In this study we tested the hypothesis that a component of the restenosis process following coronary angioplasty is dependent on the enhanced and long lasting oxidant stress that is related to baseline non-HDL cholesterol concentration, and would have the capacity to regulate inflammatory reactions and vascular smooth muscle cell proliferation after reperfusion.
Although Nunes and colleagues demonstrated persistently increased O2−
generation in injured vessels after experimental PTCA in animals,22
studies in humans have shown only transient increases in LDL oxidisability after PTCA.23,24
In fact, these studies have analysed only the period (a matter of minutes) immediately following myocardial reperfusion, and they have not compared oxidant status in subsets of patients with different outcomes after PTCA. Thus an original finding of the present study was that patients undergoing PTCA experienced a persistent increase in systemic oxidative stress, and this was significantly more obvious and prolonged in those with restenosis. Concurrent drug treatments are unlikely to have contributed to such different levels of lipid peroxidation as the treatment was similar in the two groups. Oxidant stress increased significantly in both groups 24 hours after PTCA; however, while it returned to baseline at 15 days in the patients without restenosis, in the patients with restenosis the changes persisted throughout the study. Thus the differences in oxidative burden between the two groups became apparent 24 hours after PTCA and remained significant until the final day of the study.
Our second finding was that enhanced oxidant stress after PTCA was related to the preprocedural concentration of non-HDL cholesterol, as suggested by the significant correlation between the two. Because oxidant stress has biological effects on vascular and monocyte function,4
enhanced production of oxidant compounds might mediate some of the effects of hypercholesterolaemia on important determinants of vascular occlusion, as suggested by the striking correlation between redox state and late lumen loss. Thus the ability of cholesterol dependent oxidant stress to induce and amplify monocyte activation may be relevant in settings where inflammation and enhanced lipid peroxidation coincide, such as PTCA.
Our final original finding was that the activation status of monocytes after PTCA—as reflected by spontaneous IL-1β release—was persistently higher in the group of patients who developed restenosis. That an increased oxidative burden after PTCA might have an effect on cytokine biosynthesis in influencing vascular healing was also suggested by the significant correlation between the circulating levels of indices of oxidative burden and the rate of IL-1β biosynthesis.
At least two alternative explanations could be considered for the mechanisms underlying the more impressive oxidant stress and monocyte activation following PTCA in the patients with restenosis. First, the diversity may simply have reflected differences in the degree of ischaemia/reperfusion during PTCA. However, this seems unlikely for the following reasons: all the procedures were done by the same experienced cardiologist according to standard techniques; the two groups had similar clinical and angiographic profiles at baseline; there were no significant differences in the total period of vessel occlusion during the procedure (table 1); and the differences in oxidant status between the two groups remained significant in the samples collected 15 days after PTCA. Alternatively, our data are consistent with the hypothesis that the high interindividual variability in the oxidative burden after revascularisation—which was critically related to preprocedural concentrations of non-HDL cholesterol—might, by governing early inflammatory events, contribute at least in part to the differences observed in the processes of healing, remodelling, and late lumen loss after PTCA in the presence of similar angiographic profiles.
In this study, a pronounced and long lasting systemic oxidant burden in the patients with restenosis was always characterised by a concomitant increase in oxidants and a decrease in antioxidant substances. We speculate that the reduction in vitamins C and E after PTCA in this group of patients may have reflected consumption resulting from interference of these vitamins with the pathways responsible for superoxide anion (O2−
) production or perhydroxyl radical formation.24
The striking effect of PTCA in increasing lipid peroxidation raises the question of the origin of this oxidant burden. Macrophages and smooth muscle cells, the major types of cell left in the atherosclerotic lesion after PTCA, can both elaborate active oxygen species. Furthermore, granulocytes can also produce large amounts of O2−
and other oxidants. Interestingly, it has been reported that inflammatory cells may accumulate after PTCA at sites of acute tissue trauma and thrombosis, not only in the injured intima but also in the adventitial layer surrounding the artery, which is increasingly recognised as a site of scarring and luminal constriction in restenotic lesions.25
There is increasing evidence to suggest that oxygen species might contribute to restenosis, including smooth muscle cell migration, replication, and accumulation and remodelling of the extracellular matrix. In particular, many vascular biologists now ascribe a central role to augmented transcription of several atherosclerosis related genes by the oxidant sensitive regulatory pathway involving NF-κB.26
Exposure to oxidant stress activates the NF-κB regulatory complex in vascular cells and triggers the transcription of genes that encode certain leucocyte adhesion molecules, chemoattractant cytokines, and enzymes that can influence extracellular matrix metabolism.27
Moreover, it has been proposed10
that the early acute cytokine generation in macrophages, which are a major source of IL-1β, could evoke a secondary cytokine and growth factor response from other types of cell in the lesion, including smooth muscle cells and endothelial cells. This would establish a positive, self stimulatory autocrine and paracrine feedback loop, amplifying and sustaining the proliferative response. The positive results of probucol (a lipid lowering drug with powerful antioxidant action) in preventing renarrowing after PTCA2
suggest that this agent might interrupt these or other oxidant sensitive signalling systems and thus mitigate the sustained proinflammatory state that follows balloon injury to arteries28
and ultimately reduce the restenosis rate. In contrast, the negative results observed with vitamin C and E in the same study,2
and with statins in several clinical trials,29
suggest that separate antioxidant or lipid lowering strategies are not sufficient to suppress the strong inflammatory reaction following PTCA, and that a synergistic lipid lowering and antioxidant approach is necessary to modulate this phenomenon.
Despite their originality, the results of our study must be viewed with caution because of the small sample size; thus a further study with a larger sample size will be necessary to confirm the data.
We believe that our findings have implications for both research and clinical practice. They have potential importance from a fundamental standpoint because they demonstrate the critical role of cholesterol dependent oxidant stress in the pathophysiology of restenosis after PTCA. From a practical standpoint, the findings raise the possibility that drugs capable of modulating oxidant status might provide a novel form of adjuvant treatment in patients with hypercholesterolaemia who undergo PTCA.