LUBAC assembles a ubiquitin signaling platform at mitochondria for signal amplification and transport of NF-ĸB to the nucleus (2022)
Figure 4

Figure 4. M1 ubiquitination at mitochondria protects from apoptosis

A. STS-induced mitochondrial Bax recruitment is reduced by TNF. HeLa cells were treated with STS (1 μM, 1 h) with or without a 15 min pretreatment with TNF (25 ng/ml) and then harvested. Purified mitochondrial fractions were analyzed by immunoblotting using antibodies against Bax and M1-ubiquitin. The input was immunoblotted for TIM23 (upper panel). Bax-specific signal intensities were quantified and normalized to TIM23-specific signals (lower panel). Data represent the mean with standard error of 8 independent experiments. *p < 0.05, ***p < 0.001. Kruskal-Wallis test followed by Dunn's multiple comparison test, n=8.

B. STS-induced cytochrome C release is decreased by TNF. HeLa cells were treated as described in A. The cytosolic fractions were analyzed by immunoblotting using cytochrome c antibodies. GAPDH was used as a reference. Quantification of 5 biological replicates is shown in the lower panel. Signal intensities were quantified and normalized to that of GAPDH. *p < 0.05. Kolmogorow-Smirnov normality test, paired t-test, two-tailed, n=5.

C, D. TNF treatment prevents damage of mitochondrial cristae under pro-apoptotic conditions. C. SH-SY5Y cells were treated as described in A, fixed, embedded and the mitochondrial ultrastructure was imaged by electron microscopy. Scale bar: 400 nm. D. Cristae abundance (average cristae number per mitochondrial area) and cristae length (average cristae length per mitochondrium) were analyzed by Imaris 9.8. *p < 0.05, **p < 0.01, ***p < 0.01. Cristae abundance: Shapiro-Wilk normality test, One-way ANOVA followed by Tukey's multiple comparison test, n = 27. Cristae length: Kruskal-Wallis test followed by Dunn's multiple comparison test, n=24-27. Bars represent mean ± SEM

E. The fast anti-apoptotic effect of TNF is not affected by the NF-κΒ inhibitor IκΒα. SH-SY5Y cells were transiently transfected with the NF-κΒ super-repressor IᴋBα-2S or luciferase as a control. 24 h later, the cells were treated with STS (5 µM, 2 h) with or without a 15 min pretreatment with TNF (25 ng/ml). Cells were fixed and stained by antibodies against active caspase-3. Signal intensities were quantified by immunocytochemistry and fluorescence microscopy. Expression of IᴋBα-2S was tested by immunoblotting using antibodies against the HA tag. Data represent the standard deviation of three independent experiments; at least 300 cells were counted per experiment. *p < 0.05, **p < 0.01. Student's t-test, two-tailed, n=3.

F. The protective effect of TNF is dependent on HOIP expression. HeLa cells were transiently transfected with control or HOIP siRNA. 48 h after transfection, cells were treated with STS (1 μM, 1 h) with or without a 15 min TNF pretreatment (25 ng/ml) and then harvested. The cytosolic fractions were analyzed by immunoblotting using cytochrome c antibodies. HOIP silencing efficiency was analyzed in whole cell lysates using antibodies against HOIP. GAPDH and actin were immunoblotted as input controls.

G. The protective effect of TNF is dependent on PINK1 expression. HeLa cells were transiently transfected with control or PINK1 siRNA and treated as described in F. The cytosolic fractions were analyzed by immunoblotting using cytochrome c and PINK1 antibodies. GAPDH was immunoblotted as input control.