Cystathionase mediates senescence evasion in melanocytes and melanoma cells

The development of malignant melanoma is a highly complex process, which is still poorly understood. A majority of human melanomas are found to express a few oncogenic proteins, such as mutant RAS and BRAF variants. However, these oncogenes are also found in nevi, and it is now a well-accepted fact that their expression alone leads to senescence. This renders the understanding of senescence escape mechanisms an important point to understand tumor development. Here, we approached the question of senescence evasion by expressing the transcription factor v-myc myelocytomatosis viral oncogene homolog (c-MYC), which is known to act synergistically with many oncogenes, in melanocytes. We observed that MYC drives the evasion of reactive-oxygen stress-induced melanocyte senescence, caused by activated receptor tyrosine kinase signaling. Conversely, MIZ1, the growth suppressing interaction partner of MYC, is involved in mediating melanocyte senescence. Both, MYC overexpression and Miz1 knockdown led to a strong reduction of endogenous reactive-oxygen species (ROS), DNA damage and senescence. We identified the cystathionase (CTH) gene product as mediator of the ROS-related MYC and MIZ1 effects. Blocking CTH enzymatic activity in MYC-overexpressing and Miz1 knockdown cells increased intracellular stress and senescence. Importantly, pharmacological inhibition of CTH in human melanoma cells also reconstituted senescence in the majority of cell lines, and CTH knockdown reduced tumorigenic effects such as proliferation, H2O2 resistance and soft agar growth. Thus, we identified CTH as new MYC target gene with an important function in senescence evasion.

Oncogene activation in melanocytes links reactive oxygen to multinucleated phenotype and senescence

Contrary to malignant melanoma, nevi are a benign form of melanocytic hyperproliferation. They are frequently observed as precursor lesions of melanoma, but they also feature biochemical markers of senescence. In particular, evidence for oncogene-induced melanocyte senescence as natural means to prevent tumorigenesis has been obtained in nevi with mutated B-Raf(V600E). Here, we demonstrate that strong oncogenic growth factor receptor signalling drives melanocytes into senescence, whereas weaker signals keep them in the proliferative state. Activation of oncogene-induced senescence also produces multinucleated giant cells, a long known histological feature of nevus cells. The protein levels of the senescence mediators, p53 and pRB, and their upstream activators do not correlate with senescence. However, strong oncogene signalling leads to pronounced reactive oxygen stress, and scavenging of reactive oxygen species (ROS) efficiently prevents the formation of multinucleated cells and senescence. Similarly, expression of oncogenic N-RAS results in ROS generation, DNA damage and the same multinuclear senescent phenotype. Hence, we identified oncogenic signalling-dependent ROS production as critical mediator of the melanocytic multinuclear phenotype and senescence, both of them being hallmarks of human nevus cells.

Thioredoxin and glutathione system of malaria parasite Plasmodium falciparum

Plasmodium falciparum is the causative agent of malaria tropica. Due to the increasing resistance towards the commonly used plasmodicidal drugs there is an urgent need to identify and assess new targets for the chemotherapeutic intervention of parasite development in the human host. It is established that P. falciparum-infected erythrocytes are vulnerable to oxidative stress, and therefore efficient antioxidative systems are required to ensure parasite development within the host cell. The thioredoxin and glutathione redox systems represent two powerful means to detoxify reactive oxygen species and this article summarizes some of the recent work which has led to a better understanding of these systems in the parasite and will help to assess them as potential targets for the development of new chemotherapeutics of malaria.