Lee, Jong-Sun

Cellular senescence defines an irreversible cell growth arrest state linked to loss of tissue function and aging in mammals. This transition from proliferation to senescence is typically characterized by increased expression of the cell-cycle inhibitor p16 INK4a and formation of senescence-associated heterochromatin foci (SAHF). SAHF formation depends on HIRA-mediated nucleosome assembly of histone H3.3, which is regulated by the serine/threonine protein kinase Pak2. However, it is unknown if Pak2 contributes to cellular senescence. Here, we show that depletion of Pak2 delayed oncogene-induced senescence in IMR90 human fibroblasts and oxidative stress–induced senescence of mouse embryonic fibroblasts (MEFs), whereas overexpression of Pak2 accelerated senescence of IMR90 cells. Importantly, depletion of Pak2 in BubR1 progeroid mice attenuated the onset of aging-associated phenotypes and extended life span. Pak2 is required for expression of genes involved in cellular senescence and regulated the deposition of newly synthesized H3.3 onto chromatin in senescent cells. Together, our results demonstrate that Pak2 is an important regulator of cellular senescence and organismal aging, in part through the regulation of gene expression and H3.3 nucleosome assembly.

Significance Nucleosome assembly is regulated at multiple levels to impact distinct cellular processes. Mutations in factors involved in nucleosome assembly, such as histone chaperones and histone variants, result in genome instability and gene expression defects that, in turn, promote the development of human disease including cancer and aging. Therefore, it is important to determine how nucleosome assembly of H3.3 is regulated. Our findings demonstrate a role for O-linked N -acetylglucosamine (GlcNAc) transferase in regulating H3.3 deposition/exchange and establish the O-GlcNAc modification of HIRA as a previously unidentified mechanism regulating nucleosome assembly of H3.3 and cellular senescence.