When lysosomal activity was blocked with another lysosomal inhibitor bafilomycin A1, a large number of GFP LC3 containing punctate foci accumulated in the untreated or nocoda zole treated cells as expected due to the robust basal levels of autophagy while only a few cells expressing the mutant GFP LC3 accumulate GFP punctate foci. These punctate foci more repre sent true autolysosomes formed through the autophagic machinery that are normally degraded by enzymes in lysosomes in the absence of lysosomal inhibitor. The dramatic difference in the intensities of acetylated microtubules between the untreated and nocodazole treated cells did not change the number of cells carrying GFP LC3 punctate foci. This suggested that a minimal number of intact acetylated microtubules are sufficient to meet demands of trafficking of autophago somes and lysosomes in order to achieve fusion.
Vinblastine induced accumulation of GFP LC3 punctate foci suggests a blockade of disposal of autophagosomes The vinblastine induced accumulation of GFP LC3 punc tate foci may be caused by an activation of autophagoso mal biogenesis, a blockade of autophagosomal degradation, or a blockade of conversion of LC3I to LC3II and accompanying localized aggregation of LC3I as indi cated by the paclitaxel induced accumulation of GFP LC3 punctate foci in mitotic cells. To distin guish these possibilities, lysates from cells exposed to the different drugs were analyzed by immunoblot.
Consistent with the accumulation of GFP LC3 punctate foci, vinblas tine treatment in the absence of lysosomal inhibitor caused a dramatic increase in levels of LC3II and P62, another autophagic marker directly being involved in selective autophagic degradation of ubiquitinated protein aggregates. This suggested either an activation of autophagic biogenesis or an inhibition of autophagosomal degradation. Less LC3II and P62 accumulation in the vinblastine treated cells in the presence of bafilomycin A1 confirmed an inhibition of autophagosomal Carfilzomib degradation. The cells treated with 100 uM of vinblastine contained similar levels of LC3II, but application of bafilo mycin A1 cut P62 in half. These results suggest that autophagosome degradation has been completely inhibited with the high concentration of vinblastine. The reduction in P62 may reflect alternative pathways such as the ubiquitination proteasome pathway that remains active when autophagy is blocked. In addition, since vin blastine depolymerized both acetylated and regular micro tubules, the efficiency of conversion of LC3I to LC3II was simultaneously reduced in its presence so that the total amount of LC3II generated during the block ade was reduced.