Arsenic Toxicity to Saccharomyces cerevisiae Is a Consequence of Inhibition of the TORC1 Kinase Combined with a Chronic Stress Response

Abstract

The conserved Target Of Rapamycin (TOR) growth control signaling pathway is a major regulator of genes required for protein synthesis. The ubiquitous toxic metalloid arsenic, as well as mercury and nickel, are shown here to efficiently inhibit the rapamycin-sensitive TORC1 (TOR complex 1) protein kinase. This rapid inhibition of the TORC1 kinase is demonstrated in vivo by the dephosphorylation and inactivation of its downstream effector, the yeast S6 kinase homolog Sch9. Arsenic, mercury, and nickel cause reduction of transcription of ribosome biogenesis genes, which are under the control of Sfp1, a TORC1-regulated transcriptional activator. We report that arsenic stress deactivates Sfp1 as it becomes dephosphorylated, dissociates from chromatin, and exits the nucleus. Curiously, whereas loss of SFP1 function leads to increased arsenic resistance, absence of TOR1 or SCH9 has the opposite effect suggesting that TORC1 has a role beyond down-regulation of Sfp1. Indeed, we show that arsenic activates the transcription factors Msn2 and Msn4 both of which are targets of TORC1 and protein kinase A (PKA). In contrast to TORC1, PKA activity is not repressed during acute arsenic stress. A normal level of PKA activity might serve to dampen the stress response since hyperactive Msn2 will decrease arsenic tolerance. Thus arsenic toxicity in yeast might be determined by the balance between chronic activation of general stress factors in combination with lowered TORC1 kinase activity. INTRODUCTIONThe transition metal arsenic has a long history of human exploitation as both a poison and a medicine. In more recent times EhrlichЈs discovery of the antisyphilitic drug arsphenamine (also known as salvarsan) by systematic chemical modification of arsenic derivatives marked the beginning of modern pharmaceutical research. Arsenic trioxide (ATO) is used today in cancer treatment (Evens et al., 2004;Lu et al., 2007;Wang and Chen, 2008).Exposure to arsenic evokes a broad spectrum of cellular reactions in Saccharomyces cerevisiae Haugen et al., 2004;Jin, 2008;Thorsen et al., 2007) and in higher eukaryotes (Salnikow and Zhitkovich, 2008). A number of mechanisms exist for detoxification, probably because arsenic has always been widespread in the environment. These involve reduction of influx through the aquaglyceroporin Fps1p Thorsen et al., 2006); sequestration into the vacuole in the form of glutathione conjugates, metallothionein, and other metal/protein complexes; and active extrusion (Ghosh et al., 1999). In yeast, genome-wide analysis of the transcription patterns in response to arsenic revealed a complex network of transcription factors controlling the expression of several hundred genes (Haugen et al., 2004;Wysocki et al., 2004;Thorsen et al., 2007). Mitogen-activated protein kinases mediate protective responses involving AP-1-and AP-1-like transcription factors in higher eukaryotes and in fungi (Cavigelli et al., 1996;Rodriguez-Gabriel and Russell, 2005;Thorsen et al., 2006).The mechanisms by which arsenic might influence signalin...

Experimental Evolution Reveals Interplay between Sch9 and Polyploid Stability in Yeast

“…Earlier studies have shown that, in yeast and mammalian cells, complex negative and positive feedback loops exist between Sch9 (or the mammalian Sch9 homolog, S6K1) and the TORC1 network [43, 84, 85]. In addition, some other compensatory pathways may be turned on when Sch9 or TORC1 is compromised [78, 86]. In ancestral tetraploids, Sch9 is downregulated compared to normal diploids.…”

Proteomic Analysis Identifies Ribosome Reduction as an Effective Proteotoxic Stress Response

“…In principle, this could be achieved by either a reduction in new ribosome production or an increase in post-translational degradation of ribosomes. A key observation in this regard is that microarray-based studies of the transcriptional response to arsenic have also noted broad reductions in the levels of mRNAs for ribosome components (7,20). These studies did not examine ribosome protein levels or propose that ribosome reduction might be an adaptive cellular response.…”

Direct effects of non-antifungal agents used in cancer chemotherapy and organ transplantation on the development and virulence ofCandidaandAspergillusspecies

“…Genome-wide analyses of transcription patterns in S. cerevisiae in response to treatment with arsenic have revealed a complex response that influences signaling pathways including protein kinases such as the mitogen-actived protein kinase and target of rapamycin (TOR) complex 1 (TORC1) systems. 39,40 Arsenic toxicity results from inhibition of TORC1 kinase and modulation of the activity of downstream factors, leading to activation of the transcription factors that are targets of TORC1 and protein kinase A. 40 Whether arsenic has similar effects in Candida and Aspergillus spp.…”

“…39,40 Arsenic toxicity results from inhibition of TORC1 kinase and modulation of the activity of downstream factors, leading to activation of the transcription factors that are targets of TORC1 and protein kinase A. 40 Whether arsenic has similar effects in Candida and Aspergillus spp. is unknown.…”