Zinc interference with Cd‐induced hormetic effect in differentiated Caco‐2 cells: Evidence for inhibition downstream ERK activation

Abstract

Cadmium (Cd) is a toxic metal that enters the food chain. Following oral ingestion, the intestinal epithelium represents an effective protective barrier against Cd toxicity, but it is also a target tissue that may accumulate and trap high levels of the ingested metal. Using human enterocytic‐like Caco‐2 cells, we have previously shown that Cd may induce a concentration and time‐dependent increase in 3‐(4,5‐dimethyl‐2‐thiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide assay (MTT)‐reducing activity in differentiated cultures with correlation to ERK1/2 activation. The present study shows that (a) Zn prevents the Cd‐induced hormesis effect on MTT reduction in a concentration‐dependent manner, without inhibiting Cd‐induced ERK1/2 activation; (b) Zn also induces similar hormetic stimulation of MTT‐reducing activity but without ERK1/2 activation. The effect of both metals was sensitive to inhibitors of translation during protein synthesis. There is evidence for the involvement of reactive oxygen species (ROS) in Cd‐induced ERK1/2 activation. In contrast, the Zn effect on the MTT‐reducing activity would not be triggered by ROS but it would be sensitive to the redox state of the cell. Steps downstream ERK1/2 activation by Cd does not involve eIF4E which is rather downregulated by Cd. In conclusion, Cd and Zn both can modify translation processes during protein synthesis via different signaling cascades with crosstalk, and cross‐inhibition may occur. This phenomenon is observed over a small range of metal concentrations and is characterized by a hormesis‐like response. Considering that the hormetic effect on dehydrogenase activity could reflect an adaptive response to the metals whether cross‐inhibition is beneficial is an open question.

Cadmium affects autophagy in the human intestinal cells Caco-2 through ROS-mediated ERK activation

“…The Cd-induced fivefold increase in p-ERK1/2 was partially inhibited by co-exposure with NAC but not by NAC pretreatment before incubation with Cd (Fig. 8A, C) in accordance with our previous studies showing that pretreatment did not succeed in preventing Cd-induced ROS formation (Lemaire et al 2020). Interestingly, concomitant exposure of cells with Cd and NAC (but not NAC pretreatment) led to a reduction in LC3-II/LC3-I ratio to control levels (Fig.…”

“…6H, I). As already observed in our previous studies (Gebraël and Jumarie 2015;Lemaire et al 2020), slight decreases (15%) in total ERK1/2 were observed under conditions of very high levels of activation (100 to 150%) (Fig. 6C).…”

“…Cadmium binds to intracellular SH-groups and may favor Fenton reaction leading to oxidative stress and redox imbalance. We have previously shown that a 24-h exposure to 10 µM Cd increases ROS in 21-day-old Caco-2 cells (Lemaire et al 2020) and that Cd-induced ERK1/2 activation involved redox signaling. The present study confirms that pro-oxidant conditions increase the level of p-ERK1/2, whereas the presence of antioxidants prevent Cd-induced ERK1/2 activation, further showing a positive and a negative correlation, respectively, in terms of LC3-II levels and the LC3-II/ LC3-I ratio (Fig.…”

“…In previous studies, we have shown that a 24 h exposure to subcytotoxic levels of Cd activated ERK1/2 and p38 kinase in enterocytic-like Caco-2 cells (Mantha and Jumarie 2010 ; Gebraël and Jumarie 2015 ; Lemaire et al 2020 ). This phenomenon was observed over a small range of metal concentrations and was characterized by a hormesis-like response optimal at 10 µM Cd.…”

“…The role of ROS in Cd-induced modulation of autophagy in post-confluent Caco-2 cells Cadmium is known to induce oxidative stress and we have previously shown the involvement of ROS in Cd-induced activation of ERK1/2 in Caco-2 cells (Lemaire et al 2020). Thus, the implication of ROS in Cd-dependent modulation of autophagy was investigated in cells treated with the antioxidant NAC or BSO, which inhibits glutathione synthesis and promotes pro-oxidant conditions.…”