Investigation of cellular effects of endoplasmic reticulum stress in Schizosaccharomyces pombe

The endoplasmic reticulum (ER) is an intracellular organelle in all eukaryotes and is responsible for protein synthesis and transport, lipid synthesis, and calcium storage. ER is sensitive to a variety of intracellular and extracellular factors. When ER exposed to stress, unfolded protein accumulation occurs as a result of disruption of homeostasis. This condition is called ER stress. Primarily the cell effectuates an unfolded protein response (UPR) pathway to restore homeostasis under ER stress. Three independent UPR pathways have been discovered in humans: PERK (protein kinase RNA-like ER kinase), ATF6 (activating transcription factor 6), and Ire1 (inositol essential enzyme 1). In yeast, only Ire1 (inositol essential enzyme 1) is highly conserved among these pathways. The UPR pathway, which is activated to reduce ER stress, supports the correct folding of proteins and utilizes protein degradation mechanisms to reduce unfolded protein accumulation. However, the cells, exposed to prolonged or severe stress, do not achieve homeostasis, and UPR pathway trigger the programmed cell death including apoptosis and autophagy. In the first part of this study, the cellular effects of ER stress were investigated by applying different doses of tunicamycin (TN) antibiotics (1 μg/mL and 5 μg/mL) as an ER stress agent for 1, 4 and 24 hours in Schizosaccharomyces pombe wild type strain. In the second part, the effect of tunicamycin (1 μg/mL; 1, 4 ve 24 hours) on cell viability and intracellular oxidation level was investigated by deleting the genes which are they not to involve in UPR (ire1 and gpt1), autophagy (atg8, atg15 and trs85), apoptosis (aif1), signal transduction (prz1) and cytoskeleton (myo51 and myo52) processes. It was concluded that autophagy rather than apoptosis may be effective in the regulation of cell viability under ER stress triggered by TN in S. pombe wild type strain. Also, it suggests that increased actin patch organization due to increased TN dosage and duration may be associated with phagosome formation. On the other hand, ire1Δ, gpt1Δ, atg8Δ, myo51Δ and myo52Δ mutants were found tunicamysin-sensitive in a dose-independent manner. atg15Δ and trs85Δ mutants were found tunicamysin-resistant. But, aif1Δ and prz1Δ mutans were found tunicamysin-sentitive in a dose-dependent manner. In conclusion our results fully support that the hypothesis that suggest that the cellular response to ER stress was autophagy instead of apoptosis.