Histone chaperone HIRA and PML are required for SASP expression but not for proliferation arrest

Cellular senescence, a stable proliferation arrest caused by a range of cellular stresses, is a bona fide cause of cell and tissue aging. As well as proliferation arrest, cell senescence is associated with a potent pro-inflammatory phenotype, the senescence-associated secretory phenotype (SASP). Given that SASP is regulated at various levels, gaining a comprehensive understanding of its regulation is crucial. This understanding can pave the way for potential therapeutic approaches aimed at modulating SASP, which could promote healthy aging and alleviate the burden of senescence-associated diseases and degeneration. We show here that both the Promyelocytic Leukemia (PML) protein and HIRA histone chaperone are required for SASP expression in senescent cells. PML protein is the key organizer of PML nuclear bodies, nuclear features up to 1mM in diameter, containing many proteins and previously implicated in diverse cellular processes, including control of cell senescence and cellular intrinsic anti-viral immunity. HIRA is a histone chaperone best known for its ability to incorporate histone variant H3.3 into nuclear chromatin in a DNA replication-independent manner, including in non-proliferating senescent cells. HIRA localizes to PML nuclear bodies in senescent cells. We show that both HIRA and PML are required for activation of NF-kB and SASP. We found that HIRA regulates cytoplasmic NF-kB signaling in senescent cells through CCF-cGAS-STING-TBK1 pathway and interaction with autophagy cargo receptor Sequestosome-1 (p62/SQSTM1).