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Division of Basic Reproductive Sciences, Department of Obstetrics and Gynecology, University of Colorado Health Sciences Center at Fitzsimons, Mail Stop 8309, PO Box 6511 12800 E. 19th Avenue, Aurora, Colorado 80045, USA
(Requests for offprints should be addressed to A P Bradford; Email: andy.bradford{at}uchsc.edu)
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in human endometrial tumors. To determine the functional roles of PKC and in apoptosis in endometrial cancer, Ishikawa cells were treated with selective PKC inhibitors or adenoviral constructs encoding wild-type or isoform-specific, dominant-negative mutants. Apoptosis was assessed by DNA fragmentation and caspase-mediated poly-(ADP-ribose)-polymerase cleavage. The inhibition of PKC suppressed etoposide-induced apoptosis, while overexpression of PKC enhanced it. In contrast, inhibition of PKC elevated basal levels of apoptosis and potentiated etoposide-induced cell death. Etoposide treatment also selectively activated PKC, but resulted in both cytosolic translocation and decreased activity of PKC. A fraction of PKC also underwent caspase-dependent cleavage, in response to etoposide. Our results suggest that changes in apoptosis and PKC expression in endometrial cancer are mechanistically linked, such that PKC is required for DNA damage-induced apoptosis, while PKC mediates a survival response. Thus, PKC and expression and signaling may be important in endometrial tumorigenesis and could serve as potential prognostic indicators and/or novel targets for therapeutic intervention.
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Introduction |
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The protein kinase C (PKC) family of serine/threonine kinases has been shown to regulate a broad range of key cellular pathways including growth, survival, differentiation, and apoptosis (Fishman et al. 1998, Morse-Gaudio et al. 1998, Zeidman et al. 1999, Cross et al. 2000, Musashi et al. 2000, Ventura & Maioli 2001, Brodie & Blumberg 2003, Gutcher et al. 2003, Hofmann 2004). To date, at least ten PKC isoforms have been identified, which differ in their expression patterns, substrate specificity, and response to extra-cellular stimuli (Clemens et al. 1992, Lucas & Sanchez-Margalet 1995, Jaken 1996, Nishikawa et al. 1997, Newton 2001). PKCs have been implicated in neoplastic transformation, the growth and metastasis of tumors, and response to therapy in a variety of tissues including breast, prostate, liver, colon, skin, stomach, and respiratory tract (Goodnight et al. 1994, Kiley et al. 1996, Cornford et al. 1999, Gomez et al. 1999, Spitaler et al. 1999, Watters & Parsons 1999, Musashi et al. 2000, Koivunen et al. 2006). In endometrial cancers, the total PKC activity was significantly higher when compared with normal endometrial tissue (Fujimoto et al. 1995) and differential overexpression of PKC isoforms has been linked to the proliferative potential of endometrial cancer cell lines and tumor pathogenesis (Gretz et al. 1994, Bamberger et al. 1996, 1997, 1998, Fujimoto et al. 1996, Connor et al. 1997). In an analysis of endometrial tumors, PKC was more highly expressed in higher-grade endometrial tumors exhibiting lower levels of estrogen receptor (ER) and poorer prognosis (Fournier et al. 2001). However, studies of PKC in endometrial tumors are limited, largely correlative, and the functional role of specific PKC isoforms in the pathophysiology of endometrial cancer has not been determined.
Specific PKC isoforms have also been shown to play a critical role in the regulation of apoptosis and may be either pro- or anti-apoptotic, dependent on cell type and stimulus (Leszczynski 1995, Lucas & Sanchez-Margalet 1995, Whelan & Parker 1998, Musashi et al. 2000, Brodie & Blumberg 2003). Paradoxically, studies of endometrial tumors report an increase in apoptotic index during the progression from endometrial hyperplasia, through atypical endometrial hyperplasia, to endometrial adenocarcinoma (Ioffe et al. 1998). Moreover, undifferentiated and poorly differentiated endometrial carcinomas exhibited higher apoptotic indiceswhen compared with well-differentiated tumors, and higher apoptotic indices correlated inversely with prognosis (Heatley 1997, Kokawa et al. 2001a). Thus, increased rates of apoptosis have been proposed to be a morphological indicator of potentially malignant endometrial tumors (Arends 1999, Stewart et al. 1999, Kokawa et al. 2001a).
Given the above evidence of aberrant apoptosis and changes in PKC expression in endometrial cancers and that PKCs modulate apoptosis in other tumors of epithelial origin, we investigated the potential role of PKC in the regulation of apoptosis in endometrial cancer cells. Herein, we provide evidence that PKC and differentially regulate apoptosis and survival in Ishikawa endometrial cancer cells such that PKC is a critical mediator of apoptosis, while PKC is important in cell survival. Our results demonstrate distinct functional roles for PKC and in endometrial cancer cells and provide, for the first time, a potential mechanistic link between the reported changes in apoptotic index and PKC expression and/or activity, concomitant with progression from hyperplasia to malignancy in the endometrium.
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Materials and methods |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Ishikawa endometrial adenocarcinoma cells were a generous gift from Dr K K Leslie (University of New Mexico, Albuquerque, NM, USA). Cells were grown in Dulbecco’s modified Eagle’s Medium (DMEM) supplemented with 12.5% horse serum, 2.5% fetal calf serum, 10 units/ml penicillin, 10 µg/ml streptomycin, and 200 µM L-glutamine and maintained at 37 °C in 5% CO2 in air. Before treatment, the cells were serum-deprived overnight by culturing in DMEM without added serum. Drugs used in experiments were solubilized in dimethyl sulphoxide (DMSO). Etoposide, rottlerin, Gö6976, phorbol ester (TPA), and benzyloxycarbonyl-Val-Ala-Asp (OMe) fluoro-methylketone (Z-VAD-FMK) were purchased from Calbiochem (San Diego, CA, USA).
Western blot analysis
The cells were harvested by washing twice in ice-cold PBS and lysing them in 150 µl buffer (50 mM Tris (pH 7.4), 150 mM NaCl, 0.5% Triton X-100) supplemented with protease inhibitor cocktail (Roche Diagnostics). Lysate protein concentrations were determined using the Bradford assay (Bio-Rad Laboratories) and an equal volume of Laemmli sample buffer (20% glycerol, 2% (w/v) SDS, 5% ß-mercaptoethanol, 62.5 mM Tris (pH 6.8), 1% (w/v) bromophenol blue dye) was added. Between 20 and 75 µg total cellular protein was resolved by electrophoresis on 10% polyacrylamide-SDS gels in electrophoresis buffer (25 mM Tris (pH 8.3), 192 mM glycine, 0.1% (w/v) SDS), transferred to polyvinylidene difluoride (PVDF) membrane in transfer buffer (25 mM Tris (pH 8.3), 192 mM glycine), and probed using specific antibodies. The PKC antibodies used for immunoblotting include PKC (sc-208), PKCß (sc-209), PKCßII (sc-210), PKC (sc-937), PKC
(sc-214), PKC
(sc-211), PKC
(sc-215), PKC
/
(sc-1091), PKC
(sc-212), PKC
(sc-216), and secondary anti-rabbit (sc-2004) or anti-mouse (sc-2005) IgG-HRP antibodies were obtained from Santa Cruz Biotechnology (Santa Cruz, CA, USA). The poly-(ADP-Ribose)-polymerase (PARP) cleavage fragment-specific antibody was purchased from BD Biosciences Pharmingen (San Jose, CA, USA). The actin loading controls were assessed using a monoclonal anti-ß-actin (A5316) antibody (Sigma). The primary antibody binding was visualized using species-specific secondary antibody conjugated to HRP, chemiluminescent substrate, and exposure to autoradiographic film. The molecular weight of proteins was estimated by comparison with the Full Range Rainbow recombinant protein molecular weight marker from Amersham Biosciences. Quantitation of band intensity was performed by densitometry using Quantity One software (v. 4.5.1) and a GelDoc imaging system (Bio-Rad).
Immunohistochemistry
Endometrial tumor tissue that had been formalin-fixed and paraffin-embedded was obtained from the Department of Pathology at the University of Colorado Health Science Center. The tissue was sectioned into 5 µm thick slices and blocked for endogenous peroxidase activity using 3% hydrogen peroxide. Antigen retrieval was performed in citrate buffer (20 mM, pH 6.0) for 10 min at 60 °C. The sections were incubated with antibodies specific for PKC (C-20, 0.17 µg/ml) and PKC (H-7, 1.275 µg/ml), and stained using an indirect avidin biotin immunoperoxidase method on a DAKO Autostainer (DakoCytomation, Carpinteria, CA, USA) as described (Tringler et al. 2006). The specificity of staining was verified using subclass-matched IgG1 (BD PharMingen) in place of PKC antibodies and through blocking studies with peptide antigen (Santa Cruz Biotechnology).
Chromatin condensation and DNA fragmentation assay
Nuclear DNA in healthy and apoptotic cells was stained using Hoechst 33342 dye as described (Mpoke & Wolfe 1997). Ishikawa cells were plated on glass cover slips in serum-containing media and allowed to adhere before treatments. Following treatment of cells, the cover slips were washed twice in PBS so that only the adherent cells remained. After the PBS washes, the cells were covered for 5 min with a solution containing 5 µg/ml Hoechst 33342 dye from Sigma. Following staining, the cells were washed a final time in PBS and the stained nuclei were examined at 10x magnification under u.v. light. The proportion of apoptotic cells was determined by counting cells in four random fields per condition in three separate experiments. Quantiation of extranuclear fragmented DNA was performed using the Cell Death Detection ELISA kit as per the manufacturer’s recommended protocol (Roche).
Adenoviral vectors
PKC adenoviral constructs were a generous gift of Drs Lee Carpenter and Trevor Biden (Garvan Institute of Medical Research, St Vincent’s Hospital, Sydney, Australia) and Dr Mary Reyland (University of Colorado Health Sciences Center, Denver, CO, USA). Information pertaining to the creation and use of these constructs in other cell types has been published previously (Reyland et al. 1999, Carpenter et al. 2002). Dominant-negative PKC constructs were created by introducing a single amino acid mutation in the ATP-binding site of the catalytic domain, specifically a (K376R) mutation in PKC from rat and (K368R) mutation in PKC from mouse. For adenovirus infections, 3.0 x 106 cells were plated on 60 mm dishes and allowed to adhere for 18 h or more. After removing the serum-containing media, adenovirus was added to the adherent cells in 1 ml serum-free media at a predetermined multiplicity of infection (MOI) based on viral titer. The plates were gently agitated every 10 min for 1 h, then serum free media was added to a final volume of 3 ml. The infected cells were then incubated for 18–24 h in the serum-free media to enable adequate protein expression before drug treatments.
Immunoprecipitations and PKC kinase assays
Immunoprecipitations were performed using protein A/G PLUS-agarose beads from Santa Cruz. PKC kinase assays included PKC lipid activator from Upstate (Lake Placid, NY, USA), histone (Sigma), and [-32P] ATP (Amersham). The cells were harvested by washing once in ice-cold PBS and lysing cells in lysis buffer (25 mM HEPES (pH 7.5), 0.3 M NaCl, 1.5 mM MgCl2, 0.2 mM EDTA (pH 8.0), 0.5 mM dithiothreitol, 0.1% Triton X-100) amended at each use with phosphatase and protease inhibitors (20 mM ß-glycerophosphate, 0.1 mM sodium orthovanadate, 10 mM sodium fluoride, 1 mM phenyl-methylsulphonyl fluoride, and protease inhibitor cocktail). Cell lysates were vortexed and frozen before clearing insoluble cell debris by centrifugation at 14 000 g for 10 min at 4 °C. The total cellular protein was quantitated and 1000 µg (PKC) or 500 µg (PKC) total cellular protein was combined with 5 µg PKC (sc-208) or PKC (sc-937) primary antibody in a total volume of 300 µl. After agitating overnight at 4 °C, protein A/G agarose beads were added as recommended and agitated with the samples for 2 h. The immune complexes were pelleted and washed four times in lysis buffer. After the final lysis buffer wash, the pellet was washed twice in kinase assay buffer (40 mM Tris (pH 7.4), 20 mM MgCl2, 20 µM ATP, 2.5 mM CaCl2). The final pellet was resuspended in 22 µl kinase assay buffer and a 2 µl aliquot of the suspension was acquired to be used as a loading control by western blotting. The final reaction was performed in a 40 µl volume containing the resuspended immunoprecipitates, 5 µl PKC lipid activator, 20 µg histone, and 5 µCi [-32P] ATP. Before addition to the reaction, the PKC lipid activator containing 0.05 mg/ml phosphatidylserine and 0.05 mg/ml diacylglycerol was sonicated on ice for 30 s. All reaction components were combined on ice and then moved to a 30 °C water bath. After 20 min, the reactions were immediately placed on ice and Laemmli sample buffer was added before heating the samples for 5 min at 95 °C. Phosphorylated histone protein was visualized by electrophoresis on a 12.5% polyacrylamide gel followed by drying and exposing the gel to film. Quantitation of radioactivity in each band was determined using a Storm 860 Phosphor-Imager and ImageQuant software (Molecular Dynamics, Sunnyvale, CA, USA).
Cell fractionation
Soluble and particulate cell fractions were isolated as described (Jackson et al. 2001), briefly, after treatment adherent cells were washed twice in ice-cold PBS and harvested in lysis buffer A (20 mM Tris (pH 7.5), 2 mM EDTA, 2 mM EGTA, 1 mM PMSF, 0.1% ß-mercaptoethanol, and protease inhibitor cocktail) using a cell scraper. The cells were then sonicated for 10 s (output 4, constant duty cycle) using a Branson Sonicator (Branson Ultrasonics Corp., Danburg, CT, USA) and pelleted at 70 000 g for 1.5 h. The supernatant (soluble fraction) was removed to a new tube and the remaining pellet was resuspended in lysis buffer B (lysis buffer A+1% Triton X-100) by sonicating for 10 s. Following sonication, the sample was pelleted again by centrifugation at 13 000 g for 15 min at 4 °C and the supernatant harvested as the Triton-soluble, particulate fraction.
Statistical analysis
Values shown in figures are given as the mean ± S.D. or S.E.M. The data were analyzed using a paired Student’s t-test. P values <0.05 were considered significant.
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Results |
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, Ishikawa cells express the conventional isoforms PKC, ßII, and ; novel PKCs , , , and ; and the atypical PKC. The conventional PKCßI was present at low levels, detectable after prolonged exposure, and the atypical PKC isoform / was not detected. ß-Actin was used a loading control.
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is the predominant isoform associated with cell survival or suppression of apoptosis (Whelan & Parker 1998, Li et al. 1999, Deng et al. 2001). Its expression has previously been shown in partially purified extracts of endometrial tumors (Tonetti et al. 2000) and linked to estrogen-dependent proliferation (Tonetti et al. 1998, Wu et al. 2005). PKC, the predominant pro-apoptotic isoform in other epithelial cell types (Basu 2003, Brodie & Blumberg 2003, Gutcher et al. 2003), was expressed in EnCa101 endometrial cancer cells (Tonetti et al. 1998); however, its expression had not been shown in endometrial tumors. Using immunohistochemistry, we observed the expression of both PKC and in paraffin sections of human endometrial tumors and normal tissue. PKC and were detected in each of the 24 samples of endometrial tumors and in each of the eight sections derived from proliferative, secretory, and atrophic normal endometrium (Fig. 1B and C
; data not shown). Given their functional role in modulating apoptosis in numerous epithelial cell types (Brodie & Blumberg 2003, Gutcher et al. 2003, Hofmann 2004) and expression in human endometrial tumors, we hypothesized that PKC and are important regulators of apoptosis in endometrial cancer.
To address the functional roles of PKC and in regulating apoptosis in endometrial cancer cells, we utilized selective PKC inhibitors and determined their effects on etoposide-induced apoptosis, assessed by changes in cell morphology, DNA fragmentation, and caspase activation. The topoisomerase II inhibitor etoposide has been used as a chemotherapeutic agent in endometrial cancer (Poplin et al. 1999), and is a potent inducer of DNA damage and apoptosis (Ruvolo et al. 1998). First, Ishikawa cells were stained with Hoechst fluorescent dye 33342, which is indicative of membrane disruption, organelle acidification, and chromatin condensation characteristic of the initial stages of apoptosis (Mpoke & Wolfe 1997, Chen et al. 2005, Wada et al. 2005). Cells were visualized under u.v. and visible light and results quantitated by counting multiple fields. Under basal conditions, approximately 24% of Ishikawa cells stained positive (Fig. 2A). Treatment with 50 µM etoposide increased the fraction of Hoechst 33342 staining cells to over 43%. To determine the roles of PKC and , we utilized the PKC inhibitors Gö6976 and rottlerin, which are selective for the conventional PKCs (, ßI, ßII, and ) and PKC respectively (Gschwendt et al. 1994, Keenan et al. 1997). As indicated in the representative fields (Fig. 2A) and associated quantitation (Fig. 2B), pretreatment with rottlerin had no effect on basal apoptosis in Ishikawa cells but significantly reduced the proportion of Hoechst 33342-positive cells in response to etoposide from 43 to 32%. In contrast, treatment with Gö6976 alone increased the basal apoptotic index from 23.9 to 29.1% and potentiated the effect of a lower submaximal dose (10 µM) of etoposide (Fig. 2C), significantly increasing the fraction of apoptotic cells from 31 to 47%. These results suggest that the inhibition of PKC attenuated etoposide-induced apoptosis in endometrial cancer cells, whereas inhibition of conventional PKCs increased basal levels of apoptosis and enhanced the response to etoposide.
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, treatment of Ishikawa cells with etoposide resulted in an increase in DNA fragmentation. Again, consistent with PKC being pro-apoptotic, pretreatment with rottlerin markedly reduced DNA fragmentation in a dose-dependent manner (Fig. 3A). Conversely, pre-treatment of cells with Gö6976 enhanced the amount of DNA fragmentation detectable after etoposide treatment (Fig. 3B). Neither inhibitor alone significantly affected DNA fragmentation in this assay. Thus, Ishikawa cells treated with rottlerin, a PKC-selective inhibitor, showed a reduced apoptotic response to etoposide, suggesting that PKC is pro-apoptotic. In contrast, Gö6976 enhanced etoposide-induced apoptosis, suggesting a pro-survival role for conventional PKC isoforms.
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, ß, and ), and, despite its widespread use, concern has been expressed regarding the specificity of rottlerin in vitro (Davies et al. 2000). Thus, in order to more specifically manipulate cellular activities of PKC and , we used adenoviral expression constructs expressing wild-type (WT) or dominant-negative (DN), kinase dead forms of these enzymes that have been shown to increase or suppress endogenous isoform-specific PKC activity respectively (Reyland et al. 1999, 2000). Cleavage of PARP, an essential DNA repair enzyme, is another hallmark indicator of cells undergoing apoptosis. PARP becomes inactivated when the apoptotic effector caspase-3 and caspase-7 cleave the full-length PARP protein (116 kDa) releasing an 89 kDa fragment (Soldani & Scovassi 2002). Western blotting to detect the appearance of the 89 kDa PARP cleavage fragment has thus been used as an indirect assay of intracellular caspase-3 and caspase-7 activity (Yu et al. 2001).
As shown in Fig. 4A, 48 h following transduction with adenovirus at the specified MOI of 50, both wild-type and dominant-negative PKC were overexpressed relative to the level of endogenous PKC evident in cells infected with GFP adenovirus as a control (Fig. 4A). Etoposide treatment did not significantly affect expression of endogenous or adenoviral PKC expression. Figure 4B shows Ishikawa cell extracts, treated with or without etoposide and probed for the PARP cleavage fragment. In cells transduced with WT PKC adenovirus, a marked increase in basal PARP cleavage, relative to green fluorescent protein (GFP) control is observed (Fig. 4B and C) reaching levels comparable with that observed in GFP cells treated with 50 µM etoposide. Furthermore, overexpression of PKC significantly enhanced PARP cleavage induced by etoposide. Conversely, expression of dominant-negative PKC alone suppressed any detectable basal PARP cleavage when compared with GFP controls and significantly attenuated the effects of etoposide (Fig. 4B and C). Of note, the PKC WT and DN constructs differ in only one amino acid and were expressed at similar levels. Thus, it is unlikely that the observed effects are a consequence of non-specific viral toxicity.
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overexpression in combination with etoposide, Ishikawa cells infected at a constant MOI were challenged with increasing doses of etoposide (Fig. 4D). Consistent with our initial experiment (Fig. 4B and C), overexpression of PKC alone induced PARP cleavage and this effect was increased by etoposide in a dose-dependent manner. PARP cleavage in GFP adenovirus transduced control cells was not detectable until treatment with 25 µM etoposide, while cells overexpressing PKC exhibited increased apoptotic PARP cleavage in the presence of only 2 µM etoposide. As a control, expression of dominant-negative PKC considerably suppressed PARP cleavage at both 25 and 50 µM doses of the etoposide (Fig. 4D). These results are completely consistent with observations using PKC inhibitors and suggest that PKC is a critical component of the pro-apoptotic pathway in endometrial cancer cells.
To determine the role of PKC in etoposide-mediated apoptosis, Ishikawa cells were infected with adenoviral constructs encoding wild-type or dominant-negative kinase (PKCDN). GFP adenovirus was again used as a control. Increased PARP cleavage was apparent in untreated cells overexpressing dominant-negative PKC relative to the level of basal apoptosis seen in control Ishikawa cells expressing GFP (Fig. 5A). Following etoposide treatment, PARP cleavage was increased to a greater degree in cells overexpressing dominant-negative PKC, suggesting that inhibition of PKC sensitized the cells to etoposide-induced apoptosis. Overexpression of wild-type PKC, however, elicited no change in PARP cleavage relative to GFP control cells treated with etoposide. Thus, while endogenous PKC activity is clearly an important mediator of cell survival, increased PKC expression was not sufficient to confer resistance to etoposide.
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expression (Fig. 5A, panel 2), possibly due to increased viral promoter activity or protein stabilization. No significant change in endogenous levels of PKC, in response to etoposide, was observed. However, since both wild-type and dominant-negative constructs are affected, the observed effects on apoptosis cannot be attributed to non-specific effects of viral protein overexpression. Nevertheless, to further confirm the role of PKC, we conducted a second experiment where the Ishikawa cells were challenged with a tenfold lower dose of etoposide (5 µM) and infected with adenovirus at an increasing MOI (10, 25, 50). As seen in Fig. 5C, panels 2 and 4, infecting cells with wild-type or dominant-negative PKC adenovirus at increasing MOI resulted in the expected dose-dependent increase in the level of PKC protein, but the lower dose of etoposide did not influence virally mediated PKC expression. In vehicle-treated cells expressing dominant-negative PKC, PARP cleavage increased in a MOI-dependent fashion (panel 1; lanes 4–6), whereas no increase in PARP cleavage was apparent in cells overexpressing wild-type PKC (panel 1; lanes 1–3). Treatment of cells overexpressing PKC, with 5 µM etoposide, induced minimal PARP cleavage, which was not affected by increased expression of PKC (panel 4; lanes 1–3). In contrast, etoposide resulted in a robust, MOI-dependent, increase in PARP cleavage in cells expressing dominant-negative PKC, over and above levels induced by expression of dominant-negative PKC alone (compare panels 1 and 4; lanes 3–6), indicating that inhibition of PKC dramatically sensitized the cells to this DNA damaging agent.
In summary, apoptotic PARP cleavage in untreated and etoposide-treated Ishikawa cells was consistently increased by expression of dominant-negative PKC, while overexpression of the wild-type kinase had no apparent effects. Considering our results from both the pharmacological inhibitors and the adenoviral-mediated overexpression, we conclude that PKC is mediating a pro-survival signal in Ishikawa endometrial cancer cells, such that inhibition of PKC induces apoptosis and sensitizes cells to etoposide treatment.
We next determined changes in PKC and activities in response to an apoptotic stimulus. Immunocomplex kinase assays were used to examine the effect of etoposide on PKC and activities in Ishikawa cells. As shown in Fig. 6A, in response to etoposide treatment, PKC kinase activity is rapidly increased within 15 min, reaching a maximum level at 30 min, and remains moderately elevated at 2 h. Conversely, PKC activity apparently decreases over this same period of time, reaching a nadir at 30 min and remaining below basal levels at the 2 h time point (Fig. 6B). Western blot analysis confirmed uniform PKC and loadings in the kinase assay. No histone kinase activity was detected in control immunoglobulin immunoprecipitates.
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and . Under basal conditions, the majority of PKC protein (~75%) was localized to the particulate fraction in untreated cells (Fig. 7A). While TPA treatment resulted in translocation from soluble to membrane fractions, somewhat surprisingly, etoposide induced a small but significant transient increase in PKC in cytosolic fractions (Fig. 7A). This rapid cytosolic translocation of PKC in response to etoposide occurred over a similar time course to that of the elevated kinase activity (Fig. 6A). Such stimulus-specific translocation of PKC to the cytoplasm has been reported previously and may reflect the formation of activated lipid-independent kinase activity (Rybin et al. 2004, Yang et al. 2006).
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was predominantly (~85%) localized in the soluble fraction (Fig. 7B). Consistent with the reduction in kinase activity (Fig. 6B), etoposide treatment resulted in a complete loss of PKC from the particulate fraction and a corresponding increase in soluble enzyme. Treatment with the phorbol ester TPA, a known activator of PKC (Ohno et al. 1991), resulted in a rapid and robust translocation to the particulate fraction (Fig. 7B). Thus, taken together, these results are consistent with PKC being activated in response to an apoptotic stimulus, while the activity of PKC is concomitantly decreased.
Recent evidence from other cell types suggests that PKC is not only a mediator, but also a target of the apoptotic pathway. In cells undergoing apoptosis, PKC can undergo caspase-dependent proteolytic cleavage, between the regulatory and the catalytic domains, releasing a catalytically active fragment (Emoto et al. 1995, Mizuno et al. 1997, Denning et al. 1998, Reyland et al. 1999). To determine if PKC is a target of caspases in endometrial cancer cells, lysates from etoposide-treated Ishikawa cells were harvested over a 24-h time course and probed for PKC protein using an antibody that recognizes an epitope in the C-terminal catalytic domain. As shown in Fig. 8, a 40 kDa band (indicated by the arrow), consistent with the previously reported size of the PKC catalytic fragment, increases in abundance over time in response to etoposide. It is important to note that this band is only detectable after a prolonged exposure of the Western blot, and therefore seems to represent a relatively small fraction of the total PKC protein in the cell. Treatment of Ishikawa cells with the caspase inhibitor, Z-VAD-FMK, blocked the formation of 40 kDa catalytic fragment indicating that cleavage of PKC is caspase-dependent (Fig. 8). A second minor band migrating at higher molecular weight was observed with varying intensity in some samples and was not suppressed by inhibition of caspases. Treatment with Z-VAD-FMK alone had no effect on full-length or cleaved PKC levels (data not shown).
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and can differentially regulate endometrial cancer cell survival and apoptosis. PKC is a critical component of the apoptotic pathway in Ishikawa cells, whereas PKC activity is required for cell survival. Accordingly, etoposide treatment and induction of apoptosis resulted in activation and caspase-dependent cleavage of PKC accompanied by a reduction in PKC kinase activity. Thus, the balance of PKC and expression and/or activities may be important in the survival of endometrial cancer cells and modulate response to chemotherapeutic agents. |
Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Differential overexpression of PKC isoforms has also been implicated in endometrial tumor pathogenesis and patient prognosis (Bamberger et al. 1996, 1997, 1998, Tonetti et al. 1998, 2000, Wu et al. 2005), and members of the PKC family have been shown to regulate apoptosis in a variety of other epithelial cell types and tumors (Musashi et al. 2000, Mandil et al. 2001, Brodie & Blumberg 2003). In general, PKC appears to suppress or protect against apoptosis (Whelan & Parker 1998, Li et al. 1999, Deng et al. 2001), while the majority of studies show PKC involvement in the induction of programmed cell death (Basu 2003, Brodie & Blumberg 2003, Gutcher et al. 2003). However, despite evidence that aberrant apoptosis and PKC isoform expression are phenotypic markers of carcinogenesis and tumor progression in the endometrium, the role of PKC in the regulation of apoptosis in endometrial tumors has not been elucidated.
In this study, we demonstrate, for the first time, a potential mechanistic link between the reported altered PKC expression/activity and aberrant apoptosis characteristic of endometrial tumors and suggest a functional role for specific PKC isoforms in the pathogenesis of endometrial cancer. We show that PKC and differentially modulate apoptosis in endometrial cancer cells, such that PKC is required for etoposide-induced cell death, while PKC is a critical component of a cell survival pathway. Thus, inhibition of PKC confers resistance to the chemotherapeutic drug etoposide (Figs 3 and 4), whereas inhibition of PKC is sufficient to induce apoptosis and sensitize cells to a genotoxic insult (Figs 3 and 5). Conversely, overexpression of PKC increased basal levels of apoptosis and enhanced the effect of etoposide (Fig. 4). Consistent with these observations, treatment with etoposide and induction of apoptosis resulted in the activation of PKC and the concomitant inhibition of PKC (Fig. 6). While inhibition of PKC or expression of a dominant-negative construct induced apoptosis and potentiated the response to etoposide, overexpression of wild-type PKC did not confer resistance to etoposide-induced cell death (Fig. 5). These data suggest that, while PKC is a critical component of a cell survival pathway in Ishikawa cells, it may not act directly to antagonize apoptotic signals. Our results indicate that relative expressions and/or activities of PKC and can modulate apoptosis and survival in endometrial cancer cells and that isoform-specific PKC signaling pathways may be important in endometrial tumorigenesis. These studies focus on PKC and , the principal isoforms implicated in the regulation of apoptosis. However, Ishikawa cells express additional PKCs (Fig. 1), which have also been shown to modulate cell survival and apoptosis (Musashi et al. 2000, Gutcher et al. 2003). Whilst only PKC expression has been shown to correlate with patient prognosis and response to hormonal therapy (Tonetti et al. 1998, 2000), the functional role of these additional PKC isoforms remains to be established.
The mechanism by which PKCs modulate apoptosis in the endometrium remains to be established. However, aberrant expression of Bcl-2 proteins in endometrial cancers is associated with increased malignancy and poor prognosis (Ioffe et al. 1998, Ouyang et al. 1998, Sakuragi et al. 2002). Decreased levels of the anti-apoptotic Bcl-2 and upregulation of its pro-apoptotic partner Bax, correlate with increased apoptosis and progression from hyperplasia to malignancy in the endometrium (Chieng et al. 1996, Henderson et al. 1996, Kuwashima et al. 1996, Saegusa et al. 1996, Mozzetti et al. 2000, Kokawa et al. 2001b, Peiro et al. 2001, Sakuragi et al. 2002). PKC has been shown to phosphorylate Bcl-2, which may be required for optimal anti-apoptotic function (Ruvolo et al. 1998, Deng et al. 2001). Similarly, the tumor suppressor PTEN is a negative regulator of PI3-kinase/Akt prosurvival signal transduction pathway (Kennedy et al. 1997, Vazquez & Sellers 2000) that is frequently mutated or downregulated in endometrial tumors (Maxwell et al. 1998, Ali 2000, Mutter et al. 2000). Ishikawa cells do not express functional PTEN protein (Wan et al. 2002), resulting in constitutive phosphorylation and activation of Akt (Lilja et al. 2001). The present evidence indicates that PKC and can be both regulators and targets of the PI3K/Akt signaling pathway regulating cell survival (Mandil et al. 2001, Brodie & Blumberg 2003, Lu et al. 2004, Greco et al. 2006). PI3K-dependent activation of PKC is an important mediator of invasion of mammary epithelial cells (Woods Ignatoski et al. 2003) and Akt modulates antiapoptotic PKC signaling in breast cancer cells. Conversely, PKCs have been shown to modulate PI3k-dependent signal transduction (Wen et al. 2003). PKC typically stimulates Akt phosphorylation and activity, while PKC has been shown to induce dephosphorylation and inactivation of Akt in a variety of cell types (Mao et al. 2000, Li et al. 2004, 2006, Zhu et al. 2004). In LNCap prostate cancer cells, which, like endometrial cancers, lack functional PTEN, both PKC and activation results in dephosphorylation of Akt and induction of apoptosis (Tanaka et al. 2003). We did not detect changes in the levels or phosphorylation states of Akt in response to over-expression or inhibition of PKC in Ishikawa cells (data not shown). However, potential interactions between the PTEN/Akt and PKC-dependent signaling pathways in endometrial cancer cells remain to be investigated.
The characterization of the functional roles of specific PKC isoforms in endometrial cancer cells may provide new diagnostic or prognostic markers to identify aggressive, malignant tumors and provide a rational basis for novel therapeutic strategies based upon PKC modulating drugs presently under development (Carter 2000, Goekjian & Jirousek 2001, Hofmann 2004). Given the differential effects of PKC and in endometrial cancer cells reported herein, such approaches require the development of isoform-specific PKC activators or inhibitors.
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Acknowledgements |
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Funding |
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This work was supported by NCI-CA10487 and the University of Colorado Department of Obstetrics & Gynecology. The authors have no conflicts of interest.
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References |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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