Therapeutic targeting of chemotherapy-resistant colorectal cancer stem cells with p73- and TRAIL-based small molecules

Open Access
Author:
Prabhu, Varun Vijay
Graduate Program:
Molecular Medicine
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 20, 2016
Committee Members:
  • Wafik S El Deiry, Dissertation Advisor
  • Charles H Lang, Committee Chair
  • Hong Gang Wang, Committee Member
  • Edward Joseph Gunther, Special Member
Keywords:
  • stem cell
  • cancer
  • p73
  • p53
  • TRAIL
  • small molecule
  • ONC201
Abstract:
Colorectal cancer (CRC) tumors contain a rare population of self-renewing stem cell-like cells (CRCSCs) that contribute to tumor maintenance and therapy resistance. Therapeutic targeting of CRCSCs could improve treatment response and prolong patient survival. The endogenous TNF-related apoptosis-inducing ligand (TRAIL) protein selectively induces cell death in cancer cells while sparing normal cells. Small molecule ONC201/TIC10 induces TRAIL-mediated cell death in cancer cells via transcription factor Foxo3a, which is activated by dual inhibition of Akt and ERK. ONC201/TIC10 is currently being tested in phase I/II clinical trials for patients with advanced cancer. This study investigates whether the potent anti-tumor effect of ONC201/TIC10 in CRC involves targeting CRCSCs. ONC201/TIC10 depletes CRCSC markers and prevents chemotherapy-resistant CRCSC-mediated self-renewal in colonosphere assays and xenograft tumor initiation studies in mice. ONC201/TIC10 significantly induces cell surface TRAIL in CRCSCs. ONC201/TIC10-mediated anti-CRCSC effect is significantly blocked by the TRAIL sequestering antibody RIK-2, overexpression of Akt and Foxo3a knockdown. The identification and demonstration of CRCSCs as a target of ONC201/TIC10 provides an innovative biomarker that could be monitored in the clinic. Tumor suppressor p53 is frequently mutated or inactivated in CRC. In contrast, p53 family member p73 is rarely mutated in CRC and p73 activation elicits p53-like tumor suppression. p53 restoration is known to target CRCSCs, but p73 restoration in CRCSCs has not been examined. This study investigates the effects of small molecule prodigiosin-mediated p73 activation on CRCSCs. Prodigiosin reduces chemotherapy-resistant CRCSC-mediated colonosphere formation and xenograft tumor initiation. Mechanistic studies reveal that prodigiosin increases the levels of p73 and reduces levels of the oncogenic N-terminally truncated isoform ΔNp73 in CRCSCs. Accordingly, p73 knockdown or ΔNp73 overexpression suppress prodigiosin-mediated CRCSC inhibition. Moreover, prodigiosin increases levels of the transcription factor c-Jun, a regulator of p73 and ΔNp73. c-Jun knockdown attenuates prodigiosin-mediated ΔNp73 downregulation, p73 activation and cell death. These findings have previously unrecognized implications for the use of p73 activating therapeutics and ΔNp73 inhibition for targeting CRCSCs. Together, these results demonstrate that potent small molecules targeting CRCSCs via tumor suppressive pathways such as p73 and TRAIL are a viable potential approach for robust and durable cancer therapy.