Abstract
Glioblastoma (GBM) is the most common and aggressive brain tumor and is associated with poor prognosis. GBM cells are frequently resistant to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and finding new combinatorial therapies to sensitize glioma cells to TRAIL remains an important challenge. PIM kinases are serine/threonine kinases that promote cell survival and proliferation and are highly expressed in different tumors. In this work, we studied the role of PIM kinases as regulators of TRAIL sensitivity in GBM cells. Remarkably, PIM inhibition or knockdown facilitated activation by TRAIL of a TRAIL-R2/DR5-mediated and mitochondria-operated apoptotic pathway in TRAIL-resistant GBM cells. The sensitizing effect of PIM knockdown on TRAIL-induced apoptosis was mediated by enhanced caspase-8 recruitment to and activation at the death-inducing signaling complex (DISC). Interestingly, TRAIL-induced internalization of TRAIL-R2/DR5 was significantly reduced in PIM knockdown cells. Phospho-proteome profiling revealed a decreased phosphorylation of p62/SQSTM1 after PIM knockdown. Our results also showed an interaction between p62/SQSTM1 and the DISC that was reverted after PIM knockdown. In line with this, p62/SQSTM1 ablation increased TRAIL-R2/DR5 levels and facilitated TRAIL-induced caspase-8 activation, revealing an inhibitory role of p62/SQSTM1 in TRAIL-mediated apoptosis in GBM. Conversely, upregulation of TRAIL-R2/DR5 upon PIM inhibition and apoptosis induced by the combination of PIM inhibitor and TRAIL were abrogated by a constitutively phosphorylated p62/SQSTM1S332E mutant. Globally, our data represent the first evidence that PIM kinases regulate TRAIL-induced apoptosis in GBM and identify a specific role of p62/SQSTM1Ser332 phosphorylation in the regulation of the extrinsic apoptosis pathway activated by TRAIL.
Highlights
Glioblastoma multiforme, classified by World HealthOrganization (WHO) as grade IV astrocytoma, is the most common and aggressive brain tumor in adults.Median survival of GBM patients is 14.6 months[1]
To examine the role of PIM kinases in the regulation of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) resistance in GBM cells, we initially determined the effect of the PIM kinases inhibitor SGI-1776 in apoptosis induced by TRAIL in the human GBM cell line U87MG
GBM tumors are characterized by extensive hypoxia and recurrent disease arising from therapy resistant cancer cells that very often leads to elevated mortality
Summary
Median survival of GBM patients is 14.6 months[1]. Current therapy involves surgery, followed by radiation and adjuvant alkylating chemotherapy with temozolomide[2,3]. GBM is still a challenge for medical research and new therapies are urgently required. TRAIL/Apo2L is a cytokine of the tumor necrosis factor (TNF) gene superfamily that selectively induces apoptosis in many tumor cells while leaving normal cells intact and remains an attractive candidate for antitumor therapies[4]. TRAIL induces apoptosis upon binding to death domain (DD)-containing receptors TRAIL-R1/ DR4 and TRAIL-R2/DR5. This interaction activates the Official journal of the Cell Death Differentiation Association
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