Living cells are adaptive self-sustaining systems. They strictly depend on the sufficient supply of oxygen, energy, and nutrients from the outside in order to sustain their internal organization. However, as autonomous entities they are able to monitor and appropriately adapt to any critical fluctuation in their environment. In the case of insufficient external nutrient supply or augmented energy demands, cells start to extensively digest their own interior. This process, known as macroautophagy, comprises the transport of cytosolic portions and entire organelles to the lysosomal compartment via specific double-membrane vesicles, called autophagosomes. Although extensively upregulated under nutrient restriction, a low level of basal autophagy is likewise crucial in order to sustain the cellular homeostasis. On the other hand, cells have to avoid excessive and enduring self-digestion. The delicate balance between external energy and nutrient supply and internal production and consumption is a demanding task. The complex protein network that senses and precisely reacts to environmental changes is thus mainly regulated by rapid and reversible posttranslational modifications such as phosphorylation. This review focuses on the serine/threonine protein kinases AMP-activated protein kinase, mammalian target of rapamycin (mTOR), and unc-51-like kinase 1/2 (Ulk1/2), three interconnected major junctions within the autophagy regulating signaling network. AMPK: THE ENERGY-SENSING KINASEA MP-activated protein kinase (AMPK) was initially identified as a serine/threonine kinase that negatively regulates several key enzymes of the lipid anabolism (30). Meanwhile, AMPK is regarded as the major energy-sensing kinase that activates a whole variety of catabolic processes in multicellular organisms such as glucose uptake and metabolism, while simultaneously inhibiting several anabolic pathways, such as lipid, protein, and carbohydrate biosynthesis (reviewed in reference 30).AMPK is a heterotrimeric protein complex that is precisely regulated in different ways. First, the phosphorylation of a conserved threonine residue (T172) in the activation loop of the catalytic ␣-subunit by upstream kinases is a prerequisite for the activity of AMPK. Several AMPK-phosphorylating kinases have been identified thus far. In addition to the ubiquitously expressed and constitutively active kinase LKB1 (31, 109), Ca 2ϩ -activated Ca 2ϩ /calmodulin-dependent kinase kinase ␤ (CaMKK␤) (32,43,108) and transforming growth factor ␤-activated kinase-1 (TAK1) (80) are both known as activators of AMPK. Second, AMPK activity can further be modulated by allosteric binding to the regulatory ␤-and ␥-subunit. Since the ratio of AMP to ATP represents the most accurate way to precisely measure the intracellular energy level, both AMP and ATP are able to oppositely regulate the activity of AMPK. While AMP binding to the ␥-subunit allosterically enhances AMPK kinase activity and prevents the dephosphorylation of T172, ATP is known to counteract the activating properties...

Yeast can undergo cell death accompanied by cellular markers of apoptosis. However, orthologs of classical mammalian apoptosis regulators appeared to be missing from the yeast genome, challenging a common mechanism of yeast and mammalian apoptosis. Here we investigate Yor197w, a yeast protein with structural homology to mammalian caspases, and demonstrate caspase-like processing of the protein. Hydrogen peroxide treatment induces apoptosis together with a caspase-like enzymatic activity in yeast. This response is completely abrogated after disruption and strongly stimulated after overexpression of Yor197w. Yor197w also mediates the death process within chronologically aged cultures, pointing to a physiological role in elimination of overaged cells. We conclude that Yor197w indeed functions as a bona fide caspase in yeast and propose the name Yeast Caspase-1 (YCA1, gene YCA1).

Efficient engulfment of the intact cell corpse is a critical end point of apoptosis, required to prevent secondary necrosis and inflammation. The presentation of "eat-me" signals on the dying cell is an important part of this process of recognition and engulfment by professional phagocytes. Here, we present evidence that apoptotic cells secrete chemotactic factor(s) that stimulate the attraction of monocytic cells and primary macrophages. The activation of caspase-3 in the apoptotic cell was found to be required for the release of this chemotactic factor(s). The putative chemoattractant was identified as the phospholipid, lysophosphatidylcholine. Further analysis showed that lysophosphatidylcholine was released from apoptotic cells due to the caspase-3 mediated activation of the calcium-independent phospholipase A(2). These data suggest that in addition to eat-me signals, apoptotic cells display attraction signals to ensure the efficient removal of apoptotic cells and prevent postapoptotic necrosis.

Death receptors have been recently identified as a subgroup of the TNF-receptor superfamily with a predominant function in induction of apoptosis. The receptors are characterized by an intracellular region, called the death domain, which is required for the transmission of the cytotoxic signal. Currently, five different such death receptors are known including tumor necrosis factor (TNF) receptor-1, CD95 (Fas/ APO-1), TNF-receptor-related apoptosis-mediated protein (TRAMP) and TNF-related apoptosis-inducing ligand (TRAIL) receptor-1 and -2. The signaling pathways by which these receptors induce apoptosis are rather similar. Ligand binding induces receptor oligomerization, followed by the recruitment of an adaptor protein to the death domain through homophilic interaction. The adaptor protein then binds a proximal caspase, thereby connecting receptor signaling to the apoptotic effector machinery. In addition, further pathways have been linked to death receptor-mediated apoptosis, such as sphingomyelinases, JNK kinases and oxidative stress. These pro-apoptotic signals are counteracted by several mechanisms which inhibit apoptosis at different levels. This review summarizes the current and rapidly expanding knowledge about the biological functions of death receptors and the mechanisms to trigger or to counteract cell death.Keywords : apoptosis; Bcl-2; caspase; CD95 (APO-1/Fas) ; death receptor; inhibitor of apoptosis protein; nuclear factor-κB ; tumor-necrosis factor; tumor-necrosis-factor-related apoptosis-inducing ligand; tumornecrosis-factor-receptor-related apoptosis-mediating protein.Apoptosis or programmed cell death is the innate mechanism modelling, immune regulation and tumor regression. Cells undergoing apoptosis show a sequence of cardinal morphological by which the organism eliminates unwanted cells. In contrast to necrosis, apoptosis is the most common physiological form of features including membrane blebbing, cellular shrinkage and condensation of chromatin. Biochemically, these alterations are cell death and occurs during embryonic development, tissue reassociated with the translocation of phosphatidylserine to the duced following traumatic injury or exposure to high concentra-

Although proteases of the caspase family are essential mediators of apoptosis in nucleated cells, in anucleate cells their presence and potential functions are almost completely unknown. Human erythrocytes are a major cell population that does not contain a cell nucleus or other organelles. However, during senescence they undergo certain morphological alterations resembling apoptosis. In the present study, we found that mature erythrocytes contain considerable amounts of caspase-3 and -8, whereas essential components of the mitochondrial apoptotic cascade such as caspase-9, Apaf-1 and cytochrome c were missing. Strikingly, although caspases of erythrocytes were functionally active in vitro, they failed to become activated in intact erythrocytes either during prolonged storage or in response to various proapoptotic stimuli. Following an increase of cytosolic calcium, instead the cysteine protease calpain but not caspases became activated and mediated fodrin cleavage and other morphological alterations such as cell shrinkage. Our results therefore suggest that erythrocytes do not have a functional death system. In addition, because of the presence of procaspases and the absence of a cell nucleus and mitochondria erythrocytes may be an attractive system to dissect the role of certain apoptosis-regulatory pathways. Cell Death and Differentiation (2001) 8, 1197 ± 1206.