Figure 11. Nicotinamide induces cystic epithelial cell death through p53.

(A) Western blot analysis of p53 and active caspase-3 expression in Pkd1-null MEK cells transfected or not with p53 siRNA for 24 hours and then treated or not with 10 mM nicotinamide for another 24 hours. (B) Knockdown of p53 with siRNA prevented nicotinamide-induced apoptosis, as detected by TUNEL assay, in Pkd1-null MEK cells that were transfected or not with p53 siRNA for 24 hours and then treated or not with 10 mM nicotinamide for 24 hours. (C) Overexpression of WT p53, but not mutant p53-8KR (which is mutated at 8 acetylation sites), increased apoptosis in Pkd1-null MEK cells treated with nicotinamide. Pkd1-null MEK cells were transfected with WT p53, mutant p53-8KR, or empty vector together with or without nicotinamide for 24 hours, then analyzed by TUNEL assay. (D) SIRT1-mediated pathways in Pkd1-mutant renal epithelial cells. Pkd1 knockout or mutation upregulates SIRT1 through c-MYC. Upregulated SIRT1 in Pkd1-mutant renal epithelial cells (i) is a target of nicotinamide, which decreases proliferation and induces apoptosis of cystic epithelial cells to delay cyst growth in Pkd1-null mouse kidneys; (ii) regulates the acetylation and phosphorylation of Rb and further affects Rb-E2F–mediated S-phase entry; (iii) regulates the p53 acetylation and p53-dependent apoptosis in response to nicotinamide; and (iv) can be regulated by c-MYC and induced by TNF-α. Scale bars: 50 μm. **P 0.01.