Glucocorticoid
treatment is associated with decreased expression of processed
AVP but not of proAVP, neurophysin or oxytocin in the human
hypothalamus:
Are PC1 and PC2 involved?
Z.A. Erkut, 1
B.A.Th.F. GabreÎls, 2
J. Eikelenboom, 1
F.
W. van Leeuwen1 & D.F. Swaab1
1. Netherlands Institute for Brain Research, Amsterdam, The
Netherlands.
2.
Academical Medical Center, University of Amsterdam, Department
of Internal Medicine, The Netherlands.
Submitted:
December 20, 2001
Accepted: January 7, 2002
Key
words:
vasopressin, glucocorticoids, prohormone convertases (PC1,
PC2), human hypothalamus, enzymatic processing, posttranslational,
suppression
Abstract
OBJECTIVES: We reported earlier that vasopressin (AVP) peptide
expression is significantly decreased in the postmortem hypothalamus
of glucocorticoid (GC) treated patients, while such a decrease
was not observed in AVP prohormone (proAVP) expression. This
indicated a GC-induced suppression of AVP synthesis at the
posttranslational level. Here, we investigated in detail whether
this decreased levels of AVP expression in GC treated patients
might be due to the down regulation of the prohormone convertases
PC-1 and PC-2, and the molecular chaperone 7B2, as was reported
previously in some AVP-related disorders.
MATERIALS
& METHODS: An immunocytochemical study was performed on
post-mortem hypothalami of GC exposed patients and controls,
in which quantification of proAVP, AVP, neurophysin (NP) and
oxytocin (OXT) expression were done along with the quantification
of PC1, PC2 and 7B2 expression in the paraventricular nucleus,
by using a computerized image analysis system. RESULTS: Expression
of processed AVP in GC exposed patients was significantly
decreased (p=0.021), while the amount of proAVP expression
was unchanged. Despite the strong correlation between AVP
and NP (the other cleavage product of proAVP) expression in
the GC group (r=0.917, p=0.004), the mean NP immunoreactivity
did not show a significant decrease in this group. Also the
OXT expression was similar in both groups. Although in most
of the GC treated patients, the expression intensities of
PC1 and PC2 were decreased parallel to the decrease in AVP,
the mean expression levels of neither of PC1 and PC2, nor
of 7B2 were statistically different between the groups (p=0.20-0.80).
CONCLUSION:
We conclude that the suppression of AVP expression by GCs
is not mediated solely by the down regulation of PC1, PC2
or 7B2. Other mechanisms, which may contribute to the GC-induced
posttranslational suppression of AVP, are discussed.
Introduction
The
classical mechanism of steroid action takes place at the genomic
level, regulating gene transcription [1, 2]. However, some
recent studies show that glucocorticoids (GCs) may also have
non-genomic effects, regulating posttranscriptional and posttranslational
processes [3-7].
The posttranscriptional steps of hormone synthesis are regulated
by processing enzymes and numerous regulatory factors. In
the last ten years the prohormone convertase (PC) family of
processing enzymes has been discovered and studied extensively
in mammals. Today, this family consists of furin (SPC1, PACE),
PACE4, PC1 (PC3, SPC3), PC2 (SPC2), PC4 (SPC4), PC5 (PC6,
SPC6) and PC7 (SPC7, LPC, PC8) [8-11]. In neuroendocrine cells
of the hypothalamus, the convertases PC1 and PC2 colocalize
with and cleave the precursors of corticotrophin releasing
hormone (CRH), vasopressin (AVP), neurophysin (NP), oxytocin
(OXT), thyrothropin releasing hormone (TRH), neuropeptide
Y (NPY) and neurotensin [12-17]. In addition, it has been
shown that the neuroendocrine polypeptide 7B2 can act as a
molecular chaperone for PC2, preventing premature activation
of the proenzyme in the secretory pathway and assisting it
during the processing of the prohormones [18-21].
Our group has previously shown that PC1 and PC2 are predominantly
expressed in the AVP cells of the paraventricular (PVN) and
supraoptic nucleus (SON) of the human hypothalamus. We also
reported a PC2- and 7B2-associated processing defect of AVP
in the hypothalamus of Wolfram syndrome and some Prader-Willi
patients [15, 22, 23]. These studies showed that, along with
the significant decrease in processed AVP, both PC2 and 7B2
were also diminished in the hypothalamus of these patients,
whereas the amount of AVP precursor (proAVP) was not different
from control levels. These results indicated that reduced
expression of PC2 and 7B2 could be responsible for reduced
processing of proAVP in Wolfram and a group of Prader-Willi
Syndrome patients.
In a separate study, we have shown that GC treatment was associated
with a significant decrease in the expression levels of processed
AVP in the human hypothalamus, which could be at the posttranscriptional
level [24]. In the light of previous studies indicating an
association of a defective PC1 and PC2 production with AVP
processing disturbance [15, 16, 23], and the recent evidence
showing that production of prohormone convertases can be differentially
regulated by GCs [13], we reasoned that the suppression of
AVP synthesis by GCs can be mediated through the suppression
of PC1, PC2 or 7B2 production. In the present study, therefore,
we have investigated the expression levels of PC1, PC2 and
the molecular chaperone 7B2, in relation to proAVP, the processed
AVP, NP and OXT in hypothalamic PVN neurons of GC-treated
patients and controls.
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