Mechanism underlying taurine depletion-mediated cell death.

摘要

Depletion of taurine content leads to the development of a cardiomyopathy, which is associated in part with cardiomyocyte death. However, the mechanism underlying cell death is not completely understood. This dissertation tests the hypothesis that taurine depletion induces cell death by inhibiting the ubiquitin-proteasome system (UPS) and autophagy. As both the UPS and autophagy regulate cellular homeostasis, inhibition of these processes enhances cellular damage and death. Using taurine transporter knockout (TauTKO) hearts we found that taurine depletion decreases complex I activity. A possible consequence of mitochondrial dysfunction is slower electron flux that promotes the diversion of electrons to oxygen to form superoxide, which causes oxidative stress when produced in excess. Indeed, TauTKO hearts demonstrated evidence of oxidative stress as shown by a decline in both aconitase activity and the glutathione redox ratio, as well as an increase in protein carbonylation. Oxidative stress is widely known to cause cellular oxidative damage and enhance cell death. In association with taurine depletion, the cells underwent apoptosis as shown by activation of caspase 9 and caspase 3. One event mediated by mitochondrial dysfunction and oxidative stress is impairment of the endoplasmic reticulum (ER) function. ER homeostasis is regulated by GRP78, a key ER chaperone, via the unfolded protein response (UPR). In TauTKO hearts, GRP78 levels were reduced, followed by suppression of the UPR, as shown by inhibition of the IRE1-XBP1-mediated pathway. The IRE1-mediated pathway is essential in regulating endoplasmic reticulum-associated degradation (ERAD) for misfolded protein degradation by the UPS. One adaptive mechanism in response to cellular damage is the activation of the UPS and autophagy. In TauTKO hearts, the UPS is downregulated as shown by a reduction in 26S proteasome activity and increased ubiquitinated proteins. On the other hand, autophagy is activated in TauTKO hearts, as shown by increased Beclin-1 and LC3-II levels, but autophagosome degradation is inhibited, as evidenced by decreased ATG7 and Parkin levels. In addition, chloroquine treatment does not significantly increase LC3-II levels in TauTKO hearts, which suggests impaired autophagy flux in association with taurine deficiency. These findings clearly indicate that taurine depletion induces cell death by inhibiting the UPS and autophagy, two processes that are essential to regulate cellular homeostasis.