Inorganic pyrophosphatase defects lead to cell cycle arrest and autophagic cell death through NAD+-Depletion in fermenting yeast

Abstract

Pyrophosphate removal is essential for several central biosynthetic reactions to proceed in the appropriate direction, e.g. NAD+, DNA, RNA synthesis or tRNA amino acylation. Defects in the genes encoding the pyrophosphate hydrolytic enzymes are considered inviable, although its exact nature has not been studied at the cell physiology level. Using a conditional mutant in IPP1, the Saccharomyces cerevisiae cytosolic soluble pyrophosphatase, we show that respiring cells arrest in S-phase upon pyrophosphatase deficiency but they are viable and resume growth if accumulated pyrophosphate is washed out. However, fermenting cells arrest in G1/G0 phase and suffer massive vacuolisation and eventual cell death by autophagy. Death is caused by impaired NAD+ metabolism and can be inhibited under conditions favouring accumulation of the oxidised coenzyme form. These results link pyrophosphate toxicity to the type of cell energy metabolism. Fermenting yeast cells are a good model for cancer cell metabolism (Warburg effect) and these data can help to understand the antitumoral effect of pyrophosphatase inhibitors such as bisphosphonates and the cellular targets of pyrophosphate accumulation.