Influence of cytokines on Dmt1 iron transporter and ferritin expression in insulin-secreting cells

Free intracellular ferrous iron (Fe(2+)) is essential for the generation of the extremely toxic hydroxyl radicals, which contribute to β-cell destruction by cytokines. Therefore the expression of the different divalent metal transporter 1 (Dmt1) isoforms and ferritin (Ft) subunits, responsible for iron import and chelation, was analyzed under pro-inflammatory conditions (IL1β alone or together with TNFα+IFNγ). The Dmt1 isoforms (1A/1B and +IRE/-IRE) and the total Dmt1 expression in insulin-producing cells (RINm5F and INS-1E), in primary rat islets and, for comparison, in the neuroendocrine PC12 cell line were quantified by qRT-PCR. In addition, the expression of the light (L-Ft) and heavy Ft (H-Ft) subunits and the mitochondrial Ft isoform (Mtft) in insulin-producing cells under control conditions and after cytokine treatment was estimated. The 1B isoform was the predominant Dmt1 mRNA in all insulin-producing cells, accounting for almost 100% of the 1A/1B isoform expression. For the IRE variants, +IRE expression was higher than -IRE expression. Pro-inflammatory cytokines accelerated the expression of Dmt1 isoforms significantly with an overall 2.5- to 3-fold increase in the total Dmt1 expression. In contrast, the expression of the iron-buffering ferritin subunits L- and H-Ft was unaffected by IL1β and only slightly induced by the cytokine mixture. Mtft expression was also not increased. Dmt1 expression was significantly elevated through pro-inflammatory cytokines, whereas Ft expression was marginally increased. This imbalance between the increased iron transport capacity and the almost unaffected iron storage capacity can foster cytokine-mediated formation of hydroxyl radicals and thus pro-inflammatory cytokine toxicity through elevated free iron concentrations.