Nucleo-cytoplasmic Trafficking of Metal-regulatory Transcription Factor 1 is Regulated by Diverse Stress Signals

The metal-regulatory transcription factor 1 (MTF-1) is a key regulator of heavy metal-induced transcription of metallothionein I and II and other genes in mammals and other metazoans. Transcriptional activation of genes by MTF-1 is mediated through binding to metal-responsive elements of consensus TGCRCNC in the target gene promoters. In an attempt to further clarify the mechanisms by which certain external signals activate MTF-1 and in turn modulate gene transcription, we show here that human MTF-1 has a dual nuclear and cytoplasmic localization in response to diverse stress stimuli. MTF-1 contains a consensus nuclear localization signal located just N-terminal to the first zinc finger that contributes to but is not essential for nuclear import. MTF-1 also harbors a leucine-rich, nuclear export signal. Under resting conditions, the nuclear export signal is required for cytoplasmic localization of MTF-1 as indicated by mutational analysis and transfer to the heterologous green fluorescent protein. Export from the nucleus was inhibited by leptomycin B, suggesting the involvement of the nuclear export protein CRM1. Our results further show that in addition to the heavy metals zinc and cadmium, heat shock, hydrogen peroxide, low extracellular pH (pH 6.0), inhibition of protein synthesis by cycloheximide, and serum induce nuclear accumulation of MTF-1. However, heavy metals alone (and not the other stress conditions) induce a significant transcriptional response via metal-responsive element promoter sequences, implying that nuclear import of MTF-1 is necessary but not sufficient for transcriptional activation. Possible roles for nuclear import under non-metal stress conditions are discussed.