Yoshihiro Yasuda

Minocycline sensitizes rodent and human liver mitochondria to the permeability transition: Implications for toxicity in liver transplantation

The antibiotic minocycline exerts cytoprotection in animal disease models. One proposed mechanism is modulation of the mitochondrial permeability transition (mPT), a Ca2 -dependent pathogenic event leading to necrotic and/or apoptotic cell death.1–5 A recent study in HEPATOLOGY by Theruvath et al.,6 investigating storage/ reperfusion injury following rat liver transplantation, concluded that minocycline prevented mPT and mitigated liver injury by decreasing mitochondrial Ca2 uptake without affecting mitochondrial respiration. Further, the authors argue that it could be consistent with clinical practice to (pre)treat stored livers and graft recipients with minocycline. The driving force for mitochondrial Ca2 transport is the mitochondrial membrane potential and the amount of Ca2 retained is dependent on the proton gradient and the matrix pH.7 Respiratory inhibition will decrease Ca2 retention capacity and sensitize mitochondria toward mPT.5,7 Further, endogenous inhibitors of mPT such as adenine nucleotides and Mg2 will influence the amount of Ca2 sequestered prior to mPT. In Theruvath et al., the effect of minocyline on mPT was determined in two classical assays, both using bolus additions of calcium chloride: (1) the swelling assay and (2) the Ca2 retention capacity assay. In both assays, the endpoint is Ca2 overload and induction of mPT. The authors found that minocycline prevented Ca2 induced swelling and decreased Ca2 retention and interpreted this as a specific inhibitory effect on Ca2 uptake. They excluded respiratory inhibition as the explanation to their findings by determining the respiration of mitochondria exposed to minocycline with and without Ca2 addition. However, the buffer used in the respiration assay was different from the one used in the Ca2 bolus assays, with high Mg2 concentration (Mg2 is a known endogenous inhibitor of mPT) and with the presence of the potent pharmacological mPT inhibitor cyclosporin A during Ca2 addition. We argue that minocycline at moderate to high dosing, similar to what we have shown in brain mitochondria, prevents Ca2 -uptake and mPT-induced swelling by respiratory inhibition.1,5 Further, depending on the buffer system used, the decreased Ca2 retention can be explained by minocycline-induced increase of mPT sensitivity related to (1) inhibited respiration1,5 and (2) chelating of Mg2 ,8 or (3) direct activation of mPT (even during concurrent cyclosporin A treatment) by adding Ca2 or in Ca2 loaded mitochondria, as recently shown by Kupsch et al.8 To stringently evaluate effects of minocycline during the process of Ca2 uptake, retention, and mPT, mitochondrial oxygen consumption can be monitored during a continuous Ca2 infusion (Fig. 1A,B). This assay provides information of the bioenergetic demand on mitochondria caused by Ca2 uptake as well as the respiratory inhibition triggered by mitochondrial Ca2 overload and mPT.5,7 Alternatively, the effect of minocycline on isolated mitochondria can be displayed by following changes of extramitochondrial Ca2 during a slow infusion of the cation. In these more physiologically relevant models, minocycline dose-dependently reduces Ca2 retention capacity and sensitizes rat and, importantly, human liver mitochondria to the mPT in the dose range used by Theruvath et al. (0-100 nmol/mg mitochondria; Fig. 1). In conclusion, minocycline may be a promising agent for cytoprotection at relevant dosing through mechanisms other than mPT inhibition. In the clinical setting, prevention of mitochondrial Ca2 uptake via respiratory inhibition is likely not beneficial to the organism. Further, to sensitize mitochondria to mPT by chelating Mg2 is not a viable strategy for cytoprotection. This must be kept in mind when considering the use of minocycline, even at moderate dosing, to mitigate storage/reperfusion injury during liver transplantation.