Circumventing Cisplatin Resistance In Ovarian Cancers Through Reactivation of P53 By Non-Cross-Resistant Platinum Analogs

<p><strong>Abstract</strong></p> <p><strong>CIRCUMVENTING CISPLATIN RESISTANCE IN OVARIAN CANCERS THROUGH REACTIVATION OF P53 BY NON-CROSS-RESISTANT PLATINUM ANALOGS</strong></p> <p><strong>Michelle Martinez-Rivera, B.S.</strong></p> <p><strong>Advisory Professor: Zahid H. Siddik, Ph.D.</strong></p> <p>Cisplatin (cis-Pt), an anticancer platinum (Pt) drug, is used widely in the treatment of several malignancies, such as ovarian cancer. This Pt compound induces DNA damage, which results in p53 activation through post-translational modifications, mainly phosphorylation, culminating in execution of programmed cell-death. However, despite initial therapeutic response to cis-Pt, clinical resistance to this drug emerges leading to disease progression. Pt-resistance phenotypes have been associated with dysfunction in the p53 signaling pathway. Therefore, an effort to understand molecular mechanisms that prevent p53 activity and induce cis-Pt resistance becomes vital for designing Pt-based drugs able to re-activate p53 and improve clinical management of ovarian cancer patients. To investigate the mechanism responsible for p53 inactivation, an ovarian tumor panel composed of cis-Pt sensitive (A2780) and resistant (2780CP/Cl-16, OVCAR-10, HEY and OVCA 433) cell lines was established, with two (2780CP/Cl-16 and OVCAR-10) harboring missense mutant p53. The data obtained from these cancer cell lines have identified a correlation between cis-Pt resistance, regardless of p53 status (wild-type vs. mutant), and lack of phosphorylation of p53 at Ser20 after cis-Pt treatment. Cis-Pt resistant cell lines expressed low levels of Chk2, a kinase responsible to phosphorylate p53 at Ser20, as a common feature. It was confirmed, through the generation of Chk2 knock-out clones from A2780 cells using the CRISPR/Cas9 system, that Chk2 is essential for cis-Pt to mediate phosphorylation of p53 at Ser20 and induce p53 transcriptional activity. As validation of its critical role, Chk2 knock-out in these cells leads to cis-Pt resistance. However, cis-Pt resistance was circumvented by a number of cis-Pt analogs. In this regard, oxaliplatin (oxali-Pt), a non-cross-resistant Pt analog currently used in colon cancer but not ovarian cancer, was the most effective. Interestingly, the mechanism for oxali-Pt involved restoration of p53 phosphorylation at Ser20 through a Chk2 independent pathway. RPPA analysis has identified the MAPK pathway as a possible target of activation by oxali-Pt to phosphorylate p53 at Ser20. Systematic studies using targeted inhibitors have identified MEK1/2, but not ERK1/2, as a novel biomarker important to mediate p53-Ser20 phosphorylation by oxali-Pt. Overall, the findings gathered in this research project have revealed Ser20 of p53 as a key site that induces cis-Pt resistance when its phosphorylation is not induced by cis-Pt due to loss of Chk2, whereas its phosphorylation by oxali-Pt via MEK1/2 leads to circumvention of this resistance. This knowledge may lead to repurposing oxali-Pt in ovarian cancer and improve survival of cancer patients.</p>

Name thesis:degree_name

Doctor of Philosophy (PhD)

Level thesis:degree_level

Dissertation (PhD)

Year dc:date.available

2016

Author dc:creator

• Martinez-Rivera, Michelle

Contributors dc:contributor

• Zahid H. Siddik, Ph.D.
• Gary Gallick, Ph.D.
• Peng Huang, MD, Ph.D.

• cisplatin
• platinum resistance
• oxaliplatin
• p53
• ovarian cancer
• Chk2
• MEK1/2
• Inorganic Chemicals
• Medicine and Health Sciences
• Molecular Biology
• Therapeutics

Repository record dc:identifier

https://digitalcommons.library.tmc.edu/utgsbs_dissertations/694

OAI identifier oai:identifier

oai:digitalcommons.library.tmc.edu:utgsbs_dissertations-1739