Towards a solution for saphenous vein graft failure

Polyethylene glycol-superoxide dismutase, a conjugate in search of exploitation

Without a doubt PEG-SOD has been the enzyme most studied in PEGylation. One can say that it represents the preferred model to assess chemistries for PEG activation, analytical procedures suitable for conjugate characterization, the influence of PEG size in conjugate removal from circulation and elimination of immunogenicity and antigenicity, and the effect of route of administration. The effect of PEG conjugation was studied in vitro and in vivo models in comparison with the free enzyme and the following conclusions may be drawn: (1) At the blood vessel level, PEG-SOD has been shown to provide a greater resistance to oxidant stress, to improve endothelium relaxation and inhibit lipid oxidation. (2) In the heart, PEG-SOD proved to be at least as effective as native SOD in treatment of reperfusion-induced arrhythmias and myocardial ischemia. (3) In the lung, PEG-SOD appeared to be able to reduce oxygen toxicity and E. coli-induced lung injury, but not in the treatment of lung physiopathology associated with endotoxin-induced acute respiratory failure and in the reduction of asbestos-induced cell damage. (4) On cerebral ischemia/reperfusion injuries the effect of PEG-SOD was uncertain, also due to the difficulty of cerebral cell penetration. (5) In kidney and liver ischemia both enzyme forms were found to ameliorate reperfusion damage. In view of so much positive research on PEG-SOD, it is surprising that no approved application in human therapy has been established and approved.

Macro-porosity is necessary for the reduction of neointimal and medial thickening by external stenting of porcine saphenous vein bypass grafts

BACKGROUND

placing external non-restrictive macro-porous stents around porcine vein grafts prevents neointima formation and medial thickening in both the short and long term. Whether the porosity of the stent material influences this effect, however, has not been determined. Therefore, the effect on neointimal and medial thickening of external macro-porous (polyester) and micro-porous (polytetrafluorethylene) stents of equal diameter were compared. The effect on expression of platelet-derived growth factor (PDGF), a potent mediator of vascular smooth muscle cell migration and proliferation and its receptors was also investigated.

METHODS AND RESULTS

saphenous vein-carotid artery interposition grafting was performed in Landrace pigs with external placement of 8 mm diameter macro- and micro-porous stents contralaterally. One month after surgery, graft wall dimensions, PDGF and PDGF receptor expression and cell proliferation using proliferating cell nuclear antigen (PCNA) were measured on histological sections. Macro-porous stents significantly reduced neointimal and medial thickening compared with micro-porous stents (0.1+/-0.02 vs. 0.25+/-0.03 mm, P<0.002, and 0.10+/-0.02 vs. 0.17+/-0.02 mm, P<0.014, respectively). Macro-porous stents significantly reduced the percentage of cells expressing PDGF and PCNA, compared with micro-porous stents (36+/-9 vs. 80+/-7, P < 0.002, and 11+/-3 vs. 21+/-2, P < 0.02, respectively). The percentage of cells expressing PDGF receptors was similar with both the stent types. Adventitial microvessel formation occurred across macro-porous stents but was markedly suppressed by micro-porous stents.

CONCLUSIONS

porosity is crucial to the efficacy of external stents in reducing neointima formation in porcine vein grafts. Decreases in PDGF expression and cell proliferation accompany the reduction in neointima formation. In addition, macro-porous stents allow adventitial microvessels to connect with the vasculature outside the stent, thereby potentially improving oxygenation. Although external stenting is highly effective in reducing neointima formation after vein grafting, the properties of the stent material necessary for this effect have not been defined. This study establishes that macro-porosity is one essential feature required to reduce PDGF expression cell proliferation and neointima formation.

Local treatment with recombinant tissue factor pathway inhibitor reduces the development of intimal hyperplasia in experimental vein grafts

BACKGROUND

Tissue factor (TF)-initiated thrombin generation has been implicated in the development of intimal hyperplasia after arterial injury. An increase in intimal TF expression has been shown to precede the development of intimal hyperplasia in vein grafts. This study examines the effects of local treatment with recombinant human tissue factor pathway inhibitor (rTFPI) in experimental vein grafts.

METHODS

Thirty-six male New Zealand white rabbits underwent bypass grafting of the carotid artery by use of the reversed ipsilateral jugular vein and were divided into four groups. Twenty animals had ex vivo incubation with rTFPI treatment (50 microg x mL(-1); n = 10) or placebo vehicle (control; n = 10). Sixteen animals received both ex vivo incubation and in vivo gel treatment with rTFPI (50 microg. mL(-1); n = 8) or without rTFPI (gel-control; n = 8). After operation, vein grafts were harvested at 3 days for immunohistochemical and Western analyses and at 28 days for histomorphologic study.

RESULTS

Western analysis demonstrated a 6.2-fold reduction in the expression of TF protein with rTFPI treatment in comparison to without rTFPI treatment. CD-18 leukocyte staining was diminished, whereas Tie-2 endothelial staining was increased in all rTFPI-treated vein grafts, compared with control and gel-control vein grafts. Intimal thickness was reduced by 21% with ex vivo rTFPI treatment compared with placebo (69 +/- 4 versus 87 +/- 5 microm; P <.05) and by 30% with the addition of rTFPI in vivo compared with gel-control (60 +/- 4 versus 86 +/- 5 microm; P <.01).

CONCLUSION

Local administration of rTFPI exerts early beneficial effects and limits the development of intimal hyperplasia in vein grafts. Therefore blocking TF-mediated pathway may offer new therapeutic options to reduce vein graft failure.

Neural reorganisation in porcine vein grafts: a potential role for endothelin-1

Vein grafts are associated with adventitial remodelling which may influence innervation of the graft. Since there is also evidence that endothelin-1 (ET-1) plays a role in the adventitial remodelling process, we investigated neural distribution in porcine vein grafts 1 and 6 months after implantation, as well as the localisation of immunoreactive ET-1 and its receptors in the same tissues. Saphenous vein-carotid artery interposition grafting was performed in Landrace pigs. One and 6 months after surgery, vein grafts and ungrafted saphenous veins were excised; neural tissue and ET-1 were identified by immunocytochemistry and ET receptors were identified using in vitro autoradiography. In ungrafted saphenous veins, abundant perivascular nerves were located in the outer region of the media with only a few paravascular nerves in the adventitia. In vein grafts at 1 month after implantation, there was almost complete depletion of perivascular nerves in the media. In contrast, in the neoadventitia, there was an emphatic appearance of large paravascular nerve bundles and a marked increase in small paravascular nerves. These changes were more pronounced at 6 months after surgery, although the principal changes had occurred within 1 month. Immunoreactive ET-1 (index of ET-1 content) was associated with paravascular nerve bundles, appearing as a dark, dense ring at the perineurium. Furthermore, within the nerve bundles, positive ET-1 immunoreactivity was associated with positive alpha-actin staining, indicating that ET-1 is associated with (neural) microvessels. Also, dense 125I-labelled BQ3020 (ET(B)-selective) binding to nerve bundles was observed, indicating the presence of ET(B) receptor subtypes. ET(A) receptor subtypes were not detected in neural tissue. These data demonstrate neural reorganisation in vein grafts and indicate that ET-1 content and binding may play a role in this process. The functional consequences of these changes on neointima formation, a major cause of vein graft failure, remain to be determined.

Reduction of lipid peroxidation with intraoperative superoxide dismutase treatment decreases intimal hyperplasia in experimental vein grafts

BACKGROUND

Vein graft failure is commonly attributed to the development of intimal hyperplastic lesions. Oxidative stress has been implicated in the initiation and progression of atherosclerosis. In this study we examined the effects of local intraoperative treatment with polyethylene glycolated superoxide dismutase (PEG-SOD) on lipid peroxidation and on the development of intimal hyperplasia in experimental vein grafts.

MATERIALS AND METHODS

Forty-one New Zealand White male rabbits had a right carotid interposition bypass graft using the ipsilateral reversed jugular vein. Sixteen animals received local PEG-SOD (4,100 units) treatment; 9 animals received the polyethylene glycol (PEG) vehicle without SOD; 16 animals were used as controls. Postoperatively, malondialdehyde (MDA, a product of lipid peroxidation) concentration and SOD activity were assessed in 3-day vein grafts by colorimetric spectrophotometry. To determine wall dimensions and vasomotor function, morphometric and isometric tension studies were performed on 28-day vein grafts.

RESULTS

MDA concentration was increased 5. 7-fold (P < 0.05) in 3-day control vein grafts compared to ungrafted jugular veins. Intraoperative PEG-SOD treatment raised SOD activity 5.0-fold (P < 0.05) and reduced MDA concentration 8-fold (P < 0.05) in 3-day vein grafts compared to controls. At 28 days, intimal thickness was reduced by 35% with PEG-SOD treatment (54 +/- 4 vs 83 +/- 5; P < 0.001) compared to control vein grafts, without a change in medial thickness (77 +/- 4 vs 88 +/- 5; P = ns). The vasomotor functions of 28-day PEG-SOD-treated vein grafts to norepinephrine, serotonin, bradykinin, nitroprusside, and acetylcholine were not significantly changed when compared to controls. Treatment with PEG alone did not significantly alter lipid peroxidation, wall dimensions, or vasomotor function of vein grafts.

CONCLUSION

This study demonstrates that intraoperative local treatment of vein grafts with PEG-SOD increases SOD activity and decreases lipid peroxidation for at least 3 days, resulting in reduced intimal hyperplasia at 28 days. These findings further implicate oxidative stress in the hyperplastic response of vein grafts and suggest a potential therapeutic role for PEG-SOD in the prevention of vein graft failure.

Inhibition of intracellular Ca2+ mobilisation by low antiproliferative concentrations of thapsigargin in human vascular smooth-muscle cells

Low nanomolar concentrations of thapsigargin, a modulator of intracellular Ca2+ ([Ca2+]i) pools, inhibit vascular smooth-muscle cell (VSMC) proliferation. Because the mechanisms underlying this effect have not been defined, the effect of antiproliferative concentrations of thapsigargin on [Ca2+]i in fura-2-loaded VSMCs was studied by using dynamic video imaging of [Ca2+]i. After seeding on coverslips, human VSMCs were incubated for 1-48 h with thapsigargin before loading with fura-2 or during imaging. Mobilisation of [Ca2+]i was stimulated with 1 microM ionomycin in Ca2+-free medium and the increase in [Ca2+]i detected by using Ca2+ imaging techniques. Continuous exposure of cells to low concentrations of thapsigargin (which failed measurably to increase in [Ca2+]i) reduced the ionomycin response in a time- and dose-dependent manner (100% inhibition at 10 nM thapsigargin after 1 h exposure). After exposure of cells to 10 nM thapsigargin for 1 h followed by washing and further incubation for < or = 72 h, there was a time-dependent recovery of the ionomycin response. Because the concentrations of thapsigargin and exposure times are identical to those that inhibit replication in VSMCs, it is proposed that depletion of [Ca2+]i pools mediates the inhibitory effect of thapsigargin on VSMC proliferation.

Nitric oxide, prostacyclin and cyclic nucleotide formation in externally stented porcine vein grafts

Non-restrictive, porous, external stents inhibit neointima formation in porcine vein grafts. Since the mechanisms underlying these effects are unknown we investigated the impact of this external stent on factors known to inhibit vascular smooth muscle cell proliferation: prostacyclin (PGI2), nitric oxide (NO), cAMP and cGMP formation in different regions of stented and unstented porcine vein grafts. Paired stented and unstented saphenous vein-carotid artery interposition grafting was carried out in Landrace pigs. One month after surgery, the vessels were excised and the formation of PGI2, cAMP and cGMP determined using radioimmunoassay and nitric oxide synthase (NOS) distribution studied using autoradiography and histochemistry. There were no significant differences between PGI2, cAMP and cGMP (nitroprusside-stimulated) formation in the medial/intimal regions of grafts of stented vein graft and ungrafted saphenous vein whereas all were significantly reduced in unstented vein graft. A23187-stimulated cGMP formation (mediated by NO release) and NOS content was significantly greater in the medial/intimal region of stented and unstented vein graft compared to ungrafted saphenous vein, indicating induction of endothelial NOS (eNOS) in both types of graft. This normalisation of the PGI2-cAMP axis and guanylyl cyclase activity in the medial/intimal region may contribute to the beneficial impact of the external stent on vein graft thickening. The increase in eNOS in both stented and unstented vein grafts mitigates against this isoform as playing a role in mediating the inhibitory effect of the stent on neointima formation. In the adventitia of both stented and unstented grafts there was an increase in PGI2, cAMP and cGMP formation compared to ungrafted saphenous vein, the production being greater in the stented compared to the unstented graft. In the adventitia of stented veini grafts, NOS, detected with NAPDH diaphorase staining, was associated with microvessels as well as with inflammatory cells. Taken together, these data are suggestive of a role for PGI2 and NO in promoting microangiogenesis in the adventitia of stented vein grafts which may in turn minimize graft hypoxia, an established contributory factor to neointima formation.

Cigarette smoking and erectile dysfunction

Erectile dysfunction (ED, formerly referred to as impotence, is a common (especially in diabetic and older men) and distressing condition. Several risk factors have been identified; among these are smoking, hyperlipidaemia, hypertension and diabetes mellitus. These risk factors are shared with atherosclerotic vascular disease (e.g. ischaemic heart disease). This observation underlies a common vascular pathology. Smoking may cause ED by several mechanisms, including adversely affecting intrapenile blood flow. It is important to be aware of the link between smoking and ED since this information may motivate some male smokers to quit. In this context, it is important to be aware of the link between smoking and ED since this information may motivate smokers to quit. In this context, it is relevant that there is evidence that quitting may restore/improve erectile function.

Distribution of endothelin-1 (ET) receptors (ET(A) and ET(B)) and immunoreactive ET-1 in porcine saphenous vein-carotid artery interposition grafts

Proliferation of vascular smooth muscle cells (VSMC) is a principal event in neointima formation in saphenous vein-coronary artery bypass grafts. Since endothelin-1 (ET-1) promotes VSMC replication and ET-1 receptor antagonists inhibit neointima formation in arterial injury models, it is reasonable to propose that ET-1 may be involved in neointima formation in vein grafts. However, it is not known what alterations of ET-1 and its receptors (if any) occur in vein grafts. The objective of this study, therefore, was to investigate the distribution of ET-1 and ET-1 receptor subtypes (ET(A) and ET(B)) in porcine vein grafts. Unilateral interposition saphenous vein grafting was performed by end to end anastomosis after excision of a segment of carotid artery in Landrace pigs. One month after surgery, vein grafts, ungrafted saphenous veins and carotid arteries were excised, ET-1 immunoreactivity identified by immunocytochemistry and ET(A) and ET(B) receptor subtypes studied using autoradiography. In vein grafts, there was a greater density of ET(A) compared to ET(B) receptors in both the tunica media and neointima. ET(A) binding in the tunica media of ungrafted saphenous vein was greater than that in the carotid artery or vein grafts, but greater in the vein graft compared to the carotid artery. Immunoreactive ET-1 was located in endothelial cells and throughout the neointima of the vein graft. Dense ET-1 binding (to both ET(A) and ET(B) receptors) was also associated with microvessels in the adventitia within the graft. In vein grafts, there was strong ET(B) binding to neutrophils which were present in high numbers at the subendothelium and within the adventitia. It is concluded ET(A) receptors may play a role in vein graft thickening at the medial and neointimal VSMC level, whereas ET(B) receptors may play a role in microangiogenesis. The higher levels of ET(A) receptors in the tunica media of ungrafted saphenous vein relative to the carotid artery and vein graft may also render this conduit susceptible to neointima formation. These data indicate that studies on the effect of ET receptor antagonists on the pathobiology of vein graft disease is warranted.