Project 1: Ionic Modulation of Chromatin in Cancer

Project Co-Leaders
Thomas V. O’Halloran, Chemistry, Northwestern University
Andrew P. Mazar, Pharmacology, Northwestern University

Team Members
Jack Kaplan, Biochemistry and Molecular Genetics, University of Illinois-Chicago
Vadim Backman, Biomedical Engineering, Northwestern University
Igal Szleifer, Biomedical Engineering, Northwestern University
Ming Zhao, Medicine, Feinberg School of Medicine, Northwestern University
John Marko, Molecular Biosciences, Northwestern University
Navdeep Chandel, Medicine, Feinberg School of Medicine, Northwestern University

Project Summary
Cellular potassium levels play a central role in regulating physiological processes and are maintained within narrow limits.  Recent studies reveal that aggressive and highly metastatic breast and multiple myeloma cancer cells maintain potassium concentrations that are several fold higher than matched non-tumorigenic cells. Since the strongest physiochemical interactions involved in reversible chromatin condensation are electrostatic, any perturbations in the ionic environment of the nucleus, such as those driven by a pathophysiological elevation of cellular K+ content, are anticipated to have profound effects on chromatin structure and access to transcriptional machinery.   Thus this discovery has global physiological implications.  It also has the potential to provide a mechanistic framework for a variety of reports showing that elevated potassium levels suppress cell death signaling pathways and may account for the large number of potassium channels that have been shown to play a role in cancer progression.  In this proposal we test the hypothesis that alterations in intracellular K+ and ionic strength alter chromatin structure and nuclear organization, and consequently global gene expression. These experiments will be performed across many length scales using Partial Wave Spectroscopy and STORM methods in the Nanocytometry Core: from intact living cells, to isolated nuclei and metaphase chromosomes and finally on nucleosome core particles.  To address our hypothesis we will develop new physical methods to: (a) understand the impact of potassium concentration on chromatin structure at the physical level in tumor cells; (b) probe the relationship between elevated potassium and clinical stage and grade of human tumors; and (c) probe whether this facet of cancer cell physiology can be exploited in the design of new combination chemotherapies. These advances in understanding ion imbalances in cancer may allow for the repurposing of current FDA-approved agents, such as diuretics that work by modulating intracellular potassium levels.  This project is central to the overarching framework of the CR-PSOC “Spatio-Temporal Dynamics of Chromatin and Information Transfer in Cancer” because it addresses physical changes in the nuclear environment that are important in cancer progression.  In collaboration with the Project 3 team we will determine the extent of chromatin compaction in intact nuclei, and with the Project 2 team, we will test roles for potassium accumulation in leukemia.  The role of this Project in the Center is to resolve key electrostatic aspects of chromatin and its environment in the cancer cell nucleus. These new ideas and insights will be applied to understand and ultimately intervene in disease progression. This physical chemistry approach may lead to more effective therapeutic interventions using existing drugs.

Aim 1. What subcellular potassium distributions and concentrations are characteristic of highly tumorigenic cancer cells?  The working hypothesis is that tumorigenic cells gain a series of phenotypic advantages as a result of elevated cellular potassium content.   New quantitative methods for mapping subcellular potassium distributions will be used in conjunction with pharmacological treatments that alter potassium transport to test the hypothesis that cancer cells concentrate potassium in the nucleus and, in doing so, open chromatin structure, drive aberrant patterns of gene expression and inactivate cell death mechanisms.

Aim 2. What are the cellular and molecular consequences when a cell maintains a persistent elevation of the potassium quota, and how does that relate to chromatin structure nuclear organization?  Changes in ionic strength have been central to the earliest insights into higher order chromatin structure and nucleosome organization.  We will test the hypothesis that high potassium levels in the cancer cell work in part to disrupt normal chromatin structure and nuclear architecture in a manner that supports the pathophysiological cancer phenotype.  Using a combination of force-extension measurements, physical methods and theory, we will evaluate how changes in potassium concentration alter chromatin structure in isolated nucleosomes and nuclei.  The effects of these physiological ionic strength changes on nuclear architecture, chromosome structure, glycolytic metabolism and macromolecular density will be examined in living cells.

Aim 3. Is malignancy correlated with a requirement for elevated potassium across other tumor types?  Is this inorganic phenotype an Achilles Heel of aggressive cancers that can be used to develop new therapeutic strategies?  The inorganic phenotype of breast cancer, multiple myeloma, and a series of graded and genotyped GBM cell lines from the Patient Derived Xenograft (PDX) Core will be determined.  Animal models will be evaluated using physical methods to determine how closely the potassium content scales with tumor grade. We will test the hypothesis that potassium metabolism altering drugs can be used to sensitize them to apoptosis-inducing therapeutic agents. Given the significant number of FDA-approved agents that alter potassium status, our strategic goal is to develop rational combinations of existing drugs for new clinical trials that exploit the emerging physiochemical differences between normal and highly tumorigenic cells.