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-1alpha
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-1alpha stabilization and
IRP-1 activation (e.g.,
nickel and
cobalt ions). (2) If SGM was reconstituted with a slight excess level (25 microM of FeSO(4)) of
iron, this enhancing ability was significantly decreased. (3) The effect of a high level of exogenous
iron (500 microM of FeSO(4)) 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-1alpha 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.