Under starvation conditions, eukaryotic cells recover nutrients via autophagy, a lysosome-mediated process of bulk cytoplasmic degradation. Through autophagy, long-lived proteins, organelles, and other components of the cytoplasm are non-selectively engulfed within specialized double-membraned vesicles known as autophagosomes. Subsequent fusion of the outer autophagosomal membrane with the lysosome results in a structure known as the autolysosome, in which the inner membrane and its cytoplasmic cargo are degraded. Breakdown products released from the autolysosome supply the cell with an internal source of nutrients that can support essential metabolic processes during starvation [1
]. Approximately 20 ATG
(autophagy-related) genes specifically required for autophagy have been discovered in Saccharomyces cerevisiae
], and many of these genes have functional homologs in metazoans [1
In addition to survival of starvation, autophagy has been implicated in many aspects of health and development, including aging, programmed cell death, pathogenic infection, stress responses, neurodegenerative and muscle disorders, cellular remodeling, cancer, and cell growth [1
]. As in many of these processes, evidence for a role of autophagy in cell growth control is largely correlative. Autophagy is promoted by several tumor suppressor genes including PTEN, TSC1 & 2, and beclin 1, and is inhibited by growth-promoting pathways such as type I PI3K and target of rapamycin (TOR) signaling [5
]. A variety of conditions that stimulate cell growth, such as growth factor addition, partial hepatectomy, and refeeding after starvation, also inhibit autophagy, while growth suppressive signals, such as contact inhibition and substrate detachment, induce autophagy [11
]. Together, these correlative findings are consistent with a model in which the catabolic effects of autophagy act as a brake on cell growth during development. However, this issue is complicated by the pleiotropic nature of the signaling pathways that regulate autophagy. For example, in addition to inhibiting autophagy, the TOR pathway controls cell metabolism and biosynthesis by promoting ribosome biogenesis, protein synthesis and nutrient uptake [12
]. The relevant contribution of each of these downstream effector pathways to net cell growth is poorly understood.
Autophagy is also known to be involved in programmed cell death, and distinguishes Type II (autophagic) from Type I (apoptotic) cell death. Autophagic cell death is characterized by an abundance of autophagosomes and autolysosomes in the dying cell and differs from apoptotic cell death in that dying cells are degraded by their own lysosomal enzymes, rather than by phagocytosis [13
]. It has been difficult, however, to establish whether autophagy plays a causal role in Type II cell death or represents a failed attempt at cell survival in cells undergoing programmed cell death. Death signals often induce features of both apoptosis and autophagy, and mutations that disrupt autophagy have been shown to suppress cell death in some cases [14
], and hasten it in others [2
]. Thus, as in the case for cell growth, much of the support for a direct role for autophagy in cell death rests on correlative evidence.
One approach to addressing the potential role of autophagy in functions such as cell death and growth would be to induce autophagy directly, independent of the signaling pathways that normally control it. In yeast, multiple signaling pathways, including TOR, AMPK and Ras/PKA, converge on the Ser-Thr kinase Atg1 to regulate autophagy [18
]. In addition, Atg1 interacts with multiple components of the autophagic machinery, through direct association, phosphorylation, and/or through effects on intracellular localization [18
]. Thus, Atg1 may represent a nodal point for controlling multiple steps in the autophagic process in response to various inductive cues.
Interestingly, the role of Atg1 kinase activity in yeast is the subject of some debate, and studies from different groups using ATP analog-sensitive and kinase-defective Atg1 mutants have reached conflicting conclusions. One study reported that Atg1 kinase activity is required for the autophagy-related biosynthetic cytoplasm to vacuole targeting (CVT) pathway but not for autophagy, and concluded that Atg1 plays a structural role in autophagy [23
]. However, other studies using similar reagents found a requirement for Atg1 kinase activity in both CVT and autophagy [18
We have previously shown that the Drosophila melanogaster
homolog of Atg1 is required for autophagy in the larval fat body, an organ analogous to the vertebrate liver with roles in nutrient storage and mobilization [5
]. Here, we investigate the effects of Drosophila Atg1 loss of function and overexpression on autophagy induction, cell growth control, and cell death. Our findings indicate that Atg1 expression is sufficient to effect a full autophagic response, resulting in a marked inhibition of cell growth and a rapid induction of apoptotic cell death.