Content uploaded by Paul D Lampe
Author content
All content in this area was uploaded by Paul D Lampe on Jun 10, 2014
Content may be subject to copyright.
Introduction
Gap junctions are specialized membrane domains composed of
collections of channels that directly connect neighboring cells
(Willecke et al., 2002). These pathways provide for the cell-to-
cell diffusion of small molecules, including ions, amino acids,
nucleotides and second messengers (e.g. Ca2+, cAMP, cGMP,
IP3). Recent studies on the targeted disruption of connexin
genes, which encode vertebrate gap junction channel proteins,
provide strong support for roles in cell growth control and
embryonic development, as well as the transmission of
metabolites and electrical signals between cells (Willecke et
al., 2002).
Transient changes in gap junctional communication,
probably regulated by signaling cascades, have been observed
and appear necessary for normal cell cycling. For example, gap
junctional communication was reported to be moderate during
G1/S, increased through S and decreased in G2/M (Bittman and
LoTurco, 1999; Stein et al., 1992). The downregulation of
junctional communication during G2/M has been correlated
with increased p34cdc2 kinase-dependent phosphorylation of
Cx43 (Kanemitsu et al., 1998; Lampe et al., 1998a) and
redistribution of Cx43 from gap junctions to the cytoplasm
(Lampe et al., 1998a; Xie et al., 1997). Gap junctional
structures reassemble and communication is gradually restored
as cells proceed through G1(Stein et al., 1992; Xie et al., 1997).
Cx43 is phosphorylated at multiple serine residues in vivo
(Berthoud et al., 1992; Brissette et al., 1991; Crow et al., 1990;
Kadle et al., 1991; Laird et al., 1991; Musil et al., 1990), and
upon phosphorylation, Cx43 migration in polyacrylamide gel
electrophoresis (SDS-PAGE) is reduced. Although apparently
not required for the formation of functional channels (Dunham
et al., 1992; Fishman et al., 1991), phosphorylation of gap
junction proteins appears to regulate channel function (gating)
and the rates of channel assembly and turnover (Brissette et al.,
1991; Kwak et al., 1995a; Kwak et al., 1995b; Kwak et al.,
1995c; Lampe, 1994; Lampe et al., 2000).
In the sustained absence of connexin expression,
tumorigenesis is enhanced (Laird et al., 1999; Moennikes et
al., 1999). The correlation between neoplastic transformation
and reduced gap junctional communication (Atkinson et al.,
1981; Azarnia and Loewenstein, 1984; de Feijter et al., 1990)
has led to the hypothesis that reduced cell-cell communication
is a critical step in multistage carcinogenesis (Fitzgerald and
Yamasaki, 1990; Trosko et al., 1990). PKC has received
considerable attention because PKC activators (e.g. TPA),
which promote tumorigenesis, both increase Cx43
2203
Phorbol esters such as 12-O-tetradeconylphorbol-13-
acetate (TPA) activate protein kinase C, increase
Connexin43 (Cx43) phosphorylation, and decrease cell-cell
communication via gap junctions in many cell types.
Previous work has implicated protein kinase C (PKC) in
the direct phosphorylation of Cx43 at S368, which results
in a change in single channel behavior that contributes to
a decrease in intercellular communication. We have
examined Cx43 phosphorylation in several cell lines with
an antibody specific for phosphorylated S368. We show that
this antibody detects Cx43 only when it is phosphorylated
at S368 and, consistent with previous results, TPA
treatment causes a dramatic increase in phosphorylation
at S368. However, in some cell types, the increased
phosphorylation at S368 did not cause a detectable shift
in migration as compared with the nonphosphorylated
Cx43. Immunofluorescence showed increased S368
immunolabeling in cytoplasmic and plasma membrane
structures in response to TPA. Immunoblot analysis of
synchronized cells showed increased phosphorylation at
S368 during S and G2/M phases of the cell cycle. S-phase
cells contained more total Cx43 but assembled fewer
functional gap junctional channels than G0-phase cells.
Since M-phase cells also communicate poorly and contain
few assembled gap junctions, phosphorylation at S368
appears to be negatively correlated with gap junction
assembly. Thus, both gap junctional communication and
S368 phosphorylation change during S phase and G2/M,
implying that phosphorylation at S368 might play a role in
key cell-cycle events.
Key words: Gap junctions, Connexins, Tumor promoter,
Phosphorylation, Carcinogenesis
Summary
Connexin43 phosphorylation at S368 is acute during S
and G2/M and in response to protein kinase C
activation
Joell L. Solan1, Matthew D. Fry2, Erica M. TenBroek3and Paul D. Lampe1,*
1Fred Hutchinson Cancer Research Center, Seattle, WA 98109, USA, and Department of Pathobiology, University of Washington, WA 98195, USA
2Cell Signaling Technology, Beverly, MA 01915, USA
3Genetics, Cell Biology and Development, University of Minnesota, St Paul, MN 55108, USA
*Author for correspondence (e-mail: plampe@fhcrc.org)
Accepted 12 February 2003
Journal of Cell Science 116, 2203-2211 © 2003 The Company of Biologists Ltd
doi:10.1242/jcs.00428
Research Article
2204
phosphorylation and decrease gap junction communication in
several different cell types (Berthoud et al., 1992; Berthoud et
al., 1993; Brissette et al., 1991; Lampe, 1994; Reynhout et al.,
1992). PKC has been shown to phosphorylate Cx43 at S368,
and this site has been shown to underlie a TPA-induced
reduction in intercellular communication and alteration of
single channel behavior (Lampe et al., 2000). However, in
some cell types, TPA treatment did not lead to a shift in Cx43
mobility in SDS-PAGE (which is thought to indicate increased
Cx43 phosphorylation) but did change gap junctional
communication (e.g. Rivedal and Opsahl, 2001), leading to
confusion as to the role of Cx43 phosphorylation in this
process.
Here, we report that Cx43 phosphorylation at S368 was
indeed increased by TPA treatment in all cell types tested, but
that Cx43 mobility was not significantly affected in some.
Furthermore, S368 phosphorylation was increased during key
stages of the cell cycle where gap junctional assembly is
reduced. Thus, in addition to its role in the regulation of
gap junction channel gating, phosphorylation at S368 was
negatively correlated with gap junction assembly.
Materials and Methods
Cell line maintenance and transfection
Normal rat kidney (NRK) epithelial cells (NRK-E51, American Type
Culture Collection, ATCC, Rockville, MD), Chinese Hamster Ovary
(CHO) and HeLa cells were cultured in DMEM (Mediatech,
Pittsburgh, PA) supplemented with 5% fetal calf serum and antibiotics
in a humidified 5% CO2environment. A serine-to-alanine site 368
mutant Cx43 cDNA was generated using the Chameleon double-
stranded, site-directed mutagenesis kit (Stratagene, La Jolla, CA) and
was then subcloned into the bicistronic expression vector pIREShyg
(Clontech Laboratoroes, Palo Alto, CA) and transfected into the HeLa
cell lines. Stably transfected clones were isolated by repeated dilution
subcloning in the presence of the selective antibiotic hygromycin (200
µg/ml).
Metabolic labeling and Cx43 immunoprecipitation
NRK cells were cultured, metabolically labeled with
[32P]orthophosphate (ICN, 64014L) or 35S-Trans label (ICN,
5100607), and immunoprecipitated essentially as previously
described (TenBroek et al., 2001). Briefly, cells were labeled with
[32P]orthophosphate at 1.0 mCi/ml for 3 hours in phosphate-deficient
medium (Gibco-Invitrogen, Grand Island, NY) and, where indicated,
were treated with 50 ng/ml TPA during the final 30 minutes.
Alternatively, cells were washed three times and labeled with 35S-
Trans label at 0.1 mCi/ml for 3 hours in methionine-free media
(Gibco-Invitrogen) and, where indicated, were treated with 50 ng/ml
TPA during the final 30 minutes. The cells were rinsed in PBS, lysed
in RIPA buffer [25 mM Tris-HCl, 100 mM NaCl, 10 mM EDTA,
50 mM NaF, 500 µM Na3VO4, 0.25% Triton X-100, 2 mM
phenylmethylsulphonyl fluoride (PMSF) and 1×Roche Complete
protease inhibitors], clarified with protein A beads, and
immunoprecipitated with p368 antibody [a rabbit anti-Phospho-Cx43
(Ser368) antibody #3511; Cell Signaling Technology, Beverly, MA],
rabbit antibody C6219 from Sigma (St Louis, MO) and/or monoclonal
Cx43CT1 antibody. Cx43CT1 antibody is an antibody prepared to a
peptide representing the last 23 amino acids of Cx43 (described in
Cooper and Lampe, 2002). Cx43CT1 behaves like antibody 13-8300
from Zymed, which was prepared to the same region of Cx43, in that
it immunoprecipitates primarily the ‘NP’ form of Connexin unless
cells are treated with TPA, when slower migrating forms were
detected (Cruciani and Mikalsen, 1999). After four washes in RIPA
buffer, the immunoprecipitates were treated with Laemmli sample
buffer and run via SDS-PAGE (10% polyacrylamide, Tris-glycine
gels).
Immunoblotting
Cells were lysed in sample buffer containing 50 mM NaF, 500 µM
Na3VO4, 2 mM PMSF and 1×Complete protease inhibitors (Roche
Diagnostics, Indianapolis, IN) and cellular proteins were separated by
SDS-PAGE on 10% Tris-glycine gels. For alkaline phosphatase
treatment, cells were lysed in 0.2% SDS, 2 mM PMSF and 1×protease
inhibitors, and briefly sonicated followed by addition of one-tenth
volume of 10×phosphatase buffer (M183A; Promega, Madison, WI)
and incubation with 10 units of calf intestinal alkaline phosphatase
(M182A; Promega) for 1 hour at 37°C. After electrophoresis, protein
was transferred to nitrocellulose, the membrane was blocked, and
antibodies were incubated as previously indicated (Lampe et al.,
1998a). Primary and secondary antibodies utilized were p368
antibody, mouse anti-Cx43 (Cx43NT1 described in Goldberg et al.,
2002), mouse anti-vinculin (Sigma), peroxidase-conjugated donkey
anti-mouse or mouse anti-rabbit secondary antibodies (Jackson
Immunoresearch Laboratories, West Grove, PA). Where indicated, the
blots were ‘stripped’ for 30 minutes at 50°C in 62.5 mM Tris pH 6.8,
1% SDS and 5% β-mercaptoethanol buffer followed by washing for
2 hours with at least six changes of PBS. Signal was visualized with
SuperSignal West Pico or Femto Chemiluminescent Substrate (Pierce
Chemicals, Rockford, IL) followed by exposure to Kodak Biomax
MR film. Densitometry of autoradiographs was performed on a
Macintosh G3 using a Sharp JX-325 scanner to collect the image and
the public domain NIH Image program (developed at the US National
Institutes of Health and available at http://rsb.info.nih.gov/nih-image).
Immunofluorescence
NRK cells were untreated or treated with TPA for 30 minutes at 37°C,
washed twice in PBS, and fixed in cold methanol/acetone (50:50) for
1 minute followed by blocking for 1 hour in 1% bovine serum albumin
in PBS. Cells were incubated with anti-Cx43 antibody p368 and/or
Cx43IF1 (see Cooper and Lampe, 2002; TenBroek et al., 2001) in
blocking solution for 1 hour. Following several PBS washes, the
cultures were incubated with Alexa594-conjugated goat anti-rabbit
antibody (Molecular Probes, Eugene, OR) and/or fluorescein
isothiocyanate-conjugated donkey anti-mouse antibody (Jackson
Immunoresearch Laboratories) for 30-60 minutes and counterstained
with DAPI (Molecular Probes), followed by several washes in PBS.
The coverslips were mounted onto slides with DABCO antifade
medium [25 mg/ml of 1,4-diazobicyclo-(2,2,2)octane (Sigma) diluted
in Spectroglycerol (Kodak) and 10% PBS, pH 8.6] and viewed with
a Nikon Diaphot TE300 fluorescence microscope, equipped with a
40×(1.3 n.a.) oil objective and a Princeton Instruments cooled digital
camera driven by an attached PC and Metamorph imaging software.
Cell synchronization
G0cells were prepared by contact-inhibiting NRK cells at least 3 days
past confluency without addition of fresh media. To obtain G1 cells,
confluent cells were trypsinized, then diluted to 60-80% confluency
and allowed to progress 8-10 hours for early G1and 14-16 hours for
late G1. Cell-cycle analysis showed that by 18 hours these cells begin
to enter S phase. For preparation of G1/S, S, G2and G2/M cells,
confluent cells were trypsinized then diluted to 60-80% confluency in
media containing 1 mM thymidine for 16 hours to induce a G1/S
block. Cells were released from G1/S by washing and replacement of
37°C complete media. Cell-cycle analysis showed that S phase lasts
4-6 hours in these cells and that cells cycle through G2/M to G1by 9-
11 hours after washout. We typically observed 70-90% synchrony as
Journal of Cell Science 116 (11)
2205Phosphorylation at S368 of Cx43
cells progress through S to G1. Cell-cycle analysis was performed by
fluorescence activated cell sorting. Specifically, cells were trypsinized,
then pelleted in PBS with 2% fetal bovine serum and fixed in 70%
EtOH. Cells were pelleted, washed and incubated with 5 µg/ml RNase
at 37°C for 30 minutes, and then stained with 50 µg/ml propidium
iodide on ice for 1 hour. DNA content was assessed on a Becton
Dickinson FACScalibur and data analyzed using CellQuest software.
Gap junctional communication/assembly
Gap junctional communication was assayed via dye transfer according
to published methods using either an assembly-preloading assay with
calcein-AM (Lampe et al., 1998b) or by microinjection of fluorescent
dyes. Briefly, for the preloading assay, one 10 cm plate of NRK cells
was labeled with 0.5 µM calcein-AM (Molecular Probes), the cell-
permeant ester of calcein that is cleaved to membrane-impermeant
calcein by cellular esterases. Three other culture plates were labeled
with 0.25 µM DiI (Molecular Probes). After washing twice with PBS,
the two populations of cells were each trypsin/EDTA suspended,
treated with trypsin inhibitor and pelleted. The cells were suspended
in the appropriate media, mixed, plated on culture dishes and placed
in a 37°C incubator. Cells were allowed to adhere for 2 hours then
digital images of calcein and DiI were captured. The assignment of a
cell as an acceptor of dye via transfer rather than a poorly loaded or
leaking donor is checked by digitally overlaying images of DiI and
calcein fluorescence. If a cell adjacent to a calcein-loaded, DiI-
negative cell contains both punctate DiI and more-diffuse calcein
fluorescence, gap junction assembly and dye transfer occurred. If a
DiI-labeled cell adjacent to a calcein-loaded cell does not contain
calcein, then dye transfer did not occur at that interface. A more-
complete description of this assay is published elsewhere (Lampe,
1994; Lampe et al., 1998b). The fraction of cells that transferred dye
were determined by dividing the number of DiI-labeled cells that
contained calcein (i.e. transfers) by the number of cell interfaces
between calcein-loaded and DiI-labeled cells (i.e. total).
Dye transfer in established cultures was analyzed by microinjection
of a 10 mM solution of each of the gap junction permeable dyes,
Alexa hydrazide 488 and 594 (Mr=570.5 and 758.8, respectively;
Molecular Probes) in 0.2 M KCl. The dyes were microinjected using
a 5 millisecond pulse of air at 10 psi from a General Valve Picospritzer
II, and the number of cells receiving dye was analyzed after 10
minutes using the imaging system described above.
Results
Phospho-Cx43-Ser368 (p368) antibody is specific for
phosphorylation of S368
We have developed an antibody that reacts with Cx43 when it
is phosphorylated at S368. To test the specificity of the p368
antibody, HeLa cells that did not express any Cx43 were stably
transfected with wild-type (wt) Cx43 or Cx43 containing a
serine-to-alanine substitution at position 368 (S368A) and
were examined by immunoblot analysis. Previously, we have
shown that cells treated with TPA showed increased
phosphorylation on Cx43, especially at S368 (Lampe et al.,
2000). HeLa cells, either treated with TPA for 30 minutes or
untreated, were washed and directly lysed in sample buffer, and
whole-cell lysates were immunoblotted and probed for Cx43
content using the p368 antibody (rabbit) followed by
stripping/reprobing of the blot using the mouse monoclonal
Cx43 antibody Cx43NT1 (Fig. 1). The anti-Cx43 probing of
HeLa cells expressing wt Cx43 or S368A Cx43 (Fig. 1, α-
Cx43, CON lanes) showed typical migration patterns of Cx43
in SDS-PAGE, with multiple bands representing different
phosphorylation states of Cx43. The predominant
nonphosphorylated (NP) form migrates fastest, followed by
slower-migrating phosphorylated forms, often referred to as
P1, P2, etc., which can be converted to the faster-migrating
form via alkaline phosphatase treatment (Berthoud et al., 1992;
Brissette et al., 1991; Kadle et al., 1991; Laird et al., 1991;
Lampe, 1994; Musil et al., 1990). Upon TPA treatment,
essentially all of the Cx43 migration in HeLa cells containing
wt Cx43 was shifted to slower-migrating species as has been
observed previously in many different cell types (reviewed by
Lampe and Lau, 2000). HeLa cells containing Cx43 with a
S368A site-directed mutation responded much less extensively
to TPA treatment (Fig. 1, α-Cx43), indicating that
modification/phosphorylation on S368 affects the mobility
shift, at least in these HeLa cells. Probing this same blot
with the p368 antibody (Fig. 1, α-p368) showed that cells
containing wt Cx43 had a low level of Cx43 phosphorylated at
S368 (CON) that appeared to migrate similarly to NP Cx43.
Upon TPA treatment, both a tenfold increased signal and a shift
in migration of wt Cx43 was observed with the p368 antibody.
Lysates from cells containing Cx43 with the S368A mutation
showed no p368 antibody reactivity regardless of TPA
treatment or long exposure of blots.
To verify further that this antibody recognized a
phosphorylated species of Cx43, NRK cell lysates from control
cells and TPA-treated cells were incubated with alkaline
phosphatase and analyzed by immunoblot. As above,
immunoblots were first processed with the p368 antibody, then
stripped and reprobed with the anti-Cx43 antibody, allowing
precise alignment and determination of the extent of migration
of the bands. Immunoblots incubated with anti-Cx43 showed
little change in response to TPA treatment (Fig. 2, α-Cx43
panel; compare CON and TPA lanes). When these cell lysates
were incubated with alkaline phosphatase, all of the Cx43
migrated as the NP form (Fig. 2, α-Cx43 panel, AP lanes),
which is consistent both with effective alkaline phosphatase
treatment and with what has been shown by other investigators,
as noted above. Processing of the blot with anti-p368 antibody
Fig. 1. The p368 antibody reacts with Cx43 only when S368 is
present. Shown is an immunoblot of whole cell lysates from HeLa
cells transfected with wild-type (wt) Cx43 or Cx43 containing a
S368A mutation. Cells were either incubated in the presence (TPA)
or absence (CON) of 50 ng/ml TPA for 30 minutes. The immunoblot
was probed with either an antibody to the N-terminal region of Cx43
(α-Cx43) or the anti-p368 antibody (α-p368). Positions of the
molecular weight markers are shown on the left.
2206
showed a sixfold increase in signal upon TPA treatment (Fig.
2, α-p368 panel; compare CON and TPA) and this signal was
completely lost upon alkaline phosphatase treatment (Fig. 2,
α-p368 panel, AP lanes). These data show that the p368
antibody reactivity appears to be specific for S368 only when
it is phosphorylated (i.e. it is phosphorylation-state specific)
and that a dramatic increase in phosphorylation at S368 is
generated in response to TPA.
The ‘NP’ form of Cx43 can be phosphorylated on S368
Figs 1 and 2 indicate that an isoform of Cx43 that migrated
similarly to NP Cx43 reacted with the p368 antibody. To
determine more directly whether a phosphorylated species
migrated to the same extent as the NP form of Cx43, we
performed metabolic labeling on NRK cells with
[32P]orthophosphate or [35S]methionine. Cx43 from
[32P]orthophosphate-labeled cells was immunoprecipitated,
run on SDS-PAGE and blotted to nitrocellulose. These samples
were analyzed first by autoradiography (Fig. 3, 32P panel)
and then immunoblot analysis using p368 (α-p368 panel)
and Cx43NT1 (α-Cx43) monoclonal antibodies. In the
autoradiograph, Cx43 immunoprecipitated from untreated
cells showed two band, indicated as P1 and P2, whereas cells
treated with TPA showed a more-broad phosphorylation
pattern some of which appeared to migrate at the same position
as the NP form. The α-p368 panel, which represents the
chemiluminescent signal obtained from the same blot probed
with p368 antibody, shows a dramatic TPA-dependent increase
in signal co-migrating with the NP form, whereas probing the
same blot with the α-Cx43 antibody showed minor differences
in the typical pattern for Cx43 with or without TPA treatment.
Thus, the TPA-dependent increase in Cx43 phosphorylation
levels found by autoradiography was not nearly as extensive as
that observed with the p368 antibody immunoreactivity. This
result confirms that S368 phosphorylation, in particular, is
increased dramatically via TPA treatment, whereas
phosphorylation at many other residues was not as TPA
responsive (Lampe et al., 2000), essentially diluting the p368
signal. Furthermore, the total Cx43 signal and the ratio of
‘phosphorylated’ (i.e. P1 + P2) to nonphosphorylated (Fig. 2,
α-368 panel) were quite similar regardless of TPA treatment,
in spite of the fact that dramatic changes in S368
phosphorylation occurred.
NRK cells were also labeled with [35S]methionine, and
immunoprecipitations were carried out using either the p368
antibody or an anti-Cx43 (Cx43CT1) antibody that shows a
strong preference for the NP migratory isoform. These
samples were run on SDS-PAGE and analyzed by
autoradiography. In NRK cells, the α-Cx43 antibody
immunoprecipitated a single band that did not change
significantly in intensity upon TPA treatment (Fig. 4, NRK
panel, α-Cx43). As expected, this band migrates at the same
position as NP. The p368 antibody also immunoprecipitated a
single band that migrated exactly with the band
immunoprecipitated with the α-Cx43 antibody and showed
increased signal intensity upon TPA treatment (Fig. 4, NRK
panel, α-p368). Thus, the p368 antibody was able to
immunoprecipitate a Cx43 isoform that migrates the same as
the NP form in our typical Laemmli gel system, and TPA-
treated cells contained more of this isoform than control cells.
Taken together, these metabolic labeling data show that a
phosphorylated species of Cx43 essentially co-migrates with
the NP form and that this phosphoform can be clearly detected
using the p368 antibody. Nonphosphorylated Cx43 and Cx43
phosphorylated at S368 probably could be separated given the
appropriate separation technique since these species vary in net
charge. It is noteworthy that standard isoelectric focusing and
two-dimensional analysis of Cx43 has been shown to be
Journal of Cell Science 116 (11)
Fig. 2. The p368 antibody reacts with Cx43 only when it is
phosphorylated at S368. Untreated (CON) or TPA-treated cells were
lysed in sample buffer or treated with alkaline phosphatase (+AP
lanes) prior to SDS-PAGE and immunoblotting. The blot was probed
with the p368 antibody (α-p368 panel) followed by stripping and
reprobing with the Cx43 antibody (α-Cx43 panel).
Fig. 3. TPA-treated NRK cells show increased phosphorylation at
S368 with no apparent shift in migration in SDS-PAGE. NRK cells
were metabolically labeled with [32P]orthophosphate and incubated
in the presence (TPA) or absence (CON) of 50 ng/ml TPA for 30
minutes followed by immunoprecipitation of Cx43, blotting to
nitrocellulose, and probing the blot first by autoradiography (32P) and
then using the anti-p368 antibody (α-p368) and ultimately the N-
terminal Cx43 antibody (α-Cx43).
Fig. 4. The p368 antibody immunoprecipitates more Cx43 from
either NRK or CHO cells after TPA treatment. NRK and CHO cells
were metabolically labeled with [35S]methionine and either treated
with TPA or left untreated (CON) and then lysed and
immunoprecipitated with either the Cx43 antibody (α-Cx43 lanes) or
the p368 antibody (α-p368 lanes).
2207Phosphorylation at S368 of Cx43
difficult (Stockert et al., 1999). Nevertheless, when analyzed
by a standard Tris/glycine SDS-PAGE system that has been
used by most investigators, the migration of Cx43
phosphorylated on S368 often coincided with
nonphosphorylated Cx43. For this reason, we believe that it is
probably most accurate to refer to the fastest-migrating form
as P0 rather than NP when discussing standard SDS-PAGE
separation of Cx43 from cells that have been treated with
kinase effectors or growth factors, and we do so below. This is
an interim solution since different cell types and slightly
modified gel systems appear to produce Cx43 with varying
migratory properties. A better definition of terms will probably
require a thorough understanding of the molecular events that
underlie the shift in migration.
The TPA-dependent shift in Cx43 migration, but not
phosphorylation of S368, is cell-type specific
Although the migration of Cx43 derived from NRK cells does
not shift significantly in the presence of TPA, many other cell
types can show a dramatic shift, essentially leaving little faster-
migrating species as shown for HeLa cells in Fig. 1. We found
that CHO cells also show a dramatic shift in response to TPA,
as is shown via immunoprecipitation in Fig. 4 and western
immunoblot in Fig. 5 (CHO panels). Fig. 4 shows
[35S]methionine-labeled CHO cell lysates immunoprecipitated
with α-Cx43 or α-p368 antibodies. In TPA-treated CHO cells,
α-Cx43 (Cx43CT1) immunoprecipitated both the NP/P0
migratory isoform and the slower-migrating isoforms (Fig. 4,
CHO panel, α-Cx43; see Materials and Methods for antibody
description). Immunoprecipitation of TPA-treated CHO cell
lysates with α-p368 antibody shows primarily the slower-
migrating isoforms (Fig. 4, CHO panel, α-p368) indicating
that, in this cell line, phosphorylation on S368 was coincident
with a shift in migration.
Similarly, Fig. 5 shows an immunoblot of NRK and CHO
whole cell lysates that was probed with the antibody for p368
(α-p368) and stripped/reprobed for Cx43 (α-Cx43). Consistent
with the immunoprecipitation results, Cx43 from NRK cells
did not shift its migration in response to TPA while the protein
extensively shifts to slower-migrating phosphoforms in CHO
cells. Both cell types show large TPA-dependent increases in
reactivity to the p368 antibody, but the p368 signal in NRK
cells primarily migrated at the P0 and P1 positions whereas the
p368 signal was highly shifted in the CHO cells. HeLa cells
containing wt Cx43 (Fig. 1) were intermediate between the
two, as p368 is found in the P1 and P2 forms. Notably, in all
cell lines examined, a low level of p368 was present in
untreated cells and often co-migrated with the NP/P0 isoform,
which indicates that phosphorylation of S368 is part of the
normal lifecycle of Cx43 in these cells. To examine whether
phosphorylation at S368 might be consistent with the early
phosphorylation event found in the presence of Brefeldin A
(BFA) (Laird et al., 1995), we treated NRK and CHO cell
lysates with BFA and found decreased p368 antibody labeling.
However, α-p368 binding was not eliminated, so no firm
conclusions can be drawn with respect to this event. Thus, we
have used the p368 antibody to examine TPA-induced
phosphorylation of Cx43 in NRK, CHO and Cx43-transfected
HeLa cells and found that all cell types examined show
increased phosphorylation on S368, but the degree to which
this resulted in a shift in the migration of Cx43 varied between
cell types.
TPA-induced phosphorylation of S368 occurs on both
intracellular and plasma membrane Cx43
To determine whether a specific pool of Cx43 is
phosphorylated in response to TPA, immunofluorescence was
performed on NRK cells with an antibody specific for Cx43
(Cx43IF1) and the p368 antibody. NRK cells show extensive
immunofluorescence for Cx43 at cell-cell interfaces (Fig. 6,
upper left). Upon TPA treatment, Cx43 immunofluorescence
showed no apparent change although the cells adopted a
slightly more fibroblastic appearance (Fig. 6, lower left). The
p368 antibody also showed some cell-cell interface labeling
and a light reticulate pattern throughout the cytoplasm (upper
center panel). The apparent cytoplasmic pool of p368 staining
does appear to be at least partly associated with the
endoplasmic reticulum as there was co-localization of p368
with an endoplasmic reticulum-specific dye, R6 (data not
shown). After TPA treatment, the p368 signal was greatly
increased in both cytoplasmic and interface membranes (lower
center panel). The plasma membrane pool of p368 shows co-
localization with the Cx43IF1 antibody, whereas less-distinct
co-localization of this antibody with the intracellular pool was
observed. The increase in intracellular fluorescence does
appear to be specific to the p368 epitope as co-incubation of
p368 antibody with the peptide antigen used to generate the
antibody blocked antibody binding, while co-incubation of the
antibody with a nonphosphorylated peptide representing 360-
382 of Cx43 did not block binding (data not shown). We have
observed that there is competition between Cx43IF1 and p368
antibody binding at cell-cell contacts. This was manifest by a
decrease in p368 signal when p368 and Cx43IF1 antibodies
were added together, but was reversed by inclusion of the
nonphosphorylated peptide, which removed the Cx43IF1
signal.
Fig. 5. NRK and CHO cells both show large increases in
phosphorylation at S368 upon TPA treatment but the resulting Cx43
mobilities are very different. Cells were either incubated in the
presence of no drugs (CON), 50 ng/ml TPA for 30 minutes (TPA) or
5 µg/ml brefeldin A for 4 hours (BFA), and processed for
immunoblot and separately probed with the anti-p368 antibody (α-
p368) and the N-terminal Cx43 (α-Cx43).
2208
Phosphorylation on S368 is regulated as cells progress
through the cell cycle
Given that phosphorylation on S368 appeared to be part of the
normal lifecycle of Cx43, we wanted to determine
circumstances under which this event was regulated. As it has
previously been shown that Cx43 phosphorylation increases as
cells progress through the cell cycle (Kanemitsu et al., 1998),
we looked at Cx43 and phosphorylation of S368 in cells
synchronized at different stages of the cell cycle in NRK cells.
Fig. 7 shows an immunoblot probed first for p368 (α-p368)
and then stripped/reprobed for Cx43 (α-Cx43). Vinculin was
also detected for a loading control. Densitometry was
performed for Cx43 and p368 antibody binding, and the ratio
of p368/Cx43 densitometry is shown at the bottom of the
figure. Cx43 phosphorylated at S368 was most abundant
relative to total Cx43 during S and G2/M. This result is
consistent with previous reports where gap junctional
communication was shut-down during mitosis (Stein et al.,
1992; Xie et al., 1997) and phosphorylation at S368 had been
shown to reduce communication (Lampe et al., 2000). Here,
we found that G0 cells contain very little p368 and that S368
is increasingly phosphorylated as cells approach and progress
through S phase.
Given the 7×increase in phosphorylation at S368 when G0
and S phase cells were compared (Fig. 7), we wanted to
examine Cx43 distribution and intercellular communication in
these two cell populations. G0cells showed strong plasma
membrane staining for Cx43 at cell-cell interfaces consistent
with gap junctions (Fig. 8A), while S-phase cells showed both
typical gap junctional labeling and also extensive perinuclear
staining (Fig. 8B). Immunofluorescent labeling with the p368
antibody showed both cytoplasmic and gap junctional staining
for both G0- and S-phase cells (data not shown).
Since TPA treatment of cells has been reported to decrease
intercellular communication via changes in channel gating
(e.g., Kwak et al., 1995c; Lampe et al., 2000; Moreno et al.,
1994) and gap junction assembly (Lampe, 1994), we assessed
both the ability to transfer dye and the ability to assemble
junctions in G0- and S-phase cells. When we microinjected G0-
and S-phase cells with two fluorescent dyes of the Alexa series
(A488, Mr=570.5; A594, Mr=758.8), we found that S-phase
cells transferred both dyes approximately twice as well as G0
cells (Fig. 8C). However, G0-phase cells were approximately
twice as likely to transfer dye to their neighbors than S-phase
Journal of Cell Science 116 (11)
Fig. 6. The p368 antibody binds to both junctional and cytoplasmic membranes. NRK cells that had been incubated in the presence (TPA) of 50
ng/ml TPA or absence (CON) were processed for immunofluorescence with the anti-p368 (α-pS368) and the Cx43IF1 (α-Cx43) antibodies (left
and center panels) or with the anti-p368 antibody plus the immunizing peptide (right panel, α-pS368 + peptide).
Fig. 7. The extent of p368 phosphorylation is increased through the
cell and is maximal during S and G2/M. Synchronized cells collected
at the indicated cell-cycle stage were processed for immunoblotting
and probed with antibodies to p368 (α-p368), Cx43NT1 (α-Cx43),
or vinculin (for a loading control). The molecular weight or
migration position of the Cx43 is indicated on the right, and the ratio
of the extent of p368 to Cx43NT1 antibody labeling is shown on the
bottom line.
2209Phosphorylation at S368 of Cx43
cells when the calcein/DiI assay, which requires nascent gap
junction assembly, was performed (Fig. 8D).
Discussion
Previously, we have demonstrated that phosphorylation of
Cx43 on S368 is stimulated by TPA in vivo and mediated by
PKC in vitro (Lampe et al., 2000). Furthermore,
electrophysiological studies of Cx43 and the Cx43-S368A
mutant revealed that phosphorylation at S368 is necessary for
a TPA-induced alteration of Cx43 channel behavior that
contributes to decreased gap junctional communication. Here,
we report that phosphorylation levels at S368 are high in S and
G2/M, and that cells at quiescence show only very low levels.
In addition, S-phase cells assembled gap junctions poorly
compared with G0-phase cells, implying a role for
phosphorylation at S368 in the regulation of assembly.
Cx43 phosphorylation at S368 appears to occur normally in
dividing cells. The seven- to eightfold increase in the level of
phosphorylation on S368 at S and G2/M, respectively, correlates
well with increased cytoplasmic localization of Cx43 during S
(Fig. 7) and G2/M (Lampe et al., 1998a; Xie et al., 1997),
consistent with a role for S368 phosphorylation in regulating
Cx43 trafficking/assembly into gap junctional structures.
Interestingly, we also occasionally observed a unique
and apparently nuclear envelope/endoplasmic reticulum
localization of the p368 antibody at the early stages of G2/M
(data not shown). This immunolocalization was highly
transitory because it was lost as the nuclear envelope broke
down as the cells entered mitosis. Although this localization
appeared specific for the antibody based on antigen competition
studies, Cx43IF1 antibody immunolabeling of the nuclear
envelope region of G2/M cells was not nearly as striking as the
p368 antibody. Thus, we cannot rule out the possibility that an
alternative non-connexin epitope that specifically reacts with
the p368 antibody is expressed in early mitosis.
Much of the work examining TPA-mediated downregulation
of Cx43 has been motivated by the role of PKC activators as
tumor promoters and the potential role of gap junctional
communication as a tumor suppressor. Although details are
still poorly understood, there is a wealth of data showing that
Cx43 phosphorylation is increased and gap junctional
communication is reduced upon activation of PKC (reviewed
by Lampe and Lau, 2000). However, the use of different assays
for communication, several methods for assaying Cx43
phosphorylation and various cellular systems expressing
different isoforms of PKC (Cruciani et al., 2001; Munster and
Weingart, 1993) have confused the interpretation of the role
PKC plays as a modulator of gap junctional communication.
For example, several reports have fueled the controversy as to
whether mitogen-activated protein kinase (MAPK) or PKC is
the actual kinase that phosphorylates Cx43 and reduces gap
junctional communication after growth factor or phorbol ester
treatment, or whether Cx43 phosphorylation even plays a direct
role (e.g., Hossain et al., 1999; Kanemitsu and Lau, 1993;
Rivedal and Opsahl, 2001; Vikhamar et al., 1998). One
presumption found in many of these reports that might cloud
interpretation of the data is that a shift in Cx43 migration has
been equated with increased phosphorylation. We know that
Cx43 can be phosphorylated at many (>5) sites in untreated
cells and at many more sites in growth factor-treated cells
(Lampe and Lau, 2000). At this time, we have no
understanding of the molecular events responsible for the shift
in migration, or of any of the serines involved. By comparing
two cell types where TPA led to a shift in Cx43 migration in
one but no change in another, the logical but potentially
erroneous conclusion could be that Cx43 phosphorylation
levels only changed in one of the cell types. For example, from
the α-Cx43 panel of Fig. 5, one could conclude that there was
a large change in Cx43 phosphorylation in CHO cells upon
TPA treatment, whereas NRK cells showed little change and
thus appeared unresponsive to TPA treatment; by contrast, the
α-p368 panel shows that phosphorylation was dramatically
increased at this site in NRK cells. In fact, there probably is
some correlation with the extent of shift and the overall level
of Cx43 phosphorylation. However, specific phosphorylation
events and not the overall level of phosphorylation probably
elicit a specific regulatory event such as assembly, disassembly
or gating changes. We believe re-evaluation of many of these
seemingly conflicting results might be resolved by assaying for
TPA and growth factor effects with the p368 and other
phosphorylation-site-specific antibodies.
Fig. 8. G0- and S-phase cells show different Cx43 cellular
distributions and different abilities to transfer fluorescent dyes in
established and junctional assembly assays. (A) G0cells show
extensive junctional Cx43 immunostaining. (B) S-phase cells show
extensive junctional and cytoplasmic membrane staining. (C) The
number of cells that receive either Alexa488 (A488) or Alexa594
(A594) from the injected cell is quantitated for cells in established
G0- or S-phase cultures (mean±s.d.). (D) Cells in G0or S were
assayed for the ability to assemble gap junctions and transfer calcein
(quantitated as the number of transfers per the total number of
interfaces between a calcein-loaded and a recipient cell, mean±s.d.).
2210
Phosphorylation of Cx43 appears to regulate the trafficking
of Cx43 to the plasma membrane, assembly of Cx43 into gap
junctional structures, single channel behavior and Cx43
degradation. The latter three events have been reported to be
sensitive to TPA and, therefore, could be regulated by PKC
(Kanemitsu and Lau, 1993; Kwak et al., 1995a; Kwak et al.,
1995c; Lampe, 1994). Our immunofluorescence data with the
p368 antibody and comparison of the kinetics of the mobility
shift in SDS-PAGE with decreases in gap junctional
communication (Kanemitsu and Lau, 1993) indicate that S368
phosphorylation and potentially other PKC-mediated events
can occur prior to export to the plasma membrane
(Lampe, 1994). Therefore, at least some TPA-dependent
phosphorylation at S368 occurs prior to gap junction assembly.
Intercellular communication was reduced by TPA in
quiescent but not proliferating NRK cells (Paulson et al.,
1994). Data presented here indicates that, in addition to S368
being a TPA-responsive site, there is regulation of S368
phosphorylation during the normal lifespan of Cx43 in
untreated, cycling cells. Although S-phase cells transferred dye
more rapidly than G0cells in established cultures, S-phase
cultures were less able to form new functional gap junctions in
an assembly assay (Fig. 8D). Clearly, cell-cycle regulation
plays a key role during tumorigenesis. The cell-cycle-mediated
regulation shown here might indicate a more-subtle and
physiological role for gap junctional communication through
S368-mediated effects on assembly. Cell-cycle-mediated
regulation of Cx43 has been shown during mitosis, when there
is a dramatic change in phosphorylation and Cx43 is localized
predominately to cytoplasmic membranes. A model in which
assembly is most efficient during G0/G1, and then decreases as
cells progress towards mitosis, this being partially due to
phosphorylation at S368, fits our data. During tumorigenesis
or faulty regulation of the cell cycle, this decrease in assembly
could have dramatic effects on gap junctional communication.
This work was supported by grants GM55632 (P.D.L.) and
GM46277 (R.G.J.) from the National Institutes of Health.
References
Atkinson, M. M., Menko, A. S., Johnson, R. G., Sheppard, J. R. and
Sheridan, J. D. (1981). Rapid and reversible reduction of junctional
permeability in cells infected with a temperature-sensitive mutant of avian
sarcoma virus. J. Cell Biol. 91, 573-578.
Azarnia, R. and Loewenstein, W. R. (1984). Intercellular communication and
control of cell growth. X. Alteration of junctional permeability by the src
gene. J. Membr. Biol. 82, 191-205.
Berthoud, V. M., Ledbetter, M. L. S., Hertzberg, E. L. and Saez, J. C.
(1992). Connexin43 in MDCK cells: Regulation by a tumor-promoting
phorbol ester and calcium. Eur. J. Cell Biol. 57, 40-50.
Berthoud, V. M., Rook, M., Hertzberg, E. L. and Saez, J. C. (1993). On the
mechanism of cell uncoupling induced by a tumor promoter phorbol ester
in clone 9 cells, a rat liver epithelial cell line. Eur. J. Cell Biol. 62, 384-396.
Bittman, K. S. and LoTurco, J. J. (1999). Differential regulation of connexin
26 and 43 in murine neocortical precursors. Cereb. Cortex 9, 188-195.
Brissette, J. L., Kumar, N. M., Gilula, N. B. and Dotto, G. P. (1991). The
tumor promoter 12-O-tetradecanoylphorbol-13-acetate and the ras
oncogene modulate expression and phosphorylation of gap junction
proteins. Mol. Cell. Biol. 11, 5364-5371.
Cooper, C. D. and Lampe, P. D. (2002). Casein kinase 1 regulates connexin43
gap junction assembly. J. Biol. Chem. 277, 44962-44968.
Crow, D. S., Beyer, E. C., Paul, D. L., Kobe, S. S. and Lau, A. F. (1990).
Phosphorylation of connexin43 gap junction protein in uninfected and Rous
sarcoma virus-transformed mammalian fibroblasts. Mol. Cell. Biol. 10,
1754-1763.
Cruciani, V. and Mikalsen, S. O. (1999). Stimulated phosphorylation of
intracellular connexin43. Exp. Cell Res. 251, 285-298.
Cruciani, V., Sanner, T. and Mikalsen, S. O. (2001). Pharmacological
evidence for system-dependent involvement of protein kinase C isoenzymes
in phorbol ester-suppressed gap junctional communication. Carcinogenesis
22, 221-231.
de Feijter, A. W., Ray, J. S., Weghorst, C. M., Klaunig, J. E., Goodman,
J. I., Chang, C. C., Ruch, R. J. and Trosko, J. E. (1990). Infection of rat
liver epithelial cells with c-Ha-ras: Correlation between oncogene
expression, gap junctional communication and tumorigenicity. Mol.
Carcinog. 3, 54-67.
Dunham, B., Liu, S., Taffet, S., Trabka-Janik, E., Delmar, M., Petryshyn,
R., Zheng, S. and Vallano, M. (1992). Immunolocalization and expression
of functional and nonfunctional cell-to-cell channels from wild-type and
mutant rat heart connexin43 cDNA. Circ. Res. 70, 1233-1243.
Fishman, G. I., Moreno, A. P., Spray, D. C. and Leinwand, L. A. (1991).
Functional analysis of human cardiac gap junction channel mutants. Proc.
Natl. Acad. Sci. USA 88, 3525-3529.
Fitzgerald, D. J. and Yamasaki, H. (1990). Tumor promotion: Models and
assay systems. Tetragen. Carcinog. Mutagen. 10, 89-102.
Goldberg, G. S., Moreno, A. P. and Lampe, P. D. (2002). Heterotypic and
homotypic gap junction channels mediate the selective transfer of
endogenous molecules between cells. J. Biol. Chem. 277, 36725-36730.
Hossain, M. Z., Jagdale, A. B., Ao, P. and Boynton, A. L. (1999). Mitogen-
activated protein kinase and phosphorylation of connexin43 are not
sufficient for the disruption of gap junctional communication by platelet-
derived growth factor and tetradecanoylphorbol acetate. J. Cell. Physiol.
179, 87-96.
Kadle, R., Zhang, J. T. and Nicholson, B. J. (1991). Tissue-specific
distribution of differentially phosphorylated forms of Cx43. Mol. Cell. Biol.
11, 363-369.
Kanemitsu, M. Y. and Lau, A. F. (1993). Epidermal growth factor stimulates
the disruption of gap junctional communication and connexin43
phosphorylation independent of 12-O-tetradecanoyl 13-acetate-sensitive
protein kinase C: The possible involvement of mitogen-activated protein
kinase. Mol. Biol. Cell 4, 837-848.
Kanemitsu, M. Y., Jiang, W. and Eckhart, W. (1998). Cdc2-mediated
phosphorylation of the gap junction protein, connexin43, during mitosis.
Cell Growth Differ. 9, 13-21.
Kwak, B. R., Hermans, M. M. P., de Jonge, H. R., Lohmann, S. M.,
Jongsma, H. J. and Chanson, M. (1995a). Differential regulation of
distinct types of gap junction channels by similar phosphorylating
conditions. Mol. Biol. Cell 6, 1707-1719.
Kwak, B. R., Saez, J. C., Wilders, R., Chanson, M., Fishman, G. I.,
Hertzberg, E. L., Spray, D. C. and Jongsma, H. J. (1995b). Effects of
cGMP-dependent phosphorylation on rat and human connexin43 gap
junction channels. Pflügers Arch 430, 770-778.
Kwak, B. R., van Veen, T. A. B., Analbers, L. J. S. and Jongsma, H. J.
(1995c). TPA increases conductance but decreases permeability in neonatal
rat cardiomyocyte gap junction channels. Exp. Cell Res. 220, 456-463.
Laird, D. L., Castillo, M. and Kasprzak, L. (1995). Gap junction turnover,
intracellular trafficking, and phosphorylation of connexin43 in Brefeldin A-
treated rat mammary tumor cells. J. Cell Biol. 131, 1193-1203.
Laird, D. W., Puranam, K. L. and Revel, J. P. (1991). Turnover and
phosphorylation dynamics of connexin43 gap junction protein in cultured
cardiac myocytes. Biochem. J. 273, 67-72.
Laird, D. W., Fistouris, P., Batist, G., Alpert, L., Huynh, H. T., Carystinos,
G. D. and Alaoui-Jamali, M. A. (1999). Deficiency of connexin43 gap
junctions is an independent marker for breast tumors. Cancer Res. 59, 4104-
4110.
Lampe, P. D. (1994). Analyzing phorbol ester effects on gap junction
communication: A dramatic inhibition of assembly. J. Cell Biol. 127, 1895-
1905.
Lampe, P. D. and Lau, A. F. (2000). Regulation of gap junctions by
phosphorylation of connexins. Arch. Biochem. Biophys. 384, 205-215.
Lampe, P. D., Kurata, W. E., Warn-Cramer, B. and Lau, A. F. (1998a).
Formation of a distinct connexin43 phosphoisoform in mitotic cells is
dependent upon p34cdc2 kinase. J. Cell Sci. 111, 833-841.
Lampe, P. D., Nguyen, B. P., Gil, S., Usui, M., Olerud, J., Takada, Y.
and Carter, W. G. (1998b). Cellular interaction of integrin α3β1 with
laminin 5 promotes gap junctional communication. J. Cell Biol. 143, 1735-
1747.
Lampe, P. D., TenBroek, E. M., Burt, J. M., Kurata, W. E., Johnson, R.
G. and Lau, A. F. (2000). Phosphorylation of connexin43 on serine368 by
Journal of Cell Science 116 (11)
2211Phosphorylation at S368 of Cx43
protein kinase C regulates gap junctional communication. J. Cell Biol. 126,
1503-1512.
Moennikes, O., Buchmann, A., Ott, T., Willecke, K. and Schwarz, M.
(1999). The effect of connexin32 null mutation on hepatocarcinogenesis in
different mouse strains. Carcinogenesis 20, 1379-1382.
Moreno, A. P., Saez, J. C., Fishman, G. I. and Spray, D. C. (1994). Human
connexin43 gap junction channels. Regulation of unitary conductances by
phosphorylation. Circ. Res. 74, 1050-1057.
Munster, P. N. and Weingart, R. (1993). Effects of phorbol ester on gap
junctions of neonatal rat heart cells. Pflügers Arch. 423, 181-188.
Musil, L. S., Beyer, E. C. and Goodenough, D. A. (1990). Expression of the
gap junction protein connexin43 in embryonic chick lens: Molecular
cloning, ultrastructural localization, and post-translational phosphorylation.
J. Membr. Biol. 116, 163-175.
Paulson, A. F., Johnson, R. G. and Atkinson, M. M. (1994). Intercellular
communication is reduced by TPA and Ki-ras p21 in quiescent, but not
proliferating, NRK cells. Exp. Cell Res. 213, 64-70.
Reynhout, J. K., Lampe, P. D. and Johnson, R. G. (1992). An activator of
protein kinase C inhibits gap junction communication between cultured
bovine lens cells. Exp. Cell Res. 198, 337-342.
Rivedal, E. and Opsahl, H. (2001). Role of PKC and MAP kinase in EGF-
and TPA-induced connexin43 phosphorylation and inhibition of gap
junction intercellular communication in rat liver epithelial cells.
Carcinogenesis 22, 1543-1550.
Stein, L. S., Boonstra, J. and Burghardt, R. C. (1992). Reduced cell-cell
communication between mitotic and nonmitotic cells. Exp. Cell Res. 198,
1-7.
Stockert, R. J., Spray, D. C., Gao, Y., Suadicani, S. O., Ripley, C. R.,
Novikoff, P. M., Wolkoff, A. W. and Hertzberg, E. L. (1999). Deficient
assembly and function of gap junctions in Trf1, a trafficking mutant of the
human liver-derived cell line HuH-7. Hepatology 30, 740-747.
TenBroek, E. M., Lampe, P. D., Solan, J. L., Reynhout, J. K. and Johnson,
R. G. (2001). Ser364 of connexin43 and the upregulation of gap junction
assembly by cAMP. J. Cell Biol. 155, 1307-1318.
Trosko, J. E., Chang, C. C., Madhukar, B. V. and Klaunig, J. E. (1990).
Chemical, oncogene, and growth factor inhibition of gap junctional
intercellular communication: an integrative hypothesis of carcinogenesis.
Pathobiology 58, 265-278.
Vikhamar, G., Rivedal, E., Mollerup, S. and Sanner, T. (1998). Role of
Cx43 phosphorylation and MAP kinase activation in EGF induced
enhancement of cell communication in human kidney epithelial cells. Cell
Adhes. Commun. 5, 451-460.
Willecke, K., Eiberger, J., Degen, J., Eckardt, D., Romualdi, A.,
Guldenagel, M., Deutsch, U. and Sohl, G. (2002). Structural and
functional diversity of connexin genes in the mouse and human genome.
Biol. Chem. 383, 725-737.
Xie, H., Laird, D. W., Chang, T.-H. and Hu, V. W. (1997). A mitosis-specific
phosphorylation of the gap junction protein connexin43 in human vascular
cells: biochemical characterization and localization. J. Cell Biol. 137, 203-
210.
