Effect of metal ions on HIF-1α and Fe homeostasis in human A549 cells

Several metals are carcinogenic but little is known about the mechanisms by which they cause cancer. A pathway that may contribute to metal ion induced carcinogenesis is by hypoxia signaling, which involves a disruption of cellular iron homeostasis by competition with iron transporters or iron-regulated enzymes. To examine the involvement of iron in the hypoxia signaling activity of these metal ions we investigated HIF-1α protein stabilization, IRP-1 activity, and ferritin protein levels in human lung carcinoma A459 cells exposed to various agents in serum- and iron-free salt-glucose medium (SGM) or in normal complete medium. We also studied the effects of excess exogenous iron on these responses induced by nickel ion exposure. Our results show the following: (1) SGM enhanced metals-induced HIF-1α stabilization and IRP-1 activation (e.g., nickel and cobalt ions). (2) If SGM was reconstituted with a slight excess level (25 μM of FeSO₄) of iron, this enhancing ability was significantly decreased. (3) The effect of a high level of exogenous iron (500 μM of FeSO₄) on metal-induced hypoxia and iron metabolism was highly dependent on the order of addition. If treatment with the Fe and metal ions was simultaneous (co-treatment), the effects of nickel ion exposure were overwhelmed, since the added Fe reversed HIF-1α stabilization, decreased IRP-1 activity, and increased ferritin level. Pre-treatment with iron was not able to reverse the responses caused by nickel ion exposure. These results imply that it is important to consider the available iron concentration and suitable exposure design when studying metal-induced hypoxia or metal-induced disruption of Fe homeostasis.