(Received for publication, July 25, 1995; and in revised form, September 8, 1995)
From the
The Raf-1 serine/threonine kinase transmits proliferative and developmental signals to downstream effectors in the mitogen-activated protein kinase cascade. Studies to date have concentrated on its role in growth factor-mediated activation of the cell cycle. We show here that Raf-1 kinase activity is also highly stimulated during mitosis, with an attendant distinctive electrophoretic mobility shift due to hyperphosphorylation. These results suggest that Raf-1 may play a role in traversal of at least two distinct phases of the cell cycle.
The Raf-1 kinase is an integral part of a cascade that transmits
mitogenic and developmental signals from activated growth factor (GF) ()receptor tyrosine kinases to the mitogen-activated protein
kinases (MAPKs) known as extracellular signal-regulated protein kinases
(ERKs)(1, 2) . Activation of Raf-1 following GF
stimulation involves Raf-1/Ras interaction at the plasma membrane (2) and is associated with Raf-1 hyperphosphorylation and
retarded mobility in SDS-PAGE(3) . Activated Raf-1
phosphorylates and activates the MAPK or ERK kinases (MEKs), dual
specificity kinases that in turn phosphorylate and activate the
ERKs(1, 4) . The ERKs phosphorylate a number of
cytoplasmic and nuclear targets, resulting in changes in transcription
and translation(4) .
Studies on proto-oncoproteins have
focused on their roles in the G/G
transition or
in traversal of G
. A number of proto-oncoproteins, notably
c-Src, c-Abl, and c-Mos, have previously been shown to undergo
hyperphosphorylation and modulation of activity in mitosis and/or
M-phase(5, 6, 7) . In this study we present
evidence that Raf-1 becomes activated and hyperphosphorylated in
mitosis.
Figure 2:
Characterization and duration of Raf-1
mitotic shift. A, comparison of the mobility retardations of
Raf-1 isolated from PDGF-stimulated and mitotic cells. Lysates were
prepared from NIH(pLJ) and NIH(pLJ/Raf-1) cells that had been
serum-starved for 24 h(-), serum-starved followed by 5 min, 20
ng/ml PDGF treatment (+), that had been arrested in mitosis by
nocodazole-treatment (M), or that were unsynchronized (U). Samples containing Raf-1 immunoprecipitates from 250
µg of total cell protein were separated by 8% SDS-PAGE, Western
blotted, and probed with Raf-1 antibody (upper panel). A
weaker exposure of lanes 5-8 is included (lower
panel). The apparent reduction of the Raf-1 signal in
PDGF-stimulated and mitotic NIH(pLJ) cells (lanes 2 and 3) and in PDGF-stimulated NIH (pLJ/Raf-1) cells (lanes
6) was not reproducible. B, Raf-1 is phosphorylated
predominantly on serine residues. Mitotic (M) NIH(pLJ/Raf-1)
cells were metabolically labeled in reduced phosphate medium by
arresting with nocodazole and then adding 2.0 mCi of PO
and incubating (in the continued presence
of nocodazole) for 2 more h. Unsynchronized (U) cells were
similarly labeled except without nocodazole. Raf-1 was
immunoprecipitated and Western blotted as in panel A.
Following autoradiography
P-labeled Raf-1-containing bands
were excised and subjected to phosphoamino acid analysis by partial
acid hydrolysis, thin layer chromatography, and autoradiography. The
positions of nonradioactive phosphoserine (S),
phosphothreonine (T), and phosphotyrosine (Y)
standards are indicated. C, time course showing increased
Raf-1 electrophoretic mobility following release from mitotic arrest.
Nocodazole-arrested mitotic NIH (pLJ/Raf-1) cells were replated in
drug-free medium and incubated for the indicated times (lanes
2-7). Lysates were prepared, and 30-µg fractions were
analyzed by 8% SDS-PAGE, Western blotted, and probed with a polyclonal
Raf-1 antibody. Unsynchronized (U) cell lysates (lanes 1 and 8) were used as controls. The two-three
distinct Raf-1 bands that are evident in C are preferentially
recognized by the polyclonal antibody used in this experiment. A
monoclonal antibody that recognizes an unresolved collection of
multiple bands was used in all other displayed experiments. Similar
kinetics were observed for the postmitotic disappearance of the Raf-1
electrophoretic mobility shift with both
antibodies.
We have observed that Raf-1 immunoprecipitated from nocodazole-arrested mitotic NIH 3T3 cells migrates more slowly (74-78 kDa) in SDS-PAGE than Raf-1 from unsynchronized growing cells (72-74 kDa; Fig. 1, lanes 1 and 2). Following potato-acid phosphatase treatment this mobility retardation was virtually eliminated, demonstrating that it stems from hyperphosphorylation (lanes 3 and 4). Pretreatment of potato-acid phosphatase with a mixture of phosphatase inhibitors (sodium fluoride and sodium vanadate) inhibited this effect (data not shown). To further study alterations of Raf-1 in mitosis we used the retroviral expression vector pLJ/Raf-1 to generate NIH 3T3 cell lines (designated NIH(pLJ/Raf-1)) that stably overexpress human Raf-1. NIH(pLJ) cell lines that were infected with the empty vector pLJ were used as controls. Direct comparisons with both the endogenous and overexpressed Raf-1 showed that the mitotic reduction in electrophoretic mobility was similar to that observed following 5 min of stimulation with platelet-derived growth factor (PDGF) (Fig. 2A, cf. lanes 3 and 2 and lanes 7 and 6). Multiple mobility-retarded Raf-1 bands were observed in both PDGF-treated and mitotic cells, but more highly retarded bands were more abundant in mitotic Raf-1. Raf-1 retardation increased following longer treatment with PDGF but never to the extent observed with Raf-1 from mitotic cells (data not shown).
Figure 1: The electrophoretic mobility of Raf-1 is retarded in mitotic NIH 3T3 cells due to hyperphosphorylation. Lysates were harvested from exponentially growing unsynchronized (U) and nocodazole-arrested (M) cells. Nocodazole-arrested cells were prepared as described previously(13) . Immunoprecipitated Raf-1 was incubated without(-) or with (+) potato-acid phosphatase (PAP) as described previously(35) , subjected to 8% SDS-PAGE, Western blotted, and probed with Raf-1 antibody. Bands were visualized by chemiluminescence. The broad Raf-1 band actually consists of at least six electrophoretically distinct species. The positions of molecular weight standards (in kDa) are shown.
NIH(pLJ/Raf-1) mitotic and unsynchronized cell cultures were
metabolically labeled with inorganic PO
, and
immunoprecipitated Raf-1 was analyzed by phosphoamino acid analysis (Fig. 2B). Raf-1 isolated from both mitotic and
unsynchronized cells was radiolabeled predominantly on serine residues,
but low levels of threonine phosphorylation were also detected. This is
similar to results observed following PDGF stimulation of NIH 3T3 cells (12) .
To determine the duration of Raf-1
hyperphosphorylation in mitosis, NIH(pLJ/Raf-1) cells were arrested in
mitosis using nocodazole, replated in drug-free medum, and harvested at
different intervals (Fig. 2C). The mitotic mobility
retardation of Raf-1 was partially lost by 1 h after replating (lane 4) and was negligible by 2 h (lane 5). Since
the cells undergo cytokinesis 30-45 min after release from
nocodazole arrest(13) , it appears that Raf-1
hyperphosphorylation extends into the early G phase of the
cell cycle. This contrasts with the case of c-Src, where Cdc2-catalyzed
hyperphosphorylations disappear within 30 min after replating (data not
shown and (13) ). Similar results for Raf-1 were obtained using
HeLa cells. (
)
Two different assays were used to determine
if Raf-1 kinase activity is stimulated during mitosis: 1) an autokinase
assay and 2) phosphorylation of a relevant physiological substrate, i.e. purified recombinant MEK-1. Autophosphorylation was
assayed using Raf-1 immunoprecipitated from quiescent (serum-starved),
PDGF-stimulated, mitotic, and unsynchronized NIH(pLJ/Raf-1) cells (Fig. 3A). As anticipated(3) , Raf-1
immunoprecipitated from quiescent (lane 1) or unsynchronized (lane 4) cultures exhibited negligible autokinase activity.
Importantly, autokinase activity was stimulated in mitotic cells to a
level comparable with that observed following PDGF treatment (cf. lanes 3 and 2). Phosphoamino acid
analysis revealed that both PDGF-stimulated and mitotic Raf-1 were
autophosphorylated on both serine and threonine (Fig. 3B). The MEK kinase activity of Raf-1 was also
stimulated in mitotic cells (Fig. 3C). The activity of
Raf-1 from mitotic cells was 10 /
2 (n =
4) times higher than the activity of Raf-1 from unsynchronized cells.
The level of mitotic activity was very similar to that observed
following PDGF stimulation (data not shown). Definitive investigation
of the role of the mitotic hyperphosphorylation of Raf-1 in its
activation must await mapping and mutation of the mitotic
phosphorylation sites.
Figure 3:
Raf-1 kinase activity is stimulated during
mitosis. A, autokinase assay. Lysates were prepared from
serum-starved(-), GF-stimulated (+), mitotic (M),
and unsynchronized (U) cultures of NIH (pLJ/Raf-1) cells.
Immune complex kinase assays were performed using anti-Raf-1
immunoprecipitates from 200 µg of total cell protein and 20 µCi
of [-
P] ATP. Reaction products were
analyzed by 9% SDS-PAGE and autoradiography (upper panel).
Aliquots of the immunoprecipitates were subjected to Western blot
analysis to confirm that Raf-1 loading was comparable for each sample (lower panel). For control immunoprecipitations the anti-Raf-1
primary antibody was omitted (lanes 5-8). B,
autophosphorylated PDGF-stimulated (+) and mitotic (M)
Raf-1 bands were excised from kinase gels and subjected to phosphoamino
acid analysis as described for Fig. 2B. S,
phosphoserine; T, phosphothreonine; Y,
phosphotyrosine. C, MEK kinase assay. Similar to A except that kinase reactions (upper panel) were performed
in the presence (lanes 3-6) or absence (lanes 1 and 2) of 500 ng of histidine-tagged kinase-defective (KD) MEK-1. An additional control reaction, containing
kinase-defective MEK-1 alone, was included (lane 7). D, Raf-1 activation in shake-off mitotics. Lysates were
prepared from mitotic cells collected by shake-off (S) without
nocodazole treatment as described previously(13) . Lysates were
also prepared from the adherent cells (A) that remained on the
same plates. Raf-1 MEK kinase activity (upper panel) was
assayed for these cells as well as for nocodazole-arrested mitotic (M) and unsynchronized (U) cells as described in C.
To ensure that Raf-1 hyperphosphorylation and activation were not drug-induced artifacts, we performed similar experiments using mitotic cells collected by mechanical shake-off alone (Fig. 3D). Western blots of immunoprecipitates revealed that Raf-1 mobility was retarded, although to a lesser extent than that of Raf-1 isolated from nocodazole-treated cells (Fig. 3D, lower panel). Similarly, Raf-1 kinase activity was stimulated about 3-fold in mitotic cells isolated by shake-off alone (Fig. 3D, upper panel, lane 6) compared with Raf-1 in immunoprecipitates from the adherent monolayer remaining following shake-off (lane 5) or from unsynchronized cells (lane 7). This stimulation was somewhat less than that observed in nocodazole-treated cultures (lane 8), but for reasons discussed elsewhere (13) we believe that this merely reflects heterogeneity of the shake-off population and conclude that the observed mitotic hyperphosphorylation and stimulation of Raf-1 kinase activity are authentic.
We cannot exclude the possibility that the observed enhancement of Raf-1 kinase activity comes from the mitosis-specific association or activation of another serine/threonine kinase that coprecipitates with Raf-1. We believe this is unlikely since 1) MEK-1 appears to be a target for a very restricted set of kinases (1) and 2) the lysis buffer contains 0.1% SDS and 0.5% sodium deoxycholate, which should disassociate any such activity from Raf-1 prior to Raf-1 immunoprecipitation(8) . In any case, if such an association/activation occurs, it is specific to mitosis and would therefore itself be of great interest.
Raf-1 activity is already
known to participate in the G/G
transition. The
data presented here suggest that it may also function in the transition
into or passage through mitosis. A reduction in the electrophoretic
mobility of Raf-1 in mitotic murine fibroblasts has been previously
reported, but no changes in Raf-1 activity were described(14) .
In future experiments it will be important to determine 1) if Raf-1 is
first activated before or during mitosis, 2) if the mechanism of its
mitotic activation is related to the mechanism employed following GF
stimulation (i.e. via Ras(2) ), and 3) its mitotic
targets. Interestingly, there is already evidence indicating that a
Raf-1-mediated pathway(s) participates in entry into meiosis. For
example, activated GF receptor tyrosine kinases stimulate Xenopus meiotic maturation in a Raf-1-dependent manner(15) , and
progesterone-initiated maturation is partially
Raf-1-dependent(15, 16) . Furthermore, ERK1 (which
lies downstream from Raf-1) regulates a meiotic checkpoint in Xenopus(17) . There are also hints that Raf-1-mediated
pathways may regulate mitosis. For example, Ras can regulate mitosis in Xenopus embryos and mammalian cells (18, 19) and MAPKs can regulate early embryonic
cleavage in Xenopus(17, 20) . In addition,
14-3-3 proteins, which may stabilize the Raf-1-activated
state(21) , influence the timing of mitotic entry in fission
yeast (22) .
MEK-1 is the most obvious potential target for
Raf-1 during mitosis. Raf-1-catalyzed phosphorylation and activation of
MEK-1 could explain the activation of MAPKs that occurs just before or
during mitosis in mammalian cells(23, 24) . However,
Raf-1 might phosphorylate and modulate other targets such as IkB or
p53(25, 26) . The latter possibility is of particular
interest because p53 may help regulate the G/M
transition(27, 28) . The recent finding that Raf-1
phosphorylates and activates Cdc25A, an activator of cyclin-dependent
kinases, during cell cycle activation further indicates that it may
play a significant role in cell cycle regulation(29) .
We do not yet know if the mitotic hyperphosphorylation of Raf-1 is a cause or effect of its activation. It will be important to test the possibility that Src (or other tyrosine kinases) plays a role in the mitotic activation of Raf-1. Microinjection of anti-Ras inhibitory antibodies reverts the transformed phenotypes of v-Src-transformed cells, demonstrating that v-Src acts upstream of the Ras/Raf-1 pathway (30) . Moreover, oncogenic Src can activate Raf-1 in insect and mammalian cells(9, 31, 32) . We did not detect increased tyrosine phosphorylation of Raf-1 during mitosis (Fig. 2B). However, it should be noted that enhanced tyrosine phosphorylation is confined to a small fraction of Raf-1 molecules following activation by coexpression with oncogenic Ras and Src in mammalian cells, even though tyrosine phosphorylation has been implicated in Raf-1 activation in this situation(32) . In vivo mitotic hyperphosphorylation was observed overwhelmingly at serine (Fig. 2B) while Raf-1 autophosphorylates in vitro at both serine and threonine (Fig. 3B and (31) and (33) ). MAPKs are probably responsible for at least some of the Raf-1 hyperphosphorylation seen in GF-stimulated cells (34) and might therefore also contribute to Raf-1 mitotic hyperphosphorylation in a feedback loop.
The results presented here indicate that Raf-1 must be added to the list of proto-oncoproteins whose activities become modulated in mitosis and suggest that deregulated Raf-1 activity could perturb control mechanisms at multiple points in the cell cycle.