Differential mechanical activation of mitogen-activated protein kinases in intact human blood vessels

Gene Therapy for the Extension of Vein Graft Patency: A Review

The mainstay of treatment for long-segment small-vessel chronic occlusive disease not amenable to endovascular intervention remains surgical bypass grafting using autologous vein. The procedure is largely successful and the immediate operative results almost always favorable. However, the lifespan of a given vein graft is highly variable, and less than 50% will remain primarily patent after 5 years. The slow process of graft malfunction is a result of the vein's chronic maladaptive response to the systemic arterial environment, its primary component being the uncontrolled proliferation of vascular smooth muscle cells (SMCs). It has recently been suggested that this response might be attenuated through pre-implantation genetic modification of the vein, so-called gene therapy for the extension of vein graft patency. Gene therapy seems particularly well suited for the prevention or postponement of vein graft failure since: (1) the stimulation of SMC proliferation appears to largely be an early and transient process, matching the kinetics of current gene transfer technology; (2) most veins are relatively normal and free of disease at the time of bypass allowing for effective gene transfer using a variety of systems; and (3) the target tissue is directly accessible during operation because manipulation and irrigation of the vein is part of the normal workflow of the surgical procedure. This review briefly summarizes the current knowledge of the incidence and basic mechanisms of vein graft failure, the vector systems and molecular targets that have been proposed as possible pre-treatments, the results of experimental genetic modification of vein grafts, and the few available clinical studies of gene therapy for vascular proliferative disorders.

Modulating vascular intimal hyperplasia using HSV-1 mutant requires activated MEK

Outcomes of cardiovascular procedures, such as angioplasty and stent or bypass grafting are limited by failure, predominantly caused by pathological smooth muscle cell (SMC) proliferation, known as intimal hyperplasia. Local delivery of a genetically engineered herpes simplex virus (HSV) is known to block vascular SMC proliferation while allowing for re-endothelialization. However, the mechanism this mutant virus uses to prevent SMC hyperplasia is unknown. The Ras signaling cascade is activated in SMCs undergoing hyperplasia leading to phosphorylation of the mitogen-activated protein kinase (MAPK). In this study we tested the hypothesis that MAPK kinase (MEK) activity is the molecular basis by which SMCs are susceptible to mutant HSV. We show that genetically engineered herpes simplex-1 viruses (HSV-1) can target proliferating SMCs. We demonstrate that the molecular basis of this HSV-1 anti-proliferative effect is MEK activation in SMCs. We demonstrate efficacy and practicality of the MEK-dependent HSV-1 for the treatment of intimal hyperplasia in a clinically relevant in vivo model. Important to this strategy is the ability to modulate the effects by controlling viral dose. These results propel genetically engineered HSV-1 therapy towards clinical evaluation in treatment of intimal hyperplasia.

Concomitant proliferation and caspase-3 mediated apoptosis in response to low shear stress and balloon injury

BACKGROUND

Arterial remodeling occurs as a response to hemodynamic change and direct vessel wall injury through the process of neointimal hyperplasia (NH). A concomitant response of vascular smooth muscle cell (VSMC) proliferation and apoptosis exists. The purpose of this study is to assess the cellular response of vessels following exposure to low shear stress (tau) and balloon injury in order to further elucidate the mechanisms underlying vascular injury. Our hypothesis is that the combination of low tau and balloon injury results in NH approximating that seen in clinical arterial restenosis, and that quantitative analysis of VSMC proliferation and apoptosis correlates with the associated increase in arterial remodeling.

METHODS AND RESULTS

New Zealand White rabbits underwent surgery on the carotid artery creating low tau (n =11), balloon injury (n = 11), combined low tau and balloon injury (n =11), and sham (n = 13) groups. Experiments were terminated at 1, 3, and 28 d. Day 1 and 3 arteries were analyzed with immunohistochemistry for apoptotic markers, terminal transferase dUTP nick end labeling (TUNEL), and activated caspase-3, and a cellular proliferation marker, accumulated proliferating cell nuclear antigen (PCNA), as well as immunoblot analysis for activated caspase-3 and PCNA at day 3. There was significantly greater apoptosis in the combined group as compared with the other groups assessed by quantitative TUNEL and activated caspase-3 levels at both days 1 and 3. Similarly, an increase in cellular proliferation assessed by PCNA expression, was significantly greater in the combined group as compared with the other groups. At 28 d there was no difference in NH observed in the low tau (26 +/- 3 microm) and balloon injury (51 +/- 17 microm) groups. However, significantly more NH was observed in the combined group (151 +/- 35 microm) as compared with the other groups.

CONCLUSIONS

An increase in VSMC apoptosis via a caspase-3 dependent pathway is up-regulated by 24 h in the face of combined low shear stress and balloon-induced vessel wall injury. Paradoxically, this increase in VSMC apoptosis is associated with a significant increase in neointimal thickening at 28 d. The concomitant increase of both apoptosis and proliferation are indicative of a robust arterial remodeling response.

BIOMECHANICAL CONSIDERATIONS IN THE DESIGN OF GRAFT: THE HOMEOSTASIS HYPOTHESIS

Since its inception in the 1960s, coronary artery bypass graft (CABG) evolved as one of the most common, best documented, and most effective of all major surgical treatments for ischemic heart disease. Despite its widespread use, however, the outcome is not always completely satisfactory. The objective of this review is to highlight the physical determinants of biomechanical design of CABG so that future procedures would have prolonged patency and better outcome. Our central axiom postulates the existence of a mechanical homeostatic state of the blood vessel, i.e., the variation in vessel wall stresses and strains are relatively small under physiological conditions. Any perturbation of mechanical homeostasis leads to growth and remodeling. In this sense, stenosis and failure of a graft may be viewed as an adaptation process gone awry. We outline the principles of engineering design and discuss the biofluid and biosolid mechanics principles that may have the greatest bearing on mechanical homeostasis and the long-term outcome of CABG.

Slower Onset of Low Shear Stress Leads to Less Neointimal Thickening in Experimental Vein Grafts

Vein grafts respond to low flow and shear stress (tau(w)) by generating thicker walls and smaller lumens through the processes of neointimal hyperplasia and remodeling. Clinically, however, vein grafts with obviously low tau(w), such as those distal to high-grade proximal obstructions, are not infrequently found to be widely patent and pliable. One possible explanation for this phenomenon may be that vein grafts remodel more favorably in response to changes in shear that occur gradually over time compared to abruptly. This hypothesis was tested in an experimental animal model in this report. Two separate models of experimental vein graft failure were created, causing either immediate exposure to ultralow tau(w) (<1 dyne/cm2) or delayed exposure to ultralow tau(w). Under general anesthesia and using a sterile technique, the right external jugular (EJ) veins of 28 New Zealand white rabbits were surgically exposed and isolated. An end-to-side distal EJ/common carotid artery anastomosis was created, resulting in a widely patent arteriovenous fistula. For the immediate exposure group (n = 5), the EJ was suture-ligated just proximal to the thoracic inlet, distal to a small 10-50 microm venous tributary. This created a reversed vein segment immediately and abruptly exposed to high wall tension (2.0 +/- 0.3 x 10(4) dyne/cm) and ultralow tau(w) (0.15 +/- 0.08 dyne/cm2). For the delayed exposure group (n = 22), the EJ was ligated over a 0.035 guidewire, leaving a small aperture to sustain some measure of blood flow and tau(w). This predictably resulted in slightly less wall tension (1.4 +/- 0.2 x 10(4) dyne/cm) and higher tau(w) (0.68 +/- 0.21 dyne/cm2) than the immediate exposure group. During the first week, the small outflow aperture in the delayed exposure grafts thrombosed, eventually exposing them to the same low level of tau(w) as the immediate exposure grafts. Thus, the only difference in the two models was that delayed exposure grafts enjoyed a slower decline in tau(w) than immediate exposure grafts. Fourteen rabbits in the delayed exposure group were harvested over the first 7 days to define the patency curve of the restricted outflow channel. As expected, the small aperture had thrombosed in all animals by 7 days. The remaining 14 grafts were harvested after 4 weeks, and 13/14 remained patent. Examination of the hemodynamic parameters at the time of death confirmed that wall tension and tau(w) had equalized (wall tension 0.9 +/- 0.1 vs. 1.1 +/- 0.1 x 10(4) dyne/cm, tau(w) 0.45 +/- 0.12 vs. 0.30 +/- 0.08 dyne/cm2). Histological examination revealed less neointimal hyperplasia in the delayed exposure group compared to the immediate exposure group (wall thickness 266 +/- 16 vs. 180 +/- 24 microm, p = 0.025) as well as a slightly greater luminal diameter (0.30 +/- 0.02 vs. 0.40 +/- 0.02 cm, p = 0.038). The results of this experiment suggest that slow exposure to reduced tau(w) results in more favorable remodeling (less thickening) than abrupt exposure. This finding may explain the occasional clinical observation of a widely patent vein graft even in the face of proximal arterial obstruction and very low flow; the change in tau(w) presumably occurred slowly mitigating the remodeling response.

Vein graft arterialization causes differential activation of mitogen-activated protein kinases

OBJECTIVE

Vascular injury results in activation of the mitogen-activated protein kinases-extracellular-signal regulated kinases, c-jun N-terminal kinase, and p38(MAPK)-which have been implicated in cell proliferation, migration, and apoptosis. The goal of this study was to characterize mitogen-activated protein kinase activation in arterialized vein grafts.

METHODS

Carotid artery bypass using reversed external jugular vein was performed in 29 dogs. Vein grafts were harvested after 30 minutes and 3, 8, and 24 hours, and 4, 7, 14, and 28 days. Contralateral external jugular vein and external jugular vein interposition vein-to-vein grafts were used as controls. Vein graft extracts were analyzed for extracellular-signal regulated kinases, c-jun N-terminal kinase, and p38(MAPK) activation. Proliferating cell nuclear antigen expression was investigated as a parameter of cell proliferation. Apoptosis was assessed by terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate nick end labeling staining and intimal hyperplasia by morphometric examination of tissue sections.

RESULTS

Significant intimal hyperplasia was observed at 28 days. Over the time points studied, vein graft arterialization resulted in bimodal activation of both extracellular-signal regulated kinase and p38(MAPK) (30 minutes through 3 hours; 4 days) but did not induce activation of c-jun N-terminal kinase. Proliferating cell nuclear antigen expression increased from days 1 through 28, and apoptosis increased between 8 and 24 hours.

CONCLUSION

Vein graft arterialization induces bimodal activation of extracellular-signal regulated kinase and p38(MAPK); however, in contrast with what is described in arterial injury, it does not induce c-jun N-terminal kinase activation. These results provide the first comprehensive characterization of the mitogen-activated protein kinase signaling pathways activated in vein graft arterialization and identify mitogen-activated protein kinases as potential mediators of vein graft remodeling and subsequent intimal hyperplasia.