Plasminogen activator urokinase expression reveals TRAIL responsiveness and supports fractional survival of cancer cells

V Pavet,Y Shlyakhtina,P Kohonen,T He,H Gronemeyer,D G Ceschin,M Perälä,O Kallioniemi

doi:10.1038/cddis.2014.5

Abstract

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL/TNFSF10/Apo2L) holds promise for cancer therapy as it induces apoptosis in a large variety of cancer cells while exerting negligible toxicity in normal ones. However, TRAIL can also induce proliferative and migratory signaling in cancer cells resistant to apoptosis induced by this cytokine. In that regard, the molecular mechanisms underlying the tumor selectivity of TRAIL and those balancing apoptosis versus survival remain largely elusive. We show here that high mRNA levels of PLAU, which encodes urokinase plasminogen activator (uPA), are characteristic of cancer cells with functional TRAIL signaling. Notably, decreasing uPA levels sensitized cancer cells to TRAIL, leading to markedly increased apoptosis. Mechanistic analyses revealed three molecular events taking place in uPA-depleted cells: reduced basal ERK1/2 prosurvival signaling, decreased preligand decoy receptor 2 (DcR2)-death receptor 5 (DR5) interaction and attenuated recruitment of DcR2 to the death-inducing signaling complex upon TRAIL challenge. These phenomena were accompanied by increased FADD and procaspase-8 recruitment and processing, thus guiding cells toward a caspase-dependent cell death that is largely independent of the intrinsic apoptosis pathway. Collectively, our results unveil PLAU mRNA levels as marker for the identification of TRAIL-responsive tumor cells and highlight a key role of uPA signaling in ‘apoptosis versus survival' decision-making processes upon TRAIL challenge.

Highlights

DcR2 at the cell surface, which can trigger survival signaling,[9,10,11] the recruitment of the non-functional procaspase homolog cFlip[12,13,14] and the relative levels of proteins that regulate the activation of the intrinsic mitochondrial pathway and/or modulate the activity of executor caspases.[15,16]
We observed that normal cells from epithelial and mesenchymal origins (human embryonic kidney (HEK), foreskin fibroblasts (BJ), respectively) are resistant to TRAIL-induced cell death, whereas sensitivity to apoptosis is acquired along the transformation process, with transformed cells displaying the major apoptotic rate (Figure 1b)
Acquisition of resistance to TRAIL-induced cell death during neoplastic transformation may be based on two general mechanistic principles:[23] (i) key components of the TRAIL pathway may be silenced and/or mutated, resulting in a non- or subfunctional signaling cascade and (ii) cancer cells may hijack a functional TRAIL-apoptotic pathway in vivo through the activation of survival cascades that counterbalance TRAIL-induced cell death

Summary

Introduction

DcR2 at the cell surface, which can trigger survival signaling,[9,10,11] the recruitment of the non-functional procaspase homolog cFlip[12,13,14] and the relative levels of proteins that regulate the activation of the intrinsic mitochondrial pathway and/or modulate the activity of executor caspases.[15,16]. TRAIL, cancer cells can be grouped into three classes: (i) the entire population commits to apoptosis, (ii) is inherently resistant to cell death induced by TRAIL or (iii) only a part of the cell population dies, while a significant fraction survives the treatment and may even proliferate (referred to as ‘fractional killing’).[21] From the mechanistic point of view, it was shown that activation of survival pathways by TRAIL involves assembly of a secondary signaling complex that retains the DISC components FADD and caspase-8, and recruits RIP1, TRAF2, TRADD and NEMO/IKK.[22] contrary to the well-known cellular mechanisms mediating TRAIL-induced cell death, the understanding of how TRAIL and TRAIL receptors induce the formation of this secondary complex and the triggering of non-apoptotic cascades is still in its infancy. We show that uPA depletion alters the DISC composition by reducing the preligand and TRAIL-induced DcR2–DR5 interaction These molecular events lead to the formation of an apoptosis-proficient DISC, resulting in pronounced caspase-dependent apoptosis and preventing the generation of resistant populations. Our work unveils an intricate cross-talk between uPA and TRAIL signaling, which highlights the use of PLAU mRNA as a marker of response and as a potential target for improving the apoptogenic action of TRAIL

Methods

Results

Conclusion