In the past years extensive efforts have been made to elucidate single molecular steps in pathways of transducing a lethal signal into tumor cells. Understanding the failure of apoptosis may enable the development of new therapeutic agents that interact with these pathways.
TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) was discovered in 1995 on the basis of sequence homology to tumor necrosis factor (TNF) and fas ligand (FasL). Due to the selective induction of apoptosis in many tumor cells sparing untransformed cells, TRAIL became attractive as a potential antitumor therapeutic agent. This challenging attribute stands in contrast to TNF and FasL, which indeed induce cell death in tumor cells, but provoke lethal systemic toxicity. TRAIL binds to five different receptors, whereas only the death receptor 4 (TRAIL R1) and death receptor 5 (TRAIL R2) elicit an apoptotic response. The remaining receptors are antagonizing receptors. The so-called “decoy” receptor 1 and 2 (TRAIL R3 and TRAIL R4) and the receptor named osteoprotegerin, with a weaker affinity, are capable of binding the ligand but lack a functional cytoplasmatic signal-transducing domain. It is still unclear, whether this anti-apoptotic mechanism is achieved by pure ligand depletion or an additional interaction between decoy and death receptors on the cell surface.
The study by Lee and colleagues (see article this issue) analyzes signal transduction pathways in the human carcinoma cell line, HeLa, triggered by TRAIL. Interestingly, the authors focus on anti-apoptotic effects. They describe a new mechanism to achieve a reduced rate of apoptosis. TRAIL was applied to the cell culture system, which resulted in the expected loss of cell viability up to 40 percent in a time dependent manner. Simultaneously, the concentration of activated (phosphorylaled) ERK1/2 rose. The activation of this transcription factor upregulates downstream genes such as an anti-apoptotic protein Bcl-2. In a second step, the authors induced apoptosis by TRAIL and inhibited mitogen-activated protein kinase by addition of PD 10, which abolished both the phosphorylation of ERK1/2, and, as a consequence, decreased Bcl-2 expression. In this experiment, cell death occurred earlier and overall apoptotic rate increased even more. This demonstrates a new mechanism: HeLa cells protect themselves against apoptotic stimuli through upregulation of anti-apoptotic pathways, like the previously described activation of NFêB following TNF stimulation. Afterwards, Lee et al. raised the interesting question, what kind of upstream kinase, apart from mitogen-activated protein kinase, may activate ERK1/2 and could be blocked by specific inhibitors. By means of different inhibitors, Lee et al. demonstrated that tyrosine kinase, but not protein kinase C, is involved in activation of ERK1/2. In summary, this article elucidates the activation of tyrosine kinase followed by a phosphorylation of ERK1/2 and upregulation of Bcl-2, which protects Hela cells against TRAIL-induced apoptosis.
We still do not know to what extent the expression of genes inhibiting apoptosis contribute to resistance to cancer therapy. Nevertheless, the understanding of anti-apoptotic pathways helps to provide new insights in this field.