This proposal's long-term objective is to provide a fundamental mechanistic understanding of the role of nucleus vs. mitochondria in the activation of the senescence program. Senescence is a central cellular defense mechanism that removes damaged cells to maintain tissue integrity and prevent tumorigenesis. The accumulation of senescent cells, which express a copious amount of inflammatory cytokines, termed senescence-associated secretory phenotype (SASP), is a significant contributing factor to organismal aging. In the last decade, studies have identified the disruption of nuclear membrane (lamina) integrity and subsequent release of nuclear DNA (nDNA) to the cytosol as the primary trigger of senescence and premature aging, progeria. On the other hand, experimental evidence has long implied mitochondria in senescence and aging. However, the exact role of mitochondria in senescence remains unknown. We have found that during senescence, mitochondrial DNA (mtDNA) contents were elevated significantly and mitochondria undergo elaborate fusion. Gene expression analysis of senescent cells revealed coordinated upregulation of ABAT and RRM2B, two genes that are required for mtDNA synthesis and maintenance. These genes are also elevated in mice of old age. We found that pharmacological inhibition of mtDNA synthesis suppressed SASP but did affect senescence-associated growth arrest. These findings uncover a unique signaling role of mtDNA in SASP and coordinated mitochondrial remodeling during senescence. This proposal aims to delineate the functional significance of the nuclear and mitochondrial pathway in SASP associated with senescence and aging and investigate the regulation and roles of mitochondrial dynamics in the activation of SASP.