Molecular Mechanisms For P53 Opposite Roles Under Different High Temperature Conditions

Tumor suppressor gene P53 is activated upon DNA damage to induce cell cycle arrest for DNA damage repair or to promote cell apoptosis to maintain the stability of genomic DNA. Previous study showed that apoptosis caused by higher than 41.5℃ temperature treatment is dependented on P53. Surprisingly, we found that P53 promotes cell survival at 40℃,a sub-hyperthermia condition,via suppressing chaperone mediated autophagy (CMA). Further studies showed that P53 inhibits HSF1 and HSC70 expression to repress CMA. The results raise an important question why P53 plays opposite roles in different high temperature conditions.In this thesis, a pair of P53+/+ and P53-/- isogenic colon cancer cell lines (HCT116) were used as the experimental materials to explore this scientific question. The results showed that activation of ATM (DNA damage response) is one of the most important distinctions between 43℃ and 40℃ high temperature treatments. Under the 43℃ treatment, ATM is activated to induce P53 at ser6 but not at ser15 phosphorylption to promote P53 protein accumulation. Activated P53 promotes apoptosis gene expression such as BAX to induce apoptosis. Knockdown of ATM decreases P53 protein accumulation with reduction of P53 at ser6 phosphorylation to increase cell viability, but has no significant impact on cell viability in HCT116 P53-/- cells.Under the 40℃ treatment, ATM is not activated. Knockdown of ATM has no significant effects on P53 accumulation as well as the difference of cell viability between HCT116 P53+/+ and P53-/- cells. However, overexpression of ATM can mimic 43℃ treatment, which greatly increases the mortality of P53+/+ cells and has no significant impact on P53-/- cells. On the other hand, Knockdown of HSP90 can significantly reduce the accumulation of P53 and abolish the difference of cell viability between HCT116 P53+/+ and P53-/- cells upon the 40℃ treatmnet. The results reveals the molecular mechanism of P53 oposite roles in different high temperature conditions: under hyperthermia conditions (temperature higher than 40℃), DNA damage is induced and activates ATM. ATM mediates P53 phosphorylation to induce apoptosis specifically; Under 40℃ condition, ATM is not activated due to less DNA damage. ATM dependented P53 modifications rarely occurs. P53 stablization was dependend on HSP90. Without ATM dependent modifications, P53 protein can inhibit HSF1 to suppress CMA, but not activate apoptosis gene expression to induce apoptosis, thereby to promote cell survival.Mutation of p53 is found in more than 50% of all tumors. P53-deficient tumors are also less sensitive to radio- or chemo-therapy. The fact that p53-deficient cells exhibit high mortality in the treatment of 40℃ may provide a new strategy for individualized therapy for patients with p53-deficient and CMA upregulated tumors.