Department of Gene Vectors
Cancer therapy aims to eliminate tumour cells using surgery, radio-, chemo- and immunotherapy. Except surgery, cancer therapy eliminates tumour cells by activation of the cells´ own cell death program called apoptosis. Unfortunately, tumour cells might be resistant against induction of apoptosis or might acquire apoptosis resistance during tumour therapy, e. g. upon tumour relapse. As apoptosis resistance significantly impairs the prognosis of patients suffering cancer, a more detailed understanding of apoptosis signaling is desired in order to overcome apoptosis resistance, effectively eliminate tumour cells and increase the prognosis of patients. The aim of our work is to intensify induction of apoptosis in tumour cells.
Figure 1: Cell shrinkage as typical hallmark of apoptosis
Intracellular transduction of the apoptosis signal is highly regulated. Cytotoxic drugs and gamma-irradiation activate the intrinsic apoptosis signaling pathway which leads to activation of mitochondria and the formation of the apoptosome. In addition to established cytotoxic drugs, we study betulinic acid, a natural compound of birch trees which is now tested in preclinical studies for anti-tumour therapy and which might directly activate isolated mitochondria. Body own ligands of the TNF family activate the extrinsic apoptosis pathway where apical caspase activation occurs close to the receptor and directly enables activation of downstream effector caspases. Most importantly TNF family members are TNF, the CD95 / Apo-1 / Fas ligand and TRAIL, the TNF- related apoptosis inducing ligand. We use TRAIL-induced stimulation of tumour cells as a model to study the mechanisms regulating apoptosis. TRAIL might represent a future cytotoxic drugs to treat cancer as it induces apoptosis in a vide range of tumours with extraordinarily low systemic toxicity. Upon binding, the death inducing ligands lead to the aggregation of their specific receptors TNF-RI, CD95 / Apo-1 / Fas and DR4 / DR5, respectively. After aggregation, the adaptor protein FADD (CD95 and DR4/5) or TRADD (TNF-RI) is recruited to the receptor-multimere which than directly or indirectly binds caspase-8. Upon auto aggregation, caspase-8 is activated and cleaved leading either to direct cleavage of distal caspases or to intermediate activation of mitochondria executing apoptosis. This signaling cascade is regulated by different protein families including the Bcl-2 protein family and the inhibitor of apoptosis family (IAPs). Despite of intensive research effort, many steps within the apoptosis signaling and their regulation are still unknown.
Figure 2: The extrinsic and the intrinsic apoptosis signaling pathway.
In our own work, we described that the intracellular signaling pathway of TRAIL-induced apoptosis paralleled to a great extend the apoptosis signaling pathway activated by other members of the TNF superfamily. Cytotoxic drugs were able to facilitate apoptosis induction by TRAIL leading to synergistic apoptosis induction by TRAIL and cytotoxic drugs. On the other hand, TRAIL activated the transcription factor NFkappaB in tumour cells. To our surprise, in tumour cells resistant against induction of apoptosis by TRAIL, TRAIL induced survival and even proliferation of tumour cells. TRAIL-induced proliferation was exclusively found in tumour cells which we called group P tumour cells and which contained a receptor-proximal mechanism of resistance against TRAIL-induced apoptosis such as loss of caspase-8 or dysfunction of FADD. In group AP tumour cells, either TRAIL-induced apoptosis or TRAIL-induced proliferation were found depending on the concentration of TRAIL used suggesting that both the apoptosis and the proliferation signalling pathway were present and functional in a single tumour cell. Taken together, we showed that TRAIL mediates two opposing effects on tumour cells, apoptosis and proliferation. Our aim is to characterize both signaling pathways activated by TRAIL in order to inhibit TRAIL-induced proliferation and to intensify TRAIL-induced apoptosis.
Figure 3: Aim of our research: Increase of TRAIL-induced apoptosis, inhibition of TRAIL-mediated proliferation
