Pflugers Arch - Eur J Physiol (2002) 445:60–66
DOI 10.1007/s00424-002-0873-2
ORIGINAL ARTICLE
N. Gamper · S. Fillon · Y. Feng · B. Friedrich ·
P. A. Lang · G. Henke · S. M. Huber · T. Kobayashi ·
P. Cohen · F. Lang
K
+
channel activation by all three isoforms
of serum- and glucocorticoid-dependent protein kinase SGK
Received: 6 December 2001 / Revised: 16 April 2002 / Accepted: 15 May 2002 / Published online: 28 August 2002
Springer-Verlag 2002
Abstract The serum- and glucocorticoid-dependent ki-
nase SGK1 was originally identified as a glucocorticoid-
sensitive gene. Subsequently, the two homologous kinases
SGK2 and SGK3 have been cloned, being products of
distinct genes, which are differentially expressed and
share 80% identity in amino acid sequence in their
catalytic domains. While SGK1 has been shown to
activate ion channels, including K
+
channels, the func-
tions of SGK2 and SGK3 have not been examined. The
present study was therefore performed to elucidate the
effect of SGK1, SGK2, and SGK3 on electrical properties
of renal epithelial cells. To this end human embryonic
kidney (HEK293) cells were transfected with the kinases
and ion-channel activity determined using the patch-
clamp technique. In non-transfected cells and in cells
transfected with the empty GFP construct a voltage-gated
K
+
current was observed amounting to 303€19 pA (n=13)
and 299€29 pA (n=23), respectively. Transfection with
SGK1, SGK2 or SGK3 increased the voltage-gated K
+
current to 1056€152 pA (n=17), 555€47 pA (n=17), and
775€98 pA (n=16), respectively. The K
+
current was fully
blocked by 3 mM tetraethylammonium chloride and
inhibited 45% by the Kv1 channel blocker margatoxin
(10 nM). In dual electrode voltage-clamp experiments
SGK isoforms up-regulated Kv1 voltage-gated K
+
chan-
nels expressed in Xenopus laevis oocytes. The present
observations thus reveal a powerful stimulating effect of
all three isoforms of SGK on K
+
channels. Those effects
may participate in regulation of epithelial transport, cell
proliferation, and neuromuscular excitability.
Keywords HEK293 cells · Kv1 · Margatoxin ·
Patch-clamp · Potassium channels · SGK · TEA · Voltage
clamp
Introduction
The serum- and glucocorticoid-dependent protein kinase
(SGK1) was originally identified as a glucocorticoid-
inducible gene [32]. The human isoform has subsequently
been cloned as a cell-volume-regulated gene [29]. Similar
to glucocorticoids, aldosterone up-regulates SGK1 tran-
scription, indicating a role for this kinase in the regulation
of epithelial transport [4, 21]. Recently, two new
isoforms, SGK2 and SGK3, have been identified [13].
SGK2 and SGK3 are the products of distinct genes and
share some 80% identity in amino-acid sequence within
the catalytic domain [13]. All three kinases, SGK1, SGK2
and SGK3, are activated by insulin-like growth factor 1
(IGF-1) and by oxidative stress [13]. Unlike SGK1,
expression of SGK2 and SGK3 is not increased by
glucocorticoids or serum in the cells that have so far been
tested. While the mRNA encoding SGK1 and SGK3 are
both expressed in all tissues tested, SGK2 mRNA is only
expressed in the liver, kidney and pancreas and, at lower
levels, in the brain [13].
Several observations point to a physiological role of
SGK1 in the regulation of ion channels. Voltage-clamp
N. Gamper and S. Fillon contributed equally to this study and thus
share first authorship
N. Gamper · S. Fillon · Y. Feng · B. Friedrich · P.A. Lang ·
G. Henke · S.M. Huber · F. Lang (
)
)
Department of Physiology, University of Tuebingen,
Gmelinstrasse 5, 72076 Tuebingen, Germany
e-mail: florian.lang@uni-tuebingen.de
Tel.: +49-707-12972194
Fax: +49-707-1295618
T. Kobayashi · P. Cohen
MRC Protein Phosphorylation Unit, School of Life Sciences,
University of Dundee, Dundee, UK
Present address:
N. Gamper, Sechenov Institute of Evolutionary Physiology and
Biochemistry, Thorez pr. 44, 194223 St. Petersburg, Russia
Present address:
B. Friedrich, Department of Internal Medicine,
University of Tuebingen, Otfried-Mueller-Strasse 20,
72076 Tuebingen, Germany
Present address:
T. Kobayashi, Department of Biochemistry,
Institute of Development, Aging and Cancer, Tohoku University,
Sendai 980-8575, Japan