Insights into the regulation of cellular phosphate homeostasis

Abstract

The inositol polyphosphates (InsPs) are a unique class of signaling molecules that have been implicated in the promotion of tumor growth and metastasis. In yeast and plants, the InsPs have been recognized as regulators of phosphate homeostasis. In mammalian systems however, the regulatory role of InsPs in metabolic networks has not been addressed. Although little is known about the molecular mechanisms controlling phosphate metabolism in humans, there is evidence linking phosphate availability to cancer progression. Investigating the role of phosphate in cancer metabolism and its modulation by InsPs can therefore provide a fundamental understanding of tumor cell requirements. Combining metabolomic and chemical biology approaches, we characterized the metabolic response of cancer cells to changes in phosphate availability and interrogated the modulation of these responses by InsPs. By investigating the metabolomic profiles of cells grown under phosphate deprivation, we observed a direct effect on nucleotide biosynthesis and one-carbon metabolism, as evidenced by the accumulation of purine intermediates and the impairment of serine carbon incorporation. We then applied chemically synthesized affinity reagents that led to the identification of AMP deaminase and GMP reductase as interaction partners of the small molecule messenger 5-diphosphoinositol pentakisphosphate (5PP-InsP5). Additional experiments showed that genetic deletion of the enzymes responsible for the production of 5PP-InsP5, the inositol hexakisphosphate kinases, phenocopied the metabolic effects observed under conditions of phosphate starvation. Data presented in this thesis provides a clear link between InsPs, purine biosynthesis, and phosphate homeostasis. Our long term goal is to fully annotate phosphate-dependent metabolic pathways and elucidate their potential implications in energy expenditure, cancer, and development.