Investigating Myocardial PI3Kα Signaling in Models of Ischemia Reperfusion and Cardiotoxic Cancer Therapies

• Author / Creator

  McLean, Brent A

• Background. Phosphoinositide 3-kinase (PI3K) is a lipid kinase that functions as a central intracellular signaling transduction node, receiving input from cell receptors and initiating a cascade of cellular signaling that regulates and integrates central cellular processes such as glucose uptake, growth and survival. This PhD thesis is primarily concerned with the PI3Kα isoform and its role in normal and pathological heart physiology. Background—Ischemia/reperfusion injury. PI3K signaling, including downstream activation of Akt, is regularly cited as causing cellular protective effects in experimental ischemia/reperfusion (IR) injury; however, a transgenic mouse with reduced PI3Kα activity is dramatically protected from IR injury. Objective. Elucidation of the mechanism(s) underlying this phenomenon could lead to an improved understanding of both IR injury and PI3Kα functions. Two chapters of this thesis show experiments aimed at understanding why reduced PI3Kα is protective. Methods and Results. In chapter 3 I test the contribution of the PI3Kγ isoform by using PI3Kγ knockout mice in Langendorff perfusions, metabolic substrate utilization through working heart perfusions, and downstream signaling. IR protection was not dependent on PI3Kγ or glucose oxidation but ERK activation was increased. In chapter 4 I test the importance of downstream signaling through Langendorff perfusions with selective inhibitors of the Akt and ERK pathways, and test an inducible, cardiomyocyte specific PI3Kα knockout model. Then, I investigate mechanisms of cellular injury, apoptosis and necrosis, which lead me to consider known regulators of necrotic cellular injury as possibly altered in PI3Kα deletion hearts. IR protection was not dependent on acute Akt or ERK activation. Reduced PI3Kα prevented necrotic cell death upon IR and preserved mitochondrial membrane potential, investigated with live tissue imaging. I investigated reported regulators of necrotic cell death, RIP1/3, CaMKII, Bax Bak, Cyp-D through Western blot, Co-IP cell fractionation, but did not find a correlation with the PI3Kα phenotype. Conclusions. PI3Kα deletion causes robust IR protection and maintained mitochondrial function, but not through commonly recognized mechanisms of IR protection. Background—Heart function, cardiotoxicity of PI3Kα inhibition. The role of PI3Kα in maintaining normal heart function and morphology is an ongoing question, and is topical due to the numerous PI3K inhibitors currently under development as cancer treatments. Objective. Assess heart function under both genetic and pharmacological PI3Kα inhibition in otherwise healthy adult mice, and in the context of the cytotoxic cancer therapy doxorubicin. Methods and Results. In chapter 5 I use multiple methods to delete or inhibit PI3Kα and assess heart and cardiomyocyte function. I found that PI3Kα does not directly control heart function, but it does increase the vulnerability of the heart to the cytotoxicity of Cre recombinase with tamoxifen administration. In chapter 6 I look at adverse effects of PI3Kα inhibition/deletion in the context of a commonly used cytotoxic cancer therapy doxorubicin. In the context of doxorubicin chemotherapy, PI3Kα inhibition caused severe weight loss, heart atrophy and distinct right ventricular dilation. Upon examination of underlying molecular changes, I found increased redox stress and p38 MAPK activation in combination treated hearts. Therapeutic inhibition of p38 MAPK partially ameliorated heart atrophy and dysfunction. Conclusions. PI3Kα inhibition is well tolerated by the heart, except when combined with the additional stress of cytotoxic Cre-tamoxifen or doxorubicin. PI3Kα inhibition can cause body weight and muscle loss, as well as heart atrophy, which are already common and serious morbidities present in cancer patients. The right ventricle may be particularly vulnerable to atrophy and redox stress.

• Subjects / Keywords

  • Ischemia
  • Heart Physiology

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3TH8C05G

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.