Figure 4: STUB1 protects Δ133p53α from autophagic degradation.

(a) STUB1 knockdown-induced repression of Δ133p53α is abrogated by bafilomycin A1 treatment. The siRNA transfection in MRC-5 fibroblasts was performed as in Fig. 3e. At 7 days after the initial transfection, the cells were untreated, treated with bafilomycin A1 (100 nM for 4 h) or treated with MG-132 (15 μM for 4 h), and examined in immunoblots as indicated. The results of Δ133p53α, full-length p53 and p62/SQSTM1 in untreated cells replicate those shown in Fig. 3c. The inhibition of autophagy by bafilomycin A1 was confirmed by increased levels of p62/SQSTM1 and LC3B. The inhibition of proteasomal degradation by MG-132 was confirmed by increased full-length p53. (b) STUB1 knockdown-induced repression of Δ133p53α is abrogated by knockdown of pro-autophagic proteins. MRC-5 fibroblasts (PDLs 30) were transfected for 4 days with control siRNA (−) or STUB1 siRNA (no. 1, +), in combination with control siRNA (c), p62/SQSTM1 siRNA (62), ATG5 siRNA (A5, no. 1 in Fig. 2a), ATG7 siRNA (A7, no. 1 in Fig. 2b) or Beclin-1 siRNA (B1, no. 2 in Fig. 2c), and examined in immunoblots for indicated proteins. (c) Δ133p53α is recruited to autophagosomes upon STUB1 downregulation. MRC-5 fibroblasts at early passage (PDLs 30; Y), those at or close to replicative senescence (PDLs 58; S (Rep)) and those induced to senesce by STUB1 knockdown (S (KD)) were used in immunoprecipitation (IP) with anti-STUB1, anti-Hsp70 and anti-LC3B antibodies, followed by immunoblots (IB) as indicated. Whole-protein lysates without IP (input) were also examined for Δ133p53α and β-actin levels. (d) Bafilomycin A1 stabilizes LC3B-interacting Δ133p53α in senescent cells. The same set of cells as in c were treated with bafilomycin A1 (+, 100 nM for 4 h) or untreated (−). These cells were used in IP with anti-LC3B antibody, followed by IB with anti-Δ133p53 antibody. Whole-protein lysates without IP were also examined in IB with anti-LC3B antibody. While untreated samples replicate the result shown in c, the differences between early passage and senescent cells become more evident with bafilomycin A1 treatment. (e) Schematic model of the regulation of Δ133p53α degradation by STUB1.