Abstract
p53 mediates the DNA damage response (DDR) by regulating the expression of genes implicated in cell cycle arrest, senescence, programmed cell death (PCD), and metabolism. Herein we demonstrate that human alkaline ceramidase 2 (ACER2) is a novel transcriptional target of p53 and that its transactivation by p53 mediates the DDR. We found that p53 overexpression or its activation by ionizing radiation (IR) upregulated ACER2 in cells. Two putative p53 responsive elements (p53REs) were found in its first intron of the ACER2 gene, and Chromatin Immunoprecipitation (ChIP) assays in combination with promoter activity assays demonstrated that these p53REs are the bona fide p53 binding sites that mediate ACER2 transactivation by p53. As ACER2 catalyzes the hydrolysis of ceramides into sphingosine, which in turn is phosphorylated to form sphingosine-1-phosphate (S1P), ACER2 upregulation increased the levels of both sphingosine and S1P while decreasing the levels of ceramides in cells. A moderate upregulation of ACER2 inhibited cell cycle arrest and cellular senescence in response to low-level expression of p53 or low-dose IR by elevating S1P, a pro-proliferative and pro-survival bioactive lipid, and/or decreasing ceramides whereas its robust upregulation mediated PCD in response to high-level expression of p53 or high-dose IR likely by accumulating cellular sphingosine, a pro-death bioactive lipid. ACER2 is frequently inactivated in various cancers due to its deletion or mutations, and restoring its expression inhibited the growth of tumor xenografts in mice. These results suggest that p53 mediates DDR and exerts its tumor suppressive role in part by regulating the expression of ACER2, which in turn regulates the bioactive sphingolipid lipids.