Zinc-mediated reduction of apoptosis in cardiac allografts

Background - Apoptosis is thought to occur during immune-mediated acute rejection of cardiac allografts. In vitro studies have shown that zinc inhibits the activity of the proapoptotic enzyme caspase-3. We hypothesized that ZnCl₂ would reduce acute cardiac rejection in vivo via the blockade of caspase-3-dependent apoptosis. ^99mTc-labeled annexin V was used to measure apoptosis in cardiac allografts through nuclear imaging. Annexin V binds to phosphatidylserines, which are externalized to the outer membrane of apoptotic cells. Methods and Results - Twenty-seven PVG rat hearts were transplanted heterotopically into the abdomen of untreated ACI rats as controls (group 1). Fifteen were scanned and euthanized on postoperative day 4, and 12 were assessed for graft survival. Group 2 and 3 rats (n=15 each) received 1 and 5 mg/kg ZnCl₂ BID IP, respectively. Nine of each of these groups were scanned and euthanized on postoperative day 4, and 6 were studied for allograft survival. Group 4 rats (n=3) received isografts. Region-of-interest analysis demonstrated a dose-dependent reduction in ^99mTc annexin uptake in ZnCl₂-treated allografts: 2.43±0.37% for group 1, 1.97±0.41% for group 2, 1.21±0.47% for group 3, and 0.55±0.19% for group 4 (ANOVA, P=0.001). Graft survival times of 6.4±1.7, 9.3±3.0, and 11.5±3.4 days for groups 1, 2, and 3, respectively, were also observed (ANOVA, P=0.001). Caspase-3 activity in the allografts showed a 3.7-fold reduction in group 3 animals compared with group 1 animals (P=0.004). Conclusions - Apoptosis that occurs in acute cardiac allograft rejection is reduced with ZnCl₂ in a dose-dependent manner via caspase-3 inhibition.