Loss of skeletal muscle in cancer cachexia results in asthenia, immobility and eventually
death through impairment of respiratory function. Nutritional supplementation alone
is unable to reverse this wasting process (Evans et al, 1985), suggesting that the
balance between protein synthesis and degradation is impaired, especially since visceral
protein reserves are preserved and may even increase (Fearon, 1992). Thus, while protein
synthesis in skeletal muscle of cachectic patients is impaired (Lundholm et al, 1976),
there is also an increase in protein degradation (Lundholm et al, 1982). An increased
activity of the ubiquitin–proteasome proteolytic pathway is considered to be the major
factor for this increased protein degradation (Williams et al, 1999).
Tumour production of a sulphated glycoprotein called proteolysis-inducing factor (PIF)
may be responsible for the progressive loss of skeletal muscle in cancer cachexia
(Todorov et al, 1996). Proteolysis-inducing factor is only produced by cachexia-inducing
tumours, and when purified and administered to mice it induces a specific loss of
skeletal muscle, while visceral protein is maintained or even increased, as in cancer
cachexia (Lorite et al, 1998). Using a surrogate model of skeletal muscle, PIF was
shown to inhibit protein synthesis and increase protein degradation (Smith et al,
1999). The increased protein degradation was shown to arise from an increased expression
of the ubiquitin–proteasome proteolytic pathway (Lorite et al, 2001).
Protein degradation induced by PIF was accompanied by an increased release of arachidonic
acid from membrane phospholipids and its subsequent metabolism to 15-hydroxyeicosatetraenoic
acid (15-HETE), which was shown to be related to protein catabolism. Both arachidonic
acid and lipoxygenase metabolites have been shown to activate protein kinase C (PKC)
(Fan et al, 1990), which may play a role in PIF-induced proteasome expression. Phosphorylation
of proteasome subunits may be important in the regulation of proteasome activity,
since some proteasome subunits have potential tyrosine (Tanaka et al, 1990) and serine/threonine
(Heinemeyer et al, 1994) phosphorylation sites and dephosphorylation by acid phosphatase
have been shown to lower proteasome activity significantly (Mason et al, 1996). At
least one subunit (S4) has several potential PKC phosphorylation sites (Dubiel et
al, 1992). Alternatively, PKC may play a role in activation of the transcription factor
nuclear factor-κB (NF-κB), which has been shown to be involved in the production of
interleukin-8 (IL-8), IL-6, C-reactive protein and ICAM-1 in liver cells (Watchorn
et al, 2001), and may also be involved in PIF-induced proteasome expression (Whitehouse
and Tisdale, 2003). Protein kinase C has been suggested as being an upstream activator
of the I-κB kinase complex (IKK) (Lallena et al, 1999; Trushin et al, 1999; Vertegaal
et al, 2000) leading to I-κBα phosphorylation and ubiquitination and the subsequent
processing of the 26S proteasome followed by the translocation of NF-κB into the nucleus.
The present study investigates the role of PKC in PIF-induced proteasome expression
and its relationship to activation of NF-κB in C2C12 murine myotubes.
MATERIALS AND METHODS
Materials
Foetal calf serum (FCS), horse serum (HS) and Dulbecco's modified Eagle's medium (DMEM)
were purchased from Invitrogen (Paisley, Scotland). Mouse monoclonal antibodies to
proteasome 20S α-subunits were from Affiniti Research Products (Exeter, UK). Rabbit
polyclonal antisera to ubiquitin conjugating enzyme (E214k) were a gift from Dr Simon
Wing, McGill University (Montreal, Canada). Rabbit polyclonal antisera to murine I-κBα,
β-tubulin and PKC
α
were from Calbiochem (Herts, UK) as were PMA, Ro31-8220, calphostin C, staurosporine
and Gö 6976. Rabbit polyclonal antisera to mouse actin were from Sigma-Aldridge (Dorset,
UK). Peroxidase-conjugated goat antirabbit and rabbit antimouse secondary antibodies
were from Dako Ltd (Cambridge, UK). Hybond™ nitrocellulose membranes and enhanced
chemiluminescence (ECL) were from Amersham Life Science Products (Bucks, UK). Electrophoretic-mobility
shift (EMSA) gel shift assay kits were from Panomics (California, USA). Escherichia
coli DH5α cells were from Invitrogen (Paisley, Scotland). Constitutively active and
mutant plasmids of PKC
α
were a gift from Prof. Peter Parker (Cancer Research, UK). The insert A25E PKC
α
is constitutively active due to a deletion of amino acids 22–28 in the N-terminal
region and is expressed via the pCO2 vector (Pears et al, 1990). Protein kinase C
α (T/A)3 is a dominant-negative mutant expressed in pKS1 (Bornancin and Parker, 1996).
Plasmid DNA was purified using the WIZARD® PureFection purification system (Promega,
Southampton, UK) according to the manufacturer's protocol. Primers for PCR analysis
were from MWG Biotech (Ebersberg, Germany). GeneJuice™ for transfection studies was
purchased from Calbiochem (Herts, UK).
Transformation of bacteria
E. coli DH5
α
were transformed with both constitutively active and mutant PKC
α
using heat shock, and selected with ampicillin (100 μg ml−1). Positive clones were
identified using primers with homology to bovine PKC (forward 5′-CAC CTG TGA TAT GAA
CGT GC-3′ reverse 5′-GAA GTT GAA GTC CGT GAG C-3′). The product was about 600 bp as
determined on a 2% agarose gel. Plasmid DNA was extracted from positive colonies grown
overnight in an LB medium containing ampicillin (100 μg ml−1).
Purification of PIF
PIF was purified from solid MAC16 tumours excised from mice with a weight loss between
20 and 25% as previously described (Todorov et al, 1996; Whitehouse and Tisdale, 2003).
Tumours were homogenised in 10 mM Tris-HCl, pH 8.0, containing 0.5 mM phenylmethylsulphonyl
fluoride, 0.5 mM EGTA and 1 mM dithiothreitol at a concentration of 5 ml g−1 tumour.
The supernatant obtained after addition of ammonium sulphate (40% w v−1) was subjected
to affinity chromatography using anti-PIF monoclonal antibody coupled to a solid matrix.
The immunogenic fractions were concentrated and used for further studies.
Myogenic cell culture and transfection
The C2C12 myoblast cell line was grown in DMEM supplemented with 10% FCS plus 1% penicillin
and streptomycin under an atmosphere of 10% CO2 in air. Transfection was carried out
on cells at 50% confluency using GeneJuice™ transfection reagent, according to the
manufacturer's protocol and selected by resistance to ampicillin (5 g l−1). Transfected
myoblasts were stimulated to differentiate by replacing the growth medium with DMEM
supplemented with 2% HS, when the cells reached confluence. Differentiation was allowed
to continue for 5–9 days until myotubes were clearly visible, and used for the experiments
described in results.
Measurement of proteasome ‘chymotrypsin-like activity
‘Chymotrypsin-like’ enzyme activity was determined fluorimetrically by the method
of Orino et al (1991) as previously described (Lorite et al, 2001). Myotubes were
washed with ice-cold phosphate-buffered saline (PBS) and sonicated in 20 mM Tris-HCl,
pH 7.5, 2 mM ATP, 5 mM MgCl2 and 1 mM dithiothreitol at 4°C. The supernatant formed
by centrifugation at 18 000 g for 10 min was used to measure the ‘chymotrypsin-like’
enzyme activity by the release of aminomethyl coumarin (AMC) from the fluorogenic
peptide succinyl-LLVY-AMC (0.1 mM). Activity was measured in the presence and absence
of the specific proteasome inhibitor lactacystin (10 μ
M). Only lactacystin-suppressible activity was considered to be proteasome specific.
Western blot analysis
Cytoplasmic proteins, obtained from the above assay, were also used for Western blotting,
while the pellet was dissolved in sonicating buffer containing 0.1% Nonidet P40 and
used as a source of cell membranes. Both extracts were loaded at 2–5 μg protein and
resolved on 10% sodium dodecylsulphate: polyacrylamide gels and transferred to Hybond™
nitrocellulose membrane. Membranes were blocked with 5% Marvel in PBS. The primary
antibodies for PKC
α
, E214k and β-tubulin were used at a dilution of 1 : 100, while antibodies for I-κBα
were at 1 : 1000 and 20S proteasome α-subunits at 1 : 1500. The secondary antibodies
were used at a dilution of 1 : 2000. Incubation was carried out for 2 h at room temperature,
and development was by ECL.
Electrophoresis mobility shift assay
DNA-binding proteins were extracted from myotubes by the method of Andrews and Faller
(1991), which utilises hypotonic lysis followed by high salt extraction of nuclei.
The EMSA-binding assay was carried out using a Panomics EMSA ‘gel shift’ kit according
to the manufacturer's instructions.
Statistical analysis
Differences as means between groups was determined by one-way ANOVA followed by Tukey–Kramer
multiple comparison test.
RESULTS
To evaluate the role of PKC in PIF-induced proteasome expression, the effect of excess
4α-phorbol 12-myristate 13-acetate (PMA) on ‘chymotrypsin-like’ enzyme activity, the
predominant proteolytic activity of the proteasome (Figure 1A
Figure 1
(A) Effect of PMA on PIF-induced chymotrypsin-like enzyme activity in murine myotubes.
Cells were incubated either with PIF alone (×) or in the presence of PMA (100 nM),
added 2 h prior to PIF (▪), and the chymotrypsin-like enzyme activity was determined
after 24 h, as described in Materials and methods. The experiment was repeated three
times (n=9). Differences from control are indicated as a
P<0.001, while differences from cells incubated in the presence of PIF alone are shown
as b
P<0.05 and c
P<0.001. (B) Western blot of the effect of PMA on proteasome 20S α-subunits and (C)
E214k 24 h after addition of PIF. Cells were incubated with 0 (lanes 1 and 7), 1.0
(lanes 2 and 8), 2.1 (lanes 3 and 9), 4.2 (lanes 4 and 10), 10 (lanes 5 and 11) or
20 nM PIF (lanes 6 and 12) in the absence (lanes 1–6) or presence (lanes 7–12) of
PMA (100 nM). A representative blot is shown and the experiment was repeated three
times.
), and on expression of proteasome 20S
α
subunits (Figure 1B) and the ubiquitin-conjugating enzyme (E214k) (Figure 1C) was
determined in C2C12 myotubes 24 h after PIF addition. Proteolysis-inducing factor
produced an increase in ‘chymotrypsin-like’ enzyme activity, proteasome 20S
α
subunits and E214k with a maximal effect between 2.1 and 10 nM, and this effect was
completely attenuated in myotubes pretreated with PMA. These results suggest that
PKC may be important in PIF-induced proteasome expression.
To confirm a role for PKC in this process, the effect of Ro31-8220, a competitive
and selective PKC inhibitor (Beltman et al, 1996), staurosporine, a broad-spectrum
inhibitor of protein kinases (Couldwell et al, 1994), calphostin C, a highly specific
inhibitor of PKC (Jarvis et al, 1994), and Gö 6976, which selectively inhibits PKC
α
and β, isoenzymes (Wang et al, 1998), on the PIF-induced increase in ‘chymotrypsin-like’
enzyme activity was determined (Figure 2
Figure 2
Effect of inhibitors of PKC on PIF-induced chymotrypsin-like enzyme activity. Myotubes
were incubated with PIF alone (×) or with Ro 31-8220 (1 μ
M) (A); staurosporine (300 nM) (B); calphostin C (300 nM) (C); or with Go 6976 (200 μ
M) (D) added 2 h prior to PIF, and chymotrypsin-like enzyme activity was determined
24 h after addition of PIF. The experiment was repeated three times (n=9). Differences
from control are indicated as a
P<0.05 and b
P<0.001, while differences from cells incubated in the presence of PIF alone are shown
as c
P<0.05, d
P<0.01 and e
P<0.001.
). The PIF-induced enzyme activity was completely attenuated by 10 μ
M Ro31-8220 (Figure 2A), 300 nM staurosporine (Figure 2B), 300 nM calphostin C (Figure
2C) and 200 μ
M Gö 6976 (Figure 2D). In addition, calphostin C completely attenuated the PIF-induced
increase in proteasome 20S α-subunit expression (Figure 3A
Figure 3
Western blot of the effect of calphostin C on 20S proteasome α-subunit expression
(A) and E214k (B) in the presence of PIF. Cells were incubated with 0 (lanes 1 and
6), 2.1 (lanes 2 and 7), 4.2 (lanes 3 and 8), 10 (lanes 4 and 9) or 16.8 nM PIF (lanes
5 and 10) either alone (lanes 1–5) or in the presence of calphostin C (300 nM) and
expression was determined after 24 h. A representative blot is shown and the densitometric
analysis is based on three replicate blots. Values in the presence of PIF are indicated
as ▪ and in the presence of PIF and calphostin C as □. The densitometric analysis
of the 20S subunits represents the average of the two major bands. Differences from
control are indicated as a
P<0.05, b
P<0.01 and c
P<0.001, while differences from the presence of PIF alone are shown as d
P<0.05, e
P<0.01 and f
P<0.001.
) and E214k (Figure 3B). Proteolysis-inducing factor induced a decrease in cytosolic
PKC (Figure 4A
Figure 4
Effect of PIF on activation of PKC
α
in murine myotubes in the absence or presence of calphostin C (A, B) or EPA (C). (A)
Cytoplasmic and (B) Membrane-bound PKC
α
after incubation with 0 (lanes 1 and 6), 2.1 (lanes 2 and 7), 4.2 (lanes 3 and 8),
10 (lanes 4 and 9) or 16.8 nM PIF (lanes 5 and 10) for 24 h in the absence (lanes
1–5) or presence (lanes 6–10) of calphostin C (300 nM). The densitometric analysis
was based on three replicate blots, and values in the presence of PIF are shown as
▪ and in the presence of PIF and calphostin C as □. Differences from control are shown
as c
P<0.001, while differences from PIF alone are shown as d
P<0.05, e
P<0.01 and f
P<0.001. (C) Actin loading control for the blots shown in (A, B). (C) Actin loading
control for the blots shown in (A, B). (D) Effect of EPA (50 μ
M) on membrane-bound PKC
α
in the presence of PIF. Cells were loaded with 0 (lanes 1 and 7), 1.0 (lanes 2 and
8), 2.1 (lanes 3 and 9), 4.2 (lanes 4 and 10), 10 (lanes 5 and 11) or 20 nM PIF (lanes
6 and 12) either in the absence (lanes 1–6) or after 2 h pretreatment with EPA (50 μ
M), and membrane-bound PKC
α
was determined after 24 h. (E) β-tubulin loading control for the blot shown in (D).
) and an increase in membrane-bound PKC
α
(Figure 4B) at the same concentrations as those inducing proteasome expression (Figure
1) and this effect was attenuated by both calphostin C (Figure 4A and B) and eicosapentaenoic
acid (EPA) (Figure 4D). These results confirm a role for PKC in PIF-induced proteasome
expression, and suggest another mechanism by which EPA may attenuate PIF-induced protein
degradation through inhibition of PKC.
To further substantiate a role for PKC in the induction of proteasome expression by
PIF C2C12, myoblasts were transfected with plasmids encoding constitutively active
PKC-α (pCO2) and dominant-negative PKC-α (T/A)3 (pKS1) (Bornancin and Parker, 1996;
Schonwasser et al, 1998), and induced to differentiate into myotubes. Myotubes transfected
with pCO2 showed an increased sensitivity to PIF, as determined by the ‘chymotrypsin-like’
enzyme activity (Figure 5
Figure 5
Time course for induction of chymotrypsin-like enzyme activity in wild-type myotubes
(×) and in those transfected with constitutively active (pCO2 □) and mutant (pKS1
▪) PKC
α
. Myotubes were treated with the indicated concentrations of PIF, and enzyme activity
was determined at 3 h (A), 6 h (B), 24 h (C) and 48 h (D). The experiment was repeated
three times (n=9). Differences from control or pKS1 are indicated as a
P<0.05, b
P<0.01 and c
P<0.005, while differences from wild-type myotubes are indicated as d
P<0.005.
) in comparison with wild-type myotubes, with a significant increase within 3 h of
PIF addition (Figure 5A) persisting up to 48 h (Figure 5D). In addition, the elevation
of ‘chymotrypsin-like’ enzyme activity in myotubes transfected with pCO2 greatly exceeded
that in wild type at all time points. In contrast, myotubes transfected with the dominant-negative
PKC
α
, pKS1 showed no elevation in ‘chymotrypsin-like’ enzyme activity in response to PIF
at any time point (Figure 5). These results were confirmed by Western blotting of
cellular supernatants for 20S proteasome α-subunit expression (Figure 6A
Figure 6
Western blot of the effect of PIF on 20S proteasome α-subunit expression (A) and E214k
(B) in pKS1 (lanes 1–6) and pCO2 (lanes 7–12) after treatment with 0 (lanes 1 and
7), 1.0 (lanes 2 and 8), 2.1 (lanes 3 and 9), 4.2 (lanes 4 and 10), 10 (lanes 5 and
11) and 20 nM PIF (lanes 6 and 12) determined after 24 h. (C) β-tubulin loading control.
) and E214k (Figure 6B). Proteolysis-inducing factor induced an increase in both proteasome
α-subunit expression and E214k in pCO2 but not pKS1, confirming a role for PKC in
this process. The ability of PIF to activate PKC
α
in pCO2, but not in pKS1, was confirmed by Western blotting (Figure 7
Figure 7
Western blot of the effect of PIF on cytoplasmic (A) and membrane-bound (B) PKC
α
in pKS1 and pCO2 after 24 h. The lanes are the same as in Figure 6.
). The concentrations of PIF causing maximum activation of PKC
α
were the same as those inducing 20S proteasome α-subunit expression (Figure 6A).
We have recently shown (Whitehouse and Tisdale, 2003) that PIF-induced proteasome
expression appears to require activation of NF-κB. One mechanism by which PKC may
function in the PIF signalling pathway is activation of IKK with subsequent phosphorylation
and degradation of I-κB, and translocation of NF-κB from the cytosol to the nucleus
(Vertegaal et al, 2000). Evidence for this hypothesis is provided by the following
experiments. Proteolysis-inducing factor induced a decrease in cytoplasmic I-κBα within
30 min of addition to wild-type cells (Figure 8A
Figure 8
Effect of PIF and calphostin C on cytoplasmic I-κBα (A) and nuclear-bound NF-κB (C)
determined 30 min after PIF addition to murine myotubes. (A) Myotubes were treated
with 0 (lanes 1 and 6), 2.1 (lanes 2 and 7), 4.2 (lanes 3 and 8), 10 (lanes 4 and
9) or 16.8 nM PIF (lanes 5 and 10) in the absence (lanes 1–5) or presence (lanes 6–10)
of calphostin C (300 nM) added 2 h prior to PIF. (B) Actin loading control for the
blot shown in (A). (C) Nuclear levels of NF-κB in myotubes in the absence (lanes 2–6)
or presence (lanes 7–11) of calphostin C. Lane 1 is a negative control and lane 13
a positive control and lane 12 contains excess unlabelled NF-κB. The other lanes were
the same as in (A). The densitometric analysis is an average of three replicate EMSAs.
Differences from control are indicated as a
P<0.05 and c
P<0.001, while differences in the presence of calphostin C are indicated as d
P<0.01 and e
P<0.001.
), accompanied by nuclear accumulation of NF-κB (Figure 8C) and this effect was completely
attenuated by calphostin C (Figure 8). In addition, PIF induced a decrease in I-κBα
(Figure 9A
Figure 9
Effect of PIF on cytoplasmic I-κBα (A) and nuclear-bound NF-κB (C) determined 30 min
after PIF addition to murine myotubes transfected with constitutively active (pCO2
□) and mutant (pKS1 ▪) PKC
α
. (A) Myotubes transfected with either pCO2 (lanes 1–6) or pKS1 (lanes 7–12) were
treated with 0 (lanes 1 and 7), 1.05 (lanes 2 and 8), 2.1 (lanes 3 and 9), 4.2 (lanes
4 and 10), 10 (lanes 5 and 11) or 16.8 nM PIF (lanes 6 and 12). The densitometric
analysis is an average of three replicate blots. Differences from 0 nM PIF are shown
as c
P<0.001. (B) Actin loading control for the blot shown in (A). (C) EMSA of NF-κB nuclear
binding in murine myotubes transfected with pCO2 (lanes 2 and 6) and pK1 (lanes 7–11).
Myotubes were treated with 0 (lanes 2 and 7), 2.1 (lanes 3 and 8), 4.2 (lanes 4 and
9), 10 (lanes 5 and 10) and 16.8 nM PIF (lanes 6 and 11). Lane 1 is a negative control
containing the labelled probe without a nuclear extract; lane 12 contains a 100-fold
excess of unlabelled NF-κB probe and lane 13 is a positive control for NF-κB (supplied
by the manufacturers of the kit). The densitometric analysis is an average of three
replicate EMSAs. Differences from control are indicated as b
P<0.01 and c
P<0.001.
) and an increase in DNA binding of NF-κB (Figure 9C) in myotubes transfected with
constitutively active PKC
α
(pCO2), but not in those containing dominant-negative PKC-α (pKS1). These results
suggest that activation of PKC by PIF in muscle cells leads to I-κBα degradation,
nuclear accumulation of NF-κB and an increased proteasome expression leading to increased
intracellular protein degradation (Whitehouse and Tisdale, 2003).
DISCUSSION
Although increased intracellular protein catabolism is a common feature of many disease
states, there is little knowledge of the cellular signalling pathways involved, which
may be useful in therapeutic intervention. Initial studies suggested that prostaglandin
E2 (PGE2) was involved in total protein degradation in skeletal muscle, based on the
demonstration in a variety of muscle types that tyrosine release was stimulated by
arachidonic acid and PGE2 (Rodemann and Goldberg, 1982) and blocked by prostaglandin
synthesis inhibitors (Strelkov et al, 1989). However, other studies (Hasselgren et
al, 1990) found no evidence that total or myofibrillar protein breakdown in normal
or septic muscle is regulated by PGE2. Studies with PIF showed that total protein
breakdown was related to the release of arachidonic acid and formation of PGE2, but
that PGE2 was not the eicosanoid responsible for the effect (Smith et al, 1999). Although
the arachidonic acid was converted into a range of PGs and HETEs, only one metabolite
15-HETE alone was capable of inducing protein degradation. Further studies showed
that 15-HETE induced an increase in expression of the ubiquitin–proteasome pathway,
which was responsible for the initiation of protein catabolism and that this process
involved the transcription factor NF-κB (Whitehouse et al, 2003).
The present study has investigated the possibility that PKC may act as an intermediate
in the PIF signalling pathway transmitting the rise in 15-HETE into activation of
NF-κB. The results support the suggestion that PKC plays a central role in the induction
of proteasome expression by PIF and thus protein degradation. Previous studies (Smith
and Tisdale, 2003) have shown that PIF induces activation of phospholipase C (PLC)
as an important signalling event in inducing proteasome expression. Activation of
PLC would result in the generation of diacylglycerol (DAG), which would then induce
translocation of PKC from the cytosol to the membrane, resulting in the complete activation
of the kinase. Indeed, PIF has been shown to induce translocation of PKC
α
from the cytosol to the membrane at the same concentrations as those inducing proteasome
expression. The importance of this step to the induction of proteasome expression
by PIF is shown by the attenuation of this process by a range of inhibitors of PKC.
In addition, myotubes transfected with a dominant-negative mutant of PKC
α
also showed no induction of proteasome expression in the presence of PIF. Interestingly,
myotubes transfected with constitutively active PKC
α
showed an increased induction of proteasome expression compared with their wild-type
counterparts, confirming the importance of this pathway in the signalling cascade.
At present, it is not known which particular isoenzymes of PKC are involved in this
process, or indeed whether activation of PKC occurs through production of DAG via
PLC or directly through production of 15-HETE.
Attenuation of PIF-induced activation of PKC provides another control point where
EPA may interfere with the signalling cascade leading to increased proteasome expression.
Eicosapentaenoic acid is an effective anticachectic agent both in murine models of
cachexia (Beck et al, 1991) and in weight-losing patients with pancreatic cancer (Barber
et al, 1999), and effectively attenuates PIF-induced proteasome expression in murine
myotubes (Whitehouse and Tisdale, 2003). Eicosapentaenoic acid inhibits both the release
of arachidonic acid from membrane phospholipids and formation of 15-HETE in response
to PIF (Smith et al, 1999), and stabilised the NF-κB/I-κB complex in the cytosol,
preventing nuclear accumulation of NF-κB (Whitehouse and Tisdale, 2003). This study
shows that EPA also attenuates PIF-induced activation of PKC, which may be due to
reduced generation of DAG (Sperling et al, 1993). In addition, DAG with an n-3 polyunsaturated
fatty acid (PUFA) occupying the sn-2 position were found to be less effective in activating
PKC than DAG with an n-6 PUFA, and n-3 PUFA decreased the effectiveness of activation
of PKC and binding of phosphatidyl serine in the cell membrane (Terano et al, 1996).
Protein kinase C α is an upstream activator of the I-κB kinase complex (IKK) (Vertegaal
et al, 2000), which phosphorylates I-κBα at serines-32 and -36 leading to ubiquitination
and subsequent proteasome proteolysis. This suggests a mechanism by which PIF may
induce degradation of I-κBα and stimulate nuclear binding of NF-κB (Whitehouse and
Tisdale, 2003). Nuclear factor-κB regulates the transcription of a number of genes
and has been shown (Li and Reed, 2000) to be an essential mediator of TNF-α-induced
protein catabolism in differentiated muscle cells. This study shows that degradation
of I-κBα and translocation of NF-κB to the nucleus in response to PIF is attenuated
by calphostin C and is not seen in myotubes expressing mutant PKC
α
. This suggests that PKC acts as an important mediator in activation of NF-κB in response
to PIF. It is not known whether NF-κB acts alone or in concert with other transcriptional
activators in PIF-induced proteasome expression and future studies will be aimed at
identifying the role of NF-κB in this process.