Dynamics And Functions Of The P53 Signaling Network Including MicroRNAs

The study of signal transduction is important for uncovering the molecular mecha-nisms for essential cellular activities and revealing the essence of life.Cells are contin-uously monitoring the changes in their internal and external environments and respond properly upon stress signals.The tumor suppressor p53 mediates the cellular response to various stresses,including DNA damage,oncogene activation,telomere erosion and hypoxia.In this thesis,by constructing mathematical models and performing numerical simulations,we explore how the p53-centered signaling networks respond to double-strand breaks induced by ionizing radiation and hypoxia,respectively,elucidate the molecular mechanisms for cell-fate decision by characterizing the network dynamic-s,and probe the roles for microRNAs(miRNAs)in signal transduction.The thesis consists of four chapters.In Chapter 1,we introduce some biological backgrounds.The concepts of gene expression and the underlying mechanisms are first described.miRNAs are single-strand,non-coding RNAs with 20-24 nucleotides.miRNAs can inhibit the translation of message RNAs or target them for degradation,suppressing protein synthesis.A description of the synthesis of miRNAs,their functions and correlation with diseases is presented.We describe the structure and main functions of p53,modulation of its activity,and its roles in inducing cell-cycle arrest and apoptosis.Finally,it is pointed out that miRNAs join the p53 network to modulate cellular stress responses.In Chapter 2,we explore how miR-605 and miR-34a,direct transcriptional tar-gets of p53,affect the cell-fate decision following ionizing radiation.miR-605 and miR-34a function by acting upstream and downstream of p53,respectively.miR-605 promotes p53 accumulation by repressing the expression of mdm2,while miR-34a pro-motes p53-dependent apoptosis by suppressing the expression of antiapoptotic genes such as bcl-2.What roles they play in the p53-mediated DNA damage response is less well understood.Here,we develop a four-module model of the p53 network,char-acterizing the production and repair of DNA damage,ATM activation,p53-centered feedback loops,and cell-fate decision.Results of numerical simulation indicate that the cell fate is closely associated with network dynamics.The concentration of p53 undergoes few pulses in response to repairable DNA damage,or it first oscillates and then switches to high plateau levels after irreparable damage.The amplitude of p53 pulses rises to various extents depending on miR-605 expression,and miR-605 accel-erates the switching behavior of p53 levels to induce apoptosis.In parallel,miR-34a promotes apoptosis by enhancing the accumulation of free p53AIP1,a key proapop-totic protein.Thus,both miR-605 and miR-34a can mediate cellular outcomes and the timing of apoptosis.Together,miR-605 and miR-34a collectively contribute to apop-tosis induction and enhance the sensitivity of cells to DNA damage.These results have important implications:modulating apoptosis induction via miRNAs can improve the efficacy of cancer treatments.Lack of oxygen,or hypoxia,is typical of solid tumors.In Chapter 3,we explore the dynamic mechanism for cell-fate decision under hypoxia.We develop a mathemat-ical model of the p53 and hypoxia-inducible factor-la(HIF-l?)pathways.As tran-scription factors,p53 and HIF-1? compete for binding to limiting co-activator p300;they are targeted for degradation by Mdm2.We find that the interplay between p53 and HIF-l? significantly influences cellular outcome.In mild hypoxia,HIF-la accu-mulates and induces p21 to evoke transient cell-cycle arrest,whereas p53 is kept at basal levels.Consequently,the cell adapts to mild hypoxia.Under severe hypoxia,p53 accumulates and competes with HIF-1? for binding to p300,and induces apop-tosis by repressing the expression of miR-17-92.In anoxia,p53 is fully activated,transactivating PUMA and repressing miR-17-92 simultaneously.Thus,apoptosis is quickly induced.In contrast,HIF-la loses its transcriptional activity and is degrad-ed by Mdm2.Because p300-mediated acetylation prevents Mdm2-dependent protein degradation,the competition between p53 and HIF-la for binding to p300 not only de-termines their transcriptional activity,but also regulates their abundance.These results shed new light on the molecular mechanisms for hypoxic responses.In Chapter 4,a summary of main results is presented,and some outlooks for further research are also given.