Rat heart smooth muscle cells express high and low affinity receptors for somatostatin-14, which are involved in regulation of cell proliferation

Effect of somatostatin on resistance and on capacitance rabbit isolated arteries

The effects of somatostatin, a tetradecapeptide isolated from hypothalamus extracts, were studied on the vascular reactivity of aorta and mesenteric arteries isolated from rabbits. We also investigated whether or not Ca(2+) movements were implicated in these effects. Rabbit aorta and mesenteric (fifth branch) arteries were isolated, cleaned off, and mounted in an organ bath containing Godfraind solution or physiological saline solution (PSS), respectively. Somatostatin (10(-8)-10(-4) M) produced a concentration-dependent inhibition of the contractile responses induced by high K(+) (80 mM) or noradrenaline (10(-6) M in aorta or 10(-4) M in mesenteric arteries) in both arteries studied. The inhibitory effect of somatostatin was greater in mesenteric resistance vessels (IC(50) 3.1+/-2.3x10(-5) M, and 5.2+/-4.8x10(-8) M with KCl and noradrenaline, respectively). Contractile responses produced by the addition of Ca(2+) (1-5 mM) to Ca(2+)-free high K(+) solution were also concentration dependently inhibited by somatostatin in aorta. Furthermore, somatostatin decreased noradrenaline-induced contraction attributed to intracellular Ca(2+) release in aorta, and inhibited 45Ca(2+) uptake stimulated by high K(+) or by noradrenaline. However, it did not modify 45Ca(2+) uptake in resting mesenteric resistance arteries. Taken together, these results suggest that somatostatin exerts an inhibitory effect on vascular contractions induced by some stimulating agents in different arteries isolated from rabbits, being more potent in mesenteric arteries.

Rat somatostatin sst2(a) and sst2(b) receptor isoforms mediate opposite effects on cell proliferation

We have investigated the actions of somatostatin (SRIF) and angiopeptin on cell proliferation of CHO-K1 cells expressing the recently cloned rat sst2(b) receptor (CHOsst2(b)) and compared these to their effects in cells expressing the sst2(a) receptor (CHOsst2(a)). In contrast to the sst2(a) receptor, the sst2(b) receptor did not mediate inhibition of bFGF (10 ng ml(-1))-stimulated re-growth and cell proliferation. Rather, SRIF (0.1-1000 nM) and angiopeptin (0.1-1000 nM) stimulated basal re-growth and proliferation of CHOsst2(b) cells in a concentration-dependent manner (estimated pEC50 values of 7.8 and 7.9, respectively). The opposite effects of SRIF on cell proliferation mediated through the two sst2 receptor isoforms were both abolished by 18 h pre-treatment with pertussis toxin. The proliferative effect via the sst2(b) receptor was also abolished by the tyrosine kinase inhibitor, genistein. In conclusion, the present study shows that the rat sst2(a) and sst2(b) receptor splice variants mediate opposite effects on cell proliferation.

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.

Somatostatin sst5 inhibition of receptor mediated regeneration of rat aortic vascular smooth muscle cells

1. The aim of the present study was to determine the effect of somatostatin (SRIF) on mitogen-induced regeneration of rat aortic vascular smooth muscle cells (VSMC) and for comparison Chinese hamster ovary (CHO)-K1 cells expressing human recombinant sst5 receptors (CHOsst5), following partial denudation of a confluent cell monolayer. Regeneration was assessed by measuring areas of recovery into the denuded area and by counting total cell numbers. 2. In VSMC, SRIF (0.1 nM - 1 microM) had no effect on the basal levels of regeneration but caused a concentration-dependent inhibition (pIC50 8.0-8.6) of the stimulated regeneration induced by submaximal concentrations of basic fibroblast growth factor (bFGF, 10 ng ml[-1]), platelet-derived growth factor-BB (PDGF, 5 ng ml[-1]) or endothelin-1 (ET-1, 100 nM). SRIF (pIC50 8.8) also inhibited bFGF-induced regeneration of CHOsst5 cells. 3. In VSMC, the inhibitory action of SRIF on the regeneration induced by bFGF (10 ng ml[-1]) was due to an anti-proliferative effect, rather than an effect on cell migration, as SRIF (0.1 nM - 1 microM) abolished bFGF-induced increases in total cell numbers. The bFGF-induced increase in cell numbers was also abolished by actinomycin D (0.1 microg ml[-1]). 4. The sst5 receptor-selective agonist, L-362,855 (pIC50 10.5), was about 100 times more potent than SRIF at inhibiting bFGF-induced regeneration of both VSMC and CHOsst5 cells whilst the sst2 receptor-selective agonist, BIM-23027 (pIC50 6.8), was approximately 20 times weaker than SRIF. 5. The sst5 receptor antagonist, BIM-23056 (100 nM), antagonized SRIF-induced inhibition of bFGF-induced regeneration in both VSMC and CHOsst5 cells (estimated pKB values 8.8 and 8.3, respectively). 6. SRIF-induced inhibition of bFGF-induced regeneration of VSMC and CHOsst5 cells was abolished by pretreating cells with pertussis toxin (100 ng ml[-1]) for 20 h. 7. These findings suggest that SRIF-induced inhibition of the proliferation of rat aortic VSMC is mediated via activation of receptors which are similar to human sst5 receptors. Furthermore this inhibitory effect is transduced via pertussis toxin-sensitive Gi/Go proteins.

Angiopeptin, a somatostatin-14 analogue, decreases adhesiveness of rat leukocytes to unstimulated and IL-1 beta-activated rat heart endothelial cells

We have previously demonstrated that somatostatin-14 and its octapeptide analogue, angiopeptin, decrease the ability of rat heart endothelial cells to bind leukocytes [Leszczynski, et al., Reg. Pept. 43 (1993) 131-140]. Here, we examined whether exposure of leukocytes to angiopeptin modifies their adhesiveness to the unstimulated and to IL-1 beta-activated endothelium. Monolayers of unstimulated endothelial cells bind 274 +/- 12 leukocytes/mm2. Exposure of leukocytes for 1, 4 and 24 hours to angiopeptin (1 microM) reduced significantly (p < 0.05) adhesion of leukocytes from 274 +/- 12 to 188 +/- 10, 185 +/- 8 and 172 +/- 3 cells/mm2, respectively. Stimulation of endothelial cells with Il-1 beta (100 U/ml) for 24 hours increased endothelial adhesiveness from 274 +/- 12 to 381 +/- 17 adhering leukocytes/mm2. Exposure of leukocytes for 1, 4 and 24 hours to angiopeptin (1 microM) reduced significantly (p < 0.05) binding of leukocytes to IL-1 beta-activated endothelium from 381 +/- 17 to 237 +/- 8, 254 +/- 11 and 248 +/- 13 cells/mm2, respectively. Angiopeptin had no effect on the expression of lymphocyte function-associated molecule-1 (LFA-1; CD11a/CD18) by leukocytes, as assessed by flow cytometry. This suggests that angiopeptin modulates adhesive properties of leukocytes by (1) altering the expression of other than LFA-1 adhesion molecule(s) and/or (2) modulating the affinity of adhesion molecule(s) expressed by leukocytes. In conclusion, our results demonstrate that angiopeptin reduces leukocyte adhesiveness to unstimulated and to IL-1 beta-activated endothelium. It suggests that angiopeptin may suppress immune response via modulation of the leukocyte-endothelial interaction.

Estradiol inhibition of arterial neointimal hyperplasia after balloon injury

PURPOSE

Estrogens have been shown to protect against cardiovascular disease in postmenopausal women. The mechanisms are unknown. In this study we investigated the effect of estrogen treatment on arterial neointimal proliferation elicited by balloon injury of blood vessels of the rabbit.

METHODS

The aorta and the common and external iliac arteries of the rabbit underwent balloon injury. 17-beta-Estradiol cypionate (100 micrograms/kg/day intramuscularly) was administered beginning 1 day before injury and until sacrifice at 22 to 24 days after injury. Angiopeptin (20 micrograms/kg/day) was administered for the same length of time as estrogen to a group of rabbits to use this peptide as a positive control for morphometric analysis. Angiopeptin was also combined with estradiol to determine whether it was possible to further enhance the effect of estrogen. Morphometric studies were performed to determine measurement of intimal thickening. Inhibition of cell proliferation by estrogen was evaluated by incorporation of tritiated thymidine in vitro into the balloon injured rabbit aorta 72 hours after balloon injury.

RESULTS

Treatment of male rabbits with estradiol significantly (p < 0.01) decreased the neointimal thickening of these vessels by 50% to 70%. The somatostatin analog, angiopeptin, was similarly effective in the same circumstances. Estradiol failed to further inhibit neointimal thickening when combined with angiopeptin. Treatment with estradiol for 3 days inhibited both thymidine incorporation (p < 0.01) and DNA content in injured vessels.

CONCLUSION

Estradiol treatment of rabbits undergoing balloon injury of the aorta and iliac arteries, significantly inhibits the myointimal thickening. This effect of estrogen is mediated by inhibition of vascular smooth muscle cell proliferation.