Studies on cellular autophagocytosis. The relationship of autophagocytosis to protein synthesis and to energy metabolism in rat liver and flounder kidney tubules in vitro

Abstract

The purpose of the present study was to elucidate the metabolic requirements of autophagocytosis. Two model systems were used for this purpose: a) glucagon-induced autophagocytosis in the rat liver, and b) the wave of autophagocytosis which occurs when isolated flounder kidney tubules are incubated in vitro. In the rat liver, protein synthesis was inhibited by the administration of cycloheximide (1.5 mg/kg) to rats 2 hours prior to glucagon injection. In flounder kidney tubules, protein synthesis was inhibited at least 90% by adding cycloheximide, actinomycin D, pactamycin and puromycin to the medium. In both systems the inhibition of protein synthesis failed to inhibit the formation of autophagic vacuoles or their subsequent transformation into autolysosomes, as depicted from electron microscopic histochemical preparations. In flounder kidney tubules no differences were found in the levels of p-nitrophenylphosphates, beta-DL-glycerophosphatase, N-acetyl-beta-D-glucosaminidase, arylsulphatase, beta-D-galactosidase or acid proteinase when tubules were incubated up to 5 hours in the presence or absence of protein synthesis inhibitors. When ethionine was administered to rats 2 hours before glucagon injection, a decrease of approximately 75% in the ATP levels was observed. After ethionine administration, glucagon failed to induce the formation of autophagic vacuoles. The incubation of flounder kidney tubules in the presence of cyanide or in a nitrogen atmosphere decreased the ATP levels to less than 10% of controls and blocked autophagy. On the other hand, cyanide had little effect on acid hydrolase levels at 1 hour of incubation. A wide variety of other inhibitors were also shown to block autophagy. These results further support the hypothesis that, in the formation of antophagic vacuoles, preexisting enzyme and membrane pools are utilized. On the other hand, the esotropy-exotropy membrane conformational changes occurring in the formation of autophagic vacuoles seem to be energy dependent and can therefore be blocked by lowering intracellular ATP levels.