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- Myc (2) (entfernen)
The transcription factor Myc interacts with several co-factors to regulate growth and proliferationand thereby enables normal animal development. Deregulation of Myc is associated witha wide range of human tumors. Myc binds to DNA together with its dimerization partner Max, preferentially to canonical E-box motifs, but this sequence-specific interaction is probably not sufficient for Myc’s binding to target genes.
In this work, the PAF1 complex was characterized as a novel co-factor of Myc in Drosophila melanogaster. All components of the complex are required for Myc’s recruitment to chromatin, but the subunit Atu has the strongest effect on Myc's binding to target genes through ist direct physical interaction with Myc. Unexpectedly, the impact of Atu depletion on the Expression of Myc target genes was weak compared to its effect on Myc binding. However, the influence of Atu becomes more prominent in situations of elevated Myc levels in vivo . Mycrepressed as well as Myc-activated targets are affected, consistent with the notion that Myc
recruitment is impaired.
An independent set of analyses revealed that Myc retains substantial activity even in the complete absence of Max. The overexpression of Myc in Max0 mutants specifically blocks their pupariation without affecting their survival, which raised the possibility that Myc might
affect ecdysone biosynthesis. This connection was studied in the second part of this Thesis which showed that Myc inhibits the expression of ecdysteroidogenic genes and thereby the production of ecdysone. Myc most likely affects the signaling pathways (PTTH and insulin
signaling) upstream of the PG, the organ where ecdysone is produced. By combining existing ChIPseq, RNAseq and electronic annotation data, we identified five potential Maxindependent Myc targets and provided experimental data that they might be involved in Myc's effect on Max mutant animals. Together our data confirm that some Myc functions are Max-independent and they raise the possibility that this effect might play a role during replication.
The transcription factor MYC is deregulated in over 70% of all human tumors and, in its oncogenic form, plays a major role in the cancer metabolic reprogramming, promoting the uptake of nutrients in order to sustain the biosynthetic needs of cancer cells.
The research presented in this work aimed to understand if MYC itself is regulated by nutrient availability, focusing on the two major fuels of cancer cells: glucose and glutamine.
Initial observations showed that endogenous MYC protein levels strongly depend on the availability of glutamine, but not of glucose. Subsequent analysis highlighted that the mechanism which accounts for the glutamine-mediated regulation of MYC is dependent on the 3´-untranslated region (3´-UTR) of MYC. Enhanced glutamine utilization by tumors has been shown to be directly linked to MYC oncogenic activity and MYC-dependent apoptosis has been observed under glutamine starvation. Such effect has been described in experimental systems which are mainly based on the use of MYC transgenes that do not contain the 3´-UTR. It was observed in the present study that cells are able to survive under glutamine starvation, which leads to cell cycle arrest and not apoptosis, as previously reported. However, enforced expression of a MYC transgene, which lacks the 3´-UTR, strongly increases the percentage of apoptotic cells upon starvation. Evaluation of glutamine-derived metabolites allowed to identify adenosine nucleotides as the specific stimulus responsible for the glutamine-mediated regulation of MYC, in a 3´-UTR-dependent way. Finally, glutamine-dependent MYC-mediated effects on RNA Polymerase II (RNAPII) function were evaluated, since MYC is involved in different steps of global transcriptional regulation. A global loss of RNAPII recruitment at the transcriptional start site results upon glutamine withdrawal. Such effect is overcome by enforced MYC expression under the same condition.
This study shows that the 3´UTR of MYC acts as metabolic sensor and that MYC globally regulates the RNAPII function according to the availability of glutamine. The observations presented in this work underline the importance of considering stress-induced mechanisms impinging on the 3´UTR of MYC.