Effects of the Imidazoline Binding Site Ligands

Hongwei Gao1, Mirna Mourtada1, Noel G Morgan1,2
1Cellular Pharmacology Group, School of Life Sciences, Keele University. Staffs, United
Kingdom. 2Institute of Biomedical and Clinical Science, Peninsula Medical School.
Plymouth, United Kingdom
Context Certain imidazoline drugs stimulate
insulin secretion acutely but their longer term
effects on the viability of pancreatic beta-cells
are less well characterised. Indeed, some
reports have suggested that imidazolines can
be toxic to beta-cells while others have
reported protective effects against other
cytotoxic agents.
Objective In order to address these
discrepancies, the effects of two structurally
related imidazolines, efaroxan and idazoxan,
on the viability of clonal BRIN-BD11 beta-
cells, were compared.
Design and main outcome measures BRIN-
BD11 cells were exposed to test reagents and
their viability monitored by measuring
cellular reducing ability and DNA
fragmentation. Nitric oxide was measured
indirectly via medium nitrite formation.
Results Efaroxan (up to 100 µM) did not
directly affect BRIN-BD11 cell viability in
the absence of other agents and it did not
protect these cells against the cytotoxic
effects of interleukin-1beta. Indeed, analysis
of DNA fragmentation in BRIN-BD11 cells
revealed that efaroxan enhanced the level of
damage caused by interleukin-1beta. Idazoxan
caused a time- and dose-dependent loss of
BRIN-BD11 cell viability in the absence of
other ligands. This was associated with
marked DNA degradation but was not
associated with formation of nitric oxide. The
effects of idazoxan were insensitive to
blockade of alpha2-adrenoceptors or 5-HT1A
(5-hydroxytryptamine; serotonin) receptors.
Conclusions The results confirm that
idazoxan is cytotoxic to beta-cells but show
that efaroxan is better tolerated. However,
since efaroxan enhanced the cytotoxic effects
of interleukin-1beta, it appears that this
imidazoline may sensitise BRIN-BD11 cells
to the damaging effects of certain cytokines.
It is now well accepted that a range of
imidazoline drugs (including compounds such
as efaroxan, RX871024, phentolamine,
antazoline) can stimulate insulin secretion and
that members of this class may be useful as
orally active compounds suitable for the
management of type 2 diabetes [reviewed in
1, 2, 3, 4, 5]. Despite this, the mechanisms
involved in their stimulatory effects have not
been defined fully and increasing evidence
indicates that the precise mechanism(s) may
even be variable for each compound.
Nevertheless, a consensus has emerged that
two principal actions are likely to play a role.
Firstly, the majority of imidazolines that
stimulate insulin secretion can block ATP-
sensitive potassium channels leading to
membrane depolarisation and Ca influx [6, 7,
8, 9, 10, 11]. Secondly, some members of the
class cause direct activation of the more distal

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events involved in control of insulin
exocytosis and, at least for some compounds,
this latter effect may be of greater importance
for their overall secretory activity [12, 13,
14]. It follows from this that members of the
latter group probably interact with a critical
intracellular binding site involved in the
control of insulin secretion. This site is likely
to be a member of the wider class of
imidazoline binding sites defined in other
tissues [2, 3, 15] but the molecule present in
the beta-cell exhibits an atypical
pharmacology [1, 5, 16]. It remains an
important objective to define this site and it is
encouraging that candidate molecules are now
beginning to emerge [17].
In addition to these aspects, recent studies
have added a new dimension to the potential
utility of imidazoline compounds by
providing evidence that some of them can
also alter cell viability [18, 19, 20, 21]. Most
strikingly, it has been reported that the loss of
viability resulting from exposure of ob/ob
mouse [19] or normal rat [21] pancreatic islets
to the cytokine interleukin-1beta (IL-1beta)
can be minimised by culture in the presence
of certain imidazolines. Since IL-1beta is
implicated as a causative agent in the loss of
beta-cells seen in type 1 diabetes [22], this has
raised the exciting possibility that
imidazolines may also be therapeutically
effective in preventing this condition. These
observations also suggest the intriguing
possibility that an imidazoline binding site
may regulate the sensitivity of beta-cells to
cytotoxic stimuli as well as controlling insulin
In considering the implications of these data,
it is already evident, however, that the
influence of imidazoline compounds on beta-
cell viability varies dramatically, and that
some (including idazoxan, phentolamine and
antazoline) cause the death of proliferating
beta-cells rather than exerting any protective
influence [18]. In addition, morphological
evidence indicates that idazoxan can also
damage the fully differentiated beta-cells
present in isolated islets [20]. Thus, there is
still considerable uncertainty about the effects
of imidazolines on beta-cell viability and, in
the present study, we have evaluated further
the influence of two structurally-related
imidazolines, efaroxan and idazoxan, on this
parameter. We have also investigated whether
efaroxan can alter the cytotoxic effects of IL-
1beta in clonal BRIN-BD11 beta-cells.
Clonal BRIN-BD11 cells ([23] passages 25-
35) were grown in Roswell Park Memorial
Institute (RPMI) 1640 medium supplemented
with 10% foetal calf serum, penicillin G (100
IU/mL) and streptomycin sulphate (100
µg/mL). Medium nitrite accumulation was
measured as an index of NO formation by the
cells. For these measurements, BRIN-BD11
cells were seeded into 96 well tissue culture
plates and treated with test reagents for 24-
48 h, as appropriate. After incubation,
samples of the medium were harvested for
measurement of nitrite formation. They were
incubated with a mixture (1:1 vol:vol) of 1%
sulphanilamide and 0.1% naphthylethylene-
diamine in 2% phosphoric acid and the optical
density determined at 540 nm after colour
development. Nitrite levels were determined
by reference to a standard curve constructed
using sodium nitrite.
The viability of BRIN-BD11 cells was
determined by measuring the ability of the
cells to reduce the dye, 3-(4,5-
(MTS) in the presence of the electron
coupling reagent, phenozine methosulphate
(PMS). Cells were grown in 96 well plates
and, following incubation with test reagents,
were exposed to a combination of MTS and
PMS (formulated in the CellTiter 96AQueous
Assay reagent (Promega, Southampton, UK))
according to the manufacturer's instructions.
Following a further incubation period (2-3 h
at 37 °C) the extent of MTS reduction was
determined by measuring the absorbance at
490 nm.
To monitor the integrity of cellular DNA, cell
cycle analysis was performed. BRIN-BD11
cells were harvested by brief centrifugation (3
min; 1000 g) after treatment with test reagents

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and incubation with trypsin/EDTA. The
medium was removed and the cells were
washed with phosphate buffered saline (PBS),
re-centrifuged and then fixed by resuspension
in 2 mL of a mixture of ice cold ethanol: PBS
(7:3 vol:vol). DNA integrity was determined
by fluorescence activated cell counting after
labelling with propidium iodide. Cell cycle
analysis was carried out under contract by
Babraham Technix (Babraham Bioscience
Technologies Limited, Cambridge, UK).
Efaroxan was purchased from Tocris (Bristol,
UK), idazoxan and S-nitrosoglutathione from
Sigma (Poole, Dorest, UK). Interleukin-1beta
was from Calbiochem (Nottingham, UK).
Statistical analysis of results was performed
by analysis of variance and differences were
considered significant when two-tailed P was
less than 0.05.
Previous studies have indicated that
imidazoline drugs having closely related
structures can exert profoundly differing
effects on the viability of pancreatic beta-cells
[18, 19, 20, 21]. These observations have
been confirmed in the present work which
revealed that exposure of the beta-cell line,
BRIN-BD11, to idazoxan for 24 h was
associated with a dose-dependent loss of MTS
reduction (Figure 1). By contrast, exposure to
efaroxan (up to 100 µM) was much less
effective at attenuating MTS reduction by
BRIN-BD11 cells, although modest inhibition
was occasionally seen (e.g. Figure 2). Despite
this, neither cell proliferation, insulin
secretion rate nor the overall extent of cell
viability (as judged by vital dye staining) was
compromised by exposure to efaroxan for up
to 7 days (not shown). In order to differentiate
between possible effects of idazoxan on cell
proliferation and on cell death (either of
which could result in lowered MTS reduction
after incubation) BRIN-BD11 cells were
exposed to idazoxan (100 µM) for 24 h and
then analysed by fluorescence activated cell
sorting (FACS; Becton, Dickinson and
Company, Franklin Lakes, NJ, USA) to
monitor the integrity of the cellular DNA
(Figure 3). The DNA of control cells was
principally distributed within a single G1
peak (Figure 3) although, as expected, there
was also evidence that some cells were
undergoing mitosis since a small G2 peak
(representing fully replicated DNA) was seen.
By contrast, after treatment with idazoxan,
BRIN-BD11 cells displayed a markedly
reduced G1 peak accompanied by significant
DNA fragmentation as evidenced by the
appearance of a large pre-G1 peak (Figure 3).
This suggests that idazoxan directly promotes
Figure 1. Effects of idazoxan and efaroxan on the
viability of BRIN-BD11 cells. Cultured BRIN-BD11
cells were treated with either increasing concentrations
of idazoxan or with 100 µM efaroxan, as shown. After
incubation for 24 h, the viability of the cells was
measured by virtue of their ability to promote MTS
reduction. Results are mean values±SEM (n=8).
P values vs. control
Figure 2. Effects of interleukin-1beta and efaroxan on
the viability of BRIN-BD11 cells. Cultured BRIN-
BD11 cells were treated with either interleukin-1beta
(2 ng/mL) and/or efaroxan (Efx: 100 µM), as shown.
After incubation for 48 h, the viability of the cells was
measured by virtue of their ability to promote MTS
reduction. Results are mean values±SEM (n=8).
P values vs. control

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the death of BRIN cells. Cells treated with
efaroxan alone displayed a normal
distribution of cellular DNA (see Figure 4).
To investigate the time course of these
effects, BRIN cells were exposed to 100 µM
idazoxan and harvested after 12, 16 and 20 h.
The fragmentation of cellular DNA was then
estimated by measuring the accumulation of
DNA within the pre-G1 peak (Figure 5).
Idazoxan caused a significant increase in
DNA fragmentation as early as 12 h after
exposure (P=0.006) and the extent of this
increased significantly (P=0.010) in a time-
dependent manner such that, after 20 h,
almost all of the cellular DNA was
fragmented. These results indicate that
idazoxan is acutely toxic to beta-cells but
confirm that its close structural analogue,
efaroxan, is better-tolerated. One difference in
pharmacological profile between idazoxan
and efaroxan is that, in addition to its potent
alpha2-antagonist properties, the latter also
has significant agonist activity at 5-HT1A (5-
hydroxytryptamine; serotonin) receptors [24].
We, therefore, investigated whether the ability
of idazoxan to cause beta-cell death was
subject to modulation by the selective 5-HT1A
However, this agent failed to significantly
modify the decrease in MTS reduction caused
by exposure of BRIN-BD11 cells to idazoxan
(results not presented).
In view of these data and the recent results of
Zaitsev et al. [19] and Papaccio et al. [21],
efaroxan was selected for further investigation
as a potentially protective agent that might
Figure 3. Effect of idazoxan on DNA integrity in
BRIN-BD11 cells. Cultured BRIN-BD11 cells were
exposed to either vehicle (control) or idazoxan (100
µM) for 20 h. After this time the cells were harvested
and fixed for analysis of cellular DNA integrity by
fluorescence activated cell counting. The proportion of
cells with fragmented (pre-G1), intact (G1), or
replicated (G2/M) DNA was quantified. Areas under
the peaks were integrated and are presented as mean
P values relative to idazoxan vs. control
Figure 4. Effects of interleukin-1beta and efaroxan on
the integrity of cellular DNA in BRIN-BD11 cells.
Cultured BRIN-BD11 cells were exposed to either
vehicle, efaroxan (Efx: 100 µM), interleukin-1beta (2
ng/mL) or interleukin-1beta plus efaroxan for 48 h.
After this time the cells were harvested and fixed for
analysis of cellular DNA integrity by fluorescence
activated cell counting. The proportion of cells with
fragmented (pre-G1), intact (G1), or replicated (G2/M)
DNA was quantified. Areas under the peaks were
integrated and are presented as mean values±SEM.
P value vs. interleukin-1beta alone
Figure 5. Time-dependence of the increase in BRIN-
BD11 cell DNA fragmentation induced by idazoxan
(P=0.010). Cultured BRIN-BD11 cells were exposed to
either vehicle (control) or idazoxan (100 µM) for 12-20
h, as shown. After this time the cells were harvested
and fixed for analysis of cellular DNA integrity by
fluorescence activated cell counting. Data are presented
as mean values±SEM from duplicate determinations at
the 12 and 20 h time points. In the repeat experiment
data were not collect at the 16 h time point.
P values vs. control

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reduce the extent of beta-cell death in
response to the cytokine IL-1beta. Treatment
of BRIN-BD11 cells with IL-1beta resulted in
a large increase in medium nitrite
accumulation (Figure 6) consistent with the
expected induction of an isoform of nitric
oxide synthase and an increase in the
generation of NO under these conditions.
Efaroxan alone did not lead to enhanced NO
production and it failed to attenuate the
response to IL-1beta (Figure 6). As expected,
efaroxan also had no effect on the inducible
nitric oxide synthase (iNOS) independent
production of NO arising from the use of a
chemical NO donor, S-nitrosoglutathione
(GSNO; Figure 6).
Consistent with the increased NO production,
treatment of BRIN-BD11 cells with IL-1beta
was associated with a significant reduction in
cell viability, as judged by MTS reduction
(Figure 2). Efaroxan failed to antagonise the
loss of viability mediated by IL-1beta. These
results were confirmed by FACS analysis of
cellular DNA (Figure 4) which revealed that,
whereas efaroxan failed to alter the integrity
of cellular DNA, a large increase in DNA
fragmentation resulted from exposure of the
cells to IL-1beta (Figure 4). The simultaneous
presence of efaroxan did not attenuate this
response but, surprisingly, caused a further
increase in the extent of DNA fragmentation.
Thus, these studies confirm that efaroxan does
not directly promote BRIN-BD11 cell death
and reveal that this compound fails to protect
these cells from the cytotoxic effects of IL-
1beta. Indeed, efaroxan appears to enhance
the sensitivity of the cells to this cytokine
such that the extent of DNA damage induced
by interleukin-1beta was increased in cells
exposed to both efaroxan and interleukin-
Compounds with an imidazoline structure are
showing increasing promise for use in the
management of type 2 diabetes since certain
members of this class (e.g. efaroxan) have the
capacity to stimulate insulin secretion in a
strictly glucose-dependent manner [1, 2, 3, 4,
5]. However, recent data have indicated that
efaroxan may also have a second important
functional property since, at least in ob/ob
mouse islets, it has been reported to
antagonise the induction of apoptosis
mediated by IL-1beta [19]. A second
imidazoline, RX871024, also exerted similar
effects in isolated islets [19, 21]. As a
consequence, it has been suggested that
imidazoline compounds may have the
potential to slow the progress of beta-cell loss
in type 1 diabetes, since IL-1beta-mediated
toxicity is thought to play an important role in
this disease.
The antagonism of IL-1beta responses by
RX871024 in mouse and rat islets was
reported to be due to blockade of the
induction of iNOS and was associated with a
significant reduction in medium nitrite
accumulation when islet cells were exposed to
both IL-1beta and the drug [19, 21]. It was
assumed that this response occurred primarily
in beta-cells but the possible involvement of
other cell types was not formally excluded. In
the present work, we have employed the
pancreatic beta-cell line BRIN-BD11 to study
these responses further since these cells
respond to both IL-1beta and imidazolines
and they represent a homogeneous population
of clonal beta-cells.
Figure 6. Effects of efaroxan on nitric oxide formation
in cultured BRIN-BD11 cells. Cultured BRIN-BD11
cells were incubated in the absence (control) or
presence (grey) of 100 µM efaroxan plus either
interleukin-1beta (2 ng/mL) or GSNO (250 µM) as
shown. Following incubation for 24 h the medium was
sampled and assayed for nitrite as an index of NO
production. Each point represents the mean
value±SEM (n=8).
P values vs. no addition

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Unlike the situation in ob/ob mouse islets
[19], efaroxan failed to attenuate either the
increase in medium nitrite induced by IL-
1beta or the induction of DNA damage in
BRIN-BD11 cells. Indeed, in these cells,
efaroxan was found to 'enhance' the extent of
DNA damage in IL-1beta-treated cells. Thus,
it appears that the protective effects of
efaroxan on IL-1beta-induced iNOS induction
and NO production seen in ob/ob mouse islets
are not reproduced in BRIN-BD11 beta-cells
(nor in RAW macrophages, HIT-T15 or
RINm5F cells; Gao H, Chan SLF, Morgan
NG; unpublished observations). Since these
various beta-cell types do respond to efaroxan
with changes in insulin secretion [17, 23, 25,
26], it follows that the mechanisms by which
efaroxan controls secretion are likely to be
regulated independently of any actions on cell
viability. Indeed, the possibility remains that
the protective effects of imidazolines reported
in islets, could be mediated by an indirect
mechanism that does not derive from an
interaction of the drugs with the beta-cells
themselves. If so, this would not deny the
potential importance of the response but it
would confirm that the primary target is
separate from the imidazoline binding site
involved in control of insulin secretion.
Of particular significance is the present
finding that, in BRIN-BD11 cells, efaroxan
enhanced the DNA-damaging effects of
interleukin-1beta. This was not due to any
increase in NO production when the two
agents were combined (Figure 6) but provides
evidence that, although efaroxan was not
directly cytotoxic, it sensitised the cells to the
damaging effects of interleukin-1beta. It is
unclear why this effect of imidazolines has
not been seen in normal islet cells [19, 21] but
it is unlikely to be due to any difference in the
imidazoline binding site regulating insulin
secretion since efaroxan increases secretion in
both normal islets and in BRIN-BD11 cells
[17, 23]. The mechanisms involved in this
enhancing effect now warrant further study in
order to investigate whether pancreatic beta-
cells can be sensitised to cytotoxic insults by
exposure to imidazolines under other
circumstances, including during in vivo
administration. If so, then their utility as
potential anti-diabetic agents will require
additional scrutiny.
It is important to emphasise that, despite its
enhancing effect on the response to
interleukin-1beta, efaroxan did not exert any
acute or chronic beta-cell toxicity per se [18,
19]. This result accords with the finding that
the number of apoptotic cells is not increased
in ob/ob mouse islets during culture with
efaroxan in vitro [19]. However, these results
contrast markedly with our observation that a
close structural analogue of efaroxan,
idazoxan, causes a dramatic loss of viability
in BRIN-BD11 cells. The extent of this
response was similar to that seen previously
in RINm5F and HIT-T15 cells [18] and the
results suggest that pancreatic beta-cells
exhibit a differential sensitivity to idazoxan
and efaroxan.
Examination of the profile of cellular DNA in
idazoxan-treated beta-cells by FACS
analysis, revealed that the drug induced a
large increase in DNA fragmentation within
only a few hours of exposure. This would be
consistent with the possibility that idazoxan
caused the early entry of the cells into
apoptosis. In support of this, an increased
number of BRIN-BD11 cells showed positive
surface staining with annexin-V (a marker for
the plasma membrane phosphatidylserine
translocation occurring as an early step in the
apoptotic pathway) after exposure to idazoxan
for 12 h (Gao H, Morgan NG; unpublished
The molecular mechanism by which idazoxan
induces beta-cell death has not been disclosed
but we observed that there was no increase in
medium nitrite accumulation after exposure of
BRIN-BD11 cells to the drug. This implies
that, unlike the situation with IL-1beta [22],
idazoxan-toxicity does not involve an increase
in iNOS activity or NO generation. In
addition, we have also observed that the
effects of idazoxan were not prevented by co-
incubation with either alpha2-adrenoceptor
ligands (including efaroxan [18]) or with a
specific antagonist of 5-HT1A receptors. Thus,
neither the potent alpha2-antagonist activity
nor the 5-HT1A agonist properties of the agent

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[24] can account for its cytotoxicity. In
addition, on the basis of earlier observations
[18] the ability of idazoxan to bind to I1- and
I2-imidazoline sites can also be excluded.
Thus, idazoxan appears to initiate the rapid,
apoptotic, demise of pancreatic beta-cells by a
mechanism that does not involve binding to
any of the receptor sites currently defined as
targets for this ligand. This response is not
restricted to clonal beta-cell lines since
morphological evidence indicates that the
cells of normal islets are also subject to
damage when exposed to idazoxan in culture
Taken together, the present results confirm
that structurally related imidazoline drugs can
exert markedly different effects on the
viability of pancreatic beta-cells. They do not
provide support for the view that efaroxan can
attenuate the cytotoxic actions of IL-1beta in
a pure population of clonal beta-cells but
reveal that efaroxan enhances the DNA-
damaging effects of the cytokine in BRIN-
BD11 cells. Since BRIN-BD11 cells express
an efaroxan-sensitive protein involved in
control of insulin secretion [17, 23] these
results imply that binding of ligands to this
protein does not lead to attenuation of IL-1-
induced NO formation.
In view of the fact that some imidazoline
reagents are being actively studied as
potential anti-diabetic drugs [1, 2, 3, 4, 5] and
that others are already in clinical use as
centrally-acting anti-hypertensive agents [2,
15] and as alpha2-adrenoceptor antagonists [4,
15], it is important that their effects on beta-
cell viability are defined in greater detail.
Such studies may also lead to an improved
understanding of the pathways involved in
regulating the entry of pancreatic beta-cells
into apoptosis.
Received February 6th, 2003 - Accepted
March 18th, 2003
Keywords Apoptosis; Biological Phenomena,
Cell Phenomena, and Immunity; Cell Death;
Cell Physiology; Chemicals and Drugs
Category; Dioxanes; Dioxins; Growth
Substances; Heterocyclic Compounds;
Heterocyclic Compounds, 1-Ring; Idazoxan;
Imidazoles; Interleukin-1
Abbreviations 5-HT: (5-hydroxytryptamine,
serotonin; FACS: fluorescence activated cell
sorting; IL: interleukin; GSNO: S-
nitrosoglutathione; iNOS: inducible nitric
oxide synthase; MTS: 3-(4,5-dimethylthiazol-
sulphophenyl)-2H-tetrazolium; NAN-190: 1-
phosphate buffered saline; PMS: phenozine
methosulphate; RPMI: Roswell Park
Memorial Institute
Acknowledgements Thanks are due to
BBSRC, Diabetes UK, GlaxoSmithKline and
the Royal Society (who provided a KC Wong
Fellowship to H Gao) for financial support of
this work.
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