Plk1 in DNA replication: Implication for chemoresistance
Abstract
Two critical steps during eukaryotic cell proliferation are duplication of the genome in S phase and separation of the duplicated genome into two daughter cells in mitosis. Tight regulation of the cell cycle is vital in order to maintain genome stability and cell survival. Polo-like kinase 1 (Plk1), a Ser/Thr kinase, plays many important roles in cell cycle progression. However, compared with established functions of Plk1 in almost every step of mitosis, little is known about its potential function in DNA replication. In this study, Origin Recognition Complex 2 (Orc2) was identified as a Plk1 substrate during S phase. Plk1 phosphorylates Orc2 at Ser188 in vitro and in vivo. Under conditions of replication stress, Plk1-mediated phosphorylation of Orc2 maintains the pre-replicative complex (pre-RC) on DNA replication origin, thus to ensure smooth S-phase progression. Depletion of Plk1 causes intra-S-phase checkpoint activation, likely due to lack of Orc2-Ser188 phosphorylation. Identification and characterization of Orc2 phosphorylation by Plk1 provide a unique mechanism for cells to continue DNA replication in the presence of stress Gemcitabine, a deoxycytidine analogue which can prevent DNA replication, is the standard chemotherapy treatment for advanced pancreatic cancer. However, almost all patients have either primary or eventually gain secondary resistance to gemcitabine treatment. Plk1 was found to be overexpressed in pancreatic tumors. Based on the findings of Plk1 in DNA replication, the correlation between Plk1 and gemcitabine resistance in pancreatic cancer was examined next. Plk1 expression is correlated with gemcitabine sensitivity in vitro and in vivo. Inhibition of Plk1 activity significantly enhances the antitumor effect of gemcitabine in a xenograft model. Mechanistically, Plk1-mediated phosphorylation of Orc2 at the origin of DNA replication is increased upon gemcitabine treatment to maintain DNA replication. This phosphorylation contributes to gemcitabine-resistance, as the cells expressing Plk1 unphosphorylatable mutant of Orc2 are more sensitive to gemcitabine treatment. Moreover, Plk1 phosphorylation of histone acetyltransferase binding to the origin recognition complex 1 (Hbo1) up-regulates transcription of cFos, consequently results in an elevation of its target Multi Drug Resistance 1 (MDR1). Last but not least, during Alzheimer's disease (AD) progression, researches showed that the terminally differentiated neurons in adult central neuronal system attempt to re-enter the cell cycle, as indicated by elevation of cell cycle markers and completion of DNA replication. An in vitro cell culture model to mimic the neuronal cell cycle re-entry during the pathogenesis of AD was established. Plk1 was found to be a critical player in the pathogenesis of AD. Taken together, these data provide evidence to support that Plk1 involves in the regulation of DNA replication under stressful conditions, likely open a new research direction for Plk1. Furthermore, it suggests that Plk1 might be a promising target for treatment of gemcitabine-resistant pancreatic cancer and AD.
Degree
Ph.D.
Advisors
Liu, Purdue University.
Subject Area
Molecular biology|Genetics|Cellular biology
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