Perivascular release of insulin-like growth factor-1 limits neointima formation in the balloon-injured artery by redirecting smooth muscle cell migration

Periadventitial biomaterials to improve arteriovenous fistula and graft outcomes

Periadventitial biomaterials have been employed for nearly three decades to promote adaptive venous remodeling following hemodialysis vascular access creation in preclinical models and clinical trials. These systems are predicated on the combination of scaffolds, hydrogels, and/or particles with therapeutics (small molecules, proteins, genes, and cells) to prevent venous stenosis and subsequent maturation failure. Periadventitial biomaterial therapies have evolved from simple drug delivery vehicles for traditional drugs to more thoughtful designs tailored to the pathophysiology of access failure. The emergence of tissue engineering strategies and gene therapies are another exciting new direction. Despite favorable results in experimental and preclinical studies, no periadventitial therapy has been clinically approved to improve vascular access outcomes. After conducting an exhaustive review of the literature, we identify the seminal studies and clinical trials that utilize periadventitial biomaterials and discuss the key features of each biomaterial format and their respective shortcomings as they pertain to access maturation. This review provides a foundation from which clinicians, surgeons, biologists, and engineers can refer to and will hopefully inspire thoughtful, translatable treatments to finally address access failure.

Arterial smooth muscle dynamics in development and repair

Arterial vasculature distributes blood from early embryonic development and provides a nutrient highway to maintain tissue viability. Atherosclerosis, peripheral artery diseases, stroke and aortic aneurysm represent the most frequent causes of death and are all directly related to abnormalities in the function of arteries. Vascular intervention techniques have been established for the treatment of all of these pathologies, yet arterial surgery can itself lead to biological changes in which uncontrolled arterial wall cell proliferation leads to restricted blood flow. In this review we describe the intricate cellular composition of arteries, demonstrating how a variety of distinct cell types in the vascular walls regulate the function of arteries. We provide an overview of the developmental origin of arteries and perivascular cells and focus on cellular dynamics in arterial repair. We summarize the current knowledge of the molecular signaling pathways that regulate vascular smooth muscle differentiation in the embryo and in arterial injury response. Our review aims to highlight the similarities as well as differences between cellular and molecular mechanisms that control arterial development and repair.

Local insulin application on the carotid artery inhibits neointima formation

Anti-mitogenic agents currently used to prevent restenosis in drug-eluting stents delay re-endothelialization. Delayed re-endothelialization is now considered as the main cause of late stent thrombosis with drug-eluting stents, which emphasizes the need for new treatments. We have shown that systemic insulin treatment decreases neointimal growth and accelerates re-endothelialization after arterial injury in a rat model of restenosis. However, systemic insulin treatment cannot be given to non-diabetic individuals because of the risk of hypoglycemia. Thus, we investigated whether local insulin treatment is also effective in reducing neointimal growth after arterial injury. Rats were given local vehicle or local insulin delivered via Pluronic gel applied around the carotid artery immediately following balloon injury. Plasma glucose and systemic insulin levels were not affected by local insulin treatment. Insulin decreased intimal area at 28 days (P < 0.05) and also inhibited vascular smooth muscle cell migration by 60% at 4 days (P < 0.05). NPH (a longer-lasting insulin) also decreased neointimal area. These results indicate that local insulin treatment can lead to decreased restenosis, suggesting a protective vascular effect of insulin in vivo and that local insulin treatment, possibly via insulin-eluting stents, may be clinically relevant.

IGF-I redirects doublecortin-positive cell migration in the normal adult rat brain

The migration of subventricular zone (SVZ)-derived neural precursor cells through the rostral migratory stream (RMS) to the olfactory bulb is tightly regulated by local micro-environmental cues. Insulin-like Growth Factor-I (IGF-I) can stimulate the migration of several neuronal cell types and acts as a 'departure' factor in the avian SVZ. To establish whether IGF-I can also act as a migratory factor for adult neuronal precursor cells in vivo, in addition to its well established role in precursor cell proliferation and differentiation, we used AAV2-mediated gene transfer to produce ectopic expression of IGF-I in the normal adult rat striatum. We then assessed whether the expression of IGF-I would recruit SVZ-derived neuronal precursor cells from the RMS into the striatum. Ectopic expression of IGF-I in the normal adult rat brain significantly increased the number of doublecortin (Dcx)-positive cells and the extent of their migration into the striatum 4 and 8 weeks after AAV2-IGF-I injection but did not promote neuronal differentiation. In vitro migration assays confirmed that IGF-I is an inducer of migration and directs SVZ-derived adult neuronal precursor cell migration by both chemotaxis and chemokinesis. These results demonstrate that overexpression of IGF-I in the normal adult rat brain can override the normal cues directing precursor cell migration along the RMS and can redirect precursor cell migration into a non-neurogenic region. Enhanced expression of IGF-I following brain injury may therefore act as a diffusible factor mediating precursor cell migration to areas of neuronal cell damage.

Constitutive expression of pregnancy-associated plasma protein-A in arterial smooth muscle reduces the vascular response to injury in vivo

Pregnancy-associated plasma protein-A (PAPP-A) functions to increase local IGF-I bioactivity. In this study, we used transgenic mice that constitutively express human PAPP-A in arterial smooth muscle to test the hypothesis that overexpression of PAPP-A enhances vascular smooth muscle cell (SMC) response to IGF-I in vivo. PAPP-A transgenic (Tg) and wild-type (WT) mice underwent unilateral carotid ligation, a model of injury-induced SMC hyperplasia and neointimal formation. In both WT and PAPP-A Tg mice, endogenous PAPP-A mRNA expression showed peak elevation 5 days after carotid ligation. However, PAPP-A Tg mice had 70-75% less neointima than WT at 5 and 10 days postligation, with a significant reduction in occlusion of the ligated artery. WT and PAPP-A Tg mice had equivalent increases in medial area and vessel remodeling postligation. There was little change in medial area and no evidence of neointima in the contralateral carotid of WT or PAPP-A Tg mice. Both WT and PAPP-A Tg carotids exhibited signs of dedifferentiation of SMC, which precedes the increase in proliferation and migration that results in neointimal formation. However, the number of proliferating cells in the media and neointima of the ligated PAPP-A Tg artery was reduced by 90% on day 5 postsurgery compared with WT. This decrease was associated with a significant decrease in an in vivo marker of IGF-I bioactivity and reduced IGF-I-stimulated receptor phosphorylation ex vivo. These data suggest differential effects of chronic (transgenic) and transient (endogenous) PAPP-A expression on neointimal formation following vascular injury that may be due in part to the differential impact on IGF-I signaling.

Smooth muscle cell-specific insulin-like growth factor-1 overexpression in Apoe-/- mice does not alter atherosclerotic plaque burden but increases features of plaque stability

OBJECTIVE

Growth factors may play a permissive role in atherosclerosis initiation and progression, in part via their promotion of vascular smooth muscle cell (VSMC) accumulation in plaques. However, unstable human plaques often have a relative paucity of VSMC, which has been suggested to contribute to plaque rupture and erosion and to clinical events. Insulin-like growth factor-1 (IGF-1) is an endocrine and autocrine/paracrine growth factor that is a mitogen for VSMC, but when infused into Apoe(-/-) mice it paradoxically reduces atherosclerosis burden.

METHODS AND RESULTS

To determine the effect of stimulation of VSMC growth on atherosclerotic plaque development and to understand mechanisms of IGF-1's atheroprotective effect, we assessed atherosclerotic plaques in mice overexpressing IGF-1 in smooth muscle cells (SMC) under the control of the α-smooth muscle actin promoter, after backcrossing to the Apoe(-/-) background (SMP8/Apoe(-/-)). Compared with Apoe(-/-) mice, these SMP8/Apoe(-/-) mice developed a comparable plaque burden after 12 weeks on a Western diet, suggesting that the ability of increased circulating IGF-1 to reduce plaque burden was mediated in large part via non-SMC target cells. However, advanced plaques in SMP8/Apoe(-/-) mice displayed several features of plaque stability, including increased fibrous cap area, α-smooth muscle actin-positive SMC and collagen content, and reduced necrotic cores.

CONCLUSIONS

These findings indicate that stimulation of VSMC IGF-1 signaling does not alter total atherosclerotic plaque burden and may improve atherosclerotic plaque stability.

IGF-1, oxidative stress and atheroprotection

Atherosclerosis is a chronic inflammatory disease in which early endothelial dysfunction and subintimal modified lipoprotein deposition progress to complex, advanced lesions that are predisposed to erosion, rupture and thrombosis. Oxidative stress plays a crucial role not only in initial lesion formation but also in lesion progression and destabilization. Although most growth factors are thought to promote vascular smooth muscle cell proliferation and migration, thereby increasing neointima, recent animal studies indicate that insulin-like growth factor (IGF)-1 exerts both pleiotropic anti-oxidant effects and anti-inflammatory effects, which together reduce atherosclerotic burden. This review discusses the effects of IGF-1 in models of vascular injury and atherosclerosis, emphasizing the relationship between oxidative stress and potential atheroprotective actions of IGF-1.

Stent-based Controlled Release of Intravascular Angiostatin to Limit Plaque Progression and In-Stent Restenosis

PURPOSE

To evaluate the importance of angiogenesis in plaque progression after stent placement, this study examines stent-based controlled delivery of the antiangiogenic agent, angiostatin, in a rabbit model.

MATERIALS AND METHODS

Controlled release biodegradable microspheres delivering angiostatin or polymer-only microspheres (polylactic-co-glycolic-acid-polyethylene glycol; PLGA/PEG) were loaded in channeled stents, anchored, and deployed in the aorta of adult New Zealand white rabbits (n = 6 animals per group, three each per time point). The stented aortas were harvested at 7 days and 28 days and evaluated for neovascularization, local inflammation, vascular smooth muscle cell proliferation, and in-stent plaque progression.

RESULTS

At 7 days, neovascularization was significantly decreased in the angiostatin groups (1.6 +/- 1.6 neovessels per mm(2) plaque) versus the control group (15.4 +/- 2.6 neovessels per mm(2) plaque; P =.00081), as were local inflammation where angiostatin-treated groups demonstrated significantly lower macrophage recruitment per cross section (34.9 +/- 4.9 cells per cross section) relative to the control group (55.2 +/- 3.84 cells per cross section; P =.0037). And a significant decrease in the overall vascular smooth muscle cell proliferation (143.8 +/- 26.3 Ki-67 positive cells per mm(2)) relative to the control group (263.2 +/- 16.6 Ki-67 positive cells per mm(2); P =.00074). At both 7 and 28 days, in-stent plaque progression in the angiostatin groups was successfully limited relative to the control group by 54% (0.255 +/- 0.019% of cross section; P =.00016) and 19% (1.981 +/- 0.080; P =.0033) respectively and resulted in reduction of in-stent restenosis relative to the control group.

CONCLUSION

Angiostatin-eluting stents may limit neovascularity after arterial implantation, offer insight into in-stent restenosis, and allow future refinement of bioactive stent designs and clinical strategies, particularly in light of evidence that intimal smooth muscle cells may in part be marrow-derived.

Impact of transport and drug properties on the local pharmacology of drug-eluting stents

Drugs released from stents are driven by physiological transport forces, principally solvent-driven flow (convection) and random molecular agitation (diffusion). The relative strength of these two forces determines drug penetration and distribution in the arterial wall. Drug physicochemical factors can induce critical modulations to the primary distribution, both transiently and at steady state. Hydrophobic interactions and nonspecific binding, for example, can both result in tissue drug concentrations severalfold above administered concentration. Drug interaction with native proteins may also interfere with drug transfer at the stent-artery interface. These transport forces and tissue interactions can induce local drug concentrations even at steady state to vary by one or more orders of magnitude over the span of a few cells. To account for significant local variations in drug concentrations following stent-based delivery, rational design of vascular delivery systems requires consideration of drug distribution and tissue interactions on a local, continuum basis. Continuum analysis adapts traditional pharmacokinetics to the local environment by supplementing discrete global parameters of drug content with continuous local values of concentration, transport and binding. The interplay of these parameters with local flux conditions and drug and tissue properties defines the local drug distribution in space and over time. This type of analysis may well become increasingly relevant given the trend toward stent-based drug therapy in cardiovascular care.

In vivo vascular engineering of vein grafts: directed migration of smooth muscle cells by perivascular release of elastase limits neointimal proliferation

PURPOSE

Saphenous vein bypass grafting for coronary revascularization procedures remains limited by accelerated neointima formation. It was hypothesized that creation of a modified chemotactic gradient in vivo could guide migration of smooth muscle cells (SMCs) peripherally instead of in a luminal direction and reduce intimal hyperplasia during vein graft arterialization.

MATERIALS AND METHODS

Surgical bypass vein grafting to femoral arteries was performed in adult male New Zealand White rabbits (n = 8 per treatment group; five for 7 d and three for 28 d). Controlled-release microspheres delivering elastase or buffered polymer only were administered perivascularly at the vein graft site. At 7 days, five vein grafts per group were harvested and cross-sections were immunostained with anti-proliferating cell nuclear antigen (PCNA) to determine the number and distribution of proliferating SMCs. At 28 days, three vein grafts per group were harvested and intima-to-media (I/M) ratios were calculated after staining with Verhoeff von Gieson-Masson trichrome stain.

RESULTS

Significant early outward-directed elastin degradation resulted from elastase treatment. Concurrently, proliferating SMCs migrated peripherally. PCNA(+) cells in the outer half of the wall increased 2.37 fold compared to procedural controls (P <.0001). Directional shifts in SMC migration underlie these results because overall SMC proliferation was not significantly different. At 28 days after vein graft surgery, a 38% reduction (P =.0008) in neointima was observed relative to procedural controls.

CONCLUSION

Directional guidance of SMC responses through perivascular elastase release achieves favorable vein graft remodeling characteristics, including limited neointima development. This represents practical evidence that SMC migration can be directionally guided in vivo in a vein graft model and that plaque progression can be prevented by redistributing elastin without decreasing functional vein graft wall stability.