mGCR have been detected recently in human monocytes and B cells.10
We detected mGCR by high‐sensitivity immunofluorescent staining. The validity of this technique was shown by (a) specificity controls obtained in each experiment, (b) the fact that our liposomes do not pass into the cytoplasm20,22
and (c) the confirmation by fluorescence microscopy.10
The use of this technique was driven by the idea that mGCR—like other important molecules20
—are expressed only in small numbers and therefore are not detectable by conventional methods.
mGCR are actively up regulated and transported through the cell after immunostimulation and in patients with rheumatoid arthritis.10
We examined mGCR expression in patients with SLE. We expected to find similar results, but possibly with B cells associated, as they seem to have an important role in SLE.23
Moreover, high doses of glucocorticoids are often and successfully used in the treatment of SLE. In this regard, non‐specific non‐genomic actions are assumed to have therapeutic implications, whereas the clinical relevance of mGCR‐mediated specific non‐genomic glucocorticoid actions is yet undefined.1,4
Also, patients with SLE showed a higher frequency of mGCR+ monocytes (accompanied by higher mean fluorescence intensity) than healthy controls. We found even higher maximum levels in patients with SLE (up to 35% positive cells) than in patients with active rheumatoid arthritis in comparable experiments (up to 20% positive cells).10
But as in rheumatoid arthritis, the frequency of mGCR+ B cells was neither markedly different from that in healthy controls nor correlated with SLE disease activity (fig 1). However, in contrast with rheumatoid arthritis, there was no correlation between the frequency of mGCR+ monocytes and disease activity (fig 2). One explanation for these results may be that in rheumatoid arthritis, the activation of blood monocytes has an important role.24
In SLE, however, the production of macrophage‐activating Th1‐type cytokines, such as tumour necrosis factor α and interferon γ, is decreased25
and cytokine patterns in patients with SLE are known to be generally heterogeneous according to the clinical manifestations in the patients.26
Monocytes from patients with SLE are considered to have several defective functions, such as an impaired clearance of apoptotic cells or a decreased release of arachidonic acid.27,28
A correlation of these observations with the expression of mGCR is speculative. Another interesting hypothesis is that low cortisol levels or glucocorticoid resistance may have an influence on or may result from an altered expression of mGCR. The corresponding data regarding cGCR are ambiguous. Gladman et al29
investigated cGCR in PBMC from patients with SLE who were not treated with glucocorticoid. Similar to our study, the number of cGCR was considerably higher in patients with SLE than in controls, and there was also no correlation with disease activity. Tanaka et al30
did not find any difference in cGCR levels between PBMC from patients with SLE with nephrotic syndrome and from controls, with a wide variability. Other reports on cGCR levels in patients with chronic diseases show either an increase or a decrease, even in rheumatoid arthritis.31,32
The second key result of our study is that mean frequencies of mGCR+ monocytes are lower in patients with SLE treated with medium and high doses of glucocorticoids than in those treated with
7.5 mg prednisolone‐equivalent/day (fig 3A). This observation corresponds well with data from our in vitro experiments using LPS‐stimulated monocytes. Here, we also found a trend for a decrease in the frequency of mGCR+ monocytes relative to the control at higher dexamethasone levels (fig 3C). Initial experiments with actinomycin D (inhibits RNA/DNA synthesis) and cycloheximide (inhibits protein synthesis) showed that both drugs decrease the percentage of mGCR+ monocytes compared with LPS alone. This suggests the regulation of mGCR expression to be dependent on transcription and translation, but these data must be interpreted with caution, as the viability of the cells considerably decreased in the presence of these drugs.
Taken together, we consider the most likely explanation to be that glucocorticoids down regulate the expression of mGCR. This effect is speculated to be of therapeutic relevance—especially in patients treated with higher dosages. We still do not have data supporting this assumption, but down regulation of receptors by homologous hormones in the sense of a negative feedback regulation or autoregulation is known for many hormones.33,34
In addition, a glucocorticoid‐induced down regulation is well known for cGCR.35,36,37
With regard to the underlying mechanisms, glucocorticoids are known to cause a decrease in GCR mRNA (either by cGCR promoter repression or by promotor‐independent cGCR gene repression) and to decrease the stability of cGCR mRNA and cGCR protein.38
We suggest similar mechanisms to account for the observed mGCR down regulation, but other mechanisms, such as mGCR‐mediated apoptosis, also need to be considered.
We draw attention to another detail. We found 10–11 mol/l dexamethasone to increase the frequency of mGCR+ monocytes relative to the control, but only when the control value was
50% (fig 3D). This leads us to the hypothesis that dexamethasone at very low concentrations is capable of further enhancing mGCR expression, if the LPS stimulation is not maximal. Glucocorticoids at very low (physiological) concentrations sometimes seem to have (rather small) effects opposite to those observed at high concentrations, which is occasionally called a paradox.39,40,41
Follicle‐stimulating hormone is another hormone that is capable of positively or negatively regulating its own receptor, depending on the concentration and ambient situation.42
A positive autoregulation of GCR has been shown in the human leukaemic T cell line CEM‐C7.43,44
Gametchu et al8,9
reported a positive correlation between mGCR levels and glucocorticoid‐induced apoptosis in another T cell line. Taken together, glucocorticoids at low levels may facilitate mGCR expression to sensitise the cells for mGCR‐mediated apoptosis, which prevents the immune system from over‐reaction. However, this is speculative and further experimental work is needed.
The current focus is on how membrane‐bound steroid receptors find their way to the cell surface. As a transmembrane domain is not known,10
there have to be other post‐translational modifications or transport mechanisms that facilitate membrane insertion of the mGCR protein. For the membrane oestrogen receptor, an association with the membrane protein caveolin‐1 has been found.14
Therefore, we hypothesised caveolin‐1 to be associated with the transport of mGCR to the cell surface. Our data, however, clearly indicate that caveolin‐1 is not the limiting factor of mGCR expression and that mGCR and caveolin‐1 are not colocalised in caveolae. Recently, a palmitoylation‐dependent plasma membrane oestrogen receptor recruitment has been reported.45
This mechanism needs to be investigated in further experiments regarding its relevance for mGCR transport.
- Frequencies of mGCR+ monocytes (CD14+) are considerably higher in patients with SLE than in healthy controls.
- mGCR are up regulated by inflammatory stimuli and down regulated by glucocorticoids, suggesting a negative feedback loop to control glucocorticoid action.
- mGCR are not associated with caveolin‐1 in plasma membrane caveolae.
The first two conclusions give rise to the assumption that drugs binding selectively to the mGCR may prove to be of therapeutic value in the future.