Autophagy and other vacuolar protein degradation mechanisms

Abstract

Autophagic degradation of cytoplasm (including protein, RNA etc.) is a non-selective bulk process, as indicated by ultrastructural evidence and by the similarity in autophagic sequestration rates of various cytosolic enzymes with different half-lives. The initial autophagic sequestration step, performed by a poorly-characterized organelle called a phagophore, is subject to feedback inhibition by purines and amino acids, the effect of the latter being potentiated by insulin and antagonized by glucagon. Epinephrine and other adrenergic agonists inhibit autophagic sequestration through a prazosin-sensitive alpha 1-adrenergic mechanism. The sequestration is also inhibited by cAMP and by protein phosphorylation as indicated by the effects of cyclic nucleotide analogues, phosphodiesterase inhibitors and okadaic acid. Asparagine specifically inhibits autophagic-lysosomal fusion without having any significant effects on autophagic sequestration, on intralysosomal degradation or on the endocytic pathway. Autophaged material that accumulates in prelysosomal vacuoles in the presence of asparagine is accessible to endocytosed enzymes, revealing the existence of an amphifunctional organelle, the amphisome. Evidence from several cell types suggests that endocytosis may be coupled to autophagy to a variable extent, and that the amphisome may play a central role as a collecting station for material destined for lysosomal degradation. Protein degradation can also take place in a 'salvage compartment' closely associated with the endoplasmic reticulum (ER). In this compartment unassembled protein chains are degraded by uncharacterized proteinases, while resident proteins return to the ER and assembled secretory and membrane proteins proceed through the Golgi apparatus. In the trans-Golgi network some proteins are proteolytically processed by Ca(2+)-dependent proteinases; furthermore, this compartment sorts proteins to lysosomes, various membrane domains, endosomes or secretory vesicles/granules. Processing of both endogenous and exogenous proteins can occur in endosomes, which may play a particularly important role in antigen processing and presentation. Proteins in endosomes or secretory compartments can either be exocytosed, or channeled to lysosomes for degradation. The switch mechanisms which decide between these options are subject to bioregulation by external agents (hormones and growth factors), and may play an important role in the control of protein uptake and secretion.

Biogenesis of Multilamellar Bodies via Autophagy

“…3-MA treatment results in the accumulation of inclusion bodies that morphologically resemble AV i , demonstrating that 3-MA is blocking autophagy in the GlcNAc-TV transfectants at an early stage of autophagic vacuole biogenesis. In the hepatocyte, 3-MA blocks the initial sequestration event in autophagic vacuole biogenesis and is associated with an approximate twofold reduction in autophagic sequestration (Kopitz et al, 1990;Seglen and Bohley, 1992), which is consistent with the reduction (be- tween 40 and 83%) in cytoplasmic area covered by both MLBs and inclusion bodies in 3-MA-treated M1 and M9 cells observed here ( Table 2). The demonstration that cytoplasmic FITC-dextran can be incorporated into the LAMP-2-positive MLBs provides a direct illustration that autophagic sequestration is involved in MLB biogenesis (Figure 8).…”

“…Autophagy is a normal degradative process that exists in all eukaryotic cells and is stimulated in response to a variety of environmental stresses, which necessitate the use of autophagic mechanisms to enable cellular survival (Seglen and Bohley, 1992;Dunn, 1994). Degradative autophagic vacuoles (AV d ) form after acquisition of lysosomal properties by nascent, immature autophagic vacuoles (AV i ), which present multiple limiting membranes and are considered to form by the sequestration of cytoplasm by smooth endoplasmic reticulum membranes (Dunn, 1990;Furuno et al, 1990;Ueno et al, 1991).…”

Role of autophagy in temozolomide-induced cytotoxicity for malignant glioma cells

“…Therefore, agents that inhibit autophagy at a late stage like bafilomycin A1, may be promising candidates to combine TMZ. It has long been known that 3-MA inhibits autophagy, 26 and it was recently found to be an inhibitor of PI3K. 27 PI3K is a conserved family of lipid kinases that catalyze the phosphorylation of the position 3 of the inositol ring of phosphoinositides.…”

Autophagy in the liver

“…They differ in their functional significance and the type of substrates they take in for degradation. In the lysosome system, degradation of extracellular materials is mediated by endocytosis (heterophagy), whereas degradation of intracellular components is mediated by 3 types of autophagy: macroautophagy, microautophagy, and chaperone-mediated autophagy, [1][2][3][4][5][6] which differ in how the cytoplasmic materials are delivered to the lysosome (Fig. 1).…”