The Pancreatitis-induced Vacuole Membrane Protein 1 Triggers Autophagy in Mammalian Cells

Abstract

Autophagy is a degradation process of cytoplasmic cellular constituents, which serves as a survival mechanism in starving cells, and it is characterized by sequestration of bulk cytoplasm and organelles in double-membrane vesicles called autophagosomes. Autophagy has been linked to a variety of pathological processes such as neurodegenerative diseases and tumorigenesis, which highlights its biological and medical importance. We have previously characterized the vacuole membrane protein 1 (VMP1) gene, which is highly activated in acute pancreatitis, a disease associated with morphological changes resembling autophagy. Here we show that VMP1 expression triggers autophagy in mammalian cells. VMP1 expression induces the formation of ultrastructural features of autophagy and recruitment of the microtubule-associated protein 1 light-chain 3 (LC3), which is inhibited after treatment with the autophagy inhibitor 3-methiladenine. VMP1 is induced by starvation and rapamycin treatments. Its expression is necessary for autophagy, because VMP1 small interfering RNA inhibits autophagosome formation under both autophagic stimuli. VMP1 is a transmembrane protein that co-localizes with LC3, a marker of the autophagosomes. It interacts with Beclin 1, a mammalian autophagy initiator, through the VMP1-Atg domain, which is essential for autophagosome formation. VMP1 endogenous expression colocalizes with LC3 in pancreas tissue undergoing pancreatitisinduced autophagy. Finally, VMP1 stable expression targeted to pancreas acinar cell in transgenic mice induces autophagosome formation. Our results identify VMP1 as a novel autophagy-related membrane protein involved in the initial steps of the mammalian cell autophagic process.Autophagy is an evolutionarily preserved degradation process of cytoplasmic cellular constituents, which serves as a survival mechanism in starving cells (1-3). This catabolic process is involved in the turnover of long lived proteins and other cellular macromolecules, and it might play a protective role in development, aging, cell death, and defense against intracellular pathogens (4 -8). By morphological studies, autophagy has been linked to a variety of pathological processes such as neurodegenerative diseases and tumorigenesis, which highlights its biological and medical importance (4, 9, 10). Early reports of autophagy in human disease include the ultrastructural autophagic features described in pancreas from human pancreatitis (11,12).Autophagy is characterized by sequestration of bulk cytoplasm and organelles in double-membrane vesicles called autophagosomes, which eventually acquire lysosomal-like features (13,14). Autophagy is mediated by a set of evolutionarily conserved gene products (termed the Atg proteins) originally discovered in yeast (15). In mammalian cells, Beclin 1 (3, 16 -18) promotes autophagosome formation when it functions as part of a complex with the Class III phosphatidylinositol 3-kinase (PI3K) 6 mediating the localization of other autophagic proteins to the autophagosomal membrane (19)...

Vacuole membrane protein 1, autophagy and much more

“…In this case Vmp1 is localized in autophagosomes where it interacts with Beclin 1, an essential autophagy protein. 5 Whether or not Dictyostelium Vmp1 behaves in the same way remains to be determined. localization in mammalian cells is controversial and the protein has been described to be located in autophagosomes 5 and the plasma membrane.…”

“…1,3 These vacuoles were later characterized as autophagosomes, implicating Vmp1 as a novel autophagy protein. 4,5 In a functional genomic RNA-interference screening in Drosophila, Vmp1 (known as TANGO-5) is identified as a protein required for conventional protein secretion and Golgi organization. 6 Another surprise comes from a study in kidney cancer cells where Vmp1 is localized in the plasma membrane and required in adhesion.…”

The Beclin 1 network regulates autophagy and apoptosis

“…VMP1 is induced by starvation and rapamycin treatment, and its overexpression triggers conversion of LC3-I to LC3-II and autophagosome formation in mammalian cells. 57,58 Moreover, VMP1 hydrophilic C-terminal region (Atg domain) has proved to be essential for Beclin 1 interaction and autophagy induction. Atg mutant VMP1 expression fails to induce LC3 recruitment, and the triple colocalization as well as interaction of VMP1 and Beclin 1 is abolished.…”

Vesicular trafficking and autophagosome formation

“…VMP1 has been shown to colocalize with LC3 and Beclin1 on autophagosome membranes. 51 Overexpression of VMP1 induces autophagosomes even under nutrient-rich conditions and this seems to be dependent on its direct binding to Beclin1. Importantly, depletion of VMP1 by RNAi inhibits autophagosome formation after starvation or rapamycin treatment.…”