EDITORIAL FOCUS
HSP27 and signaling to the actin
cytoskeleton Focus on "HSP27 expression
regulates CCK-induced changes of the actin cytoskeleton in
CHO-CCK-A cells"
Curtis T.
Okamoto
Department of Pharmaceutical Sciences, School of Pharmacy,
University of Southern California, Los Angeles, California 90089-1921
 |
ARTICLE |
DIVERSE CELLULAR PROCESSES such as cell migration,
cell-cell and cell-substrate adhesion, phagocytosis, cytokinesis,
secretion, endocytosis, and bacterial invasion depend on actin-based
motility. For most of these processes, an extracellular signal must be
transduced either by or to the actin cytoskeleton. Some of these
transducers are relatives of well-characterized signaling modules. For
example, recent attention has been focused on the small ras-like
GTPases, Rho, Rac, and Cdc42, as regulators of actin-based motility
(24). Another member of the family of small GTPases, ADP-ribosylation factor 6, and its exchange factor may regulate the actin cytoskeleton at the plasma membrane (5, 6). In addition, some signals to the
cytoskeleton are transduced by the products of phosphatidylinositol metabolism (17). With respect to the effectors, the apparent targets of
the signal transduction cascade would be proteins that regulate the
polymerization, depolymerization, anchoring, or bundling of actin microfilaments.
In the current article in focus, Schäfer et al. (Ref. 22, see
page C1032 in this issue) present data that characterize an alternate signal transduction cascade involving the gastrointestinal hormone cholecystokinin (CCK), its cell surface receptor, and the
modulation of the actin cytoskeleton. They used a heterologous expression system, a Chinese hamster ovary (CHO) cell line stably transfected with the G protein-coupled CCK-A receptor, to characterize further the effectors downstream of CCK-A receptor stimulation. In
previous work, Groblewski et al. (8) showed that stimulation of
pancreatic acinar cells by CCK results in activation of
mitogen-activated protein kinase (MAPK)-activated protein kinase 2, which in turn phosphorylates the small heat shock protein HSP27, a
chaperone protein. Subsequently, Schäfer et al. were able to show
in acinar cells that CCK stimulates p38 MAPK (which is upstream of
MAPK-activated protein kinase 2), resulting in HSP27 phosphorylation
and changes in the actin cytoskeleton (23). In this new study,
Schäfer et al. were able to recapitulate in the transfected CHO
cells the CCK-stimulated p38 MAPK signaling cascade down to the
phosphorylation of HSP27 and were able to use CCK-induced morphological
changes in the actin cytoskeleton as the "readout." They found
that stimulation of the CCK-A receptor resulted in dose-dependent
changes in the actin cytoskeleton, activation of p38 MAPK, and
phosphorylation of HSP27. Overexpression of either wild-type HSP27 or a
mutated form of HSP27 that apparently mimics the phosphorylated form of HSP27 in CCK-A receptor-expressing CHO cells resulted in modulation of
the response of the actin cytoskeleton to CCK stimulation; cells
overexpressing mutated forms of HSP27 that cannot be phosphorylated did
not acquire this phenotype. Thus these data provided a direct demonstration of the link between phosphorylated HSP27 and the response
of the actin cytoskeleton to the CCK-p38 MAPK signaling pathway. They
were also able to show that the diphosphorylated form of HSP27 is the
form that regulates its effector function relative to the CHO cell
cytoskeleton. Overall, the Schäfer study is another significant
contribution to the burgeoning field of the characterization of the
action of HSP27 on the actin cytoskeleton. To date, several studies in
other systems have shown separately that HSP27 is phosphorylated by the
p38 MAPK pathway (1, 14) or that phosphorylated HSP27 regulates the
dynamics of the actin cytoskeleton (15, 16, 25, 27, 28). However, the
current work by Schäfer et al. and their previous
study (23) are two of the few studies (9, 10, 12) that link the
stimulation of a cell surface receptor to the p38 MAPK pathway, with
the consequent phosphorylation of HSP27 and its modulation of the actin cytoskeleton.
The results of Schäfer et al. are also significant in
understanding the action of CCK on its target cells in vivo and suggest that CCK-induced signaling to the actin cytoskeleton via p38 MAPK and
HSP27 may be a common pathway in cells expressing the CCK receptor. A
major function of CCK in vivo is to stimulate exocytosis from
pancreatic acinar cells by activation of a signaling cascade that is
initiated by the mobilization of intracellular
Ca2+. Perhaps in the alternate,
but simultaneously activated p38 MAPK pathway, activated HSP27 may
modulate the actin cytoskeleton to facilitate some aspect of the
zymogen granule secretory cycle, from the transport and/or fusion of
zymogen granules to the endocytic reuptake of zymogen granule membrane
from the plasma membrane. Several other secretory cells have shown a
dependence on the actin cytoskeleton to facilitate exocytosis (3, 20,
26). In addition, endocytosis has been shown to depend on the actin
cytoskeleton (3, 7, 13, 26). It would be of interest to determine whether, in general, HSP27 plays a role in these plasma
membrane-associated trafficking events via its cytoskeletal effector
function. In addition to stimulation of exocrine pancreatic acinar
cells, CCK stimulates gallbladder contractility and gastrointestinal
smooth muscle motility. With respect to smooth muscle function, a
recent report has shown that the activation of the p38 MAPK pathway in cultured tracheal myocytes by platelet-derived growth factor, interleukin-1
, or transforming growth factor-
results in the phosphorylation of HSP27 and stimulation of cell migration (10).
In identifying several key events in the CCK-stimulated pathway to the
actin cytoskeleton, this study by Schäfer et al. has also opened
the door to several new questions. First, the small GTPases Rac and
Cdc42 have been shown to be upstream of p38 MAPK (2, 18, 27). It will
be of interest to identify the upstream components in the
CCK-stimulated signaling cascade that leads to the activation of p38
MAPK and HSP27. Second, with respect to cytoskeletal remodeling, HSP27
has been shown in vitro to have actin "barbed end" capping
activity (19), suggesting that it may act stoichiometrically to
regulate actin microfilaments. However, it is not yet clear whether
HSP27 in vivo is acting either stoichiometrically or catalytically or
both. As speculation, for a putative catalytic function, HSP27 may
activate other cytoskeletal effector proteins via its chaperone
activity, much in the way that HSP90 is considered to be a chaperone
for proteins in signal transduction pathways (11). Third, there appear
to be differences in the types of cytoskeletal changes induced by the
activation of HSP27; these differences may be cell type specific and
depend on the levels of HSP27 expression. These issues will be
important to resolve in future studies. Fourth, like many G
protein-coupled receptors, ligand binding stimulates the endocytosis of
the CCK-A receptor as a first step in desensitization and
downregulation (21). Recently, endocytosis and postendocytotic
trafficking have been shown to regulate signaling to the ras-dependent
MAPK pathway by the
2-adrenergic receptor (4). It
may be worthwhile to entertain the possibility that the CCK-A receptor
may modulate its own endocytosis and postendocytotic trafficking by
localized regulation of the cortical actin cytoskeleton via p38 MAPK
and HSP27. In summary, the p38 MAPK/HSP27 pathway appears to be an exciting new pathway bridging the gap between cellular stimulation by
an extracellular cue and its dynamic response through actin-based motility.
 |
ACKNOWLEDGEMENTS |
This work was supported by National Institute of Diabetes and
Digestive and Kidney Diseases Grant DK-51588.
 |
FOOTNOTES |
Address for reprint requests and other correspondence: C. T. Okamoto,
Department of Pharmaceutical Sciences, School of Pharmacy, Univ. of
Southern California, Los Angeles, CA 90080-9121 (E-mail:
cokamoto{at}hsc.usc.edu).
 |
REFERENCES |
1.
Ahlers, A.,
C. Belka,
M. Gaestel,
N. Lamping,
C. Sott,
F. Herrmann,
and
M. A. Brach.
Interleukin-1-induced intracellular signaling pathways converge in the activation of mitogen-activated protein kinase and mitogen-activated protein kinase-activated protein kinase 2 and the subsequent phosphorylation of the 27-kilodalton heat shock protein in monocytic cells.
Mol. Pharmacol.
46:
1077-1083,
1994[Abstract].
2.
Bagrodia, S.,
B. Derijard,
R. J. Davis,
and
R. A. Cerione.
Cdc42 and PAK-mediated signaling leads to Jun kinase and p38 mitogen-activated protein kinase activation.
J. Biol. Chem.
270:
27995-27998,
1995[Abstract/Free Full Text].
3.
Black, J. A.,
T. M. Forte,
and
J. G. Forte.
The effects of microfilament disrupting agents on HCl secretion and ultrastructure of piglet gastric oxyntic cells.
Gastroenterology
83:
595-604,
1982[Medline].
4.
Daaka, Y.,
L. M. Luttrell,
S. Ahn,
G. J. Della Rocca,
S. S. G. Ferguson,
M. G. Caron,
and
R. J. Lefkowitz.
Essential role for G protein-coupled receptor endocytosis in the activation of mitogen-activated protein kinase.
J. Biol. Chem.
273:
685-688,
1998[Abstract/Free Full Text].
5.
Franco, M.,
P. J. Peters,
J. Boretto,
E. van Donselaar,
A. Neri,
C. D'Souza-Schorey,
and
P. Chavrier.
EFA6, a sec7 domain-containing exchange factor for ARF6, coordinates membrane recycling and actin cytoskeleton organization.
EMBO J.
18:
1480-1491,
1999[Abstract/Free Full Text].
6.
Frank, S. R.,
J. C. Hatfield,
and
J. E. Casanova.
Remodeling of the actin cytoskeleton is coordinately regulated by protein kinase C and the ADP-ribosylation factor nucleotide exchange factor ARNO.
Mol. Biol. Cell
9:
3133-3146,
1998[Abstract/Free Full Text].
7.
Gottlieb, T. A.,
I. E. Ivanov,
M. Adnesnik,
and
D. D. Sabatini.
Actin microfilaments play a critical role in endocytosis at the apical but not the basolateral surface of polarized epithelial cells.
J. Cell Biol.
120:
695-710,
1993[Abstract].
8.
Groblewski, G. E.,
T. Grady,
N. Mehta,
H. Lambert,
C. D. Logsdon,
J. Landry,
and
J. A. Williams.
Cholecystokinin stimulates heat shock protein 27 phosphorylation in rat pancreas both in vivo and in vitro.
Gastroenterology
112:
1354-1361,
1997[Medline].
9.
Guay, J.,
H. Lambert,
G. Gingras-Breton,
J. N. Lavoie,
J. Huot,
and
J. Landry.
Regulation of actin filament dynamics by p38 map kinase-mediated phosphorylation of heat shock protein 27.
J. Cell Sci.
110:
357-368,
1997[Abstract/Free Full Text].
10.
Hedges, J. C.,
M. A. Dechert,
I. A. Yamboliev,
J. L. Martin,
E. Hickey,
L. A. Weber,
and
W. T. Gerthoffer.
A role for p38MAPK/HSP27 pathway in smooth muscle cell migration.
J. Biol. Chem.
274:
24211-24219,
1999[Abstract/Free Full Text].
11.
Hunter, T.,
and
R. Y. C. Poon.
Cdc37: a protein kinase chaperone?
Trends Cell Biol.
7:
157-161,
1997.
12.
Huot, J.,
F. Houle,
S. Rousseau,
R. G. Deschesnes,
G. M. Shah,
and
J. Landry.
SAPK2/p38-dependent F-actin reorganization regulates early membrane blebbing during stress-induced apoptosis.
J. Cell Biol.
143:
1361-1373,
1998[Abstract/Free Full Text].
13.
Lamaze, C.,
L. M. Fujimoto,
H. L. Yin,
and
S. L. Schmid.
The actin cytoskeleton is required for receptor-mediated endocytosis in mammalian cells.
J. Biol. Chem.
272:
20332-20335,
1997[Abstract/Free Full Text].
14.
Larsen, J. K.,
I. A. Yamboliev,
L. Weber,
and
W. T. Gerthoffer.
Phosphorylation of the 27-kDa heat shock protein via p38 MAP kinase and MAPKAP kinase in smooth muscle.
Am. J. Physiol.
273 (Lung Cell. Mol. Physiol. 17):
L930-L940,
1997[Abstract/Free Full Text].
15.
Lavoie, J. N.,
G. Gingras-Breton,
R. M. Tanguay,
and
J. Landry.
Induction of Chinese hamster hsp27 gene expression in mouse cells confers resistance to heat shock. Hsp27 stabilization of the microfilament organization.
J. Biol. Chem.
268:
3420-3429,
1993[Abstract/Free Full Text].
16.
Lavoie, J. N.,
E. Hickey,
L. A. Weber,
and
J. Landry.
Modulation of actin microfilament dynamics and fluid phase pinocytosis by phosphorylation of heat shock protein 27.
J. Biol. Chem.
268:
24210-24214,
1993[Abstract/Free Full Text].
17.
Martin, T. F. J.
Phosphoinositide lipids as signaling molecules: common themes for signal transduction, cytoskeletal regulation, and membrane trafficking.
Ann. Rev. Cell Dev. Biol.
14:
231-264,
1998[Medline].
18.
Minden, A.,
A. Lin,
F. X. Claret,
A. Abo,
and
M. Karin.
Selective activation of the JNK signaling cascade and c-Jun transcriptional activity by the small GTPases Rac and Cdc42Hs.
Cell
81:
1147-1157,
1995[Medline].
19.
Miron, T.,
K. Vancompernolle,
J. Vandekerckhove,
M. Wilchek,
and
B. Geiger.
A 25-kD inhibitor of actin polymerization is a low molecular mass heat shock protein.
J. Cell Biol.
114:
255-261,
1991[Abstract].
20.
Muallem, S.,
K. Kwiatkowska,
X. Xu,
and
H. L. Yin.
Actin filament disassembly is a sufficient final trigger for exocytosis in non-excitable cells.
J. Cell Biol.
128:
589-598,
1995[Abstract].
21.
Roettger, B. F.,
D. I. Pinon,
T. P. Burghardt,
and
L. J. Miller.
Regulation of lateral mobility and cellular trafficking of the CCK receptor by a partial agonist.
Am. J. Physiol.
276 (Cell Physiol. 45):
C539-C547,
1999[Abstract/Free Full Text].
22.
Schäfer, C.,
P. Clapp,
M. J. Welsh,
R. Benndorf,
and
J. A. Williams.
HSP27 expression regulates CCK-induced changes of the actin cytoskeleton in CHO-CCK-A cells.
Am. J. Physiol.
277 (Cell Physiol. 46):
C1032-C1043,
1999[Abstract/Free Full Text].
23.
Schäfer, C.,
S. E. Ross,
M. J. Bragado,
G. E. Groblewski,
S. A. Ernst,
and
J. A. Williams.
A role for the p38 mitogen-activated protein kinase/hsp27 pathway in cholecystokinin-induced changes in the actin cytoskeleton in rat pancreatic acini.
J. Biol. Chem.
273:
24173-24180,
1998[Abstract/Free Full Text].
24.
Schmidt, A.,
and
M. N. Hall.
Signaling to the actin cytoskeleton.
Ann. Rev. Cell Dev. Biol.
14:
305-338,
1998[Medline].
25.
Schneider, G. B.,
H. Hamano,
and
L. F. Cooper.
In vivo evaluation of hsp27 as an inhibitor of actin polymerization: hsp27 limits actin stress fiber and focal adhesion formation after heat shock.
J. Cell. Physiol.
177:
575-584,
1998[Medline].
26.
Valentijn, K. M.,
F. D. Gumkowski,
and
J. D. Jamieson.
The subapical actin cytoskeleton regulates secretion and membrane retrieval in pancreatic acinar cells.
J. Cell Sci.
112:
81-96,
1999[Abstract/Free Full Text].
27.
Zhang, S.,
J. Han,
M. A. Sells,
J. Chernoff,
U. G. Knaus,
R. J. Ulevitch,
and
G. M. Bokoch.
Rho family GTPases regulate p38 mitogen-activated protein kinase through the downstream mediator Pak1.
J. Biol. Chem.
270:
23934-23936,
1995[Abstract/Free Full Text].
28.
Zhu, Y.,
S. O'Neill,
J. Saklatvala,
L. Tassi,
and
M. E. Mendelsohn.
Phosphorylated HSP27 associates with the activation-dependent cytoskeleton in human platelets.
Blood
84:
3715-3723,
1994[Abstract/Free Full Text].
Am J Physiol Cell Physiol 277(6):C1029-C1031
0002-9513/99 $5.00
Copyright © 1999 the American Physiological Society