Altered iron homeostasis involvement in arsenite-mediated cell transformation

Abstract

Chronic exposure to low doses of arsenite causes transformation of human osteogenic sarcoma (HOS) cells. Although oxidative stress is considered important in arsenite-induced cell transformation, the molecular and cellular mechanisms by which arsenite transforms human cells are still unknown. In the present study, we investigated whether altered iron homeostasis, known to affect cellular oxidative stress, can contribute to the arsenite-mediated cell transformation. Using arsenite-induced HOS cell transformation as a model, it was found that total iron levels are significantly higher in transformed HOS cells in comparison to parental control HOS cells. Under normal iron metabolism conditions, iron homeostasis is tightly controlled by inverse regulation of ferritin and transferrin receptor (TfR) through iron regulatory proteins (IRP). Increased iron levels in arsenite transformed cells should theoretically lead to higher ferritin and lower TfR in these cells than in controls. However, the results showed that both ferritin and TfR are decreased, apparently through two different mechanisms. A lower ferritin level in cytoplasm was due to the decreased mRNA in the arsenitetransformed HOS cells, while the decline in TfR was due to a lowered IRP-binding activity. By challenging cells with iron, it was further established that arsenite-transformed HOS cells are less responsive to iron treatment than control HOS cells, which allows accumulation of iron in the transformed cells, as exemplified by significantly lower ferritin induction. On the other hand, caffeic acid phenethyl ester (CAPE), an antioxidant previously shown to suppress As-mediated cell transformation, prevents As-mediated ferritin depletion. In conclusion, our results suggest that altered iron homeostasis contributes to arsenite-induced oxidative stress and, thus, may be involved in arsenite-mediated cell transformation.

Impairment of antioxidant defenses as a contributor to arsenite-induced cell transformation

“…Our previous results showed that chronic exposures to low As doses induce malignant HOS cell transformation. Concomitantly, increases in bioavailable cellular iron and decreases in both inflammatory cytokines and antioxidant enzymes levels were observed, thus, indicating that these factors could be involved in the cell transformation process (Wu et al 2006; Yang et al 2005). …”

“…The variability in ferritin subunit composition could affect the rates of iron uptake and release in different tissues (Proulx-Curry and Chasteen 1995) and, consequently, could further increase the content of bioavailable iron within the cells. In fact, we previously showed that arsenite-treated HOS cells contain much higher “free” iron levels (bound to low molecular weight cellular chelators, such as citrate) than the parental HOS cells do (Wu et al 2006). The As-induced decline in ferritin and concomitant increase in iron levels could create oxidative stress and consequent cellular vulnerability and/or DNA damage.…”

“…Here, we presented evidence that ferritin also is present in HOS cell nuclei and that its levels are affected by submicromolar arsenic exposure. It was reported that nuclear ferritin has the ability to protect DNA from certain types of damage, such as that induced by UV(Cai et al 1998), but the exact mechanism by which nuclear ferritin damage is involved in arsenic’s carcinogenesis needs to be further investigated (Wu et al 2006). …”

“…Our study here employed a long-term (8-week) chronic exposure to low submicromolar doses of arsenite (0.1 or 0.5 μM), while the other published studies often utilized short-term (12–16 h) treatments with 100-fold higher arsenic doses (12 μM) in different cell models. We have previously shown that short-term exposures to arsenite stimulate antioxidant defense genes while long-term exposures lead to their suppression (Wu et al 2006). Our experiments here were designed to better mimic human chronic long-term, low-dose exposure to arsenic.…”

Zinc transport by respiratory epithelial cells and interaction with iron homeostasis

“…Alteration of iron homeostasis can affect both transport and biological effect exerted by several different metals (Kang et al 2006;Martelli and Moulis 2004;Wu et al 2006). Iron is an essential micronutrient utilized in almost every aspect of normal cell function.…”