Angiotensin II stimulates tyrosine phosphorylation and activation of insulin receptor substrate 1 and protein-tyrosine phosphatase 1D in vascular smooth muscle cells

Angiotensin II (Ang II) and insulin-like growth factor I (IGF I) stimulate intracellular signaling events through binding to their respective G-protein-coupled and growth factor receptors. In rat aortic vascular smooth muscle cells, IGF I (20 ng/ml) induced a sustained (>30 min) increase in the tyrosine phosphorylation of both Src-homology 2 domain-docking insulin receptor substrate 1 (IRS-1) and Src-homology 2-binding tyrosine phosphatase 1D (PTP-1D). In addition, IGF I stimulated PTP-1D phosphatase activity. Ang II (10(-7) M) also increased the tyrosine phosphorylation of IRS-1 (4-fold), PTP-1D (5-fold), and PTP-1D activity (3-4-fold), but with a more transient time course. Ang II also induced PTP-1D.IRS-1 complex formation. These Ang II-induced events were not affected by preincubation with an anti-IGF I antibody, suggesting that Ang II's actions were not mediated via the autocrine secretion of IGF I. Anti-PTP-1D antibody electroporation attenuated Ang II-induced PTP-1D.IRS-1 complex formation and PTP-1D tyrosine phosphorylation and activation. Our findings show that the tyrosine phosphorylation of IRS-1 and PTP-1D represents a convergent intracellular signaling cascade stimulated by both growth factor (i.e. IGF I) and G-protein-coupled (i.e. AT1) receptors.

Reactive oxygen species stimulate insulin-like growth factor I synthesis in vascular smooth muscle cells

OBJECTIVES

The objective was to study potential regulation of insulin-like growth factor I (IGF I), its binding proteins, and the IGF I receptor by reactive oxygen species in vascular smooth muscle cells.

METHODS

We used cultured rat aortic smooth muscle cells exposed to xanthine (100 microM) and xanthine oxidase (5 microU/ml) or H2O2 (200 microM) and measured IGF I mRNA levels by solution hybridization/RNase protection assays, IGF I protein levels by radioimmunoassay, and IGF binding proteins by Western ligand blotting. Additionally, we measured the effect of anti-IGF I antiserum on xanthine/xanthine oxidase- and H2O2-stimulated [3H]thymidine incorporation.

RESULTS

Xanthine/xanthine oxidase and H2O2 stimulated increases in IGF I mRNA and protein levels and reduced IGF binding protein-4 levels in conditioned medium. The effect of xanthine/xanthine oxidase was inhibited by the scavengers superoxide dismutase and catalase. Xanthine/xanthine oxidase- and H2O2-stimulated DNA synthesis was completely inhibited by a neutralizing anti-IGF I antiserum.

CONCLUSION

Reactive oxygen species increased vascular smooth muscle cell synthesis of IGF I and reduced levels of the inhibitory IGF binding protein-4. Furthermore, reactive oxygen species-induced DNA synthesis was inhibited by an anti-IGF I antiserum. These findings suggest that the autocrine IGF I system plays an important role in vascular smooth muscle cell growth responses to reactive oxygen species. Furthermore, the findings have important implications for understanding biological responses to changes in redox state.

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.

Insulin-like growth factor 1 binding protein 3 synthesis by aortic endothelial cells is a function of cell density

Proliferating bovine aortic endothelial cells in culture do not express the insulin-like growth factor 1 binding protein 3 gene, in contrast to the genes encoding binding proteins 2, 4, 5, and 6. The binding protein 3 gene is activated only at cell confluency with continual transcription thereafter, for at least an eight-day post-confluent period, both in the presence or absence of serum. Secretion of protein product into the medium parallels transcription. DNA synthesis is inversely related to the amount of total insulin-like growth factor 1 binding protein 3 in the culture medium. The addition of anti-binding protein 3 antibody to media significantly increases the rate of DNA synthesis by extensively post-confluent cells. These data suggest (1) aortic endothelial cells are an important source of circulating binding protein 3, (2) binding protein 3 has an inhibitory effect upon the growth of endothelial cells, (3) the triggering of binding protein 3 synthesis after confluency suggests a role in maintaining the growth-arrested monolayer state of the cells in vivo.

Transcriptional regulation of the insulin-like growth factor-I receptor gene: evidence for protein kinase C-dependent and -independent pathways

An important mechanism whereby growth factors stimulate vascular smooth muscle cell proliferation is by increasing insulin-like growth factor (IGF)-I receptor binding. To characterize the mechanisms involved, we studied transcription of the IGF-I receptor gene in rat aortic smooth muscle cells. Angiotensin II (100 nM) and basic fibroblast growth factor (5 ng/ml) caused a marked increase in IGF-I receptor messenger RNA (mRNA) levels, peaking at 3 h (215 +/- 16.8% and 85 +/- 7.4% above control, respectively). Nuclear run-on assays indicated that angiotensin II and fibroblast growth factor stimulated IGF-I receptor gene transcription by 2.1- and 2.5-fold, respectively. Down-regulation of protein kinase C, a serine/threonine kinase that is important in growth factor-activated signal transduction, completely inhibited fibroblast growth factor- but not angiotensin II-mediated up-regulation of IGF-I receptor mRNA. The protein kinase C inhibitors chelerythrine (3 microns), calphostin C (100 nM), and staurosporine (10 nM) also blocked fibroblast growth factor but not angiotensin II induction of IGF-I receptor mRNA. Thus, angiotensin II and fibroblast growth factor transcriptionally regulate the IGF-I receptor gene by protein kinase C-independent and -dependent pathways, respectively. In view of our prior data indicating that IGF-I receptor density is a critical determinant of vascular smooth muscle cell growth, our findings have particular relevance to understanding mechanisms whereby growth factors regulate vascular proliferation in vivo.

Characterization of insulin-like growth factor-I and its receptor and binding proteins in transected nerves and cultured Schwann cells

The insulin-like growth factors (IGFs) are trophic factors whose growth-promoting actions are mediated via the IGF-I receptor and modulated by six IGF binding proteins (IGFBPs). In this study, we observed increased transcripts of both IGF-I and IGF-I receptor after rat sciatic nerve transection. Schwann cells (SCs) were the main source of IGF-I and IGFBP-5 immunoreactivity until 7 days after nerve transection, when invading macrophages in the distal nerve stumps were strongly IGF-I positive. In vitro, IGF-I promoted SC mitogenesis. Northern analysis revealed that SCs expressed IGF-I receptor and IGFBP-5. IGF-I treatment increased the intensity of IGFBP-5 without affecting gene expression. Des(1-3)IGF-I, an IGF-I analogue with low affinity for IGFBP, had no such effect. Incubation of recombinant human IGFBP-5 with SC conditioned media revealed IGF-I protection of IGFBP-5 from proteolysis, implying the presence of an IGFBP-5 protease in SC conditioned media. Collectively, these data support the concept that, in response to nerve injury, invading macrophages produce IGF-I and SC express the IGF-I receptor, to facilitate regeneration. This regenerative process may be augmented further by the ability of SC to secrete IGFBPs, which in turn may increase local IGF-I bioavailability.

G-protein and tyrosine kinase receptor cross-talk in rat aortic smooth muscle cells: thrombin- and angiotensin II-induced tyrosine phosphorylation of insulin receptor substrate-1 and insulin-like growth factor 1 receptor

Insulin-like growth factor I is an autocrine/paracrine factor for vascular smooth muscle cells and is required for angiotensin II- and thrombin-induced mitogenesis. The insulin-like growth factor I-triggered signaling pathway involves autophosphorylation of the beta-subunit of its tyrosine kinase receptor and phosphorylation of insulin receptor substrate-1, the latter providing binding sites for proteins with src homology-2 domains. In rat aortic smooth muscle cells we observed that both angiotensin II and thrombin induced rapid tyrosine phosphorylation of insulin receptor substrate-1. Our results also demonstrated that these mitogens rapidly stimulated phosphorylation of the insulin-like growth factor 1 receptor beta-chain. These data demonstrate a novel interaction between the G-protein coupled angiotensin II and thrombin receptors and the tyrosine-kinase insulin-like growth factor I receptor.

G-protein coupled and tyrosine kinase receptors: evidence that activation of the insulin-like growth factor I receptor is required for thrombin-induced mitogenesis of rat aortic smooth muscle cells

IGF I is an ubiquitous peptide that activates a membrane tyrosine kinase receptor and has autocrine/paracrine effects on vascular smooth muscle cells. Thrombin activates a G-protein coupled receptor and is also a mitogen for vascular smooth muscle cells. To assess the potential role of IGF I as a mediator of thrombin's effects, we characterized expression of IGF I and of its receptor on vascular smooth muscle cells exposed to thrombin. Thrombin dose-dependently decreased IGF I mRNA levels and caused a delayed decrease in IGF I secretion from vascular smooth muscle cells. This effect was mimicked by the hexapeptide SF-FLRN (that functions as a tethered ligand) and was inhibited by hirudin. In contrast, thrombin doubled IGF I receptor density on vascular smooth muscle cells, without altering binding affinity (Kd). An anti-IGF I antiserum markedly reduced thrombin-induced DNA synthesis, whereas nonimmune serum and an anti-fibroblast growth factor antibody were without effect. Cell counts confirmed these results. Downregulation of IGF I receptors by antisense phosphorothioate oligonucleotides likewise markedly inhibited thrombin-induced DNA synthesis. These data demonstrate that a functional IGF I-IGF I receptor pathway is essential for thrombin-induced mitogenic signaling and support the concept of cross talk between G-protein coupled and tyrosine kinase receptors.

Insulin-like growth factor I and its binding proteins in the cardiovascular system

A large body of evidence has conclusively shown that IGF I is an essential regulator of developmental growth. Thus mice bearing a null mutation for the IGF IR gene invariably die shortly after birth, and mice bearing a null mutation for the IGF I gene have a high neonatal mortality rate and marked growth retardation [158,159]. The ubiquitous effects of IGF I make it likely that this autocrine/endocrine system plays an important role in cardiovascular development. Its potential role in cardiovascular pathophysiology has raised considerable interest over the last several years. There is strong evidence that IGF I is a critical determinant of vascular growth responses in vitro and in vivo. Regulation of VSMC IGF IR availability appears to be crucial for the control of VSMC growth, and as such is at a convergence point for the effects of multiple growth factors. Clinical studies relating to IGF I in hypertension are extremely limited but significant data from animal studies now suggest a role for IGF I as a mediator of hypertrophic/hyperplastic responses in hypertension. Furthermore, significant animal data now exist implicating IGF I as an important mediator of cardiac hypertrophic responses. The development of a specific pharmacologic inhibitor of the IGF IR should allow rational clinical trials to address the function of IGF I as a mediator of cardiovascular growth responses. Specifically, areas of great interest will include the potential prevention of post-angioplasty restenosis, of atherosclerotic lesion development and progression, and of the complications of hypertensive vascular disease. The use of IGF I to ameliorate myocardial growth and function post infarction, to promote angiogenesis and collateral artery formation in the setting of peripheral vascular disease, are other important directions for future research. The use of IGF I to improve wound healing, improve recovery from acute renal failure and improve glucose control is currently under investigation. Clearly ongoing studies addressing the mechanisms whereby IGF I interacts with its receptor and binding proteins to produce its effects in cardiovascular tissues, will provide a rationale for novel and pertinent clinical research.

Thrombin stimulates phosphorylation of insulin-like growth factor-1 receptor, insulin receptor substrate-1, and phospholipase C-gamma 1 in rat aortic smooth muscle cells

It has recently been reported that protein-tyrosine kinase activity is required for thrombin-induced growth in vascular smooth muscle cells (VSMC). In the present study, we have identified several phosphoproteins that are tyrosine-phosphorylated in response to thrombin in quiescent VSMC. These proteins are insulin-like growth factor-1 receptor beta-subunit (IGF-IR beta), insulin receptor substrate-1 (IRS-1), and phospholipase C-gamma 1 (PLC-gamma 1). Thrombin-stimulated phosphorylation of these proteins was rapid; it was maximal at 1 min and reduced thereafter. Thrombin also activated mitogen-activated protein kinases (MAPK) in quiescent VSMC in a biphasic manner with a rapid and larger peak at 10 min (6-fold) followed by a sustained smaller second peak at 2 h (2-fold). Inhibition of protein-tyrosine kinase activity by the use of two structurally different protein-tyrosine kinase inhibitors, genistein and herbimycin A, significantly blocked the thrombin-induced tyrosine phosphorylation of IGF-1R beta, IRS-1, and PLC-gamma 1 and decreased thrombin-stimulated DNA synthesis. In contrast, however, inhibition of protein-tyrosine kinase activity had no effect on thrombin activation of MAPK. Collectively, these findings suggest a role for tyrosine phosphorylation of IGF-IR beta, IRS-1, and PLC-gamma 1 in thrombin-induced mitogenic signaling events in VSMC. Furthermore, while protein tyrosine phosphorylation is essential for thrombin-induced DNA synthesis, it is not required for thrombin-stimulated MAPK activation. Since thrombin rapidly activated Src in VSMC, Src may be involved in the cross-talk between the G-protein-coupled receptor agonist and a tyrosine kinase receptor such as IGF-1R.

Regulation of vascular smooth muscle cell insulin-like growth factor I receptors by phosphorothioate oligonucleotides. Effects on cell growth and evidence that sense targeting at the ATG site increases receptor expression

We have recently shown that insulin-like growth factor I (IGF I) is a mediator of angiotensin II-induced mitogenesis in vascular smooth muscle cells (Delafontaine, P., and Lou H. (1993) J. Biol. Chem. 268, 16866-16870). To study the role of the IGF I receptor in vascular smooth muscle cell growth, phosphorothioate oligonucleotides were used to modulate IGF I receptors. An antisense oligonucleotide targeting the ATG site inhibited basal and serum-induced DNA synthesis in vascular smooth muscle cells. Mismatch oligonucleotide had no effect, while surprisingly sense oligonucleotide increased IGF I receptor number and basal and serum-induced DNA synthesis. A 51% reduction in IGF I receptor number following exposure to 5 microM antisense oligonucleotide markedly inhibited angiotensin II-induced mitogenesis. A 70% increase in IGF I receptor number following exposure to 5 microM sense oligonucleotide resulted in a 4-fold increase in basal [3H]thymidine incorporation, and angiotensin II (1-1000 nM) had no additive stimulatory effect. An antisense oligonucleotide targeting a sequence starting at +109 base pairs (relative to ATG) also reduced IGF I receptor number, however, the corresponding sense oligonucleotide was without effect. These findings demonstrate that alterations in vascular smooth muscle cell IGF I receptor density play a critical role in the proliferative response of vascular smooth muscle cells to serum and to angiotensin II. In addition, the surprising observation that an ATG-directed sense oligonucleotide up-regulates IGF I receptors identifies a novel effect of oligonucleotides on gene expression.

Inhibition of vascular smooth muscle cell growth through antisense transcription of a rat insulin-like growth factor I receptor cDNA

Insulin-like growth factor I (IGF I) is an autocrine/paracrine growth factor that is produced in multiple tissues and is essential for normal developmental growth. Its effects are mediated by activation of a membrane-bound tyrosine kinase receptor, IGF IR. On the basis of the partial rat IGF IR alpha-chain cDNA sequence previously reported, we cloned cDNA encoding the full-length rat IGF IR. The deduced amino acid sequence predicts a 1370-amino acid receptor precursor, which includes signal sequence, a 707-amino acid alpha-chain, a 4-Arg cleavage site, and a 629-amino acid beta-chain. Overall, similarity to human IGF IR is 89% and 98% at the nucleotide and amino acid levels, respectively. Antisense IGF IR expression constructs in vectors incorporating Epstein-Barr virus replicative signals and the cytomegalovirus promoter/enhancer or the inducible human metallothionein IIa promoter/enhancer were assembled and stably transfected into cultured rat aortic smooth muscle cells. Clone CA9 (constitutively expressing abundant antisense IGF IR transcripts), clones MA5 and MA7 (expressing antisense IGF IR transcripts inducibly), and clones ME8 and ME10 (expressing vector alone) were characterized. There was a 57% reduction in IGF IR mRNA levels in clone CA9 after confluence compared with clone ME10. This resulted in a 51% decrease in IGF I binding sites in clone CA9, without a change in binding affinity (Kd), and a 55% and 57% reduction in DNA synthesis rates, basally and in response to 10 ng/mL IGF I, respectively. Clones MA5/MA7 similarly showed a 54% reduction in IGF IR number after confluence following exposure to 100 mumol/L ZnSO4 and a 44% and 58% reduction in DNA synthesis, basally and in response to 10 ng/mL IGF I, respectively. Growth curves indicated that proliferation of clone CA9 in the presence of 10% serum was reduced by 60% compared with clone ME10. Thus, cloning of cDNA encoding the full-length rat IGF IR indicates that this receptor is highly conserved. Antisense targeting of this receptor in vascular smooth muscle cells (VSMCs) demonstrates that a decrease in IGF IR density results in marked inhibition of VSMC proliferation. These findings indicate an important role for this ligand-receptor system in regulating VSMC growth. Specifically, they suggest that modulation of VSMC IGF IR density may be an important mechanism whereby growth of these cells is controlled.

Direct stimulation of Jak/STAT pathway by the angiotensin II AT1 receptor

The peptide angiotensin II is the effector molecule of the reninangiotensin system. All the haemodynamic effects of angiotensin II, including vasoconstriction and adrenal aldosterone release, are mediated through a single class of cell-surface receptors known as AT1 (refs 1, 2). These receptors contain the structural features of the G-protein-coupled receptor superfamily. We show here that angiotensin II induces the rapid phosphorylation of tyrosine in the intracellular kinases Jak2 and Tyk2 in rat aortic smooth-muscle cells and that this phosphorylation is associated with increased activity of Jak2. The Jak family substrates STAT1 and STAT2 (for signal transducers and activators of transcription) are rapidly tyrosine-phosphorylated in response to angiotensin II. We also find that Jak2 co-precipitates with the AT1 receptor, indicating that G-protein-coupled receptors may be able to signal through the intracellular phosphorylation pathways used by cytokine receptors.

Identification of two positive transcriptional elements within the 91-base pair promoter for mouse testis angiotensin converting enzyme (testis ACE)

Testis angiotensin-converting enzyme (testis ACE) is an isozyme of ACE only expressed by male germ cells during spermiogenesis. It is the result of a strong sperm-specific promoter found within the 12th intron of the somatic ACE gene. Previous studies have localized the boundaries of the mouse testis ACE promoter as being from -91 to -9, relative to the transcriptional start site, and have suggested two important DNA regulatory elements starting at positions -55 and -32. DNA constructs were made in which these motifs were either eliminated or substituted. Each construct was tested for its ability to promote transcription in vitro, using a rat testis nuclear extract. Disruption of either motif reduced in vitro transcription to about 30% of control levels, while mutations of both elements abolished transcription. Two sites were selected inside each motif and altered by point mutation. Each of four constructs, containing a mutation at -51, -48, -30, or -28, transcribed at 29% or less the efficiency of the parent construct. The DNA element at -55, TGAGGTCA, is homologous to a consensus cyclic AMP response element. The motif at -32, TCTTAT, is located at a position analogous to a TATA box. Substitution of the -32 motif with a consensus TATA box sequence, TATAAA, stimulated transcriptional activity about 3-fold. As measured by gel mobility shift, oligonucleotides encompassing the -32 motif and the consensus TATA box formed different DNA-protein complexes. However, the -32 motif oligonucleotide was recognized by nuclear proteins prepared from either liver or testis nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)

Epitope mapping of the alpha-chain of the insulin-like growth factor I receptor using antipeptide antibodies

Insulin-like growth factor I (IGF I) is an important mitogen for vascular smooth muscle cells (VSMC). The IGF I receptor (IGF IR) is a heterotetramer composed of two cross-linked extracellular alpha-chains and two membrane-spanning beta-chains that contain a tyrosine-kinase domain. It has a high degree of sequence similarity to the insulin receptor (IR), and the putative ligand-specific binding site has been localized to a cysteine-rich region (CRR) of the alpha-chain. To obtain insights into antigenic determinants of the IGF IR, we raised a panel of site-specific polyclonal antibodies against short peptide sequences N-terminal to and within the CRR. Several antibodies raised against linear epitopes within the CRR bound to solubilized and native rat and human IGF IR by ELISA, did not cross-react with IR, but unexpectedly failed to inhibit 125I-IGF I binding. A polyclonal antibody directed against a 48-amino acid synthetic peptide, corresponding to a region of the CRR postulated to be essential for ligand binding, failed to react with either solubilized, reduced or intact IGF IR. Three antibodies specific for the N-terminus of the alpha-chain reacted with solubilized and native IGF IR. One of these, RAB 6, directed against amino acids 38-44 of the IGF IR, inhibited 125I-IGF I binding to rat aortic smooth muscle cells (RASM) and to IGF IR/3T3 cells (overexpressing human IGF IR) by up to 45%. Immunohistochemical analysis revealed strong IGF IR staining in the medial smooth muscle cell layer of rat aorta. These findings are consistent with a model wherein conformational epitopes within the CRR and linear epitopes within the N-terminus of the alpha-chain contribute to the IGF I binding pocket. These antibodies should provide a valuable tool to study structure-function relationships and in vivo regulation of the IGF IR.

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

Insulin-like growth factor-I (IGF-I) is an endocrine and autocrine/paracrine growth factor. Recently, we have demonstrated that interrenal aortic coarctation in the rat increases IGF-I mRNA levels in the thoracic aorta, consistent with a role for this mitogen in hypertensive vascular remodeling. The effects of IGF-I are modulated by several IGF binding proteins including IGFBP-3, the main circulating carrier of IGF-I, and IGFBP-4, the main IGF binding protein produced by vascular smooth muscle cells in vitro. To obtain insights into the regulation of IGF-I and more specifically to study potential changes in IGF binding proteins in high-renin hypertension, we studied male Sprague-Dawley rats that had undergone abdominal aortic coarctation. Compared with sham-operated rats, the study rats showed a rapid increase in IGFBP-4 mRNA levels in the hypertensive (thoracic) aorta, reaching a plateau at 3 days (2.5-fold increase) and persisting for at least 14 days. In striking contrast, IGFBP-4 mRNA decreased slightly in the normotensive (abdominal) aorta at 14 days. IGFBP-3 mRNA levels did not change in either vascular bed after coarctation. Study of hepatic tissue indicated that in coarcted rats IGFBP-4 and IGFBP-3 mRNA levels decreased transiently (approximately 50% at 7 days compared with sham). Circulating IGF-I in coarcted animals decreased slightly (P = .08), and Western ligand analysis indicated that circulating levels of IGF binding proteins were not altered.(ABSTRACT TRUNCATED AT 250 WORDS)

The angiotensin II AT1 receptor is tyrosine and serine phosphorylated and can serve as a substrate for the src family of tyrosine kinases

Angiotensin II AT1 receptor signal transduction has recently been shown to function through the phospholipase C isozyme, PLC-gamma. Since PLC-gamma is known to interact with phosphotyrosine containing proteins through SH2 domains, we examined the phosphorylation state of the AT1 receptor. Immunoprecipitation of the [32P] labeled AT1 receptor from rat aortic smooth muscle cells followed by alkali hydrolysis demonstrated the presence of tyrosine phosphorylation. Phosphoamino acid analysis of the excised bands demonstrated the presence of phosphoserine and phosphotyrosine residues. A fusion protein comprising the intracellular tail of the AT1 receptor was used to screen for candidate kinases, and the src kinase family displayed high activity. In summary, this study shows that the AT1 receptor is serine and tyrosine phosphorylated in vivo and suggests that a soluble kinase related to the src family may be responsible for the tyrosine phosphorylation.

Endothelin A and B receptors are down-regulated in the hearts of hypertensive rats

Endothelins are vasoactive peptides that have been implicated in the development and maintenance of systemic arterial hypertension. The biologic effects of endothelins result from activation of either or both of the two known endothelin receptor subtypes, A and B [ET-R(A) and ET-R(B)], which are present not only in blood vessels but also throughout the cardiovascular and central nervous systems. To investigate the potential role and regulation of myocardial endothelin receptors in hypertension, we examined the expression of ET-R(A) and ET-R(B) receptors in the hearts of normotensive and hypertensive rats. A cDNA probe for the ET-R(A) receptor was obtained by polymerase chain reaction amplification of rat aortic smooth muscle cell mRNA, using degenerate primers specific for intramembrane domains III and VI of G-coupled receptors. Moderate stringency hybridization screening of a rat aortic smooth muscle cell cDNA library yielded a partial clone for the ET-R(B) receptor. These two clones were used to examine expression of the ET-R(A) and ET-R(B) receptors in heart, brain, and kidney tissues from Wistar-Kyoto (normotensive), spontaneously hypertensive, salt-hypertensive sensitive, and salt-hypertensive resistant rats by Northern analysis. ET-R(A) and ET-R(B) mRNA were present in the hearts of normal rats. Spontaneously hypertensive rat hearts did not express either ET-R(A) or ET-R(B) mRNA, whereas both salt-hypertensive sensitive and resistant rats fed a high-salt diet expressed both ET-R(A) and ET-R(B) receptor mRNAs. Conversely, in the brain of spontaneously hypertensive rats, mRNAs for both ET-R(A) and ET-R(B) mRNA were present.(ABSTRACT TRUNCATED AT 250 WORDS)

Fibroblast growth factor regulates insulin-like growth factor-binding protein production by vascular smooth muscle cells

Insulin-like growth factor I is an important mitogen for vascular smooth muscle cells, and its effects are regulated by several binding proteins. Western ligand blotting of conditioned medium from rat aortic smooth muscle cells detected a 24 kDa binding protein and a 28 kDa glycosylated variant of this protein, consistent with insulin-like growth factor binding protein-4 by size. Low amounts of a glycosylated 38 to 42 kDa doublet (consistent with binding protein-3) and a 31 kDa non-glycosylated protein also were present. Basic fibroblast growth factor markedly increased secretion of the 24 kDa binding protein and its 28 kDa glycosylated variant. This effect was dose- and time-dependent and was inhibited by co-incubation with cycloheximide. Crosslinking of [125I]-insulin-like growth factor I to cell monolayers revealed no surface-associated binding proteins, either basally or after agonist treatment. Induction of binding protein production by fibroblast growth factor at sites of vascular injury may be important in vascular proliferative responses in vivo.

Sequence of a cDNA encoding dog insulin-like growth factor I

Polymerase chain reaction amplification of a cDNA derived from dog left ventricular myocardium, using primers specific for rat insulin-like growth factor I (IGFI), exons 3 and 6, yielded the dog clone, IGFI5.1. This clone includes the signal peptide sequence, the entire coding sequence for mature dog IGFI and the C-terminal extension sequence. By analogy with the organization of the rat and human IGFI genes which encode two extension peptides, we have termed this cDNA, dog IGFIa. The deduced amino acid sequence of mature dog IGFI is identical to that of human IGFI.

Angiotensin II regulates insulin-like growth factor I gene expression in vascular smooth muscle cells

The potent vasoconstrictor peptide angiotensin II (ang II) has been shown to promote growth of vascular smooth muscle cells (VSMC) in vitro and in vivo. We have previously demonstrated that VSMC synthesize insulin-like growth factor I (IGF I), an important autocrine/paracrine growth factor. Exposure of quiescent VSMC to ang II caused a marked increase in IGF I mRNA levels, peaking at 6 h (199 +/- 26% above control) and sustained for at least 24 h. This increase was completely inhibited by actinomycin D. Nuclear run-on assays indicated that ang II stimulated IGF I gene transcription 3.6-fold. Protein synthesis inhibition with cycloheximide increased basal IGF I mRNA levels but blocked ang II-induced IGF I expression. Immunoreactive IGF I levels in VSMC-conditioned medium were increased by 2.7-fold 24 h following ang II exposure. Measurements of [3H]thymidine incorporation showed that ang II caused a 117% increase in DNA synthesis at 24 h that was almost completely inhibited in the presence of an anti-IGF I antibody. Thus, ang II regulates transcription of the IGF I gene in VSMC and IGF I is required for ang II-induced DNA synthesis. These findings suggest a potentially important role for IGF I as a mediator of the vascular growth responses induced by activation of the renin-angiotensin system in vivo.

Induction of cardiac insulin-like growth factor I gene expression in pressure overload hypertrophy

Molecular mechanisms regulating the cardiac hypertrophic response to increased hemodynamic load are understood poorly. Insulin-like growth factor I (IGF I) is a mitogen that is thought to play a key role in pre- and postnatal growth. To investigate a possible role of IGF I in the cardiac response to pressure overload, rats underwent banding of the ascending aorta immediately above the aortic valve using a hemoclip, or a sham procedure. An analysis of left-ventricular RNA by Northern hybridization using a 32P-labeled IGF I cDNA revealed four messenger ribonucleic acid transcripts of 7.6, 4.6, 1.7, and 0.9 to 1.2 Kb. Insulin-like growth factor I messenger ribonucleic acid was quantitated by ribonuclease protection assays using a rat exon 3 riboprobe. There was a sustained increase in IGF I mRNA levels that correlated temporally with the development of left ventricular hypertrophy. These results indicate that left ventricular pressure overload is associated with an induction of cardiac IGF I gene expression. Insulin-like growth factor I may play a role in the response to increases in wall stress and likely contribute to cardiac hypertrophy.

Regulation of insulin-like growth factor I receptors on vascular smooth muscle cells by growth factors and phorbol esters

Insulin-like growth factor I (IGF I) is an important mitogen for vascular smooth muscle cells. To characterize regulation of vascular IGF I receptors, we performed radioligand displacement experiments using rat aortic smooth muscle cells (RASMs). Serum deprivation for 48 hours caused a 40% decrease in IGF I receptor number. Exposure of quiescent RASMs to platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), or angiotensin II (Ang II) caused a 1.5-2.0-fold increase in IGF I receptors per cell. After FGF exposure, there was a marked increase in the mitogenic response to IGF I. IGF I downregulated its receptors in the presence of platelet-poor plasma. Stimulation of protein kinase C (PKC) by exposure of quiescent RASMs to phorbol 12-myristate 13-acetate caused a biphasic response in IGF I binding; there was a 42% decrease in receptor number at 45 minutes and a 238% increase at 24 hours. To determine the role of PKC in growth factor-induced regulation of IGF I receptors, we downregulated PKC by exposing RASMs to phorbol 12,13-dibutyrate (PDBu) for 48 hours. PDGF- and FGF- but not Ang II-mediated upregulation of IGF I receptors was completely inhibited in PDBu-treated cells. Thus, acute PKC activation by phorbol esters inhibits IGF I binding, whereas chronic PKC activation increases IGF I binding. PDGF and FGF but not Ang II regulate vascular IGF I receptors through a PKC-dependent pathway. These data provide new insights into the regulation of vascular smooth muscle cell IGF I receptors in vitro and are of potential importance in characterizing vascular proliferative responses in vivo.