Modulation of glycogen synthase kinase-3β following TRAIL combinatorial treatment in cancer cells
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Sreevidya Santha1, Gantulga Davaakhuu1, Aninda Basu1, Rong Ke1, Subhasis Das1, Ajay Rana1,2,3, Basabi Rana1,2,3
1Department of Surgery, Division of Surgical Oncology, University of Illinois at Chicago, Chicago, IL 60612, USA
2University of Illinois Hospital and Health Sciences System Cancer Center, University of Illinois at Chicago, Chicago, IL 60612, USA
3Jesse Brown VA Medical Center, Chicago, IL 60612, USA
Basabi Rana, email: [email protected]
Keywords: GSK3β, AKT, PPARγ, AMPK, apoptosis-resistance
Received: July 26, 2016 Accepted: August 29, 2016 Published: September 02, 2016
Glycogen Synthase Kinase-3β (GSK3β) is a serine/threonine kinase, known to regulate various cellular processes including proliferation, differentiation, survival, apoptosis as well as TRAIL-resistance. Thus pathways that can modulate GSK3β axis are important targets for cancer drug development. Our earlier studies have shown that combinatorial treatment with Troglitazone (TZD) and TRAIL can induce apoptosis in TRAIL-resistant cancer cells. The current studies were undertaken to investigate whether GSK3β pathway was modulated during this apoptosis. Our results indicated an increase in inhibitory GSK3βSer9 phosphorylation during apoptosis, mediated via AKT. At a later time, however, TZD alone and TRAIL-TZD combination produced a dramatic reduction of GSK3β expression, which was abolished by cycloheximide. Luciferase assays with GSK3β-luc promoter reporter showed that TZD can effectively antagonize GSK3β promoter activity. Since TZD is a ligand for transcription factor PPARγ and can activate AMPK, we determined their roles on antagonism of GSK3β. Knockdown of PPARγ was unable to restore GSK3β expression or antagonize GSK3βSer9 phosphorylation. Although pretreatment with Compound C (pharmacological inhibitor of AMPK) partially rescued GSK3β expression, knockdown of AMPKα1 or α2 alone or in combination were ineffective. These studies suggested a novel PPARγ-AMPK-independent mechanism of targeting GSK3β by TZD, elucidation of which might provide newer insights to improve our understanding of TRAIL-resistance.
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