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Advances in Brief |
Cancer Research Campaign Academic Unit of Clinical Oncology, University of Nottingham, City Hospital, Nottingham NG5 1PB [J. M. C., J. L. E., P. J. W.], and Veterinary Pathology, University of Edinburgh, Summerhall, Edinburgh, EH9 1QH [J. P. Q.], United Kingdom
| ABSTRACT |
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| Introduction |
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We have previously characterized transcription factors that interact
with the proximal vasopressin promoter, which contains the major
elements that determine the expression of vasopressin in lung tumor
cell lines (8, 9, 10)
. We recently identified a motif with
homology to the NRSE around the transcriptional start site of
vasopressin, suggesting a role for NRSF (REST/X box repressor 1) in the
normal restriction of expression to neuronal cells (11)
.
The NRSF is a large repressor protein (12
, 13)
that can
silence the expression of neuronal genes in nonneuronal tissues via
NRSE motifs (14)
. The complex secondary structure of NRSF
includes nine zinc fingers, eight of which form a DNA binding domain,
and there are two repressor domains at the NH2-
and COOH-terminals (illustrated in Fig. 1
). The COOH-terminal domain ninth zinc finger was recently reported to
interact with a corepressor (15)
, whereas the
NH2-terminal repressor domain is now thought to
be involved in histone deacetylation (16)
. We have shown
that overexpression of NRSF could switch off the vasopressin promoter
in some SCLCs (11)
. However, rather than observing a loss
in binding of NRSF to the vasopressin promoter NRSE motif, we noted
that several SCLC complexes were present in electrophoretic mobility
shift analysis. We postulated that these multiple complexes
could be modified forms of NRSF, such as the splice variants recently
reported by others in a rat model (17)
. We now report that
a novel splice variant of NRSF is very highly expressed in SCLCs
relative to NSCLCs or normal lung tissue. We propose that the encoded
isoform antagonizes the function of normal NRSF, thus derepressing
genes such as vasopressin. This isoform may represent a key factor
early in establishment of neuroendocrine SCLCs and can be exploited as
a specific clinical marker for detecting the majority of these tumors.
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| Materials and Methods |
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Tissue Samples.
Primary lung tumor cells were PW3 (SCLC pretreatment, passage 4), PW5
(SCLC posttreatment, passage 5), 011 (SCLC, pleural fluid, passage 6),
and 002 (NSCLC, lymph node, passage 6). These were derived from lymph
node and pleural aspirates by culture in selective media
(HITESA = RPMI 1640 with L-glutamine,
supplemented with 10 nM hydrocortisone, 5 µg/ml insulin,
10 µg/ml transferrin, 10 nM estradiol, 30 nM
selenite, and 0.25% BSA). Normal brain biopsies were collected at
Nottingham City Hospital from fresh postmortem material, these tissues
were snap-frozen in liquid nitrogen. PNET cDNA was a gift from Dr. P.
Scotting (Queens Medical Center, Nottingham, United Kingdom).
Isolation of mRNA from Cultured Cells and Tissues.
Total cellular RNA was prepared from 2 x 107 cultured cells using a Purescript RNA kit
(Gentra Systems, Inc.). Total RNA was prepared from human tissue using
Trizol (Life Technologies, Inc.). The concentration and integrity of
all RNAs were determined spectrophotometrically and
electrophoretically.
RT-PCR.
Total cellular RNA (1 µg for cell lines or 2 µg for primary tissue)
was used for reverse transcription (Promega, Southampton, United
Kingdom), and the cDNA was denatured and diluted to 100 µl; 2 µl of
this were used for PCR. Dynazyme II polymerase was used in
magnesium-free buffer (Flowgen) supplemented to a final concentration
of 1.5 mM MgCl2. As described
previously, GAPDH primers (8)
and ß-actin primers that
produce a band of 626 bp (18)
were used as controls for
semiquantification. Amplification of 28 (GAPDH) or 35 (ß-actin)
cycles was used for cell lines, and amplification of 33 or 37 cycles,
respectively, was used for primary samples. NRSF primer sequences
NRSFfor (5'-GAATCTGAAGAACAGTTTGTGCAT-3') and NRSFrev
(5'-TTTGAAG-TTGCTTCTATCTGCTGT3'), which amplify 554-1180
(19)
of the human NRSF/REST cDNA (HSU22314), were used for
35 cycles (cell lines) or for up to 45 cycles (primary material). A
SCLC-specific NRSF primer, sNRSFrev (5'-ATCACACTCTAGTAAATATTACC-3'),
was designed against the insert sequence and used in seminested RT-PCR
to detect the splice variant in tissues where it was not seen in the
standard PCR. The first round of amplification (25 cycles) was
performed with NRSFfor/NRSFrev; 2 µl were then used as the template
in the second round with NRSFfor/sNRSFrev (25 cycles, 521 bp). All PCR
products were electrophoretically separated on 2% agarose gels, and
AMRESCO 3:1 agarose (Anachem, Bedfordshire, United Kingdom) was used
for high resolution (Fig. 1b
). Products were excised and
purified using QIAquick columns (Qiagen, Crawley, United Kingdom) for
direct automated sequencing or cloned into PCR2.1 (InVitrogen) before
sequencing on an ABI Prism 377 (DNA Sequencing Facility,
Queens Medical Center).
| Results and Discussion |
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Interestingly, the one SCLC cell line (COR-L88) in which the splice
variant was not evident (Fig. 1a
) is a morphological variant
cell line that we have found to express low levels of vasopressin by
nested RT-PCR (data not shown). Repeated PCR and increased resolution
of the NRSF amplification products by the use of AMRESCO 3:1 agarose in
fact reproducibly demonstrated that some sNRSF isoform was expressed in
this cell line (Fig. 1b
). However, the normal NRSF product
was much more abundant than the isoform, in contrast to the other SCLC
cell lines, in which the two forms were expressed at similar levels.
This finding in a variant cell line has implications in terms of both
neuroendocrine profiles and clinical exploitation, as discussed below.
To confirm that the SCLC splice variant (sNRSF) was not an artifact of
established tumor cell lines cultured in vitro, we have also
examined primary tumors for the splice variant. Four low-passage
primary cultures of lung tumors established from lymph node or pleural
aspirates were screened by RT-PCR (Fig. 2
). Whereas sNRSF and the wild-type repressor were both seen in 011
(SCLC, passage 6), only the latter was evident in the NSCLC isolate
(002, passage 6). We also studied a pair of cultures (PW3, passage 4
and PW5, passage 5) established from the same patient at the early
(pretreatment) and late (posttreatment) stages of disease. sNRSF was
detected in the two samples, and equivalent amounts of normal NRSF and
sNRSF were seen in PW5; however, in the PW3 sample, sNRSF was the major
product. These data imply that high-level expression of the aberrant
splice variant is a feature of SCLC tumors but not of NSCLC tumors and
that this expression is seen pretreatment and may therefore represent
an early change in the development of SCLC. It has been
suggested that variant SCLCs represent recurrent or drug-resistant
tumors, which have reduced neuroendocrine characteristics. The
reduction in the ratio of sNRSF:NRSF posttreatment (PW5) compared to
pretreatment (PW3) and in a variant cell line (COR-L88) compared to
other SCLC lines is consistent with this hypothesis.
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Interestingly, nested PCR with the sNRSF-specific primer amplified two products from the NSCLC cell line NCI-H460. Sequencing of these products revealed that one corresponded to amplification of sNRSF, and the other corresponded to the normal product of the first-round amplification, which must have been highly abundant. These data imply that although the sNRSF splice variant was present at greatly elevated levels in SCLCs, it could also be detected, albeit at substantially lower levels, in NSCLCs. This lends support to the theory of a common epithelial origin for SCLC and NSCLC lung tumor types.
Structure and Functions of NRSF and Its Isoforms.
The SCLC splice variant, sNRSF, incorporates an additional 50-bp exon
at the boundary of exons 5 and 6, which introduces a stop codon
resulting in a truncated isoform of NRSF with a novel 13-amino acid
sequence not present in wild-type NRSF. We have submitted the
nucleotide sequence to GenBank (accession number AF228045). Sequence
comparison (Fig. 4
) shows that this is distinct from but related to the neural-specific
NRSF isoforms (rREST4 and rREST5) previously described in rat brain
(17)
. The 3' portion of the sNRSF insert (16 bp) has
homology to the common region of the rat neural variants, with only one
mismatched base. However, sNRSF also has an additional 34-bp at
sequence 5' to this, which has not been described in rat. The
presence of this splice variant in SCLCs, which is generally believed
to be an epithelial-derived tumor, is of great interest because rREST4
and rREST5 have been detected exclusively in neuronal tissue. We failed
to detect sNRSF in most brain tissue studied, and where the
corresponding PCR product was amplified, this was of very low
abundance. These data concur with the reported expression of rREST4 and
rREST5, which were present at low levels in rat neuronal tissue
(17)
.
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A Potential Role for sNRSF in Neuropeptide Expression.
The rat neural-specific splice variants were originally described as
retaining repressor activity (17)
. However, a recent
publication (21)
describes an antisilencer mechanism for
murine REST4 that prevents NRSF silencing of the cholinergic gene locus
through a protein-protein interaction, implying a role in neuronal
transcription. This recent description of an antagonistic role for at
least one NRSF isoform (21)
and our discovery of a human
NRSF splice variant in SCLCs provide strong support for our earlier
hypothesis that abnormal forms of NRSF lead to derepression of
vasopressin expression in these tumors (11)
. We propose
that sNRSF antagonizes the function of normal NRSF and that this
isoform could potentially derepress a number of neuroendocrine genes
contributing to the neuroendocrine pathology of SCLC and the expression
of a number of autocrine growth factors. Because it has been suggested
that relative concentrations of NRSF and REST4 control expression
levels (21)
, those tissues with very low levels of sNRSF,
for example, NSCLC (Fig. 3b
), would be unlikely to express
sufficient variant for antisilencer activity. We have also shown that
ratios of sNRSF:wild-type NRSF vary between classical and variant (Fig. 1b
) or pre- and posttreatment SCLCs (Fig. 2
), implying that
this isoform may be less important in later stage or drug-resistant
SCLC tumors, which often show reduced neuroendocrine properties.
Application of sNRSF in SCLC Diagnosis.
This human SCLC NRSF splice variant was not detected by standard RT-PCR
in normal lung or in other nonneuronal tissue. However, we have
detected high relative levels of sNRSF in most primary SCLC tumors.
From the data presented here, this represents a largely tumor-specific
clinical marker that is highly expressed in the majority of SCLCs. We
propose to use sNRSF in the detection of these tumors
(22)
. Our data imply that high-level sNRSF expression
would be a useful clinical marker in early detection of SCLC, whereas
the loss of sNRSF in a tumor may be an indicator of recurrent or
resistant disease.
| Acknowledgments |
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| FOOTNOTES |
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1 Supported by the Association for International
Cancer Research, St Andrews, Scotland. ![]()
2 To whom requests for reprints should be
addressed, at CRC Academic Unit of Clinical Oncology, City Hospital,
Hucknall Road, Nottingham NG5 1PB, United Kingdom. Phone:
44-0-115-969-1169, ext. 47320; Fax: 44-0-115-962-7923; E-mail: judy.coulson{at}nott.ac.uk ![]()
3 The abbreviations used are: SCLC, small cell
lung cancer; NSCLC, non-SCLC; NRSE, neuron-restrictive silencer
element; NRSF, neuron-restrictive silencer factor; REST, RE-1 silencing
transcription factor; PNET, primitive neuroectodermal tumor; GAPDH,
glyceraldehyde-3-phosphate dehydrogenase; RT-PCR, reverse
transcription-PCR. ![]()
4 M. Garcia-Ocejo, S. Ahmed, J. M. Coulson,
and P. J. Woll. Mitogenic neuropeptides and receptors in lung
cancer: potential autocrine circuits and early markers, submitted for
publication. ![]()
Received 12/ 9/99. Accepted 2/17/00.
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