SIRT1 regulates DNA damage signaling through PP4 phosphatase complex

The Sirtuin family of NAD+-dependent enzymes play an important role in the maintenance of genome stability upon stress. Several mammalian Sirtuins have been linked directly or indirectly to the regulation of DNA damage during replication through Homologous recombination (HR). The role of one of them, SIRT1, is intriguing as it seems to have a general regulatory role in DNA damage response (DDR) that has not been addressed. SIRT1-deficient cells show impairment of DDR reflected in a decrease in repair capacity, increased genome instability and decreased levels of H2AX. Here we unveil a close functional antagonism between SIRT1 and the PP4 phosphatase multiprotein complex in the regulation of DDR. Upon DNA damage SIRT1 interacts specifically with PP4C and promotes its inhibition through deacetylation of the domain WH1 of the PP4R3/ regulatory subunits. This in turn, regulates H2AX signal and RPA2 phosphorylation, two key events in the signaling of DNA damage and repair by HR. We propose a mechanism whereby during stress SIRT1 signaling ensures a global control of DNA damage through PP4.