Loss of ERAD bridging factor UBX2 modulates lipid metabolism and leads to ER stress-associated apoptosis during cadmium toxicity in Saccharomyces cerevisiae.

Abstract

The endoplasmic reticulum (ER) stress potentially activates the unfolded protein response (UPR) and ER-associated protein degradation (ERAD) as quality-control mechanisms. During ERAD process, the ERAD adaptor protein Ubx2 serves as a bridging factor and transports the misfolded proteins from the ER to the cytosol for subsequent ubiquitylation and proteasomal degradation. Cadmium (Cd) is a toxic metal that initiates ER stress and has an impact on lipid homeostasis and this study focuses on the synergistic impact of Cd exposure and ERAD (using ubx2∆ strain). With Cd exposure in ubx2∆ strain, we observed stunted growth and induction of ER stress. The ER stress was confirmed by measuring the expression of UPR marker (Kar2p), and mRNA expression of ER stress-responsive genes (HAC1, IRE1, ERO1, and PDI1), heat shock responsive genes (HSP104 and HSP60), ERAD pathway genes (DOA10, CDC48, HRD1, and YOS9), and proteasome regulators (UBI14, and RPN4). We also observed aberrant membrane morphology with DiOC6 staining, and interrupted mitochondria with mitotracker dye using microscopic analysis. The cell's inability to relieve stress through adaptive response results in apoptosis and was assessed using acridine orange (AO)-ethidium bromide (EtBr) staining. In ubx2∆ strain, there was reduction in triacylglycerol (TAG) and lipid droplets (LDs), and increase in the phospholipids. The mRNA expression of lipid metabolic genes (LRO1, DGA1, ARE1, ARE2, and OLE1) supported the lipid pattern observed. Collectively, our data suggest that in Saccharomyces cerevisiae, the Cd exposure on ubx2∆ strain induced cellular stress and has an impact on ERAD, UPR, and LD homeostasis.