Regulation of Autophagy by the p300 Acetyltransferase

Autophagy is a regulated process of intracellular catabolism required for normal cellular maintenance, as well as serving as an adaptive response under various stress conditions, including starvation. The molecular regulation of autophagy in mammalian cells remains incompletely understood. Here we demonstrate a role for protein acetylation in the execution and regulation of autophagy. In particular, we demonstrate that the p300 acetyltransferase can regulate the acetylation of various known components of the autophagy machinery. Knockdown of p300 reduces acetylation of Atg5, Atg7, Atg8, and Atg12, although overexpressed p300 increases the acetylation of these same proteins. Furthermore, p300 and Atg7 colocalize within cells, and the two proteins physically interact. The interaction between p300 and Atg7 is dependent on nutrient availability. Finally, we demonstrate that knockdown of p300 can stimulate autophagy, whereas overexpression of p300 inhibits starvation-induced autophagy. These results demonstrate a role for protein acetylation and particularly p300 in the regulation of autophagy under conditions of limited nutrient availability.Macro-autophagy, herein referred to as autophagy, is an evolutionary conserved process first characterized in lower organisms (1). In yeast, over 20 separate genes (designated ATG1, ATG2, etc.) have been demonstrated to be essential to carry out the autophagy program. This process is thought to provide a mechanism for the efficient removal of both long lived proteins and damaged cellular organelles. This regulated degradation provides several essential functions for the cell. First, it allows for the removal of damaged and potentially harmful cellular contents. In addition, in breaking down various intracellular components, the autophagy process provides essential building blocks for the cell to use in the re-synthesis of necessary macromolecules. To accomplish this recycling effort, the coordinated actions of various Atg gene products are required. In particular, the Atg gene products together orchestrate the formation of a double membrane structure known as the autophagosome that engulfs the intended cellular cargo targeted for degradation. The autophagosome eventually fuses with the vacuole in yeast or the lysosome in mammals.In both yeast and mammalian cells, autophagy can be stimulated by the withdrawal of nutrients. Under these conditions, autophagic degradation of nonessential components may be essential to meet ongoing energetic needs in the presence of limited extracellular nutrients. This point was underscored by the analysis of mice containing a targeted deletion of Atg5 (2). In the absence of Atg5, there is a lack of both basal and starvation-induced autophagy. Mice lacking Atg5 are born normally but succumb within the 1st day of life. This post-natal lethality is thought to be due in large part for the requirement of autophagy to supply the energetic needs of neonates. These needs are particularly critical during the small window of time where the animal no longer has a placental circulation and before the pup can begin to nurse and thus obtain external nutrients.Relatively little is known regarding how signals such as nutrient availability are able to be transduced to ultimately regulate the level of cellular autophagy. One important pathway that impinges on the process is signaling thorough the target of rapamycin (TOR)2 network (3). Evidence suggests that TOR signaling inhibits autophagy, and indeed agents such as rapamycin that can inhibit TOR are known to result in increased autophagy. We recently have observed that in addition to this mode of regulation, the NAD-dependent deacetylase Sirt1 is also a regulator of autophagy in mammalian cells and tissues (4). In particular, we demonstrated that in the absence of Sirt1 levels of acetylation for various components of the autophagy machinery are increased and that starvation-induced autophagy is impaired. Interestingly, like the Atg5 knock-out animals, Sirt1^-/- mice are also born normally but die within the few hours to days after birth. Consistent with a defect in autophagy, electron micrographs of hearts from Sirt1^-/- mice demonstrated an accumulation of abnormal appearing organelles, including mitochondria, a phenotype previously observed in Atg-deficient animals (5). Here we have further characterized the role of acetylation in the regulation of autophagy, and in particular, we demonstrate a role for the p300 acetyltransferase in this process.