Prevention by cycloheximide of neutral red-induced formation of autophagic vacuoles and krinom granules in mouse pancreatic acinar cells

Translation inhibitors induce formation of cholesterol ester-rich lipid droplets

Lipid droplets (LDs) in non-adipocytes contain triglycerides (TG) and cholesterol esters (CE) in variable ratios. TG-rich LDs are generated when unsaturated fatty acids are administered, but the conditions that induce CE-rich LD formation are less well characterized. In the present study, we found that protein translation inhibitors such as cycloheximide (CHX) induced generation of CE-rich LDs and that TIP47 (perilipin 3) was recruited to the LDs, although the expression of this protein was reduced drastically. Electron microscopy revealed that LDs formed in CHX-treated cells possess a distinct electron-dense rim that is not found in TG-rich LDs, whose formation is induced by oleic acid. CHX treatment caused upregulation of mTORC1, but the CHX-induced increase in CE-rich LDs occurred even when rapamycin or Torin1 was given along with CHX. Moreover, the increase in CE was seen in both wild-type and autophagy-deficient Atg5-null mouse embryonic fibroblasts, indicating that mTORC1 activation and suppression of autophagy are not necessary to induce the observed phenomenon. The results showed that translation inhibitors cause a significant change in the lipid ester composition of LDs by a mechanism independent of mTORC1 signaling and autophagy.

Dynamics of vinblastine-induced autophagocytosis in murine pancreatic acinar cells: influence of cycloheximide post-treatments

Accumulation of autophagic vacuoles (AVs) was monitored by electron microscopic morphometry in murine pancreatic acinar cells during the 5-hr period after a single injection of vinblastine (VBL). The expansion of the autophagic compartment (AC) occurred in two waves. AVs accumulated in the first 90 min and regressed in the next hour, but thereafter AC expanded again, and 5 hr following the VBL injection, as much as 5.3% of the cytoplasmic volume was found sequestered into the AC. The high rates of accumulation of AVs indicated that VBL stimulated AV formation (segregation) during both expansion phases. To have a deeper insight into the dynamics of the process segregational inhibitor cycloheximide (CHI) was given 1 and 3 hr after VBL and the subsequent regression of the AC and its subcompartments (i.e., early, advanced, and late AVs) were measured during the next 90 min. We found that regression of AVs was fast in the first expansion and slowed down in the second expansion phase during which only early AVs regressed. CHI proved to be a fast and effective inhibitor of autophagic segregation, whether it was given before, simultaneously, or after the VBL injection. The aforementioned results argue for a dual mode of action of VBL (i.e., a prompt stimulation of segregation and a delayed retardation of AV maturation). The two effects of the alkaloid prevail differently along the time course. A further analysis of the behavior of the AC subcompartments showed that CHI perhaps inhibits segregational step(s) occurring prior to the actual formation of the autolysosomes.

Effect of microtubular or translational inhibitors on general cell protein degradation. Evidence for a dual catabolic pathway

Rat embryo fibroblasts were grown in Eagle's minimal essential medium with 10% serum and cell proteins prelabelled with L-[1-(14)C]leucine, followed by a 24h chase. When transferred to medium deprived of serum these cells showed a 2--3-fold increase in the production of trichloroacetic acid-soluble radioactivity during a 4h observation period. The microtubular poisons vinblastine, vincristine and colchicine partially inhibited this induced proteolysis, but had no effect on the proteolytic rate of cells maintained in medium with 10% serum. A similar discriminating effect on induced proteolysis was observed with cycloheximide, puromycin and insulin. The inhibitory effects of cycloheximide and vinblastine were not additive. These data support the hypothesis that, in addition to the basal turnover of cell proteins, a second mechanism of protein degradation involving cytoplasmic autophagy can be activated by nutritional step-down and is selectively inhibited by agents that interfere with microtubular function and protein synthesis.