The Crosstalk between ROS and Autophagy in the Field of Transplantation Medicine

Abstract

Many factors during the transplantation process influence posttransplant graft function and survival, including donor type and age, graft preservation methods (cold storage, machine perfusion), and ischemia-reperfusion injury. Successively, they will lead to cellular and molecular alterations that determine cell and ultimately organ fate. Oxidative stress and autophagy are implicated in posttransplant outcome since they are both affected by the stress responses triggered in each step (donor, preservation, and recipient) of the transplantation process. Furthermore, oxidative stress influences autophagy and vice versa. Interestingly, both processes have positive as well as negative effects on graft outcome, suggesting they are tightly linked during the transplantation process. In this review, we discuss the importance, regulation and crosstalk of oxidative signals, and autophagy in the field of transplantation medicine.

Figure 1

Overview of the autophagy process. Autophagy is initiated by the ULK1 complex, which is negatively regulated by mTOR, but positively by AMPK. This way it responds to nutrient or energy deprivation. In addition, class III PI3K complex requires Beclin 1, which is inhibited by Bcl-2. During elongation, Atg5-Atg12 and LC3-II are required. The latter is attached to the autophagosomal membranes. LC3-II will be delipidated on the outer membrane by Atg4 (a process inhibited by H₂O₂) but remains on the inner membrane and will be degraded inside the lysosomes. Mitochondria can be also degraded (mitophagy), via recruitment of Sqstm1/p62. The latter protein also recruits Keap1 for degradation, thereby enabling Nrf-2-dependent antioxidant transcription. Eventually, autophagosomes fuse with lysosomes, the cellular structures in which degradation takes place. The levels of autophagy determine the outcome on cellular injury and need to stay balanced in order not to provoke death.

Figure 2

Regulation of oxidative signals and autophagy by transplantation-related factors. Excessive autophagy and/or oxidative stress can lead to increased graft injury and tilt the balance to the right. To tilt the balance towards survival, excessive signals need to be reduced towards protective levels of oxidative signals and autophagy.

Figure 3

Gender differences in autophagy activation in response to ischemia-reperfusion injury. Western blot expression of autophagy-related proteins LC3-I, LC3-II, and Sqstm1/p62 in female and male Sprague-Dawley rat kidneys subjected to 45 min of warm ischemia (WI) followed by 3 h of reperfusion (3 h R). Each lane represents an independent experiment (N = 6). Quantification of p62 or LC3-II over GAPDH levels, compared to the mean of the corresponding Sham group. LC3-II and p62 are clearly lower in the WI group of females compared to the male WI group. Results are presented as mean ± SD (N = 6 per group) (^∗p < 0.05; ^∗∗∗p < 0.001).

Figure 4

Posttranscriptional reduction of apoptosis and induction of autophagy in the liver of brain-dead rats following T₃ treatment, yet no effects in the kidney. Western blot expression of proapoptotic protein Bax and autophagy-related proteins LC3-II and SQSTM1/p62 in the liver (a, c) and kidney (b, d) and renal injury markers in plasma (e) of T3- or vehicle-pretreated brain-dead rats. Each lane represents an independent experiment. Results are presented as mean ± SD (N = 7 per group) (^∗∗p < 0.01).

Figure 5

Proposed mechanism of T₃ preconditioning during brain death in the liver.