Somatostatin receptors and the potential use of Sandostatin to interfere with vascular remodelling

Deletion of the FHL2 gene attenuates intima-media thickening in a partially ligated carotid artery ligated mouse model

The four and a half LIM domain protein 2 (FHL2) is a member of the four and a half LIM domain (FHL) gene family, and it is associated with cholesterol‐enriched diet‐promoted atherosclerosis. However, the effect of FHL2 protein on vascular remodelling in response to hemodynamic alterations remains unclear. Here, we investigated the role of FHL2 in a model of restricted blood flow‐induced atherosclerosis. To promote neointimal hyperplasia in vivo, we subjected FHL2^+/+ and FHL2^−/− mice to partial ligation of the left carotid artery (LCA). The expression of p‐ERK and p‐AKT was decreased in FHL2^−/− mice. FHL2 bound to AKT regulated AKT phosphorylation and led to Rac1‐GTP inactivation. FHL2 silencing in human aortic smooth muscle cells down‐regulated the PDGF‐induced phosphorylation of ERK and AKT. Furthermore, FHL2 silencing reduced cytoskeleton conformational changes and caused cell cycle arrest. We concluded that FHL2 is essential for the regulation of arterial smooth muscle cell function. FHL2 modulates proliferation and migration via mitogen‐activated protein kinase (MAPK) and PI3K‐AKT signalling, leading to arterial wall thickening and thus neointimal hyperplasia.

Cortistatin inhibits migration and proliferation of human vascular smooth muscle cells and decreases neointimal formation on carotid artery ligation

RATIONALE

Proliferation and migration of smooth muscle cells (SMCs) are key steps for the progression of atherosclerosis and restenosis. Cortistatin is a multifunctional neuropeptide belonging to the somatostatin family that exerts unique functions in the nervous and immune systems. Cortistatin is elevated in plasma of patients experiencing coronary heart disease and attenuates vascular calcification.

OBJECTIVE

To investigate the occurrence of vascular cortistatin and its effects on the proliferation and migration of SMCs in vitro and in vivo and to delimitate the receptors and signal transduction pathways governing its actions.

METHODS AND RESULTS

SMCs from mouse carotid and human aortic arteries and from human atherosclerotic plaques highly expressed cortistatin. Cortistatin expression positively correlated with the progression of arterial intima hyperplasia. Cortistatin inhibited platelet-derived growth factor-stimulated proliferation of human aortic SMCs via binding to somatostatin receptors (sst2 and sst5) and ghrelin receptor, induction of cAMP and p38-mitogen-activated protein kinase, and inhibition of Akt activity. Moreover, cortistatin impaired lamellipodia formation and migration of human aortic SMCs toward platelet-derived growth factor by inhibiting, in a ghrelin-receptor-dependent manner, Rac1 activation and cytosolic calcium increases. These effects on SMC proliferation and migration correlated with an inhibitory action of cortistatin on the neointimal formation in 2 models of carotid arterial ligation. Endogenous cortistatin seems to play a critical role in regulating SMC function because cortistatin-deficient mice developed higher neointimal hyperplasic lesions than wild-type mice.

CONCLUSIONS

Cortistatin emerges as a natural endogenous regulator of SMCs under pathological conditions and an attractive candidate for the pharmacological management of vascular diseases that course with neointimal lesion formation.

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.

Octreotide inhibits growth factor-induced and basal proliferation of lens epithelial cells in vitro

PURPOSE

To evaluate whether a synthetic analog of somatostatin, octreotide, has an inhibitory effect on lens epithelial cell (LEC) proliferation induced by basal and basic fibroblast growth factor as well as insulin-like growth factor 1.

METHODS

Confluent LEC cultures were kept in serum-free defined medium containing [(3)H]-thymidine in the presence of octreotide alone at the concentration range of 10(-7) M to 10(-10) M or in combination with either basic fibroblast growth factor or insulin-like growth factor 1. Additionally, the expression of somatostatin receptors (1-5) in LECs were analyzed by reverse transcription-polymerase chain reaction.

RESULTS

Octreotide decreased the proliferation of human LECs in a dose-dependent manner, exhibiting a maximal inhibitory concentration at 10(-9) M (P<.03). Moreover, octreotide (10(-9) M) potently inhibited basic fibroblast growth factor- and insulin-like growth factor 1-induced bovine lens epithelial cell proliferation (P<.0001). The respective products of all 5 subtypes of somatostatin receptors were found in human LECs and somatostatin receptor type 2 in bovine LECs.

CONCLUSION

The data show that, depending on the concentration, octreotide is able to decrease proliferative responses of LECs. Moreover, the cell proliferation induced by growth factors was potently inhibited by octreotide. Therefore, octreotide could be a potential drug after cataract surgery in prevention of growth factor-dependent proliferative disorders such as posterior capsule opacification and anterior capsule contraction in diabetic patients.

Somatostatin analogues: multiple roles in cellular proliferation, neoplasia, and angiogenesis

Angiogenesis, the development of new blood vessels is a crucial process both for tumor growth and metastatic dissemination. Additionally, dysregulation in angiogenesis has been implicated in the pathogenesis of cardiovascular disease, proliferative retinopathy, diabetic nephropathy, and rheumatoid arthritis (RA). The neuropeptide somatostatin has been shown to be a powerful inhibitor of neovascularization in several experimental models. Furthermore, somatostatin receptors (sst) are expressed on endothelial cells; particularly, sst2 has been found to be uniquely up-regulated during the angiogenic switch, from quiescent to proliferative endothelium. The present manuscript reviews the anti-angiogenic activity of somatostatin and its analogues in neoplastic and nonneoplastic disease. The role of sst subtypes particularly sst2 in mediating its angioinhibitory activity is described. Somatostatin agonists may also exert their anti-angiogenic activity indirectly by inhibition of growth factors like vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis or through its immunomodulatory effects. However, the therapeutic utility of somatostatin agonists as anti-angiogenic drugs in these diseases remains confusing because of conflicting results from different studies. More basic research, as well as patient-oriented studies, is required to firmly establish the clinical potential of somatostatin agonists in therapeutic angiogenesis. The currently available somatostatin agonists have high affinity of sst2 with lower affinities for sst3 and sst5. The emergence of novel somatostatin agonists especially bispecific analogues (agonists targeting multiple cellular receptors) and conjugates (synthesized by chemically linking somatostatin analogues with other antineoplastic agents) with improved receptor specificity signify a new generation of anti-angiogenics, which may represent novel strategies in the treatment of neovascularization-related diseases.

Somatostatin, acting at receptor subtype 1, inhibits Rho activity, the assembly of actin stress fibers, and cell migration

Somatostatin regulates multiple biological functions by acting through a family of five G protein-coupled receptors, somatostatin receptors (SSTRs) 1-5. Although all five receptor subtypes inhibit adenylate cyclase activity and decrease intracellular cAMP levels, specific receptor subtypes also couple to additional signaling pathways. In CCL39 fibroblasts expressing either human SSTR1 or SSTR2, we demonstrate that activation of SSTR1 (but not SSTR2) attenuated both thrombin- and integrin-stimulated Rho-GTP complex formation. The reduction in Rho-GTP formation in the presence of somatostatin was associated with decreased translocation of Rho and LIM kinase to the plasma membrane and fewer focal contacts. Activation of Rho resulted in the formation of intracellular actin stress fibers and cell migration. In CCL39-R1 cells, somatostatin treatment prevented actin stress fiber assembly and attenuated thrombin-stimulated cell migration through Transwell membranes to basal levels. To show that native SSTR1 shares the ability to inhibit Rho activation, we demonstrated that somatostatin treatment of human umbilical vein endothelial cells attenuated thrombin-stimulated Rho-GTP accumulation. These data show for the first time that a G protein-coupled receptor, SSTR1, inhibits the activation of Rho, the assembly of focal adhesions and actin stress fibers, and cell migration.