Induction of Apoptosis by Sphingoid Long-Chain Bases inAspergillus nidulans

Abstract

Sphingolipid metabolism is implicated to play an important role in apoptosis. Here we show that dihydrosphingosine (DHS) and phytosphingosine (PHS), two major sphingoid bases of fungi, have potent fungicidal activity with remarkably high structural and stereochemical specificity against Aspergillus nidulans. In fact, only naturally occurring DHS and PHS are active. Further analysis revealed that DHS and PHS induce rapid DNA condensation independent of mitosis, large-scale DNA fragmentation, and exposure of phosphatidylserine, all common morphological features characteristic of apoptosis, suggesting that DHS and PHS induce apoptosis in A. nidulans. The finding that DNA fragmentation requires protein synthesis, which implies that an active process is involved, further supports this proposition. The induction of apoptosis by DHS and PHS is associated with the rapid accumulation of reactive oxygen species (ROS). However, ROS are not required for apoptosis induced by DHS and PHS, as scavenging of ROS by a free radical spin trap has no effect. We further demonstrate that apoptosis induced by DHS and PHS is independent of metacaspase function but requires mitochondrial function. Together, the results suggest that DHS and PHS induce a type of apoptosis in A. nidulans most similar to the caspase-independent apoptosis observed in mammalian systems. As A. nidulans is genetically tractable, this organism should be an ideal model system for dissecting sphingolipid signaling in apoptosis and, importantly, for further elucidating the molecular basis of caspase-independent apoptosis.In recent years, mounting evidence has indicated that sphingolipid metabolism is an important cell signaling system. Rapid and transient changes in sphingolipid metabolism are closely associated with a wide range of cellular activities, including the stress response, apoptosis, inflammation, cell cycle regulation, and cancer development (19,20,33). For instance, stress signals rapidly and transiently elevate the level of cellular ceramide; conversely, growth factors stimulate the rapid, transient generation of sphingosine-1-phosphate (S-1-P). Importantly, the treatment of cells with short-chain ceramide analogs or the expression of sphingomyelinases reproduces most of the effects of endogenous ceramide, particularly in the stress response and apoptosis (19,20,33). Furthermore, S-1-P, as a highaffinity ligand of the edg family of G-protein-coupled receptors, promotes cell survival and proliferation by antagonizing ceramide-mediated apoptosis (52, 61). Thus, ceramide, as an important regulatory component in the stress response and apoptosis, and S-1-P, involved in cell survival and proliferation, have attracted enormous scientific interest in recent years. In particular, a large body of evidence has now accumulated to implicate an important role of ceramide in the stress response and apoptosis. It is proposed that the relative levels of cellular ceramide and S-1-P determine whether cells undergo apoptosis or continue to proliferate (61).The sphingoid ...

Transcriptional analysis of the response of Neurospora crassa to phytosphingosine reveals links to mitochondrial function

“…Both nuclear and mitochondrial genes encoding the organelle proteins were extremely enriched among thẽ 100 genes that showed a decrease in relative expression following PHS exposure. This is in line with the findings that mitochondria have a central role in PHS-induced PCD (Castro et al, 2008;Cheng et al, 2003;Woodcock, 2006). Altered transcription during PCD has been noticed before.…”

“…Moreover, the ongoing project for the disruption of all predicted genes in the N. crassa genome (Colot et al, 2006) allows direct testing of the sensitivity of specific mutants to PCDinducing stimuli, including mutants in genes identified by expression profiling. In filamentous fungi, the sphingolipid phytosphingosine (PHS) induces PCD with the involvement of mitochondria (Castro et al, 2008;Cheng et al, 2003;Woodcock, 2006). In this article, we present our analysis of the transcriptional response to the deathinducer PHS in N. crassa and its major effect on genes encoding mitochondrial proteins.…”

Isc1p Plays a Key Role in Hydrogen Peroxide Resistance and Chronological Lifespan through Modulation of Iron Levels and Apoptosis

“…Sphingolipids regulate cellular redox homeostasis, and the activity of sphingomyelinases and ceramidase can be modulated by ROS and glutathione levels (Won and Singh, 2006). In Aspergillus nidulans, LCBs induce apoptosis associated with the accumulation of ROS, although ROS scavenging does not impair the induction of apoptosis (Cheng et al, 2003). In mammalian cells, ceramide has also been shown to increase the production of ROS, by directly inhibiting the mitochondrial complex III (Gudz et al, 1997) and via inhibition of Bcl2 mediated by the ceramide-activated protein phosphatase 2A (Ruvolo et al, 1999).…”

Transcriptome changes initiated by carbon starvation in Aspergillus nidulans

“…According to the microarray data (Table S2), neither aifA (Savoldi et al, 2008;Dinamarco et al, 2010), nor casA (Cheng et al, 2003) and prpA (Semighini et al, 2006), were induced. However, repression of ndeA and ndiA, which are both putative anti-apoptotic NADH-dehydrogenases (Dinamarco et al, 2010), was observed in the microarray experiment (Table S2).…”