Riboflavin transporter SLC52A1, a target of p53, suppresses cellular senescence by activating mitochondrial complex II.

Cellular senescence is a state of permanent proliferative arrest induced by a variety of stresses, such as DNA damage. The transcriptional activity of p53 has been known to be essential for senescence induction. It remains unknown, however, whether among the downstream genes of p53, there is a gene that has antisenescence function. Our recent studies have indicated that the expression of SLC52A1 (also known as GPR172B/RFVT1), a riboflavin transporter, is up-regulated specifically in senescent cells depending on p53, but the relationship between senescence and SLC52A1 or riboflavin has not been described. Here, we examined the role of SLC52A1 in senescence. We found that knockdown of SLC52A1 promoted senescence phenotypes induced by DNA damage in tumor and normal cells. The senescence suppressive action of SLC52A1 was dependent on its riboflavin transport activity. Furthermore, elevation of intracellular riboflavin led to activation of mitochondrial membrane potential (MMP) mediated by the mitochondrial electron transport chain complex II. Finally, the SLC52A1-dependent activation of MMP inhibited the AMPK-p53 pathway, a central mediator of mitochondria dysfunction-related senescence. These results suggest that SLC52A1 contributes to suppress senescence through the uptake of riboflavin and acts downstream of p53 as a negative feedback mechanism to limit aberrant senescence induction.

LY6D-induced macropinocytosis as a survival mechanism of senescent cells.

Although senescent cells display various morphological changes including vacuole formation, it is still unclear how these processes are regulated. We have recently identified the gene, lymphocyte antigen 6 complex, locus D (LY6D), to be upregulated specifically in senescent cells. LY6D is a glycosylphosphatidylinositol-anchored cell-surface protein whose function remains unknown. Here, we analyzed the functional relationship between LY6D and the senescence processes. We found that overexpression of LY6D induced vacuole formation and knockdown of LY6D suppressed the senescence-associated vacuole formation. The LY6D-induced vacuoles were derived from macropinocytosis, a distinct form of endocytosis. Furthermore, Src family kinases and Ras were found to be recruited to membrane lipid rafts in an LY6D-dependent manner, and inhibition of their activity impaired the LY6D-induced macropinocytosis. Finally, reduction of senescent-cell survival induced by glutamine deprivation was recovered by albumin supplementation to the culture media in an LY6D-dependent manner. Because macropinocytosis acts as an amino acid supply route, these results suggest that LY6D-mediated macropinocytosis contributes to senescent-cell survival through the incorporation of extracellular nutrients.