Proteo-stress refers to cellular stress caused by disturbances in proteostasis, such as an increase in unfolded or aggregated proteins, whereas its roles in biofunctions of phytochemicals remain to be fully elucidated. Zerumbone has previously been shown to bind to cellular proteins in a non-specific manner to cause proteo-stress and activate the protein quality control (PQC) systems. In this study, we aimed to identify other phytochemicals that have properties similar to zerumbone. The formation of p62/SQSTM1 protein oligomers was evaluated in a screening assay. Among 28 nutrients and phytochemicals, curcumin markedly increased the formation of p62/SQSTM1 oligomers. Treatment with curcumin increased denatured proteins in an insoluble protein fraction. Moreover, two housekeeping proteins degraded or insolubilized by curcumin and zerumbone, and they significantly induced the formation of aggresomes. The amounts of curcumin-generated abnormal proteins increased in a time-dependent manner and thereafter decreased, suggesting the activation of adaptive mechanisms for proteostasis. In fact, both zerumbone and curcumin markedly upregulated the expressions of heat shock proteins. They markedly suppressed cell death induced by 4-hydroxy-2-nonenal. Our results suggest that curcumin induces proteo-stress toward cellular proteins and the resultant activation of the PQC systems may protect the cell from proteo-toxic stimuli.

In this study, we attempted to elucidate the effects of ally isothiocyanates (AITC) on stress resistance. RGM1 cells, derived from the normal gastric mucosa of rats, were pretreated with AITC (0-30 μM) 24 hr or 48 hr prior to posttreatment with AITC (0 or 20 μM), or both. While approximately 90% of the vehicle-pretreated cells died by a posttreatment with AITC, pretreatments with AITC (10-30 μM), especially 24 hr prior and double pretreatments, exhibited striking cytoprotective effects. AITC, as a xenobiotic, increased the amounts of reactive oxygen species and insoluble proteins. On the other hand, double pretreatments with AITC markedly upregulated the mRNA expression levels of anti-oxidative, detoxification, and molecular chaperone genes for homeostasis. Interestingly, pretreatments with AITC (10 and 15 μM) significantly mitigated low-pH, but not high-pH, stress conditions, which may involve the activation of phosphoinositide 3-kinase and Na⁺/H⁺ exchanger. Taken together, we show here that multiple exposures to AITC can confer a stress resistance phenotype, including adaptation to acidic pH, by upregulating the expressions of self-defensive enzymes. Therefore, this study implies the importance of continuous ingestion of phytochemicals for efficiently increasing the stress resistance capacity against harmful chemicals.