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Advances in Brief |
Nelson Institute of Environmental Medicine, and Kaplan Comprehensive Cancer Center, New York University School of Medicine, New York, New York 10016 [K. S., M. C.]; Medicine Branch, National Cancer Institute, NIH, Bethesda, Maryland 20892 [M. B.]; and Department of Biology, Center for Molecular Genetics, University of California, San Diego, La Jolla, California 92093-0366 [H. R., R. J.]
| ABSTRACT |
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| Introduction |
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Nickel treatment produced accumulation of
HIF3
-1
in both A549 and MCF-7 cells (12)
. Additionally,
nickel-transformed cells had elevated HIF-1 activity relative to
p53, suggesting the importance of this transcription factor
in nickel carcinogenesis (12)
. The HIF-1 transcription
factor was isolated from hypoxic cells and was shown to be involved in
the regulation of hypoxia-inducible genes (13)
. This
factor has been shown to be heterodimeric, basic helix-loop-helix
protein composed of HIF-1
and HIF-1ß subunits (14)
.
HIF-1
was a short-lived protein that was stabilized under hypoxic
conditions and was unique to HIF-1, whereas HIF-1ß (ARNT) was less
inducible and dimerized with several other basic helix-loop-helix
proteins (15)
. We also found that the Cap43
gene was induced by hypoxic conditions in three human cell lines with a
time course in the A549 cell line similar to nickel-dependent
induction. The induction of Cap43 gene expression by hypoxic
conditions or nickel was highly dependent on HIF-1 transcription
factors because this gene was not induced in fibroblasts that
originated from HIF-1
knockout mice. The Cap43 mRNA was
increased over 30-fold more than control in cells rendered hypoxic or
treated with nickel. The mechanism of this stimulation involved both an
increase in the rate of transcription and an increase in mRNA
stability. We have shown that stabilization of Cap43 mRNA
occurred in cells exposed to hypoxia conditions or Ni,2+
suggesting a similarity of response to both inducers at the
transcriptional as well as posttranscriptional levels. Another
gene, GAPDH, known to be induced by hypoxia was also
induced by nickel treatment in A549 cells. This gene was only induced
in HIF-1
-proficient fibroblasts by nickel, cobalt, and hypoxia but
not in HIF-1
-deficient fibroblasts. The signaling pathway(s)
involved in the transduction of hypoxic signals in mammalian cells are
still largely unknown. One hypothetical pathway included an oxygen
sensor that was considered to be a heme protein (16)
that
in turn activated HIF-1 transcription factor, possibly by
phosphorylation. However, the protein kinases involved in the hypoxic
response in mammalian cells have not been identified. We have shown
previously that serine/threonine but not tyrosine phosphorylation was
involved in nickel-stimulated Cap43 gene expression
(11)
. Because hypoxia affected intracellular calcium
homeostasis (17)
, we tested whether changes in
intracellular calcium also affected Cap43 gene expression by
hypoxia. The intracellular calcium chelator BAPTA-AM attenuated
hypoxia-induced Cap43 gene expression in three different
human cell lines. A similar effect was found for Cap43 gene
expression induced by nickel (18)
. These data suggested
that intracellular calcium was essential for signaling gene induction
in cells rendered hypoxic or exposed to nickel.
The similarity of gene induction by nickel or hypoxia suggested that exposure of cells to nickel produced a hypoxia-like state of gene expression that may be involved in the progression of normal cells to a neoplastic state.
| Materials and Methods |
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+/+ and
HIF-1
-/- fibroblasts were obtained from C57
B mice with wild-type, normal, or knockout HIF-1
gene
(19)
and were maintained in DMEM supplemented with 10%
fetal bovine serum, 100 units/ml penicillin, and 100 µg/ml
streptomycin. All cells were incubated at 37°C in the presence of 5%
of CO2 in air. Cells were rendered hypoxic in a
chamber with a gas mixture of 0.5% of O2, 5% of
CO2 balanced with N2 at
37°C. The level of oxygen in a chamber was verified using a gas
monitor (SKC, Inc., Eighty Four, PA).
Northern Blot Analysis.
Total RNA was extracted from cells immediately after exposure using an
ULTRASPEC RNA isolation system (Biotecx), and the RNA was
electrophoresed (15 µg of total RNA/lane) in 1.0%
agarose/formaldehyde gels. Probes were labeled with
[
-32P]dCTP using a Random Primed DNA
Labeling kit (Boehringer Mannheim).
Western Blot Analysis.
Cells were lysed in TNES buffer [50 mM Tris-HCl (pH 7.5),
2 mM EDTA, 100 mM NaCl, 1 mM sodium
orthovanadate, and 1% NP40 containing protease inhibitors (20 µg/ml
aprotinin, 20 µg/ml leupeptin, and 1 mM
phenylmethylsulfonyl fluoride). The detection of HIF-1
was as
described previously (12)
.
| Results |
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in response to hypoxia in A549 cells. HIF-1
was an
inducible subunit of HIF-1 transcription factor, and its accumulation
correlated with the activation of HIF-1 transcriptional activity
(15)
. A significant amount of HIF-1
protein was found
in A549 cells after an exposure of cells to hypoxia (Fig. 1C)
was observed in
nickel-treated A549 cells (12)
. To confirm that the HIF-1
transcription factor was involved in the Cap43 gene
expression, we compared the Cap43 gene expression in
response to hypoxia, as well as nickel and cobalt in mouse fibroblasts
that originated from normal or HIF-1
knockout mice. An induction of
Cap43 by hypoxia was found in fibroblasts that originated
from HIF-1
+/+ mice but not in those from
HIF-1
-/- mice (Fig. 2A)
+/+ mice but not in those
from HIF-1
-/- mice (Fig. 2A)
+/+ mice and not in cells from
HIP-1
-/- mice (Fig. 2B)
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protein under hypoxic conditions declined after 6 h
and reached baseline levels by 24 h. At the same time, the amount
of mRNA for Cap43 continued to increase, suggesting the
involvement of posttranscriptional stabilization of mRNA. It was
conceivable that the accumulation of large amounts of Cap43
mRNA in response to both hypoxia and nickel resulted from
posttranscriptional stabilization of mRNA. To test whether the
Cap43 mRNA was stabilized, cells were exposed to hypoxia or
nickel for 20 h to induce Cap43, and cells were then
transferred to fresh medium under normal oxygen conditions in the
presence or absence of 4 µM AD (Fig. 3)
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| Discussion |
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We have studied the regulation of expression of a human nickel-inducible gene Cap43 to compare signaling pathway(s) activated by hypoxia or NiCl2. We have found that Cap43 gene expression was induced by hypoxia in human cells. This finding will add another gene to the family of hypoxia-inducible genes and further support the idea that exposure of cells to nickel activated a signaling pathway similar to the pathway activated by hypoxia. The time course, the magnitude of gene induction, and the effects of specific activators or inhibitors were found to be similar for nickel and hypoxia.
Nickel induced the expression of a number of genes including GAPDH,
VEGF, and Epo (22, 23, 24)
. Although the molecular mechanism
of induction by nickel was not studied, a common feature of these genes
was that they were also induced by hypoxia. A transcription factor
(HIF-1) involved in the regulation of VEGF or Epo
genes was isolated from hypoxic cells (13
, 14)
. We already
have shown that nickel significantly induced HIF-1
protein
(12)
. Here, using fibroblasts that originated from
HIF-1
-/- mice, we have shown that the
expression of Cap43 and GAPDH by nickel or hypoxia required
HIF-1. This finding supported the hypothesis that nickel activated the
expression of these genes by mimicking hypoxic conditions.
Because the level of gene expression in mammalian cells can be determined not only by transcriptional activation but also by the rate of mRNA degradation, we compared the degradation rates of Cap43 mRNA induced by nickel or hypoxia. We found that Cap43 mRNA could be stabilized by AD, suggesting the existence of factors that destabilized this mRNA. This stabilization occurred for the induced Cap43 mRNA (i.e., nickel or hypoxia). The basal level of expression, however, was not affected by AD, but basal levels could be induced by cycloheximide (11) . The stabilization of mRNA that was shown for other hypoxia-inducible genes, including VEGF, Epo, and tyrosine hydroxylase (25, 26, 27) , is probably an important part of the cellular response to hypoxic stress. Stabilization of hypoxia-induced VEGF and Epo mRNA by cycloheximide was used to suggest a common mechanism of regulation of these hypoxia-inducible genes (28) . In this study, we found that nickel- or hypoxia-induced Cap43 mRNA was stabilized by AD or cycloheximide, providing additional evidence that nickel or hypoxia exposures produced a similar type of response with respect to these genes.
Previously, we have shown that the induction of Cap43 by nickel involved serine/threonine phosphorylation and mobilization of intracellular calcium (11) , because the calcium ionophore A23187 induced Cap43 gene expression and the intracellular calcium chelator BAPTA-AM attenuated its expression (18) . Although the protein kinases that were activated by calcium in response to nickel exposure were not identified, it was conceivable that the same protein kinase cascade was involved in hypoxia-activated Cap43 gene expression. In this study, we found that the induction of Cap43 gene expression by hypoxia was also attenuated by BAPTA-AM in three human cell lines confirming the involvement of intracellular calcium signaling during hypoxia. This further supported the hypothesis that the same or similar signaling pathway involving calcium was activated by both nickel and hypoxia.
The exposure of cells to nickel-stimulated hypoxia, even under normal oxygen conditions, produced cells that had high levels of HIF-1-dependent transcription and decreased levels of p53-dependent transcription (12) . It was conceivable that selection against p53-dependent cell death occurred during nickel exposure, and cells that survived severe hypoxic stress acquired resistance to diverse physiological and nonphysiological insults including radio- and chemotherapy. It is tempting to speculate that similar changes occur in nickel-transformed cells. Additional experiments are required to understand the acquisition of a hypoxic state in cells exposed to nickel and undergoing cellular transformation.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 This work was supported by Grants ES05512 and
ES00260 from the National Institute of Environmental Health Sciences
and Grant CA13687 from the National Cancer Institute. ![]()
2 To whom requests for reprints should be
addressed, at New York University School of Medicine, Department of
Environmental Medicine, 550 First Avenue, New York, NY 10016. Phone:
(914) 731-3516; Fax: (914) 351-2118; E-mail: salnikow{at}env.med.nyu.edu ![]()
3 The abbreviations used are: HIF,
hypoxia-inducible transcription factor; GAPDH,
glyceraldehyde-3-phosphate dehydrogenase; BAPTA-AM,
bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic
acid tetra-(acetoxymethyl)-ester; AD, actinomycin D; VEGF, vascular
endothelial growth factor; Epo, erythropoietin. ![]()
Received 10/ 4/99. Accepted 11/11/99.
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