Age-Associated YBX1 Phosphorylation Regulates the Keratinocyte Senescence-Associated Secretory Phenotype Through Translational Control.

Skin aging is characterized by epidermal atrophy, reduced keratinocyte proliferation, and the accumulation of senescent cells that sustain a chronic, low-grade inflammatory secretome known as the senescence-associated secretory phenotype (SASP). We previously showed that Y-box binding protein 1 (YBX1) limits keratinocyte senescence in human epidermis by acting as a translational repressor of SASP chemokines, including CXCL1 and IL8. How this brake is regulated during skin aging, however, remains undefined. Here we demonstrate that, although total YBX1 protein is reduced in keratinocytes from aged human epidermis, the fraction of phosphorylated YBX1 (pYBX1) is increased relative to young donors, resulting in an elevated pYBX1/total YBX1 ratio that correlates with chronological age. Because pYBX1 is predominantly nuclear, whereas unphosphorylated YBX1 is cytoplasmic, this shift is predicted to deplete the cytoplasmic pool available for translational repression of CXCL1 and IL8. Consistent with this model, treatment of immortalized (Ker-CT) and primary human keratinocytes (HK) with the PI3K inhibitors PI-103 and GDC-0941 suppressed YBX1 phosphorylation without altering total YBX1 abundance, thus retaining YBX1 in the cytoplasm. This allows cytoplasmic YBX1 to re-engage and repress the translation of