The proteasome degradation machinery is essential for a variety of cellular processes including senescence and T‐cell immunity. Decreased proteasome activity is associated with the aging process; however, the regulation of the proteasome in CD4+ T cells in relation to aging is unclear. Here, we show that defects in the induction of the proteasome in CD4+ T cells upon T‐cell receptor (TCR) stimulation underlie T‐cell senescence. Proteasome dysfunction promotes senescence‐associated phenotypes, including defective proliferation, cytokine production and increased levels of PD‐1+ CD44High CD4+ T cells. Proteasome induction by TCR signaling via MEK‐, IKK‐ and calcineurin‐dependent pathways is attenuated with age and decreased in PD‐1+ CD44High CD4+ T cells, the proportion of which increases with age. Our results indicate that defective induction of the proteasome is a hallmark of CD4+ T‐cell senescence.

Summary The proteasome is a therapeutic target in cancer, but resistance to proteasome inhibitors often develops owing to the induction of compensatory pathways. Through a genome-wide siRNA screen combined with RNA sequencing analysis, we identified hexokinase and downstream O-GlcNAcylation as cell survival factors under proteasome impairment. The inhibition of O-GlcNAcylation synergistically induced massive cell death in combination with proteasome inhibition. We further demonstrated that O-GlcNAcylation was indispensable for maintaining proteasome activity by enhancing biogenesis as well as proteasome degradation in a manner independent of Nrf1, a well-known compensatory transcription factor that upregulates proteasome gene expression. Our results identify a pathway that maintains proteasome function under proteasome impairment, providing potential targets for cancer therapy.

Maintaining protein homeostasis is central to cell survival. The ubiquitin-proteasome system and autophagy play pivotal roles in protein quality control through protein degradation. Activities of these degradative pathways are carefully orchestrated, and autophagy is up-regulated during proteasome dysfunction for cellular homeostasis. However, the mechanism by which proteasome impairment induces compensatory autophagy has remained largely elusive. Here, we show that FAM48A mediates autophagy induction during proteasome inhibition. FAM48A is degraded by the proteasome and accumulates in cells by proteasome inhibition. Knockdown of FAM48A led to defective induction of autophagy during proteasome inhibition and accompanied by defective localization of Atg9 on recycling endosomes. Our results indicate that FAM48A is a kind of sensor that is required for compensatory autophagy induction upon proteasome impairment. K E Y W O R D S autophagy, proteasome 560 | Genes to Cells ARATA eT Al.