Abstract Although numerous pathogenic changes within the mitochondrial respiratory chain (RC) have been associated with an elevated occurrence of apoptosis within the affected tissues, the mechanistic insight into how mitochondrial dysfunction initiates apoptotic cell death is still unknown. In this study, we show that the specific alteration of the cytochrome c oxidase (COX), representing a common defect found in mitochondrial diseases, facilitates mitochondrial apoptosis in response to oxidative stress. Our data identified an increased ceramide synthase 6 (CerS6) activity as an important pro-apoptotic response to COX dysfunction induced either by chemical or genetic approaches. The elevated CerS6 activity resulted in accumulation of the pro-apoptotic C 16 : 0 ceramide, which facilitates the mitochondrial apoptosis in response to oxidative stress. Accordingly, inhibition of CerS6 or its specific knockdown diminished the increased susceptibility of COX-deficient cells to oxidative stress. Our results provide new insights into how mitochondrial RC dysfunction mechanistically interferes with the apoptotic machinery. On the basis of its pivotal role in regulating cell death upon COX dysfunction, CerS6 might potentially represent a novel target for therapeutic intervention in mitochondrial diseases caused by COX dysfunction.
The BH3-only protein NOXA represents one of the critical mediators of DNA-damage-induced cell death. In particular, its involvement in cellular responses to cancer chemotherapy is increasingly evident. Here, we identify a strategy of cancer cells to escape genotoxic chemotherapy by increasing proteasomal degradation of NOXA. We show that the deubiquitylating enzyme UCH-L1 is a key regulator of NOXA turnover, which protects NOXA from proteasomal degradation by removing Lys48-linked polyubiquitin chains. In the majority of tumors from patients with melanoma or colorectal cancer suffering from high rates of chemoresistance, NOXA fails to accumulate because UCH-L1 expression is epigenetically silenced. Whereas UCH-L1/NOXA-positive tumor samples exhibit increased sensitivity to genotoxic chemotherapy, downregulation of UCH-L1 or inhibition of its deubiquitylase activity resulted in reduced NOXA stability and resistance to genotoxic chemotherapy in both human and C. elegans cells. Our data identify the UCH-L1/NOXA interaction as a therapeutic target for overcoming cancer chemoresistance.
Apoptosis, the physiological cell death, is an essential component of cellular homeostasis and tissue regeneration, and the dysregulation of apoptosis culminates in multiple human diseases. Whereas the failure to execute timely programmed cell death in renovating tissues contributes to cancer, excessive apoptosis in post-mitotic tissues precipitates degenerative states, aging and aging-associated diseases.
Research work within the last two decades has shown that tissue degeneration caused by mitochondrial oxidative phosphorylation (OXPHOS) defects is associated with excessive apoptosis. However, it is unclear how OXPHOS-dysfunctions interfere with the apoptotic machinery and how this impacts on tissue degeneration.
In this context we demonstrate that solely the chemical induction of cytochrome c oxidase (COX) deficiency, which is among the most common defects found in mitochondrial diseases, exclusively and dramatically increases the Bcl-2 dependent apoptotic response towards oxidative stress. Furthermore, we could widen and reconfirm our findings in COX-deficient cybrids harboring an mtDNA-encoded deletion of COX subunit I as well as in murine fibroblast devoid of COX-assembly factor COX10. These findings indicate a general mechanism of COX-deficiency induced apoptosis by oxidative stress. Moreover, our data highlight that COX-deficiency is accompanied by an increased de novo synthesis and accumulation of the sphingolipid species ceramide. Due to their chemical properties especially ceramides with an acyl side-chain length of C16:0 have conclusively and repeatedly been demonstrated to induce apoptosis by pore-formation and by contributing to pro-apoptotic Bax and Bak induced permeabilization of the mitochondrial outer membrane (MOMP). Correspondingly, we show that the inhibition of endoplasmic reticulum-resident specific ceramide synthases abrogates enhanced apoptosis induced by mitochondrial respiratory chain-dysfunction. In particular, we identify ceramide synthase 6 (CerS6) as the key mediator of C16:0 ceramide induced apoptosis to oxidative stress: While down-regulation of CerS6 protects COX-deficient cells from oxidative stress, overexpression of CerS6 introduces susceptibility towards oxidative stress in COX-functional cells.
In summary, our findings identify ceramide accumulation and in particular CerS6 as important components of the apoptotic response of COX-deficient cells towards oxidative stress and provide new insights into how mitochondrial dysfunction interferes with the apoptotic machinery and may impact on tissue homeostasis.
