Inositol pyrophosphates modulate hydrogen peroxide signalling

Inositol pyrophosphates are involved in a variety of cellular functions, but the specific pathways and/or downstream targets remain poorly characterized. In the present study we use Saccharomyces cerevisiae mutants to examine the potential roles of inositol pyrophosphates in responding to cell damage caused by ROS (reactive oxygen species). Yeast lacking kcs1 [the S. cerevisiae IP6K (inositol hexakisphosphate kinase)] have greatly reduced IP₇ (diphosphoinositol pentakisphosphate) and IP₈ (bisdiphosphoinositol tetrakisphosphate) levels, and display increased resistance to cell death caused by H₂O₂, consistent with a sustained activation of DNA repair mechanisms controlled by the Rad53 pathway. Other Rad53-controlled functions, such as actin polymerization, appear unaffected by inositol pyrophosphates. Yeast lacking vip1 [the S. cerevisiae PP-IP5K (also known as IP7K, IP₇ kinase)] accumulate large amounts of the inositol pyrophosphate IP₇, but have no detectable IP₈, indicating that this enzyme represents the physiological IP₇ kinase. Similar to kcs1Δ yeast, vip1Δ cells showed an increased resistance to cell death caused by H₂O₂, indicating that it is probably the double-pyrophosphorylated form of IP₈ [(PP)₂-IP₄] which mediates the H₂O₂ response. However, these inositol pyrophosphates are not involved in directly sensing DNA damage, as kcs1Δ cells are more responsive to DNA damage caused by phleomycin. We observe in vivo a rapid decrease in cellular inositol pyrophosphate levels following exposure to H₂O₂, and an inhibitory effect of H₂O₂ on the enzymatic activity of Kcs1 in vitro. Furthermore, parallel cysteine mutagenesis studies performed on mammalian IP6K1 are suggestive that the ROS signal might be transduced by the direct modification of this evolutionarily conserved class of enzymes.