Δ40p53 is involved in the inactivation of autophagy and contributes to inhibition of cell death in HCT116-Δ40p53 cells

Delta 40p53 is an isoform of wild-type p53 (wtp53). Here, we assessed whether Delta 40p53 has the same functions as wild-type p53 in the regulation of cell death and autophagy. First, we used HCT116 (p53+/+) and H1299 (p53-free) cells to produce two cell lines (HCT116-Delta 40p53 and H1299-Delta 40p53) that express exogenous Delta 40p53 but not wtp53. By using these cell lines, we determined that Delta 40p53 inhibited starvation-induced autophagy, as does wtp53. This inhibition arises from both Delta 40p53 and wtp53 having 3'-5' exonuclease activity, which reduces the levels of double-stranded RNA (dsRNA) and then inhibits PKR/eIF2a-induced autophagy in cells exposed to starvation. Like wtp53, the translocation of Delta 40p53 to the nucleus increased in cells in response to Methyl methane sulfonate (MMS) treatment-induced DNA damage. Previous studies have shown that nuclear wtp53 can induce DRAM expression and DRAM-induced autophagy in cells in response to DNA damage, thereby contributing to apoptotic cell death as DRAM-induced autophagy is a proapoptotic factor. Here, nuclear Delta 40p53 did not individually induce DRAM-induced autophagy and cell death in response to DNA damage. However, nuclear Delta 40p53 inhibited wtp53-induced DRAM expression and cell death. Thus,Delta 40p53 and wtp53 have 3'-5' exonuclease activity and inhibit starvation-induced autophagy in the cytoplasm; however, nuclear Delta 40p53 inhibits wtp53-induced cell death by impairing the transactivation activity of wtp53. Because wtp53 inhibits tumor and viral infection by inhibiting autophagy and promoting degradation of viral dsRNA, it is reasonable to believe that Delta 40p53 has the similar functions. A deeper study of these functions of Delta 40p53 is needed in the future.