Insulin-like growth factor-I and IGF-I receptors in diabetic patients with neuropathy

MACHINE LEARNING ANALYSIS OF THE UK BIOBANK REVEALS PROGNOSTIC AND DIAGNOSTIC IMMUNE BIOMARKERS FOR POLYNEUROPATHY AND NEUROPATHIC PAIN IN DIABETES

AIMS

We assessed the health data of 11,047 people with diabetes in the UK Biobank to rank 329 risk factors for diabetic polyneuropathy (DPN) and DPN with chronic neuropathic pain without a priori assumption.

METHODS

The Integrated Disease Explanation and Risk Scoring (IDEARS) platform applies machine learning algorithms to multimodal data to determine individual disease risk, and rank risk factor importance using mean SHapley Additive exPlanations (SHAP) score.

RESULTS

IDEARS models showed discriminative performances with AUC > 0.64. Lower socioeconomic status, being overweight, poor overall health, cystatin C, HbA1C, and immune activation marker, C-reactive protein (CRP), predict DPN risk. Neutrophils and monocytes were higher in males and lymphocytes lower in females with diabetes that develop DPN. Neutrophil-to-Lymphocyte Ratio (NLR) was increased and IGF-1 levels decreased in people with type 2 diabetes that later develop DPN. CRP was significantly elevated in those with DPN and chronic neuropathic pain compared to DPN without pain.

CONCLUSIONS

Lifestyle factors and blood biomarkers predict the later development of DPN and may relate to DPN pathomechanisms. Our results are consistent with DPN as a disease involving systemic inflammation. We advocate for the use of these biomarkers clinically to predict future DPN risk and improve early diagnosis.

IGF-1 Via PI3K/Akt/S6K Signaling Pathway Protects DRG Neurons with High Glucose-induced Toxicity

Hyperglycemia-induced toxicity of neurons contributes to the pathogenesis and progression of diabetic neuropathy (DNP). High concentration glucose triggered reactive oxygen species (ROS) overproduction and induced cell apoptosis of neurons from dorsal root ganglion (DRG) in vitro. Currently, there is no effective therapeutic method to retard this devastating complication or neurotoxicity induced by high glucose. Insulin-like growth factor-1 (IGF-1) has multi-neurotrophic actions which need to be explored regarding its actions and mechanisms on relieving high glucose induced neurotoxicity. Herein, high concentration glucose was exposed to the DRG neurons in vitro. The effects of IGF-1 on relieving high glucose-induced neurotoxicity were evaluated. We illustrated that IGF-1 enhanced regeneration of neurites sent from DRG neuronal cell bodies and increased neuronal viability which inhibited by high glucose challenge. IGF-1 alleviated neuronal apoptosis caused by high glucose exposure. IGF-1 also suppressed the intracellular ROS overproduction and ATF3 expression upregulation which was induced by high glucose insult. The anti-neurotoxic effects of IGF-1 might be through restoration of prosurvival PI3K/Akt/S6K signaling. These data shed some light on the treatment of intractable DNP and suggested that IGF-1 might be a potential effective agent on relieving high glucose induced neurotoxicity.

PAPP-A activity is increased in cerebrospinal fluid from patients with diabetic polyneuropathy and correlates with peripheral nerve impairment

OBJECTIVE

Insulin-like growth factors (IGFs) have neuroprotective effects. IGF activity is partly controlled by pregnancy-associated plasma protein-A (PAPP-A), an enzyme which enhances IGF-action by cleavage of IGF-binding protein-4 (IGFBP-4). To study the role of PAPP-A and the IGF system in diabetic polyneuropathy (DPN), we measured immunoreactive (total) concentrations of IGF-I and IGF-II, bioactive IGF by cell-based bioassay, PAPP-A, as well as intact and PAPP-A-cleaved IGFBP-4 in cerebrospinal fluid (CSF) and serum from patients with type 2 diabetes (T2D) with and without DPN.

DESIGN

Twenty-three patients with T2D were included. Based on clinical examination, vibratory perception thresholds and nerve conduction studies, patients were diagnosed with (n = 9) or without (n = 14) DPN.

RESULTS

In CSF, PAPP-A activity, as estimated by IGFBP-4 fragment levels, was higher in patients with than without DPN (34.57 vs 13.79 μg/L, p = .003) and concentrations correlated with peripheral nerve impairment measures (r = 0.73, p < .01). Furthermore, serum bioactive IGF was lower in patients with than without DPN (0.8 vs 1.3 μg/L, p = .006) and correlated inversely to the severity of DPN (r = -0.67, p < .01).

CONCLUSIONS

In both CSF and serum, members of the IGF system correlated with measures of peripheral nerve impairment in patients with T2D. This supports a relationship between the IGF system and the development of DPN. Further studies are needed to clarify if these changes are causally linked to the pathogenesis of DPN.

IGF-1 and IGF-1R modulate the effects of IL-4 on retinal ganglion cells survival: The involvement of M1 muscarinic receptor

Trophic factors are involved in different cellular responses. Previously we demonstrated that IL-4 treatment induces an increase in retinal ganglion cell survival (RGCS) and regulates cholinergic differentiation of retinal cells in vitro. Data from literature show that IGF-1 also promotes RGCS, an effect mediated by PI-3K/AKT pathway. The aim of this study was to investigate the role of IGF-1 and IGF-1R on RGCS mediated by IL-4 treatment and the role of M1 acetylcholine receptors in this effect. Here we show that the effect of IL-4 on RGCS depends on IGF-1 and IGF-1R activation, the PI-3K/AKT and NFkB intracellular pathways and depends on M1 mAChRs activation. IGF-1 increases the levels of M1 mAChRs in 15min, 45min, 24 h and 48 h in mixed retinal cells culture, modulates the levels of IL-4, pIGF-1R, IGF-1R. IL-4 modulates IGF-1, pIGF-1R and IGF-1R levels in different time intervals. These results put in evidence a crosstalk between IL-4 and IGF-1 and a role of M1 mAChRs, IGF-1 and IGF-1R in RGCS mediated by IL-4.

Peripheral pain is enhanced by insulin-like growth factor 1 and its receptors in a mouse model of type 2 diabetes mellitus

BACKGROUND

Insulin-like growth factor 1 (IGF1) is a neurotrophic factor with many actions, including a possible hyperalgesic effect. This study investigated the effects of IGF1 on the overall behavior of diabetic mice and explored the possible mechanisms underlying IGF1-induced pain.

METHODS

Mice were divided into five groups (db/m, db/db, vehicle-treated db/db, IGF1-treated db/db, and IGF1 + JB1-treated db/db mice). Behavioral studies were conducted using the hot plate and Von Frey tests after intraplantar injection of recombinant (r) IGF1 (50 μg/kg) and the IGF1 receptor (IGF1R) antagonist JB1 (6 μg/mouse). Morphological changes in dorsal root ganglia (DRG) were evaluated using electron microscopy. Immunofluorescence was used to detect IGF1R expression and colocalisation with pain mediators in the DRG. Changes in the expression of IGF1R, extracellular signal-regulated kinase (ERK), and ras-associated factor-1 (c-raf) in the DRG were evaluated using western blotting.

RESULTS

Intraplantar injection of rIGF1 resulted in a hyperalgesic effect after 2 hours. This IGF1-induced hypersensitivity was attenuated by prior intraplantar injection of the IGF1R antagonist. There was no significant change in neuronal structure in the db/m group, whereas neuronal structure was impaired in the other four groups. Moreover, IGF1R was colocalised with pain mediators in the DRG of mice. Intraplantar injection of rIGF1 resulted in increased IGF1R, phosphorylated (p-) ERK, and c-raf expression in the DRG; prior intraplantar injection of the IGF1R antagonist attenuated rIGF1-induced increases in p-ERK and c-raf.

CONCLUSIONS

The results indicate that IGF1-induced acute hyperalgesia may be associated with the IGF1R/c-raf/ERK pathway. The IGF1-induced hypersensitivity was attenuated by an IGF1R antagonist.

Risk Factors and Comorbidities in Diabetic Neuropathy: An Update 2015

Distal symmetric sensorimotor polyneuropathy (DSPN) is the most common neurological manifestation in diabetes. Major risk factors of DSPN include diabetes duration, hyperglycemia, and age, followed by prediabetes, hypertension, dyslipidemia, and obesity. Height, smoking, insulin resistance, hypoinsulinemia, and others represent an additional risk. Importantly, hyperglycemia, hypertension, dyslipidemia, obesity, and smoking are modifiable. Stringent glycemic control has been shown to be effective in type 1, but not to the same extent in type 2 diabetes. Antilipidemic treatment, especially with fenofibrate, and multi-factorial intervention have produced encouraging results, but more experience is necessary. The major comorbidities of DSPN are depression, autonomic neuropathy, peripheral arterial disease, cardiovascular disease, nephropathy, retinopathy, and medial arterial calcification. Knowledge of risk factors and comorbidities has the potential to enrich the therapeutic strategy in clinical practice as part of the overall medical care for patients with neuropathy. This article provides an updated overview of DSPN risk factors and comorbidities.

Mechanisms of diabetic complications

It is increasingly apparent that not only is a cure for the current worldwide diabetes epidemic required, but also for its major complications, affecting both small and large blood vessels. These complications occur in the majority of individuals with both type 1 and type 2 diabetes. Among the most prevalent microvascular complications are kidney disease, blindness, and amputations, with current therapies only slowing disease progression. Impaired kidney function, exhibited as a reduced glomerular filtration rate, is also a major risk factor for macrovascular complications, such as heart attacks and strokes. There have been a large number of new therapies tested in clinical trials for diabetic complications, with, in general, rather disappointing results. Indeed, it remains to be fully defined as to which pathways in diabetic complications are essentially protective rather than pathological, in terms of their effects on the underlying disease process. Furthermore, seemingly independent pathways are also showing significant interactions with each other to exacerbate pathology. Interestingly, some of these pathways may not only play key roles in complications but also in the development of diabetes per se. This review aims to comprehensively discuss the well validated, as well as putative mechanisms involved in the development of diabetic complications. In addition, new fields of research, which warrant further investigation as potential therapeutic targets of the future, will be highlighted.

Insulin and insulin-like growth factor prevent brain atrophy and cognitive impairment in diabetic rats

There are an estimated 36 million dementia patients worldwide. The anticipated tripling of this number by year 2050 will negatively impact the capacity to deliver quality health care. The epidemic in diabetes is particularly troubling, because diabetes is a substantial risk factor for dementia independently of cerebrovascular disease. There is an urgent need to elucidate the pathogenesis of progressive brain atrophy, the cause of dementia, to allow rational design of new therapeutic interventions. This review summarizes recent tests of the hypothesis that the concomitant loss of insulin and insulin-like growth factors (IGFs) is the dominant cause for age-dependent, progressive brain atrophy with degeneration and cognitive decline. These tests are the first to show that insulin and IGFs regulate adult brain mass by maintaining brain protein content. Insulin and IGF levels are reduced in diabetes, and replacement of both ligands can prevent loss of total brain protein, widespread cell degeneration, and demyelination. IGF alone prevents retinal degeneration in diabetic rats. It supports synapses and is required for learning and memory. Replacement doses in diabetic rats can cross the blood-brain barrier to prevent hippocampus-dependent memory impairment. Insulin and IGFs are protective despite unabated hyperglycemia in diabetic rats, severely restricting hyperglycemia and its consequences as dominant pathogenic causes of brain atrophy and impaired cognition. These findings have important implications for late-onset alzheimer's disease (LOAD) where diabetes is a major risk factor, and concomitant decline in insulin and IGF activity suggest a similar pathogenesis for brain atrophy and dementia.

Insulin-like growth factors in the peripheral nervous system

Insulin-like growth factors (IGFs) play an integral role in development, growth, and survival. This article details the current understanding of the effects of IGFs in the peripheral nervous system (PNS) during health and disease, and introduces how the IGF system regulates PNS development and impacts growth and survival of PNS cells. Also discussed are implications of IGF signaling in neurodegeneration and the status and prospects of IGF therapies for PNS conditions. There is substantial support for the application of IGF therapies in the treatment of PNS injury and disease.

The effects of maternal diabetes on expression of insulin-like growth factor-1 and insulin receptors in male developing rat hippocampus

Diabetes during pregnancy causes neurodevelopmental and neurocognitive abnormalities in offspring. Insulin and insulin-like growth factor-1 (IGF-1) are important regulators of developmental and cognitive functions in the central nervous system. We examined the effects of maternal diabetes on insulin-like growth factor-1 receptor (IGF-1R) and insulin receptor (InsR) expression in the developing rat hippocampus. Female rats were maintained diabetic from a week before pregnancy through parturition and male offspring was killed at P0, P7, and P14. We found a significant bilateral upregulation of both IGF-1R and InsR transcripts in the hippocampus of pups born to diabetic mothers at P0, as compared to controls. However, at the same time point, the results of western blot analysis revealed only a slight change in their protein levels. At P7, there was a marked bilateral reduction in mRNA expression and protein levels of IGF-1R, although not of InsR in the diabetic group. We also found a downregulation in IGF1-R transcripts, especially in left hippocampus of the diabetic group at P14. Moreover, at the same time point, InsR expression was significantly decreased in both hippocampi of diabetic newborns. When compared with controls, we did not find any difference in hippocampal IGF-1R or InsR mRNA and protein levels in the insulin-treated group. The present study revealed that diabetes during pregnancy strongly influences the regulation of both IGF-1R and InsR in the right/left developing hippocampi. Furthermore, the rigid control of maternal glycaemia by insulin administration normalized these effects.

Changes in serotoninergic and noradrenergic descending pain pathways during painful diabetic neuropathy: the preventive action of IGF1

Painful diabetic neuropathy (PDN) induces neuronal hyperactivity at the spinal cord and periaqueductal gray (PAG), a key area in descending nociceptive modulation. Since the PAG uses relay stations at serotoninergic and noradrenergic brainstem areas, we determined the serotonin and noradrenaline levels at the spinal cord of streptozotocin-diabetic rats and at those brainstem areas (serotoninergic rostroventromedial medulla and noradrenergic A(5) and A(7) cell groups). Since, during diabetes, the levels of insulin growth factor 1 (IGF1) decrease, reducing its neurotrophic effect in the brain, we also studied the effects of IGF1 treatment. One week after diabetes induction, subcutaneous injections of IGF1 (2.5mg/kg) were performed during 3 weeks. Body weights, glycemia, and mechanical nociception were weekly evaluated until the end of the study, the time when the animals were subjected to a modified formalin test to study chemical allodynia. Serotonin and noradrenaline levels were quantified by ELISA at the spinal cord, whereas at the brainstem, the quantification was performed by immunohistochemistry against, respectively, tryptophan hydroxylase (TpH) or tyrosine hydroxylase (TH). STZ-diabetic rats exhibited mechanical hyperalgesia and chemical allodynia, along with higher spinal levels of serotonin and noradrenaline and higher numbers of neurons expressing TpH at the RVM and TH at the A(5) noradrenergic cell group. Treatment with IGF1 prevented the behavioral signs of PDN and reversed the neuronal hyperactivity at the spinal cord and ventrolateral PAG and the neurochemical changes at the spinal cord and at the brainstem. Based on the facilitatory role of serotoninergic and noradrenergic descending modulation during chronic pain, the increased serotonin and noradrenaline innervation of the dorsal horn in STZ-diabetic rats may probably account for enhanced pain during PDN. The benefits of IGF1 in PDN are probably due to blockade of the increased peripheral input to the somatosensory system, but direct central actions cannot be discarded. The value of IGF1 in PDN treatment deserves further evaluation.

Insulin-like growth factors in the peripheral nervous system

IGF-I and -II are potent neuronal mitogens and survival factors. The actions of IGF-I and -II are mediated via the type I IGF receptor (IGF-IR) and IGF binding proteins regulate the bioavailability of the IGFs. Cell viability correlates with IGF-IR expression and intact IGF-I/IGF-IR signaling pathways, including activation of MAPK/phosphatidylinositol-3 kinase. The expression of IGF-I and -II, IGF-IR, and IGF binding proteins are developmentally regulated in the central and peripheral nervous system. IGF-I therapy demonstrates mixed therapeutic results in the treatment of peripheral nerve injury, neuropathy, and motor neuron diseases such as amyotrophic lateral sclerosis. In this review we discuss the role of IGFs during peripheral nervous system development and the IGF signaling system as the potential therapeutic target for the treatment of nerve injury and motor neuron diseases.

Diabetic neuropathy: mechanisms to management

Neuropathy is the most common and debilitating complication of diabetes and results in pain, decreased motility, and amputation. Diabetic neuropathy encompasses a variety of forms whose impact ranges from discomfort to death. Hyperglycemia induces oxidative stress in diabetic neurons and results in activation of multiple biochemical pathways. These activated pathways are a major source of damage and are potential therapeutic targets in diabetic neuropathy. Though therapies are available to alleviate the symptoms of diabetic neuropathy, few options are available to eliminate the root causes. The immense physical, psychological, and economic cost of diabetic neuropathy underscore the need for causally targeted therapies. This review covers the pathology, epidemiology, biochemical pathways, and prevention of diabetic neuropathy, as well as discusses current symptomatic and causal therapies and novel approaches to identify therapeutic targets.

Inflammatory mediators modulating the transient receptor potential vanilloid 1 receptor: therapeutic targets to treat inflammatory and neuropathic pain

The transient receptor potential vanilloid 1 receptor (TRPV1) plays an important role in inflammatory heat hyperalgesia. TRPV1 is a non-selective cation channel gated by noxious heat, protons and capsaicin, thus being regarded as a polymodal molecular integrator in nociception. Abundant evidence has demonstrated that TRPV1 is also modulated by numerous inflammatory mediators, including growth factors, neurotransmitters, peptides or small proteins, lipids, chemokines and cytokines. By activating multiple protein kinases to increase the phosphorylation of TRPV1, pronociceptive inflammatory mediators sensitise the TRPV1 response to noxious heat, protons and capsaicin, thus augmenting thermal hyperalgesia. In contrast, by inhibiting protein kinases or other mechanisms, antinociceptive inflammatory mediators suppress the response of TRPV1 to these stimuli, thus damping thermal hyperalgesia. The positive modulation of TRPV1 by inflammatory mediators may constitute a novel mechanism underlying sustained inflammatory or neuropathic pain. Blocking pronociceptive inflammatory mediator-exerted sensitising effects or boosting antinociceptive inflammatory mediator-induced suppressing effects on TRPV1 should be considered as sources of novel potential therapies to more effectively treat chronic pain conditions.

Insulin and insulin-like growth factor type-I up-regulate the vanilloid receptor-1 (TRPV1) in stably TRPV1-expressing SH-SY5Y neuroblastoma cells

The capsaicin receptor, transient receptor potential, vanilloid type 1 (TRPV1), is a Ca(2+)-permeable ion channel activated by noxious stimuli eliciting pain. Several reports have shown modulation of TRPV1 activity and expression by neuronal growth factors. Here, we study the long-term effects on TRPV1 expression mediated by insulin-like growth factor type-I (IGF-I) and insulin in a stably TRPV1-expressing SH-SY5Y neuroblastoma cell line. We show that, after 72 hr of 10 nM IGF-I or insulin exposure, the TRPV1 protein level was up-regulated 2.5- and 2-fold, respectively. By blocking phosphatidylinositol-3-kinase [PI(3)K] or mitogen-activated protein kinase (MAPK) signaling, we concluded that the increase in total TRPV1 protein content induced by IGF-I was controlled by PI(3)K signaling, whereas insulin seemed to regulate TRPV1 protein expression via both PI(3)K and MAPK pathways. Inhibiting protein kinase C (PKC) blocked the effects of both IGF-I and insulin. Furthermore, the concentrations causing a 50% Ca(2+) increase (EC(50)) after insulin and IGF-I treatments were significantly lowered compared with untreated cells. We conclude that IGF-I and insulin enhance TRPV1 protein expression and activity, and impaired pain sensation might result from distorted TRPV1 regulation in the peripheral nervous system.

Identification of changes in gene expression in dorsal root ganglia in diabetic neuropathy: correlation with functional deficits

This study aimed to correlate the onset of functional deficits in diabetic neuropathy with changes in gene expression in rat dorsal root ganglia (DRG). After 1, 4, or 8 weeks of streptozotocin-induced diabetes, sensory and motor nerve conduction velocities (NCV) were measured as an indicator of neuropathy and changes in gene expression were measured using Affymetrix oligonucleotide microarrays. No significant changes in NCV were found after 1 week of diabetes, but after 4 and 8 weeks, there was a significant reduction in both sensory and motor NCV. Global gene expression changes in diabetic rat DRG were evident from principal component analysis of microarray data after 1, 4, and 8 weeks. Expression changes in individual genes were relatively small in line with a gradual degenerative neuropathy indirectly resulting from diabetes. Sets of differentially expressed genes have been identified and quantitative reverse transcriptase-polymerase chain reaction has been used to confirm the microarray data for several genes. Gene ontology overrepresentation analysis was performed on the microarray data to identify biologic processes altered in diabetic DRG. The genes identified in this study may be responsible for causing the functional deficits and suggest pathways/processes that require further investigation as possible targets for therapeutic intervention.

Systemic IGF-I treatment inhibits cell death in diabetic rat retina

Diabetic retinopathy can result in apoptotic cell death of retinal neurons, as well as significant visual loss. It is further known that insulin-like growth factor (IGF) levels are reduced in diabetes and that IGF-I can prevent cell death in many cell types. In this study, we tested the hypothesis that systemic treatment with IGF-I could inhibit death of neuroretinal cells in diabetic rats by examining the expression of proapoptotic markers. In diabetic rat retina, the number of TUNEL-immunoreactive cells increased approximately sixfold in the photoreceptor layer (P<.001) and eightfold in the inner nuclear layer (INL; P<.001); phospho-Akt (p-Akt; Thr 308) immunoreactivity increased eightfold in the ganglion cell layer (GCL; P<.001) and threefold in the INL (P<.01). Subcutaneous IGF-I treatment significantly reduced the number of TUNEL (P<.001) and p-Akt immunoreactive retinal cells (P<.05) in diabetic rats approximately to the level of the nondiabetic group. Qualitative results showed that caspase-3 and BAD immunoreactivities were also elevated in diabetes and reduced in IGF-I-treated animals. Elevated TUNEL and p-Akt immunoreactivities were localized to distinct cell layers in the retina of diabetic rats. Early intervention with systemic IGF-I reduced the presence of proapoptotic markers indicative of neuroretinal cell death, despite ongoing hyperglycemia and weight loss. The eye is a special sensory organ, and these data show that cell loss in the nervous system, even in uncontrolled diabetes, can be prevented by IGF-I administration.

Effect of IGF-I on DNA, RNA, and protein loss associated with brain atrophy and impaired learning in diabetic rats

Brain atrophy in diabetic dementia (DD) may be due to a catabolic state with DNA loss. Insulin-like growth factor (IGF) levels are reduced in diabetes, and IGF replacement may ameliorate brain protein loss. Subcutaneous minipumps released vehicle (Db + Veh) or IGF-I (Db + IGF-I) in diabetic rats. Brain wet, dry, and water weights as well as DNA, rRNA, poly(A)+ RNA, and protein contents per brain were significantly reduced in diabetic rats. In the remaining brain cells, there was a significant decline in ratios of (rRNA/DNA) and (protein/DNA). IGF-I administration partially prevented the loss of brain protein content independently of hyperglycemia (P < 0.03). This is the first demonstration of a severe disturbance in the brain protein regulatory pathway together with DNA loss in diabetes. Because Alzheimer's Disease (AD) is associated with a diabetes-like brain environment, a catabolic state may contribute to brain atrophy in sporadic AD as well as DD.

The insulin-like growth factor system and its pleiotropic functions in brain

In recent years, much interest has been devoted to defining the role of the IGF system in the nervous system. The ubiquitous IGFs, their cell membrane receptors, and their carrier binding proteins, the IGFBPs, are expressed early in the development of the nervous system and are therefore considered to play a key role in these processes. In vitro studies have demonstrated that the IGF system promotes differentiation and proliferation and sustains survival, preventing apoptosis of neuronal and brain derived cells. Furthermore, studies of transgenic mice overexpressing components of the IGF system or mice with disruptions of the same genes have clearly shown that the IGF system plays a key role in vivo.

Insulin detemir, does a new century bring a better basal insulin?

The treatment of diabetes was revolutionised shortly after the turn of the twentieth century by the extraction and purification of insulin. Methods to protract (i.e. prolong) the action of insulin were developed in the 1930s; little changed in the technology of insulin protraction until the turn of this century when, with renewed interest in the importance of basal insulin in controlling diabetes and thus preventing or delaying complications, technology advanced again. Two new long-acting insulin analogues have come to the market; some may be familiar with insulin glargine, which has been widely used for some years now. This review attempts to describe the novel method of protraction that insulin detemir (launched last summer) employs by albumin binding, to discuss the possible therapeutic benefits of this method of protraction and to describe the findings of studies comparing insulin detemir with other currently available long-acting insulin preparations. The intention of this article is not to review all of the currently available long-acting insulin analogues.

Sensitization and translocation of TRPV1 by insulin and IGF-I

Insulin and insulin-like growth factors (IGFs) maintain vital neuronal functions. Absolute or functional deficiencies of insulin or IGF-I may contribute to neuronal and vascular complications associated with diabetes. Vanilloid receptor 1 (also called TRPV1) is an ion channel that mediates inflammatory thermal nociception and is present on sensory neurons. Here we demonstrate that both insulin and IGF-I enhance TRPV1-mediated membrane currents in heterologous expression systems and cultured dorsal root ganglion neurons. Enhancement of membrane current results from both increased sensitivity of the receptor and translocation of TRPV1 from cytosol to plasma membrane. Receptor tyrosine kinases trigger a signaling cascade leading to activation of phosphatidylinositol 3-kinase (PI(3)K) and protein kinase C (PKC)-mediated phosphorylation of TRPV1, which is found to be essential for the potentiation. These findings establish a link between the insulin family of trophic factors and vanilloid receptors.

Experimental rat models of types 1 and 2 diabetes differ in sympathetic neuroaxonal dystrophy

Dysfunction of the autonomic nervous system is a recognized complication of diabetes, ranging in severity from relatively minor sweating and pupillomotor abnormality to debilitating interference with cardiovascular, genitourinary, and alimentary dysfunction. Neuroaxonal dystrophy (NAD), a distinctive distal axonopathy involving terminal axons and synapses, represents the neuropathologic hallmark of diabetic sympathetic autonomic neuropathy in man and several insulinopenic experimental rodent models. Although the pathogenesis of diabetic sympathetic NAD is unknown, recent studies have suggested that loss of the neurotrophic effects of insulin and/or insulin-like growth factor-I (IGF-I) on sympathetic neurons rather than hyperglycemia per se, may be critical to its development. Therefore, in our current investigation we have compared the sympathetic neuropathology developing after 8 months of diabetes in the streptozotocin (STZ)-induced diabetic rat and BB/ Wor rat, both models of hypoinsulinemic type 1 diabetes, with the BBZDR/Wor rat, a hyperglycemic and hyperinsulinemic type 2 diabetes model. Both STZ- and BB/Wor-diabetic rats reproducibly developed NAD in nerve terminals in the prevertebral superior mesenteric sympathetic ganglia (SMG) and ileal mesenteric nerves. The BBZDR/Wor-diabetic rat, in comparison, failed to develop superior mesenteric ganglionic NAD in excess of that of age-matched controls. Similarly, NAD which developed in axons of ileal mesenteric nerves of BBZDR/Wor rats was substantially less frequent than in BB/Wor- and STZ-rats. These data, considered in the light of the results of previous experiments, argue that hyperglycemia alone is not sufficient to produce sympathetic ganglionic NAD, but rather that it may be the diabetes-induced superimposed loss of trophic support, likely of IGF-I, insulin, or C-peptide, that ultimately causes NAD.

Nitrosative Injury and Antioxidant Therapy in the Management of Diabetic Neuropathy

Strong evidence implicates oxidative stress as a mediator of diabetes-induced microvascular complications, including distal symmetric polyneuropathy. Dorsal root ganglia neurons are particularly susceptible to glucose-mediated oxidative stress and die by apoptotic mechanisms in animal and cell culture models of diabetes. Key mediators of glucose-induced oxidative injury are superoxide anions and nitric oxide (NO). Superoxides are believed to underlie many of the oxidative changes in hyperglycemic conditions, including increases in aldose reductase and protein kinase C activity. Superoxides can also react with NO, forming peroxynitrite (ONOO-), which rapidly causes protein nitration or nitrosylation, lipid peroxidation, deoxyribonucleic acid (DNA) damage, and cell death. ONOO- formation is dependent on both superoxide and NO concentrations; therefore, cells that constitutively express NO synthase, such as endothelial cells and neurons, may be more vulnerable to ONOO–induced cell death in conditions favoring the production of superoxides. Although NO and ONOO- can cause endothelial and neuronal cell death in vitro, in animal models of diabetes, reductions in endothelial NO production can inhibit vasodilatation and cause nerve ischemia. Therefore, ideal therapeutic approaches should limit the formation of superoxides and ONOO while preventing reductions in vascular NO. Despite strong evidence that oxidative stress is associated with complications of diabetes, including neuropathy, the results of clinical trials of antioxidants have shown some promise but not established therapeutic efficacy. Clinical studies of several antioxidants, including α-lipoic acid, vitamins C and E, aldose reductase inhibitors, and growth factors, in diabetic neuropathy are discussed.

Systemic insulin-like growth factor-I administration prevents cognitive impairment in diabetic rats, and brain IGF regulates learning/memory in normal adult rats

Diabetic patients have impaired learning/memory, brain atrophy, and two-fold increased risk of dementia. The cause of cognitive disturbances that progress to dementia is unknown. Because neurotrophic insulin-like growth factor (IGF) levels are reduced in diabetic patients and rodents, and IGF can cross the blood-central nervous system barrier (B-CNS-B), the hypothesis was tested that IGF administered systemically can prevent cognitive disturbances, independently of hyperglycemia and a generalized catabolic state. Latency to escape to a hidden platform in the Morris Water Maze is used widely to test spatial memory, a hippocampus-dependent task. Adult rats were rendered diabetic with streptozotocin and implanted 4 weeks later with subcutaneous pumps that released either vehicle (D + Veh) or 20 microg/day IGF-I (D + IGF). Latency to escape to the hidden platform was prolonged in (D + Veh) versus non-diabetic rats (P < 0.003) 10.5 weeks after the onset of diabetes. Such prolongation was prevented in (D + IGF) versus (D + Veh) rats (P < 0.03). The data show that IGF-I can act across the B-CNS-B to prevent loss of cognition-related performance in the water maze independently of ongoing hyperglycemia and reduction in brain (P < 0.001) and whole body weight (P < 0.001) in diabetic rats. The hypothesis that brain IGF contributes to learning/memory was tested. An anti-IGF antibody, or preimmune serum, was infused into the lateral ventricles in non-diabetic rats. Learning in a passive avoidance task was impaired significantly in the IGF antibody versus preimmune serum-treated groups on test Days 1, 2, and 3 (P = 0.04, 0.02 and 0.004, respectively). The data together are consistent with a model in which brain IGF is essential for learning/memory, and a loss of IGF activity due to diabetes may contribute to cognitive disturbances.

Analysis of the Zucker Diabetic Fatty (ZDF) type 2 diabetic rat model suggests a neurotrophic role for insulin/IGF-I in diabetic autonomic neuropathy

Dysfunction of the autonomic nervous system is a recognized complication of diabetes. Neuroaxonal dystrophy (NAD), a distinctive axonopathy involving distal axons and synapses, represents the neuropathologic hallmark of diabetic sympathetic autonomic neuropathy in human and several insulinopenic experimental rodent models. Recent studies have suggested that loss of the neurotrophic effects of insulin and/or IGF-I on sympathetic neurons and not hyperglycemia per se, may underlie the development of sympathetic NAD. The streptozotocin (STZ)-diabetic and BB/W rat, the most commonly used experimental rodent models, develop marked hyperglycemia and concomitant deficiency in both circulating insulin and IGF-I. These animals reproducibly develop NAD in nerve terminals in the prevertebral sympathetic ganglia and the distal portions of noradrenergic ileal mesenteric nerves. The Zucker Diabetic Fatty (ZDF) rat, an animal model of type 2 diabetes, also develops severe hyperglycemia comparable to that in the STZ- and BB/W-diabetic rat models, although in the presence of hyperinsulinemia. In our study, ZDF rats maintained for 6 to 7 months in a severely diabetic state, as assessed by plasma glucose and glycated hemoglobin levels, maintained significant hyperinsulinemia and normal levels of plasma IGF-I at sacrifice. NAD did not develop in diabetic ZDF rat sympathetic ganglia and ileal mesenteric nerves as assessed by quantitative ultrastructural techniques, which is in dramatic contrast to neuropathologic findings in comparably hyperglycemic 6-month STZ-diabetic insulinopenic rats. These data combined with our previous results argue very strongly that hyperglycemia is not the critical and sufficient element in the pathogenesis of diabetes-induced NAD, rather that it is the loss of trophic support, most likely of IGF-I or insulin, that causes NAD.

Implications of diabetes mellitus in urology

The incidence of DM is rapidly growing among Americans. DM will rival cancer and heart disease in terms of cost and suffering. The National Institute of Health is tripling the research dollars that are spent on diabetic-related research in an attempt to combat this disease. Urologists are on the front line in the diagnosis and treatment of the complications of DM. The complications of DM that we reviewed in this article, diabetic cystopathy and diabetic ED, can occur in the early stage of DM and often progress in a silent fashion. More awareness and interest are needed to improve our understanding of diabetic complications in urology. Exciting new approaches in the treatment of diabetic cystopathy and ED are being investigated.

Neuropathology and pathogenesis of diabetic autonomic neuropathy

Autonomic neuropathy is a significant complication of diabetes resulting in increased patient morbidity and mortality. A number of studies, which have shown correspondence between neuropathologic findings in experimental animals and human subjects, have demonstrated that axonal and dendritic pathology in sympathetic ganglia in the absence of significant neuron loss represents a neuropathologic hallmark of diabetic autonomic neuropathy. A recurring theme in sympathetic ganglia, as well as in the pot-ganglionic autonomic innervation of various end organs, is the involvement of distal portions of axons and nerve terminals by degenerative or dystrophic changes. In both animals and humans, there is a surprising selectivity of the diabetic process for subpopulations of autonomic ganglia, nerve terminals within sympathetic ganglia and end organs, from end organ to end organ, and between vascular and other targets within individual end organs. Although the involvement or autonomic axons in somatic nerves may reflect an ischemic pathogenesis, the selectivity of the diabetic process confounds simple global explanations of diabetic autonomic neuropathy as the result of diminished blood flow with resultant tissue hypoxia. A single unifying pathogenetic hypothesis has not yet emerged from clinical and experimental animal studies, and it is likely that diabetic autonomic neuropathy will be shown to have multiple causative mechanisms, which will interact to result in the variety of presentations of autonomic injury in diabetes. Some of these mechanisms will be shared with aging changes in the autonomic nervous system. The role of various neurotrophic substances and the polyol pathway in the pathogenesis and treatment of diabetic neuropathy likely represents a two-edged sword with both salutary and exacerbating effects. The basic neurobiologic process underlying the diabetes-induced development of neuroaxonal dystrophy, synaptic dysplasia, defective axonal regeneration, and alterations in neurotrophic substance may be mechanistically related.

IGFs and IGFBPs: role in health and disease

The insulin-like growth factors (IGFs), IGF-binding proteins (IGFBPs), and IGFBP proteases are the main regulators of somatic growth and cellular proliferation. IGFs are involved in growth pre-natally and post-natally. Dysregulation of the IGF axis can lead to growth disorders such as growth hormone deficiency and acromegaly. Pre-natally, this dysregulation can lead to IUGR or macrosomia. IGFs also have an important mitogenic action and play a role in tumorigenesis and cancer. These actions are regulated by co-interactions with IGFBPs, especially IGFBP-3. In addition to somatic growth and mitogenic activity, IGFs have hypoglycaemic and insulin sensitizing actions, and their dysregulation is involved in diabetes and its complications. In this chapter, we examine the role of IGFs and IGFBPs in growth, tumorigenesis and diabetes, and discuss treatment modalities for each disease involving the GH-IGF-IGFBP axis, including discussion of current in vitro and in vivo investigations in this field.

Evaluation of sensorimotor polyneuropathy in children and adolescents with type I diabetes: associations with microalbuminuria and retinopathy

To investigate diabetic polyneuropathy, we measured peroneal motor conduction velocity, sural sensory nerve conduction velocity and vibratory sense threshold (biothesiometry) in 28 children and adolescents with insulin-dependent diabetes (type 1), and in 28 age- and sex-matched, normal controls. Age varied from 8 to 19 yr (mean +/- SD = 13.04 +/- 2.61); age at the onset of diabetes from 9 months to 12 yr (4.53 +/- 2.42 yr); and the duration of diabetes from 5 to 16 yr (8.48 +/- 2.98). Eight patients (28%) fulfilled the minimal criteria for the diagnosis of polyneuropathy. Four of these patients showed symptoms while three had clinical signs of neuropathy. Eight patients had abnormal, sural sensory nerve conduction velocities. The presence of polyneuropathy did not correlate with the duration of diabetes or degree of metabolic control of diabetes. The prevalence of microvascular complications (microalbuminuria and retinopathy) was 32%. The presence of microvascular complications did not correlate with metabolic control but did with the duration of diabetes. The relationship between polyneuropathy and microvascular complications was 34%.

Insulin-like growth factor type 1 upregulates uncoupling protein 3

In this study the expression of uncoupling protein 3 (UCP3) and its regulation by insulin-like growth factor 1 (IGF-I) and insulin in human neuroblastoma SH-SY5Y cells were characterized. Reverse transcriptase-PCR, Western blot, and immunofluorescence analysis showed that SH-SY5Y cells express UCP3 natively. IGF-I induced a time- and concentration-dependent induction of UCP3 protein reaching a twofold expression after 72 h with 10 nM IGF-I. Extremely high insulin concentrations (860 nM) and 10 nM trIGF-I, a truncated form of IGF-I with the same affinity for the IGF-I receptor as the full-length IGF-I, but with lower activity on the insulin receptor, also upregulated UCP3. We conclude that SH-SY5Y cells express UCP3 natively and that the expression is regulated by IGF-I via the IGF-I receptor.

New trends in the etiopathogenesis of diabetic peripheral neuropathy

Neuropathy is well recognized as a major complication of insulin-dependent diabetes mellitus in adults, resulting in significant morbidity and possibly an increased mortality. Both the peripheral and autonomic nervous systems can be involved, and adolescents with diabetes can show early evidence of neuropathy. The pathogenesis of diabetic neuropathy remains unclear but is thought to involve various mechanisms. This complication can be traced to the metabolic effects of hyperglycemia and/or other effects of insulin deficiency on the various constituents of the peripheral nerve. The polyol pathway and/or nonenzymatic glycation affecting one or more cell types in the multicellular constituents of the peripheral nerve appear likely to have an inciting role. The role of other factors, such as possible direct neurotrophic effects of insulin and insulin-related growth factors, seems to be relevant.

Insulin treatment enhances expression of IGF-I in sural nerves of diabetic patients

We studied the expression of insulin-like growth factor I (IGF-I) and its receptor in sural nerves from 8 diabetic patients divided into insulin-treated (IT) and non-insulin-treated (NIT) groups, compared with 5 patients with axonal neuropathies and 4 control patients (undergoing biopsies for diagnostic purposes). Insulin-like growth factor I mRNA levels did not differ in diabetic cases compared with control subjects. In sural nerves from IT patients and axonal neuropathies, IGF-I expression was higher than in NIT subjects and diagnostic controls. Changes in IGF-I receptor mRNA levels paralleled those of the ligand. Insulin-like growth factor I immunoreactivity was higher in nerves undergoing axonal degeneration and higher in IT than NIT diabetic patients and diagnostic controls. These findings suggest that insulin treatment increases IGF-I expression in diabetic nerves. Our data do not support the hypothesis of an absolute IGF-I deficiency in human diabetic neuropathy. A Schwann cell's incapacity to increase IGF-I expression after severe nerve damage, as happens in axonal neuropathies, may be a cofactor in the pathogenesis of diabetic neuropathy.

Effect of IGF-I and neurotrophin-3 on gracile neuroaxonal dystrophy in diabetic and aging rats

Neuroaxonal dystrophy (NAD), a distinctive axonopathy characterized by dramatic swelling of preterminal axons and nerve terminals by the accumulation of a variety of subcellular organelles, develops in the central projections of sensory neurons to medullary gracile nuclei in aged animals and man, and in a number of diseases and experimental conditions. Although its pathogenesis is unknown, proposed mechanisms include abnormalities of axonal regeneration, collateral sprouting and synaptic plasticity which may reflect alteration in neurotrophic support. In the current study, we have demonstrated quantitatively that aging causes the expected marked increase in the frequency of gracile NAD; however, substantial numbers of dystrophic axons develop between 6 and 10 months of age, earlier than expected. Although diabetes has been reported to increase the frequency of NAD in the central processes of sensory neurons in the gracile fasciculus of genetically diabetic BB rats, we have found that 8-10 months of streptozotocin-induced diabetes results in fewer dystrophic axons in the gracile nucleus than in age-matched controls. Administration of neurotrophin-3 (NT-3) and insulin-like growth factor-I (IGF-I), which have been shown to affect synaptic plasticity (implicated in the pathogenesis of NAD), for the last two months before sacrifice did not affect the frequency of gracile NAD in controls or diabetics. The sensory terminals in the gracile nuclei provide a simple, well-characterized experimental system in which questions of pathogenesis and prevention of neuroaxonal dystrophy can be addressed.

Insulin-like growth factor-I in diabetes mellitus: its physiology, metabolic effects, and potential clinical utility

Type 1 diabetes mellitus (DM) is a disease of insulin deficiency, resulting from the autoimmune-mediated destruction of pancreatic beta cells. However, as a likely consequence of intraportal insulin deficiency, patients with type 1 DM also exhibit abnormalities of the growth hormone (GH)/IGF/IGF-binding protein (IGFBP) axis, including GH hypersecretion, reduced circulating levels of insulin-like growth factor-I (IGF-I) and IGFBP-3, and elevated levels of IGFBP-1. These abnormalities not only exacerbate hyperglycemia in patients with type 1 DM, but may contribute to the pathogenesis of diabetes-specific complications, including diabetic neuropathy, nephropathy, and retinopathy. Therefore, therapeutic modalities aimed at restoring the GH-IGF-IGFBP axis are being considered. Herein, we review the efficacy of one such therapy, specifically IGF-I replacement therapy. To date, short-term beneficial metabolic effects of recombinant human IGF (rhIGF)-I therapy have been demonstrated in numerous diabetic conditions, including type 1 DM, type 2 DM, and type A insulin resistance. However, the long- term safety and metabolic efficacy of rhIGF-I therapy remains to be established. Moreover, the potential impact of rhIGF-I on the natural history of diabetic complications has yet to be explored.

Insulin-like growth factor I reverses experimental diabetic autonomic neuropathy

Recent studies have suggested a role for neurotrophic substances in the pathogenesis and treatment of diabetic neuropathy. In this study, the effect of insulin-like growth factor I (IGF-I) on diabetic sympathetic autonomic neuropathy was examined in an experimental streptozotocin-induced diabetic rat model. Two months of IGF-I treatment of chronically diabetic rats with established neuroaxonal dystrophy (the neuropathological hallmark of the disease) involving the superior mesenteric ganglion and ileal mesenteric nerves resulted in nearly complete normalization of the frequency of neuroaxonal dystrophy in both sites without altering the severity of diabetes. Treatment with low-dose insulin (to control for the transient glucose-lowering effects of IGF-I) failed to affect the frequency of ganglionic or mesenteric nerve neuroaxonal dystrophy or the severity of diabetes. The striking improvement in the severity of diabetic autonomic neuropathy shown with IGF-I treatment in these studies and the fidelity of the rat model to findings in diabetic human sympathetic ganglia provide promise for the development of new clinical therapeutic strategies.