Antagonistic Controls of Autophagy and Glycogen Accumulation by Snf1p, the Yeast Homolog of AMP-Activated Protein Kinase, and the Cyclin-Dependent Kinase Pho85p

Abstract

In the yeast Saccharomyces cerevisiae, glycogen is accumulated as a carbohydrate reserve when cells are deprived of nutrients. Yeast mutated in SNF1, a gene encoding a protein kinase required for glucose derepression, has diminished glycogen accumulation and concomitant inactivation of glycogen synthase. Restoration of synthesis in an snf1 strain results only in transient glycogen accumulation, implying the existence of other SNF1-dependent controls of glycogen storage. A genetic screen revealed that two genes involved in autophagy, APG1 and APG13, may be regulated by SNF1. Increased autophagic activity was observed in wild-type cells entering the stationary phase, but this induction was impaired in an snf1 strain. Mutants defective for autophagy were able to synthesize glycogen upon approaching the stationary phase, but were unable to maintain their glycogen stores, because subsequent synthesis was impaired and degradation by phosphorylase, Gph1p, was enhanced. Thus, deletion of GPH1 partially reversed the loss of glycogen accumulation in autophagy mutants. Loss of the vacuolar glucosidase, SGA1, also protected glycogen stores, but only very late in the stationary phase. Gph1p and Sga1p may therefore degrade physically distinct pools of glycogen. Pho85p is a cyclin-dependent protein kinase that antagonizes SNF1 control of glycogen synthesis. Induction of autophagy in pho85 mutants entering the stationary phase was exaggerated compared to the level in wild-type cells, but was blocked in apg1 pho85 mutants. We propose that Snf1p and Pho85p are, respectively, positive and negative regulators of autophagy, probably via Apg1 and/or Apg13. Defective glycogen storage in snf1 cells can be attributed to both defective synthesis upon entry into stationary phase and impaired maintenance of glycogen levels caused by the lack of autophagy.Cells constantly abstract information about their environment and modify their cellular and metabolic programs to cope with the prevailing conditions. For unicellular organisms like the budding yeast, Saccharomyces cerevisiae, much of the information about nutritional status is carried by the nutrients themselves. Depending on the type and availability of carbon, nitrogen, sulfur, and other requirements, the appropriate metabolic and cellular programs are elicited. Exhaustion of a preferred carbon source, like glucose, signals the induction of numerous genes needed to change to other metabolic regimes, in part by derepression of glucose-repressed genes and in part by cyclic AMP (cAMP) pathway control of gene expression (18,30). Another decision linked to nutritional deprivation is the synthesis of storage compounds like glycogen and trehalose, which are the primary carbohydrate reserves of the yeast (15). Glycogen is a branched polymer of glucose units that acts as a reserve of glucose and energy (47). In yeast growing on glucose, glycogen is synthesized late in the logarithmic phase and begins to be utilized when cells enter the stationary phase (7,8,37). However, glycogen stores can be p...

Autophagy in the liver

“…43 Although this suppressive effect of AMPK indicated by the use of AICAR as well as metformin, another indirect AMPK activator, could be confirmed in both hepatocytes and nonhepatocytes, complementary approaches employing an AMPK inhibitor, compound C, or a dominant-negative AMPK mutant instead indicate that AMPK is required for autophagy induced by amino acid deprivation. 44 This latter finding that AMPK is a positive regulator of autophagy is supported by other studies in yeast 45 and in mammalian cells 46,47 under various conditions, including ischemia. In general, these latest observations are consistent with the dogma that AMPK can suppress TOR and TOR suppresses autophagy (Fig.…”

Autophagy and signaling: their role in cell survival and cell death

“…Yeast cells contain the AMP kinase homolog, snf1p, which is, indeed, required for autophagy. 49 In contrast, the activation of AMP kinase by 5-aminoimidazole-4-carboxamide riboside (AICAR) or by other pharmacological interventions in hepatocytes inhibits autophagy. 50 In addition to its effects on mTOR phosphorylation (see previous paragraph), AMPK activation also results in the phosphorylation of Thr 421 and Ser 424 in the tail region of S6K1 in a rapamycin-insensitive manner; in contrast, amino acids stimulate the phosphorylation of Thr 421 and Ser 424 and Thr 389 (required for S6K1 activity) in a manner sensitive to rapamycin.…”

Life in the midst of scarcity: adaptations to nutrient availability in Saccharomyces cerevisiae

“…Deletion of PHO85 results in slow growth with a G 1 -delay on rich medium and a severe growth defect on poor carbon and nitrogen sources (Lee et al 2000). More specifically, mutant pho85D cells display a background-dependent hyperaccumulation of glycogen (Timblin et al 1996;Lee et al 2000), morphology and polarity defects (Measday et al 1997;Tennyson et al 1998), constitutive expression of phosphate-responsive or so-called PHO genes, CWI defects and hypersensitivity to stress conditions (Huang et al 2002), sporulation defects (Gilliquet and Berben 1993), aberrant expression profiles during the diauxic shift (Nishizawa et al 2004) and a hyperinduction of nutrient starvation-induced autophagy (Wang et al 2001). Consistent with its multiple functions, Pho85 can interact with ten different cyclins that can be divided into two different subfamilies according to their sequence similarities (Measday et al 1997).…”