Steroid hormones affect binding of the sigma ligand 11C-SA4503 in tumour cells and tumour-bearing rats
Anna A. Rybczynska
0
Philip H. Elsinga
0
Jurgen W. Sijbesma
0
Kiichi Ishiwata
0
Johan R. de Jong
0
Erik F. de Vries
0
Rudi A. Dierckx
0
Aren van Waarde
0
0
K. Ishiwata Positron Medical Center, Tokyo Metropolitan Institute of Gerontology
,
Tokyo, Japan
1
) Nuclear Medicine and Molecular Imaging, University of Groningen Medical Center, University of Groningen
, Hanzeplein 1, 9713GZ Groningen,
The Netherlands
Purpose Sigma receptors are implicated in memory and cognitive functions, drug addiction, depression and schizophrenia. In addition, sigma receptors are strongly overexpressed in many tumours. Although the natural ligands are still unknown, steroid hormones are potential candidates. Here, we examined changes in binding of the sigma-1 agonist 11C-SA4503 in C6 glioma cells and in living rats after modification of endogenous steroid levels. Methods 11C-SA4503 binding was assessed in C6 monolayers by gamma counting and in anaesthetized rats by microPET scanning. C6 cells were either repeatedly washed and incubated in steroid-free medium or exposed to five kinds of exogenous steroids (1 h or 5 min before tracer addition, respectively). Tumour-bearing male rats were repeatedly treated with pentobarbital (a condition known to result in reduction of endogenous steroid levels) or injected with progesterone. Results Binding of 11C-SA4503 to C6 cells was increased (~50%) upon removal and decreased (~60%) upon addition of steroid hormones (rank order of potency: progesterone > allopregnanolone = testosterone = androstanolone > dehydroepiandrosterone-3-sulphate, IC50 progesterone 33 nM).
-
The use of positron emission tomography (PET) for
imaging of cancer has a successful history spanning more
than three decades. Suitable radiopharmaceuticals have
been designed to track molecular events in the body, to
monitor the time course of disease and to assess treatment
outcome. Although 18F-FDG and other metabolic PET
tracers (18F-FLT, 11C-methionine, 11C-choline, etc.) are
widely available for the diagnosis of cancer and monitoring
of treatment response, these tracers are not tumour specific
and/or have only moderate cellular uptake [1, 2]. For this
reason, there is much interest in the development and
validation of novel radiopharmaceuticals with greater
tumour selectivity. Attractive candidates are sigma ligands
since sigma receptors are strongly overexpressed in rapidly
proliferating cells [36].
Sigma receptors are proteins with a highly conserved
sequence (8792% identity and 9093% homology for
sigma-1 receptor) [7]. They are found in kidney, liver,
immune, endocrine and reproductive organs [8].
Furthermore, sigma receptors are widely distributed in the brain
and are implied in memory function, cognition, drug
addiction, depression and schizophrenia [9]. Importantly,
sigma receptors were demonstrated to play a role in tumour
cell proliferation and cancer cell death. These binding sites
are therefore important targets in the development of novel
anti-cancer drugs [10]. Although the endogenous ligands
for sigma receptors have not yet been identified, steroid
hormones (in particular progesterone) are potential
candidates [11, 12].
Recent work has demonstrated regulation of sigma-1
receptor availability by endogenous steroids. Such steroids
modulate the efficacy of a sigma-1 receptor agonist in stress
and depression [12]. Binding of neurosteroids to sigma
receptors affects the release of substance P from nociceptor
endings in mice [13] and the release of glutamate and
norepinephrine in rat prelimbic cortex and hippocampus
[14, 15]. Furthermore, such binding reduces subjective
craving in cocaine addiction [16]. Based on their effects
on substance P and norepinephrine release, progesterone
was proposed to be a sigma antagonist, whereas
dehydroepiandrosterone-3-sulphate (DHEA-S) and
pregnenolone sulphate were considered as sigma agonists [13, 15].
Multiple sequence analyses revealed the presence of two
steroid-like binding domains (SBDLI and SBDLII) in the
guinea pig sigma-1 receptor (amino acids 91109 and 176
194, respectively) [17, 18]. Recently, it was shown that
administration of steroids reduces in vivo binding of the
sigma-1 receptor ligand 18F-FPS in rodent brain [19].
Although an interaction between cerebral sigma
receptors and steroids has been established, our knowledge about
competition between sigma ligands and endogenous
steroids in cancer cells is still rudimentary. Moreover, it
remains necessary to examine how steroid hormones affect
the binding of radiopharmaceuticals other than 18F-FPS,
since sigma ligands may bind to different receptor subtypes
[20] and to different binding sites within the sigma-1
receptor molecule (e.g. the agonist binding site and the
phenytoin-binding site [21]).
Here, we examine the impact of steroid competition on
binding of the sigma-1 agonist 11C-SA4503. 11C-SA4503
has been used extensively for PET studies of animal
tumours [2, 2224] and the human brain [2528]. For in
vitro tes (...truncated)