Hypertension increases insulin-like growth factor binding protein-4 mRNA levels in rat aorta

The insulin-like growth factor family: molecular mechanisms, redox regulation, and clinical implications

Insulin-like growth factor (IGF)-induced signaling networks are vital in modulating multiple fundamental cellular processes, such as cell growth, survival, proliferation, and differentiation. Aberrations in the generation or action of IGF have been suggested to play an important role in several pathological conditions, including metabolic disorders, neurodegenerative diseases, and multiple types of cancer. Yet the exact mechanism involved in the pathogenesis of these diseases by IGFs remains obscure. Redox pathways involving reactive oxygen species (ROS) and reactive nitrogen species (RNS) contribute to the pathogenetic mechanism of various diseases by modifying key signaling pathways involved in cell growth, proliferation, survival, and apoptosis. Furthermore, ROS and RNS have been demonstrated to alter IGF production and/or action, and vice versa, and thereby have the ability to modulate cellular functions, leading to clinical manifestations of diseases. In this review, we provide an overview on the IGF system and discuss the potential role of IGF-1/IGF-1 receptor and redox pathways in the pathophysiology of several diseases.

Aortic coarctation hypertension induces fibroblast growth factor-2 immunoreactivity in the stimulated nucleus tractus solitarii

The actions of neurotrophic factors i.e. basic fibroblast growth factor (bFGF, FGF-2) to neurons are related not only to neuronal development and maintenance but also to synaptic plasticity regarding neurotransmission. We analyzed here the levels of FGF-2 immunoreactivity in the nucleus tractus solitarii (NTS) of Wistar Kyoto rats in response to alterations of neuronal activity promoted by the stimulation of the baroreceptor reflex following an aortic coarctation-induced-hypertension. The FGF-2 immunoreactivity (IR) was found in the cytoplasm of the neurons and in the nuclei of the glial cells in the NTS. A large number of NTS neurons expressed FOS immunoreactivity 4 h after coarctation, as an indication of neuronal activity. Stereological methods showed an increased number of FGF-2 immunoreactive (ir) neuronal profiles (90%) and glial profiles (149%) in the NTS of the 72 h aortic coarctated rats. 1-week later, FGF-2 ir neurons were still increased (54%) but no change was found in the number of FGF-2 ir glial profiles. The double immunoperoxidase method revealed that the majority of the FGF-2 ir glial cells was glial fibrillary acidic protein (GFAP) positive astrocytes. GFAP immunohistochemistry showed an astroglial reaction at 72 h time-interval (55%) but not 1 week after stimulation. The number of the cresyl violet positive neurons and OX42 ir profiles (marker of activated microglia) in the NTS of coarctated rats were not different from control by 1 week and 1 month after the surgery, indicating a lack of NTS injury in this period following coarctation hypertension. FGF-2 may be an important neurotrophic factor in areas involved in the control of blood pressure. The increased FGF-2 IR in the NTS cells following neuronal stimulation may represent trophic and plastic adaptive responses in this nucleus in an autocrine/paracrine fashion.

Expression, regulation, and function of IGF-1, IGF-1R, and IGF-1 binding proteins in blood vessels

The vascular insulin-like growth factor (IGF)-1 system includes the IGFs, the IGF-1 receptor (IGF-1R), and multiple binding proteins. This growth factor system exerts multiple physiologic effects on the vasculature through both endocrine and autocrine/paracrine mechanisms. The effects of IGF-1 are mediated principally through the IGF-1R but are modulated by complex interactions with multiple IGF binding proteins that themselves are regulated by phosphorylation, proteolysis, polymerization, and cell or matrix association. During the last decade, a significant body of evidence has accumulated, indicating that expression of the components of the IGF system are regulated by multiple factors, including growth factors, cytokines, lipoproteins, reactive oxygen species, and hemodynamic forces. In addition, cross-talk between the IGF system and other growth factors and integrin receptors has been demonstrated. There is accumulating evidence of a role for IGF-1 in multiple vascular pathologies, including atherosclerosis, hypertension, restenosis, angiogenesis, and diabetic vascular disease. This review will discuss the regulation of expression of IGF-1, IGF-1R, and IGF binding proteins in the vasculature and summarize evidence implicating involvement of this system in vascular diseases.

Targeted expression of a protease-resistant IGFBP-4 mutant in smooth muscle of transgenic mice results in IGFBP-4 stabilization and smooth muscle hypotrophy

The insulin-like growth factor-binding protein 4 (IGFBP-4), the most abundant IGF-binding protein produced by rodent smooth muscle cells (SMC), is degraded by specific protease(s) potentially releasing IGF-I for local bioactivity. IGFBP-4 protease(s) recognizes basic residues within the midregion of the molecule. We constructed a mutant IGFBP-4 with the cleavage domain substitution 119-KHMAKVRDRSKMK-133 to 119-AAMAAVADASAMA-133. Myc-tagged native and IGFBP-4.7A retained equivalent IGF-I binding affinity. Whereas native IGFBP-4 was cleaved by SMC-conditioned medium, IGFBP-4.7A was completely resistant to proteolysis. To explore the function of the protease-resistant IGFBP-4 in vivo, expression of the mutant and native proteins was targeted to SMC of transgenic mice by means of a smooth muscle alpha-actin promoter. Transgene expression was confined to SMC-rich tissues in all lines. Bladder and aortic immunoreactive IGFBP-4/transgene mRNA ratios in SMP8-BP4.7A mice were increased by 2- to 4-fold relative to SMP8-BP4 mice, indicating that the IGFBP-4.7A protein was stabilized in vivo. SMP8-BP4.7A mice had lower aortic, bladder, and stomach weight and intestinal length relative to SMP8-BP4 counterparts matched for protein expression by Western blotting. Thus, IGFBP-4.7A results in greater growth inhibition than equivalent levels of native IGFBP-4 in vivo, demonstrating a role for IGFBP-4 proteolysis in the regulation of IGF-I action.

Mechanical regulation of IGF-I and IGF-binding protein gene transcription in bladder smooth muscle cells

Mechanical forces are well known to modulate smooth muscle cell growth and synthetic phenotype. The signals controlling this process are complex and potentially involve changes in the expression of peptide growth factor genes such as those of the insulin-like growth factor (IGF) system. This study was designed to investigate the mechanical regulation of IGF-I and the binding proteins for IGF (IGFBPs) in smooth muscle cells cultured on a deformable surface and subjected to cyclic stretch. Using the RNase protection assay, we found that the application of a cyclic biaxial strain to cells induced a 2.5- to 4-fold increase in IGF-I mRNA levels after 8 h and an even greater increase after 16-24 h of stretch. This change was not affected by variations in the magnitude of the applied strain but was attenuated ( approximately 40%) when cells were treated with antagonists for angiotensin II receptors. Furthermore, the transcript levels of the three major IGF binding proteins produced in smooth muscle cells, e.g., IGFBP-2, IGFBP-4, and IGFBP-5, varied between stretched and control cells. Both IGFBP-2 and IGFBP-4 mRNA levels were consistently reduced in stretched cells but remained comparable to those of the control cells when the angiotensin II transducing pathway was blocked by inhibitors prior to the application of mechanical strain. Conversely, the gene expression of IGFBP-5 was upregulated in stretched cells, and neutralizing antibodies to IGF-I blocked this activation. Similarly, pharmacologic inhibition of the phosphatidylinositol 3-kinase, an important component of the IGF receptor transduction pathway, inhibited IGFBP-5 gene expression in stretched cells. These results suggest that the downstream effects of mechanical strain on IGF-I and IGFBP transcript levels are mediated, to greater or lesser extent, either through an angiotensin II tranducing pathway or via a feedback loop involving the autocrine secretion of IGF-I itself.

Vascular smooth muscle growth: autocrine growth mechanisms

Vascular smooth muscle cells (VSMC) exhibit several growth responses to agonists that regulate their function including proliferation (hyperplasia with an increase in cell number), hypertrophy (an increase in cell size without change in DNA content), endoreduplication (an increase in DNA content and usually size), and apoptosis. Both autocrine growth mechanisms (in which the individual cell synthesizes and/or secretes a substance that stimulates that same cell type to undergo a growth response) and paracrine growth mechanisms (in which the individual cells responding to the growth factor synthesize and/or secrete a substance that stimulates neighboring cells of another cell type) are important in VSMC growth. In this review I discuss the autocrine and paracrine growth factors important for VSMC growth in culture and in vessels. Four mechanisms by which individual agonists signal are described: direct effects of agonists on their receptors, transactivation of tyrosine kinase-coupled receptors, generation of reactive oxygen species, and induction/secretion of other growth and survival factors. Additional growth effects mediated by changes in cell matrix are discussed. The temporal and spatial coordination of these events are shown to modulate the environment in which other growth factors initiate cell cycle events. Finally, the heterogeneous nature of VSMC developmental origin provides another level of complexity in VSMC growth mechanisms.

Impaired insulin-like growth factor I vasorelaxant effects in hypertension

Insulin-like growth factor I (IGF-I) can be considered a factor potentially involved in arterial hypertension not only for its growth-promoting features but also for its effects on vascular tone. Nevertheless, the actions of the hormone on vascular reactivity are still unexplored in hypertension. Therefore, the vasodilation induced by increasing doses of IGF-I and the modulation of norepinephrine vasoconstriction induced by low levels of the hormone were tested on aortic rings of spontaneously hypertensive and normotensive rats. The results indicate that the vasodilation evoked by IGF-I is impaired in hypertensive rats (Delta% of maximal vasorelaxation, 30+/-1 versus 41+/-1; P<0.01), and after the removal of endothelium or the inhibition of endothelial NO synthase, the vasodilation evoked by the hormone was blunted in both rat strains and became similar between hypertensive and normotensive rats (Delta% of maximal vasorelaxation, 21+/-1 versus 20+/-1; P=NS). Moreover, IGF-I does not show any effect on norepinephrine vasoconstriction in hypertensive rats, and this alteration may depend on the lack of sensitizing effect exerted by IGF-I on alpha(2)-adrenergic-evoked NO vasorelaxation. The defect in IGF-I vascular action is also present in young spontaneously hypertensive rats (age 5 weeks). In conclusion, our data demonstrate that IGF-I vasorelaxant properties are impaired in spontaneously hypertensive rats, suggesting that such defect may play a causative or permissive role in the development of hypertensive conditions.

Insulin-like growth factor binding protein-4 expression is decreased by angiotensin II and thrombin in rat aortic vascular smooth muscle cells

Insulin-like growth factor-I (IGF-I) is a ubiquitous peptide that regulates cellular growth and differentiation and is involved in vascular proliferative responses. The effects of IGF-I are modulated by several IGF-I binding proteins (IGFBPs), including IGFBP-4, the main IGFBP produced by vascular smooth muscle cells (VSMCs). We have previously shown that angiotensin II (Ang II)-induced and thrombin-induced mitogenesis in VSMCs is dependent on autocrine IGF-I. In addition, we have demonstrated that IGF-I and IGFBP-4 mRNA levels are upregulated in the hypertensive aorta of abdominally coarcted rats, a high-renin hypertension model. To obtain further insight into the IGF-I system and to specifically study changes in IGFBP-4, a known inhibitor of IGF-I action, VSMCs were incubated with Ang II or thrombin. Compared with control, Ang II induced an 87+/-2% downregulation of IGFBP-4 mRNA levels at 24 hours, with a 61+/-6% decrease of IGFBP-4 levels, as determined by Western ligand blot analysis. Thrombin had the same depressor effects (87+/-2% for the mRNA levels and 61+/-3% for the protein levels). Ang II and thrombin coincubation with (125)I-IGFBP-4 in the conditioned media failed to reveal any increase in fragmentation, indicating that proteolytic cleavage of IGFBP-4 was not involved in the observed effects. Exogenous recombinant human IGFBP-4 decreased thrombin-induced DNA synthesis of human aortic VSMCs by 64%, whereas anti-IGFBP-4 antibody potentiated thrombin-induced DNA synthesis. These data suggest that downregulation of IGFBP-4 expression in VSMCs may play a critical role in vascular growth response to Ang II and thrombin in normal and diseased states, by increasing the bioavailability of IGF-I for its cell-surface receptor.

Interaction of angiotensin II and the insulin-like growth factor system in vascular smooth muscle cells

We studied the effects and interactions of ANG II and the insulin-like growth factor (IGF) system in cultured rat aortic smooth muscle cells. ANG II (1 microM) and IGF-I (10 nM) stimulated both DNA and protein synthesis. The effects of the two peptides in combination were additive or more than additive. The AT(1) receptor antagonist losartan (10 and 100 microM) blocked their synergistic effect on DNA synthesis. IGF binding protein (IGFBP)-1 inhibited the effect of IGF-I but not that of ANG II on DNA synthesis. IGF-I stimulated gene expression of IGFBP-2 and IGFBP-4. ANG II decreased IGF-I, IGFBP-2, and IGFBP-4 transcripts but increased the IGF-I receptor transcript. IGF-I and ANG II in combination had similar effects on gene expression as ANG II alone. The IGFBP-2 and IGFBP-4 peptides could be detected in the conditioned medium. Our results show that ANG II and IGF-I have synergistic effects on vascular smooth muscle cells and that they interact in several ways.

Effects of antihypertensive treatment on cardiac IGF-1 during prevention of ventricular hypertrophy in the rat

There is some evidence that cardiac rather than circulating insulin-like growth factor-1 (IGF-1) levels contribute to the development of renovascular hypertensive left ventricular hypertrophy (LVH), remaining unknown the effects of antihypertensive drugs on IGF-1 levels. We have assessed here the preventive effects of enalapril, losartan, propanolol and alpha-methyldopa on left ventricle (LV) and circulating IGF-1 levels in a rat model of hypertension and LVH (Goldblatt, GB). Our results show that relative LV mass and the LV content of IGF-1 were significantly lower with all antihypertensive drugs in GB rats (p<0.001). Serum concentrations of IGF-1 were lower in GB rats treated with enalapril, alpha-methyldopa and propanolol (p<0.01), but not in those treated with losartan. These results support the hypothesis that local rather than seric IGF-1 contributes to the development of left ventricular hypertrophy induced by pressure overload in the rat.

Angiotensin II causes weight loss and decreases circulating insulin-like growth factor I in rats through a pressor-independent mechanism

The renin-angiotensin system regulates normal cardiovascular homeostasis and is activated in certain forms of hypertension and in heart failure. Angiotensin II has multiple physiological effects and we have shown recently that its growth-promoting effects on vascular smooth muscle require autocrine activation of the IGF I receptor. To study the effect of angiotensin II on circulating IGF I, we infused rats with 500 ng/kg/min angiotensin II for up to 14 d. Angiotensin II markedly reduced plasma IGF I levels (56 and 41% decrease at 1 and 2 wk, respectively) and IGF binding protein-3 levels, and increased IGF binding protein-2 levels, a pattern suggestive of dietary restriction. Compared with sham, angiotensin II-infused hypertensive rats lost 18-26% of body weight by 1 wk, and pair-feeding experiments indicated that 74% of this loss was attributable to a reduction in food intake. The vasodilator hydralazine and the AT1 receptor antagonist losartan had comparable effects to reverse angiotensin II-induced hypertension, but only losartan blocked the changes in body weight and in circulating IGF I and its binding proteins produced by angiotensin II. Moreover, in Dahl rats that were hypertensive in response to a high-salt diet, none of these changes occurred. Thus, angiotensin II produces weight loss through a pressor-independent mechanism that includes a marked anorexigenic effect and an additional (likely metabolic) effect. These findings have profound implications for understanding the pathophysiology of conditions, such as congestive heart failure, in which the renin-angiotensin system is activated.

The insulin-like growth factor system in vascular smooth muscle: interaction with insulin and growth factors

Vascular smooth muscle cells (SMCs) occur throughout the vascular tree and have important physiological functions. They are also involved in pathological processes such as development and progression of atherosclerotic lesions, restenosis following angioplasty, and in hypertension. This review is focused on the role of the insulin-like growth factor (IGF) system in proliferation, migration, and hypertrophy of vascular SMCs and its interaction with insulin and other growth factors. The IGF-I receptor is highly expressed in SMCs in intact arteries and in cultured SMCs and is activated by binding of IGF-I to the two alpha-subunits. Insulin and IGF-II from the circulation can interact with the IGF-I receptor at higher concentrations. Insulin receptors are few or absent in SMCs and circulating insulin concentrations in vivo are probably too low for a direct action of insulin on the IGF-I receptor in SMCs. Receptor activation initiates a number of signal transduction pathways. Increased phosphatidylinositol turnover and calcium mobilization correlates with actin filament reorganization and stimulation of directed migration of the SMC in a gradient of IGF-I. The effects of IGF-I receptor activation on signal transduction pathways (eg, the MAP kinase cascade) implicated in DNA synthesis and proliferation are weak and this correlates with the meager mitogenic activity of IGF-I in SMC. Several components of the IGF-system in SMC are regulated by growth factors such as platelet-derived growth factor (PDGF)-BB and basic fibroblast growth factor (bFGF).(ABSTRACT TRUNCATED AT 250 WORDS)