Probucol Does not Inhibit Myeloperoxidase-Dependent Low-Density Lipoprotein Oxidation as a Potent Protective Effect in Atherosclerosis

Inhibition of suicidal erythrocyte death by probucol

Probucol, an antioxidant and anti-inflammatory agent counteracting atherosclerosis and restenosis, is partially effective by influencing suicidal cell death or apoptosis. In analogy to apoptosis of nucleated cells, suicidal death of erythrocytes or eryptosis is characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the erythrocyte surface. Eryptosis is stimulated by increase in cytosolic Ca(2+) activity, for example, after energy depletion or oxidative stress. The present study explored whether probucol influences eryptosis. Phosphatidylserine exposure was estimated from annexin-V-binding, cell volume from forward scatter (FSC), and cytosolic Ca(2+) concentration from fluo-3 fluorescence in flow cytometry. As a result, energy depletion (48-hour glucose removal) increased annexin-V-binding, decreased FSC, and increased fluo-3 fluorescence. Probucol (≤30 μM) did not significantly modify annexin-V-binding, FSC, or fluo-3 fluorescence in the presence of glucose but (at ≥5 μM) blunted the effect of glucose depletion on annexin-V-binding. Probucol (≥20 μM) only slightly blunted the effects of glucose depletion on FSC and fluo-3 fluorescence. Ca(2+) ionophore ionomycin (1 μM) and oxidative stress (30-minute exposure to 0.3 mM of tert-butylhydroperoxide) increased annexin-V-binding, effects again blunted by 30 μM of probucol. In conclusion, probucol blunts cell membrane scrambling after energy depletion and oxidative stress, effects primarily because of interference with the scrambling effects of increased cytosolic Ca(2+) concentration.

Actions of "antioxidants" in the protection against atherosclerosis

This review addresses the role of oxidative processes in atherosclerosis and its resulting cardiovascular disease by focusing on the outcome of antioxidant interventions. Although there is unambiguous evidence for the presence of heightened oxidative stress and resulting damage in atherosclerosis, it remains to be established whether this represents a cause or a consequence of the disease. This critical question is complicated further by the increasing realization that oxidative processes, including those related to signaling, are part of normal cell function. Overall, the results from animal interventions suggest that antioxidants provide benefit neither generally nor consistently. Where benefit is observed, it appears to be achieved at least in part via modulation of biological processes such as increase in nitric oxide bioavailability and induction of protective enzymes such as heme oxygenase-1, rather than via inhibition of oxidative processes and lipid oxidation in the arterial wall. Exceptions to this may be situations of multiple/excessive stress, the relevance of which for humans is not clear. This interpretation is consistent with the overall disappointing outcome of antioxidant interventions in humans and can be rationalized by the spatial compartmentalization of cellular oxidative signaling and/or damage, complex roles of oxidant-producing enzymes, and the multifactorial nature of atherosclerosis.

Molecular mechanisms underlying the antiatherosclerotic and antidiabetic effects of probucol, succinobucol, and other probucol analogues

PURPOSE OF REVIEW

New therapies for the management of cardiovascular disease remain highly desirable, yet the recently developed agents, such as the cholesterylester transfer protein inhibitor torcetrapib, the antidiabetic agent rosiglitazone, and anti-inflammatory inhibitors of cyclooxygenase-2, have failed. In this review, the more recent developments in the molecular mechanisms underlying the beneficial activities of probucol and related compounds are described.

RECENT FINDINGS

In-vivo and in-vitro studies have revealed that several of the protective activities of probucol can be explained by the ability of this drug to induce the enzyme heme oxygenase-1. It is now apparent that the sulfur atoms, rather than the phenol moieties of probucol, are required for its antiatherogenic and antirestenotic activities. Compounds related to probucol that have improved efficacy without the adverse effects offer promise as novel therapies of cardiovascular disease. Recent results suggest these compounds may also be used for the prevention of type-2 diabetes, a disease that is increasing in prevalence and importance worldwide.

SUMMARY

The development of derivatives of probucol targeting anti-inflammatory and antioxidant processes, perhaps via induction of heme oxygenase-1, may add to the armamentarium of current agents used in treatment of atherosclerotic disease and diabetes.