B cell lymphoma 2 (Bcl-2) proteins are the central regulators of apoptosis. The two bcl-2 genes in Drosophila modulate the response to stress-induced cell death, but not developmental cell death. Because null mutants are viable, Drosophila provides an optimum model system to investigate alternate functions of Bcl-2 proteins. In this report, we explore the role of one bcl-2 gene in nutrient stress responses.
We report that starvation of Drosophila larvae lacking the bcl-2 gene, buffy, decreases survival rate by more than twofold relative to wild-type larvae. The buffy null mutant reacted to starvation with the expected responses such as inhibition of target of rapamycin (Tor) signaling, autophagy initiation and mobilization of stored lipids. However, the autophagic response to starvation initiated faster in larvae lacking buffy and was inhibited by ectopic buffy. We demonstrate that unusually high basal Tor signaling, indicated by more phosphorylated S6K, was detected in the buffy mutant and that removal of a genomic copy of S6K, but not inactivation of Tor by rapamycin, reverted the precocious autophagy phenotype. Instead, Tor inactivation also required loss of a positive nutrient signal to trigger autophagy and loss of both was sufficient to activate autophagy in the buffy mutant even in the presence of enforced phosphoinositide 3-kinase (PI3K) signaling. Prior to starvation, the fed buffy mutant stored less lipid and glycogen, had high lactate levels and maintained a reduced pool of cellular ATP. These observations, together with the inability of buffy mutant larvae to adapt to nutrient restriction, indicate altered energy metabolism in the absence of buffy.
All animals in their natural habitats are faced with periods of reduced nutrient availability. This study demonstrates that buffy is required for adaptation to both starvation and nutrient restriction. Thus, Buffy is a Bcl-2 protein that plays an important non-apoptotic role to promote survival of the whole organism in a stressful situation.
Keywords: autophagy, Bcl-2, metabolism, non-apoptotic, nutrient restriction, S6K, starvation, Tor