Elevated intracellular calcium increases ferritin H expression through an NFAT-independent post-transcriptional mechanism involving mRNA stabilization.
An increase in intracellular Ca2+ is one of the initiating events in T-cell activation. A calcium-mediated signalling cascade in T-cells involves activation of calcineurin and the dephosphorylation and translocation of NFAT (nuclear factor of activated T-cells), resulting in the transcriptional activation of target genes such as IL-2 (interleukin-2). In the present study, we found that increased intracellular calcium leads to induction of the antioxidant protein ferritin H. We previously reported that the ferritin H gene is transcriptionally activated under oxidative stress conditions through an ARE (antioxidant-responsive element). The facts that the ferritin H ARE contains a composite AP-1 (activator protein 1) site and that NFAT collaborates with AP-1 transcription factors led us to test whether calcium-activated NFAT is involved in the ferritin H induction through the ARE. Treatment of Jurkat T-cells with the calcium ionophore, ionomycin, increased ferritin H mRNA and protein expression. Although NFAT translocated to the nucleus and bound a consensus NFAT sequence located in the IL-2 promoter after ionomycin treatment, it did not activate ferritin H transcription despite the presence of a putative NFAT-binding sequence in the ferritin H ARE. In addition, the calcineurin inhibitor cyclosporin A treatment blocked ionomycin-mediated NFAT nuclear translocation but failed to abrogate the increase in ferritin H mRNA. Analysis of mRNA stability after actinomycin D treatment revealed that ionomycin prolongs ferritin H mRNA half-life. Taken together, these results suggest that ionomycin-mediated induction of ferritin H may occur in an NFAT-independent manner but through post-transcriptional stabilization of the ferritin H mRNA.
MacKenzie
Elizabeth L
EL
Department of Environmental and Molecular Toxicology, North Carolina State University, Campus Box 7633, Raleigh, NC 27695-7633, USA.
Tsuji
Yoshiaki
Y
D016475:3T3 Cells; D000818:Animals; D001052:Apoferritins / Q000235:genetics*; D019703:Calcineurin / Q000378:metabolism; D002118:Calcium / Q000502:physiology*; D005786:Gene Expression Regulation; D006801:Humans; D015759:Ionomycin / Q000494:pharmacology; D019169:Jurkat Cells; D051379:Mice; D050778:NFATC Transcription Factors / Q000378:metabolism*; D020871:RNA Stability; D020218:Response Elements
D050778:NFATC Transcription Factors; D015759:Ionomycin; D001052:Apoferritins; D019703:Calcineurin; D002118:Calcium