insulin sensitivity into old age. Strikingly, we observe major and broad improvements in blood glucose homeostatic control by IF: IF improves ad libitum-fed hyperglycemia, glucose tolerance, and insulin sensitivity, and reduces hepatic gluconeogenesis, in aging mutant and normal mice. These results on correction of aging-resultant blood glucose dysregulation have potentially important clinical and public health implications for our ever-graying global population, and are consistent with the Longevity Dividend concept.

Preferential impact of pregnancy-associated plasma protein-A deficiency on visceral fat in mice on high-fat diet

Accumulation of visceral fat, more so than subcutaneous fat, is strongly associated with severe metabolic complications. However, the factors regulating depot-specific adipogenesis are poorly understood. In this study, we show differential expression of pregnancy-associated plasma protein-A (PAPP-A), a secreted regulator of local insulin-like growth factor (IGF) action, in adipose tissue of mice. PAPP-A mRNA expression was fivefold higher in visceral (mesenteric) fat compared with subcutaneous (inguinal, subscapular), perirenal, and brown fat of mice. To investigate the possible role of depot-specific PAPP-A expression in fat accumulation, wild-type (WT) and PAPP-A knockout (KO) mice were fed a high-fat diet (HFD) for up to 20 wk. Adipocyte size increased in subcutaneous and perirenal depots similarly in WT and PAPP-A KO mice. However, fat cell size and in vivo lipid uptake were significantly reduced in mesenteric fat of PAPP-A KO compared with WT mice. After 20 wk on HFD, phosphorylation of AKT, a downstream signaling intermediate of IGF-I and insulin receptor activation, was significantly decreased by 50% in mesenteric compared with subcutaneous fat in WT mice, but was significantly increased threefold in mesenteric compared with subcutaneous fat in PAPP-A KO mice. This appeared to be because of enhanced insulin-stimulated signaling in mesenteric fat of PAPP-A KO mice. These data establish fat depot-specific expression of PAPP-A and indicate preferential impact of PAPP-A deficiency on visceral fat in the mouse that is associated with enhanced insulin receptor signaling. Thus, PAPP-A may be a potential target for treatment and/or prevention strategies for visceral obesity and related morbidities.

Chronic kidney disease: a clinical model of premature aging

Premature aging is a process associated with a progressive accumulation of deleterious changes over time, an impairment of physiologic functions, and an increase in the risk of disease and death. Regardless of genetic background, aging can be accelerated by the lifestyle choices and environmental conditions to which our genes are exposed. Chronic kidney disease is a common condition that promotes cellular senescence and premature aging through toxic alterations in the internal milieu. This occurs through several mechanisms, including DNA and mitochondria damage, increased reactive oxygen species generation, persistent inflammation, stem cell exhaustion, phosphate toxicity, decreased klotho expression, and telomere attrition. Because recent evidence suggests that both increased local signaling of growth factors (through the nutrient-sensing mammalian target of rapamycin) and decreased klotho expression are important modulators of aging, interventions that target these should be tested in this prematurely aged population.

PAPP-A in cardiac and non-cardiac conditions

Pregnancy-associated plasma protein-A (PAPP-A), a newly discovered member of insulin like growth factors (IGFs) axis, has been reported to be a biomarker in both cardiac and non-cardiac conditions. PAPP-A mainly acts as a protease cleaving IGF inhibitors - IGF binding proteins (IGFBPs), thereby setting free IGFs. In cardiac conditions, PAPP-A plays an important role in progressive atherosclerosis. As a biomarker, PAPP-A is not only sensitive, specific and early for diagnosis of acute coronary syndrome, but also an independent risk factor for all-cause mortality or combined cardiovascular events. In non-cardiac conditions, PAPP-A is a new anti-aging target. PAPP-A knock out (KO) mice have a prolonged lifespan than the wild type. In addition, PAPP-A is also a biomarker associated with malignant cancer and end stage renal disease.

The effect of walking on postprandial glycemic excursion in patients with type 1 diabetes and healthy people

OBJECTIVE

Physical activity (PA), even at low intensity, promotes health and improves hyperglycemia. However, the effect of low-intensity PA captured with accelerometery on glucose variability in healthy individuals and patients with type 1 diabetes has not been examined. Quantifying the effects of PA on glycemic variability would improve artificial endocrine pancreas (AEP) algorithms.

RESEARCH DESIGN AND METHODS

We studied 12 healthy control subjects (five males, 37.7 ± 13.7 years of age) and 12 patients with type 1 diabetes (five males, 37.4 ± 14.2 years of age) for 88 h. Participants performed PA approximating a threefold increase over their basal metabolic rate. PA was captured using a PA-monitoring system, and interstitial fluid glucose concentrations were captured with continuous glucose monitors. In random order, one meal per day was followed by inactivity, and the other meals were followed by walking. Glucose and PA data for a total of 216 meals were analyzed from 30 min prior to meal ingestion to 270 min postmeal.

RESULTS

In healthy subjects, the incremental glucose area under the curve was 4.5 mmol/L/270 min for meals followed by walking, whereas it was 9.6 mmol/L/270 min (P = 0.022) for meals followed by inactivity. The corresponding glucose excursions for those with type 1 diabetes were 7.5 mmol/L/270 min and 18.4 mmol/L/270 min, respectively (P < 0.001).

CONCLUSIONS

Walking significantly impacts postprandial glucose excursions in healthy populations and in those with type 1 diabetes. AEP algorithms incorporating PA may enhance tight glycemic control end points.

Insulin-like growth factors and insulin: at the crossroad between tumor development and longevity

Numerous lines of evidence indicate that insulin-like growth factor signaling plays an important role in the regulation of life span and tumor development. In the present paper, the role of individual components of insulin-like growth factor signaling in aging and tumor development has been extensively analyzed. The molecular mechanisms underlying aging and tumor development are frequently overlapping. Although the link between reduced insulin-like growth factor signaling and suppressed tumor growth and development is well established, it remains unclear whether extended life span results from direct suppression of insulin-like growth factor signaling or this effect is caused by indirect mechanisms such as improved insulin sensitivity.

IGF-1 and atherothrombosis: relevance to pathophysiology and therapy

IGF-1 (insulin-like growth factor-1) plays a unique role in the cell protection of multiple systems, where its fine-tuned signal transduction helps to preserve tissues from hypoxia, ischaemia and oxidative stress, thus mediating functional homoeostatic adjustments. In contrast, its deprivation results in apoptosis and dysfunction. Many prospective epidemiological surveys have associated low IGF-1 levels with late mortality, MI (myocardial infarction), HF (heart failure) and diabetes. Interventional studies suggest that IGF-1 has anti-atherogenic actions, owing to its multifaceted impact on cardiovascular risk factors and diseases. The metabolic ability of IGF-1 in coupling vasodilation with improved function plays a key role in these actions. The endothelial-protective, anti-platelet and anti-thrombotic activities of IGF-1 exert critical effects in preventing both vascular damage and mechanisms that lead to unstable coronary plaques and syndromes. The pro-survival and anti-inflammatory short-term properties of IGF-1 appear to reduce infarct size and improve LV (left ventricular) remodelling after MI. An immune-modulatory ability, which is able to suppress 'friendly fire' and autoreactivity, is a proposed important additional mechanism explaining the anti-thrombotic and anti-remodelling activities of IGF-1. The concern of cancer risk raised by long-term therapy with IGF-1, however, deserves further study. In the present review, we discuss the large body of published evidence and review data on rhIGF-1 (recombinant human IGF-1) administration in cardiovascular disease and diabetes, with a focus on dosage and safety issues. Perhaps the time has come for the regenerative properties of IGF-1 to be assessed as a new pharmacological tool in cardiovascular medicine.

PAPP-A: a new anti-aging target?

This article focuses on the role of PAPP-A in mammalian aging. It introduces PAPP-A and a little of its history, briefly discusses the function of PAPP-A in the insulin-like growth factor (IGF) system and the regulators of PAPP-A expression, and then reviews data concerning PAPP-A in aging and age-related diseases especially in regard to the PAPP-A knockout (KO) mouse. The PAPP-A KO mouse is a valuable new model to test hypotheses concerning the control of the tissue availability of IGF, independent from systemic levels, on healthspan as well as lifespan.

Longevity is not influenced by prenatal programming of body size

Insulin-like growth factor (IGF) signaling is essential for achieving optimal body size during fetal development, whereas, in the adult, IGFs are associated with aging and age-related diseases. However, it is unclear as to what extent lifespan is influenced by events that occur during development. Here, we provide direct evidence that the exceptional longevity of mice with altered IGF signaling is not linked to prenatal programming of body size. Mice null for pregnancy-associated plasma protein-A (PAPP-A), an IGF-binding protein proteinase that increases local IGF bioavailability, are 60-70% the size of their wild-type littermates at birth and have extended median and maximum lifespan of 30-40%. In this study, PAPP-A(-/-) mice whose body size was normalized during fetal development through disruption of IgfII imprinting did not lose their longevity advantage. Adult-specific moderation of IGF signaling through PAPP-A inhibition may present a unique opportunity to improve lifespan without affecting important aspects of early life physiology.

In vivo analysis of gene expression in long-lived mice lacking the pregnancy-associated plasma protein A (PappA) gene

Mice lacking the pregnancy-associated plasma protein A (PappA) gene exhibit diminished localized IGF-1 bioavailability and a 30% increase in mean life span. However, it is uncertain which tissues exhibit reduced IGF-1 signals in the PappA(-/-) mouse, and whether effects of this mutation parallel those of mutations that diminish IGF-1 in serum. Across a panel of 21 tissues, we used RT-PCR to evaluate the effects of the PappA(-/-) mutation on expression of Igfbp5, which served as an in vivo indicator of IGF-1 signaling. Among these tissues, expression of Igfbp5 was significantly reduced by PappA(-/-) only in kidney. A broader survey of IGF-associated genes in six organs identified five other genes responsive to PappA(-/-) in kidney, with stronger effects in this organ relative to other tissues. Renal expression of Irs1 and Mt1 was increased by PappA(-/-) as well as by mutations that reduce IGF-1 in serum (i.e., Ghr(-/-), Pit1(dw/dw) and Prop1(df/df)), and we demonstrate that expression of these genes is regulated by growth hormone-treatment and calorie restriction. These results provide in vivo data on an important new model of mammalian aging, and characterize both similar and contrasting expression patterns between long-lived mice with reduced local IGF-1 availability and diminished IGF-1 in serum.

Longevity and age-related pathology of mice deficient in pregnancy-associated plasma protein-A

The pregnancy-associated plasma protein-A knockout (PAPP-A KO) mouse is a model of reduced local insulin-like growth factor (IGF)-I activity with normal circulating IGF-I levels. In this study, PAPP-A KO mice had significantly increased mean (27%), median (27%), and maximum (35%) life span compared with wild-type (WT) littermates. End-of-life pathology indicated that the incidence of neoplastic disease was not significantly different in the two groups of mice; however, it occurred in older aged PAPP-A KO compared with WT mice. Furthermore, PAPP-A KO mice were less likely to show degenerative changes of age. Scheduled pathologies at 78, 104, and 130 weeks of age indicated that WT mice, in general, had more degenerative changes and tumors earlier than PAPP-A KO mice. This was particularly true for abnormalities in heart, testes, brain, kidney, spleen, and thymus. In summary, the major contributors to the extended life span of PAPP-A KO mice are delayed occurrence of fatal neoplasias and decreased incidence of age-related degenerative changes.

Resistance to age-dependent thymic atrophy in long-lived mice that are deficient in pregnancy-associated plasma protein A

Pregnancy-associated plasma protein A (PAPPA) is a metalloproteinase that controls the tissue availability of insulin-like growth factor (IGF). Homozygous deletion of PAPPA in mice leads to lifespan extension. Since immune function is an important determinant of individual fitness, we examined the natural immune ecology of PAPPA(-/-) mice and their wild-type littermates reared under specific pathogen-free condition with aging. Whereas wild-type mice exhibit classic age-dependent thymic atrophy, 18-month-old PAPPA(-/-) mice maintain discrete thymic cortex and medulla densely populated by CD4(+)CD8(+) thymocytes that are capable of differentiating into single-positive CD4 and CD8 T cells. Old PAPPA(-/-) mice have high levels of T cell receptor excision circles, and have bone marrows enriched for subsets of thymus-seeding progenitors. PAPPA(-/-) mice have an overall larger pool of naive T cells, and also exhibit an age-dependent accumulation of CD44(+)CD43(+) memory T cells similar to wild-type mice. However, CD43(+) T cell subsets of old PAPPA(-/-) mice have significantly lower prevalence of 1B11 and S7, glycosylation isoforms known to inhibit T cell activation with normal aging. In bioassays of cell activation, splenic T cells of old PAPPA(-/-) mice have high levels of activation antigens and cytokine production, and also elicit Ig production by autologous B cells at levels equivalent to young wild-type mice. These data suggest an IGF-immune axis of healthy longevity. Controlling the availability of IGF in the thymus by targeted manipulation of PAPPA could be a way to maintain immune homeostasis during postnatal development and aging.