Somatostatin receptor expression in rat iliac arteries after balloon injury

Somatostatin receptors in fibrotic myocardium

A patient with a neuroendocrine tumor and history of coronary artery disease underwent PET with 68Ga-DOTATATE PET tracer for tumor visualization. Analysis of the scan showed uptake of 68Ga-DOTATATE in the left ventricle corresponding to previous myocardial infarct. 68Ga-DOTATATE binds by somatostatin receptors (SSTR) and it has been proposed that it may be useful for the detection of cardiac inflammatory lesions. We aimed to test whether SSTR could be upregulated in cardiac fibrotic scar. We analyzed SSTR in cardiac samples from patients with end-stage ischemic cardiomyopathy (ICM, n = 8) and control hearts (n = 5). In mature ICM tissue, SSTR1 and SSTR2 expression was unchanged and SSTR5 expression was significantly decreased in ICM samples vs. control. Immunohistochemistry showed increased SSTR1 and SSTR2 in ICM. Areas with SSTR1 or SSTR2 staining were often adjacent to fibrotic areas. The majority of SSTR1 and SSTR2 staining localized in cardiomyocytes in fibrotic scar-rich areas where CD68 macrophage staining was not present. SSTR are occasionally upregulated in cardiac fibrotic areas. When using 68Ga-DOTATATE PET tracer to detect cardiac sarcoidosis or atherosclerotic plaque, the possibility of tracer uptake in fibrotic areas should be considered.

Cushing's disease and hypertension: in vivo and in vitro study of the role of the renin-angiotensin-aldosterone system and effects of medical therapy

OBJECTIVE/METHODS

Cushing's disease (CD) is often accompanied by hypertension. CD can be treated surgically and, given the expression of somatostatin subtype 5 and dopamine 2 receptors by corticotroph pituitary adenomas, pharmacologically. Indeed, we recently observed that stepwise medical combination therapy with the somatostatin-analog pasireotide, the dopamine-agonist cabergoline, and ketoconazole (which directly suppresses steroidogenesis) biochemically controlled CD patients and lowered their blood pressure after 80 days. Glucocorticoids (GC) modulate the renin-angiotensin-aldosterone system (RAAS) among others by increasing hepatic angiotensinogen expression and stimulating mineralocorticoid receptors (MR). This study therefore evaluated plasma RAAS components in CD patients before and after drug therapy. In addition, we studied whether cabergoline/pasireotide have direct relaxant effects in angiotensin II (Ang II)-constricted iliac arteries of spontaneously hypertensive rats, with and without concomitant GR/MR stimulation with dexamethasone or hydrocortisone.

RESULTS

Baseline concentrations of angiotensinogen were elevated, while renin and aldosterone were low and suppressed, respectively, even in patients treated with RAAS-blockers. This pattern did not change after 80 days of treatment, despite blood pressure normalization, nor after 4 years of remission. In the presence of dexamethasone, pasireotide inhibited Ang II-mediated vasoconstriction.

CONCLUSIONS

The low plasma renin concentrations, even under RAAS blockade, in CD may be the consequence of increased GC-mediated MR stimulation and/or the elevated angiotensinogen levels in such patients. The lack of change in RAAS-parameters despite blood pressure and cortisol normalization suggests persisting consequences of long-term exposure to cortisol excess. Finally, pasireotide may have a direct vasodilating effect contributing to blood pressure lowering.

Intimal Hyperplasia in Rats after Subcutaneous Injection of a Somatostatin Analog

The somatostatin analog octreotide was administered to male and female Sprague-Dawley rats by subcutaneous injection for thirteen weeks at 0 (saline control), 0 (placebo control [mannitol and lactic acid; pH 4.2]), 1.25 mg/kg/day and 2.5 mg/kg/day to explore its potential effect on cutaneous vascular morphology. The placebo caused an increase in the incidence of intimal hyperplasia compared to saline controls in female rats; octreotide increased the incidence and severity of intimal hyperplasia in males and females. Intimal hyperplasia consisted of increased numbers of cells located between the endothelial cell layer and the internal elastic lamina. Severity was based on the degree of compromise of the vascular lumen (regardless of vessel size and number), with severely affected vessels having no visible lumen. Intimal hyperplasia in rats treated with octreotide was considered to be an unexpected and adverse finding, given that this compound and other somatostatin analogs have been investigated as reducers of intimal proliferation or restenosis after angioplasty in humans and that no such lesion has been reported in the literature for this class of compound to date. The induction of intimal hyperplasia by the placebo is also a notable finding; this may be because of the low pH of the formulation.

A selective somatostatin type-2 receptor agonist inhibits neointimal thickening and enhances endothelial cell growth and morphology following aortic balloon injury in the rabbit

Somatostatin analogs have been shown to inhibit vascular smooth muscle cell (VSMC) proliferation and attenuate neointimal thickening following experimental balloon catheter injury. In this study, the effects of a selective agonist for the somatostatin receptor subtype 2, PRL-2486, on neointimal thickening and endothelial cell regrowth 2 weeks following balloon catheterization of male New Zealand White rabbits were determined. Rabbits treated 2 days prior to and 2 weeks after catheter injury with 10 microg/kg/day PRL-2486 (PRL-tx) had decreased I/M ratios (intimal area/medial area x 100; p < 0.05) but had no effect at lower (5 microg/kg/day) or higher (20 microg/kg/day) doses. PRL-tx had significantly decreased VSMC proliferation compared to untreated animals. PRL-tx increased endothelial regrowth by over 2-fold (p < 0.002) and improved endothelial cell morphology. Endothelial-dependent relaxation responses to acetylcholine were attenuated by catheter injury, and were not improved with PRL-tx. These data suggest that the PRL-2486-mediated inhibition of neointimal thickening exhibits a bell-shaped dose-response curve. This inhibition may be due in part to decreased VSMC proliferation, which may be a function of enhanced endothelial regrowth, but not the return of endothelium-dependent vascular function.

Endovascular photodynamic therapy with aminolaevulinic acid prevents balloon induced intimal hyperplasia and constrictive remodelling

BACKGROUND AND OBJECTIVE

intimal hyperplasia (IH) and constrictive remodelling are important causes of restenosis following endovascular interventions, such as percutaneous transluminal angioplasty. Photodynamic therapy (PDT) with 5-aminolaevulinic (ALA) may prevent restenosis by cellular depletion and the elimination of cholinergic innervation.

STUDY DESIGN/MATERIALS AND METHODS

rats (n=90) were subdivided into 4 main groups. In the experimental group (n=36: 3 replications x 4 doses x 3 examination time-points), ALA was administered (200mg/kg i.v.) 2-3h before balloon injury (BI) of the common iliac artery followed by endovascular illumination with 633nm at either 12.5, 25, 50 or 100J/cm diffuser length (dl BI+PDT group). As control groups served the BI+Light only (LO) group (n=36) that received no ALA, the BI only group (n=9) (BI), and a group (n=9) that received a Sham procedure (Sham group).

RESULTS

planimetric analysis showed IH of 0.28+/-0.12mm(2) (BI), 0.27+/-0.12mm(2) (BI+LO at 100J/cmdl) in contrast to 0.02+/-0.02mm(2) after BI+PDT at 100J/cmdl at 16 weeks (p<0.05). In the BI+PDT groups, a light-dose increase of a factor 2 led to an IH decrease of 17% (p<0.05). In the BI and BI+LO groups constrictive remodelling was found, in contrast to BI+PDT treated groups at 16 weeks. The staining of cholinergic innervation of the tunic media of the blood vessel wall in BI+PDT showed no damage at the highest fluence.

CONCLUSION

endovascular ALA-PDT prevents IH and constrictive remodelling after BI without damage of cholinergic innervation of the tunica media. The effective light fluence rate in the rat is 50-100J/cmdl.

Somatostatin receptor subtype expression and function in human vascular tissue

In animal models the somatostatin analog angiopeptin inhibits intimal hyperplasia by acting primarily through somatostatin receptor 2 (SSTR-2). However, the results of clinical trials using angiopeptin have been disappointing. In this study we showed that human blood vessels express high levels of SSTR-1 with significantly lower levels of SSTR-2 and -4. Samples of normal veins and arteries, as well as atherosclerotic arteries, expressed predominantly SSTR-1. In addition, the levels of SSTR-1 varied between individuals, indicating that the vascular disease process may have affected SSTR gene expression. Immunocytochemical studies demonstrated that SSTR-1 was present in endothelial but not vascular smooth muscle cells. No evidence of SSTR-3 or -5 expression was detected in normal or diseased blood vessels. Two endothelial cell preparations, ECV304 and human umbilical vein endothelial cells, were investigated and shown to express only SSTR-1 and -4. Exposure of these cells to 10 nM somatostatin or 10 nM SSTR-1-specific agonist resulted in alterations to the actin cytoskeleton, as characterized by a loss of actin stress fibers coupled with an increase in lamellipodia formation at the plasma membrane. These results suggest that the lack of effectiveness of angiopeptin in humans may be due to the differential expression of SSTR-1 by human endothelial cells.

Signal transduction of somatostatin receptors negatively controlling cell proliferation

Somatostatin acts as an inhibitory peptide of various secretory and proliferative responses. Its effects are mediated by a family of G-protein-coupled receptors (sst1-5) that can couple to diverse signal transduction pathways such as inhibition of adenylate cyclase and guanylate cyclase, modulation of ionic conductance channels, and protein dephosphorylation. The five receptors bind the natural peptide with high affinity but only sst2, sst5 and sst3 bind the short synthetic analogues. Somatostatin negatively regulates the growth of various normal and tumour cells. This effect is mediated indirectly through inhibition of secretion of growth-promoting factors, angiogenesis and modulation of the immune system. Somatostatin can also act directly through sst receptors present on target cells. The five receptors are expressed in various normal and tumour cells, the expression of each receptor being receptor subtype and cell type specific. According to the receptor subtypes, distinct signal transduction pathways are involved in the antiproliferative action of somatostatin. Sst1, 4 and 5 modulate the MAP kinase pathway and induce G1 cell cycle arrest. Sst3 and sst2 promote apoptosis by p53-dependent and -independent mechanisms, respectively.

Effect of endothelial and adventitial injury on somatostatin receptor expression

BACKGROUND

The somatostatin analog, angiopeptin, inhibits intimal hyperplasia formation; although the specific somatostatin receptor (SSTR) subtypes transducing this effect are unknown. The purpose of this study was to determine the expression of SSTR subtypes in rat iliac arteries after balloon catheter endothelial injury and perivascular dissection.

METHODS

Male rats received balloon endothelial injury to their left common and external iliac arteries with or without circumferential arterial dissection. The right arteries served as controls. At 1 and 2 months after intimal injury, animals were killed and their iliac arteries harvested and studied for SSTR expression by using immunocytochemical and molecular techniques. Quantitative polymerase chain reaction was used to determine the level of SSTR expression.

RESULTS

Normal rat iliac arteries expressed only SSTR2 and 3. After balloon endothelial injury, there was significant upregulation of SSTR2 messenger RNA at 1 and 2 months after injury as compared with controls (1 month, 1.8 +/- 0.3 vs 0.4 +/- 0.1 zmol, P < .001; 2 months, 2.7 +/- 0.5 vs 1.1 +/- 0.2 zmol, P < .001). The addition of adventitial dissection to endothelial injury also showed a significant increase in SSTR2 expression (1 month, 2.4 +/- 0.4 vs 0.8 +/- 0.2, P < .05; 2 months, 1.3 +/- 0.3 vs 0.7 +/- 0.3, P < .05), but not significantly greater than that seen after balloon endothelial injury alone. Immunocyto-chemical studies also demonstrated an increase in SSTR2 immunoreactivity on the luminal surface of the endothelial cells in the balloon catheter-injured arteries.

CONCLUSIONS

These findings show that SSTR2 is the primary SSTR that is upregulated after injury and likely mediates the effects of somatostatin analogs on intimal hyperplasia.

Octreotide (somatostatin analog) treatment reduces endothelial cell dysfunction in patients with diabetes mellitus

Octreotide is a long-acting somatostatin analog that has been shown to have various effects in diabetes. This study was performed to evaluate whether octreotide affects the vascular complications of diabetes mellitus. Albuminuria and serum thrombomodulin were used as markers of vascular and renal dysfunction. We studied the effect of octreotide in 27 patients with insulin-dependent diabetes mellitus (IDDM). They received 200 microg octreotide per day over a period of 6 months. As a marker of endothelial cell damage, we measured the serum thrombomodulin level. We also measured urinary albumin excretion, hemoglobin A1c (HbA1c), insulin-like growth factor-1 (IGF-1), and other parameters. IGF-1 decreased from 123 ng/mL before treatment to 114 ng/mL after 6 months of octreotide treatment (P = .009), while no significant change was observed in the unblinded control group (from 103 ng/mL to 102 ng/mL after 6 months of treatment). Urinary albumin excretion in patients with macroalbuminuria declined from 1,124 mg/L before octreotide treatment to 556 mg/L after 6 months of treatment (P < .05), whereas no change was observed in the control group. There was also a reduction of the plasma thrombomodulin level from 61.8 ng/mL to 46.1 ng/mL (P < .07) after 6 months of treatment. Furthermore, HbA1c decreased from 8.75% +/- 1.27% to 8.12% +/- 1.23% (P < .07) after octreotide treatment.

Differential regulation of somatostatin receptor types 1-5 in rat aorta after angioplasty

Treatment of restenosis after angioplasty with octapeptide somatostatin (SST) analogs has met with variable success. These analogs bind with high affinity to only two SST receptor (SSTR) subtypes (2 and 5), display moderate affinity for SSTR3, and low affinity for SSTR1 and 4. To optimize the vasculoprotective effect of SST, we have investigated the pattern of expression of all five SSTRs in rat thoracic aorta in the resting state and at 15 min, 3, 7, and 14 days after balloon endothelial denudation. SSTR1-5 were analyzed as mRNA by semiquantitative reverse transcriptase-polymerase chain reaction and as protein by immunocytochemistry. All five SSTRs were expressed in rat aorta both as mRNA and protein and displayed a time-dependent, subtype-selective response to endothelial denudation. mRNA for SSTR1 and 2 increased acutely (SSTR1 > SSTR2) on days 3 and 7, coincident with smooth muscle cell (SMC) proliferation, and declined to basal levels by day 14. SSTR3 and 4 displayed a different pattern with a delayed, more gradual increase in mRNA beginning at days 3-7 and continued to increase thereafter. SSTR5 mRNA was constitutively expressed at a low level and showed no change during the 2 wk postinjury period. By immunohistochemistry, SSTR1-5 antigens were localized predominantly in SMC that were present in the media or had migrated into the intima; antigen expression correlated with receptor mRNA expression. Notably, only SSTR1,3,4 were expressed in the intima: SSTR1 and 4 during the proliferative burst and SSTR3 and 4 after proliferation, when SMC migration into the intima continues. These results demonstrate dynamic changes in SSTR1-5 expression after vascular trauma localized to areas of vascular SMC migration and replication. In view of their early and prominent induction, SSTR1 may be the optimal subtype to target for inhibition of myointimal proliferation, and SSTR3 and 4 for migration and remodeling.

Differential effects of somatostatin and angiopeptin on cell proliferation

1. Somatostatin (SRIF) exerts antiproliferative effects, and angiopeptin (an sst2/sst5 receptor-selective analogue) has recently been evaluated in clinical trials for the prophylaxis of restenosis following coronary angioplasty. Using an in vitro model of cell growth we have examined the effects of SRIF and angiopeptin on cell proliferation in CHO-K1 cells stably transfected with the human or rat recombinant sst2 or sst5 receptor and compared these with their effects on rat aortic vascular smooth muscle cells (VSMC) expressing endogenous somatostatin receptors. 2. In CHO-KI cells, expressing either human or rat recombinant sst2 or sst5 receptors, or in rat aortic VSMC, SRIF and angiopeptin (0.1-1000 nM) had no effect on basal re-growth of cells into a denuded area of a previously confluent monolayer. In contrast, basic fibroblast growth factor (bFGF, 10 ng ml(-1)) stimulated re-growth of these cells. 3. SRIF (0.1-1000 nM) caused a concentration-dependent inhibition of the bFGF-stimulated re-growth in CHO-K1 cells expressing human sst2 (h sst2) or sst5 (h sst5) receptors (pIC50=8.05+/-0.03 and 8.56+/-0.12, respectively). In contrast, angiopeptin (0.1-1000 nM) acted as a partial agonist at the h sst2 receptor (44.6+/-2.7% inhibition of the bFGF-stimulated re-growth at 100 nM; pIC50=8.69+/-0.25) but was devoid of any agonist activity at the h sst5 receptor. 4. In CHO-K1 cells stably expressing rat recombinant sst2 (r sst2) or sst5 (r sst5) receptors, SRIF (0.1-1000 nM) was able to inhibit the bFGF-stimulated re-growth (pIC50=7.98+/-24 and 8.50+/-0.12, respectively). Angiopeptin (0.1-1000 nM) caused a concentration-dependent inhibition of bFGF-stimulated re-growth at the r sst2 receptor (pIC50=8.08+/-0.24) but acted as a partial agonist at the r sst5 receptor (maximum response= 57.7+/-3.6% inhibition of bFGF-stimulated re-growth at 100 nM; pIC50=8.60+/-0.16). 5. Although angiopeptin was inactive as an agonist at the h sst5 receptor, 100 nM angiopeptin potently antagonized the SRIF-induced inhibition of proliferation in CHO h sst5 (estimated pKB= 10.4+/-0.3). 5-Hydroxytryptamine (0.1 nM-10 microM) also inhibited bFGF-stimulated re-growth (pIC50=8.36+/-0.11) and angiopeptin had no effect on this response (pKB<7). 6. SRIF (0.1-1000 nM) caused a concentration-dependent (pIC50=8.04+/-0.08) inhibition of bFGF-stimulated re-growth in VSMC, whereas angiopeptin displayed weak agonist activity, only inhibiting bFGF-stimulated re-growth at concentrations greater than 100 nM. Angiopeptin (100 nM) caused a rightward displacement of the concentration-effect curve to SRIF with an estimated pKB value of 7.70+/-0.12. 7. These findings suggest that the low intrinsic activity of angiopeptin at the h sst2 receptor, combined with its lack of agonist activity at the h sst5 receptor, may explain the poor clinical efficacy of angiopeptin in trials for coronary artery restenosis, which contrasts with encouraging data found in equivalent in vivo animal studies.