Background
Hepatocellular carcinoma (HCC) is one of the most prevalent tumor types worldwide, especially in several areas of Asia and Africa [
1,
2]. HCC leads to approximately 662,000 deaths worldwide every year, and the mortality rate is increasing [
3,
4]. In spite of improvements in diagnosis and clinical treatment methods, HCC remains an aggressive malignant tumor due to the nonspecific symptoms, invasiveness, resistance to chemotherapy and high rate of tumor recurrence [
3]. HCC is closely associated with chronic liver disease, particularly cirrhosis due to hepatitis B virus or hepatitis C virus infection [
1,
5]. Patients with liver cirrhosis and HCC are often poor candidates for surgery, even if the HCC is detected at an early stage, as they generally lack a hepatic reserve as a result of the coexisting advanced cirrhosis [
1]. Therefore, new treatments against this aggressive neoplasm are urgently needed.
Cantharidin, the active constituent of the mylabris Chinese blister beetle, has been used as a traditional Chinese medicine for more than 2000 years and is still used as a folk medicine. Cantharidin has an affinity for the liver [
6], and has demonstrated therapeutic effects against HCC in clinical trials without suppressing bone marrow function, even in patients at an advanced stage [
6,
7]. Cantharidin is a potent and selective inhibitor of protein phosphatase 2A (PP2A). The core enzyme of PP2A consists of a catalytic subunit (PP2Ac) and a regulatory A subunit (PP2Aa). A third regulatory B subunit can be associated with this core structure, and this modulates the substrate specificity of PP2A. At present, two isoforms of the α and β catalytic subunits have been identified [
8,
9]. In previous studies, we proved that cantharidin repressed cancer cell proliferation and triggered apoptosis in a mechanism dependent on the inhibition of PP2A, suggesting that PP2A inhibition may provide a novel approach for hepatoma therapy [
7,
10,
11]. However, the cytotoxicity of cantharidin in normal hepatic tissue and the urinary system restricts its clinical application [
6], indicating that a cancer tissue-specific therapy strategy should be developed for the inhibition of PP2A.
Gene therapy using tumor- or tissue-specific promoter-driven suicide genes, immunosuppressors, antiangiogenic genes or tumor suppressor genes is a promising approach for the treatment of cancer. Expression of the α-fetoprotein (
AFP) gene is reactivated in HCC cells; however, the therapeutic results of
AFP promoter-driven gene therapy are unsatisfactory, as the transcriptional activity of this promoter is usually weak. It has been proven that the enhancer and silencer regions located upstream of the
AFP gene play a critical role in the selective expression of AFP in HCC. Additionally, the
AFP enhancer fragment may provide HCC-specific activity to the promoter of the non-tissue-specific, housekeeping phosphoglycerate kinase (
pgk) gene, and this novel strategy may be useful for HCC-specific cancer gene therapy [
12].
Therefore, in the present study, we attempted to develop a HCC-specific gene therapy system by expressing a dominant negative mutant form of the PP2A catalytic subunit α (DN-PP2Acα) [
13] under direct transcriptional control of the
AFP enhancer/
pgk promoter, and investigated the therapeutic effects of this system in HCC
in vitro and
in vivo.
Methods
Cell lines and culture
The AFP-positive human hepatoma cell lines, HepG2 and Hep3B, the AFP-negative human hepatoma cell line SK-HEP-1, and the normal human liver cell line L-02 were purchased from the American Type Culture Collection (Manassas, VA, USA). The cells were maintained in RPMI-1640 medium (DMEM; Gibco, Grand Island, NY, USA) supplemented with 10% fetal calf serum (FCS; Hyclone, Logan, UT, USA), 100 U/ml penicillin and 100 mg/ml streptomycin. The cultures were incubated at 37°C in a humidified atmosphere containing 5% CO2, and passaged every 2–3 days to maintain exponential growth.
MTT assay
Cellular growth was evaluated using the 3-[4,5-dimethyltiazol-2-yl] 2,5-diphenyl-tetrazolium bromide (MTT) assay [
14]. The cells were seeded in 96-well plates at 5×10
3 cells/well. After treatment, MTT (Sigma, St. Louis, MO, USA) was added to each well at a final concentration of 0.5 mg/ml and incubated at 37°C for 4 h. The media was removed, 200 μl dimethyl sulphoxide (DMSO) was added to each well and the absorbance was measured at 490 nm using a microplate ELISA reader (Bio-Rad Laboratories, Hercules, CA, USA). The inhibition rate was calculated as follows: inhibition rate = [(mean control absorbance-mean experimental absorbance)/mean control absorbance] × 100 (%). The concentration which caused a 50% growth inhibition (IC
50) was calculated using the modified Kärbers method [
15] according to the formula: IC
50 = lg
− 1[Xk − i(∑p − 0.5)], where Xk represents the logarithm of the highest drug concentration; i is the ratio of the adjacent concentration; and ΣP is the sum of the percentage growth inhibition at various concentrations. The relative cell viability was calculated as follows: relative cell viability = (mean experimental absorbance/mean control absorbance) × 100 (%).
Serine/threonine phosphatase assay
PP2A activity was analyzed using the nonradioactive serine/threonine-phosphatase assay kit (Promega, Madison, WI, USA) according to the manufacturer’s protocol. In brief, the cell lysate supernatant was passed twice through a Sephadex G-25 spin column to remove free phosphate, the eluate was placed into 96-well plates, and the assay was performed in the presence of a PP2A-specific serine/threonine phosphopeptide substrate (RRApTVA, in which pT represents phosphothreonine). Molybdate dye solution was added to the wells, incubated for 30 min at room temperature, color development was observed, absorbance was measured at 630 nm, and the amount of phosphate released was calculated using a standard curve. The relative activity of PP2A was calculated according to the following equation: PP2A activity = (mean experimental phosphate amount/mean control phosphate amount) × 100 (%).
Site-directed mutagenesis
Wild-type PP2A catalytic subunit α (PP2Acα) was cloned as previously described [
10]. The dominant negative mutant form of PP2Acα (DN-PP2Acα) was PCR-amplified from wild-type PP2Acα (WT-PP2Acα) using site-directed mutagenesis to mutate Leu 199 to Pro [
13]. Site-directed mutagenesis was performed by primed PCR amplification of the plasmid [
16]. Plasmid template DNA (10 ng) was added to a PCR cocktail containing PrimerSTAR HS DNA polymerase (TAKARA Biochemicals, Dalian, China) and the mutagenic oligonucleotide primers: sense: 5’-CCAATGTGTGACTTG
CCGTGGTCAGATCCAGATG-3’; anti-sense: 5’-CATCTGGATCTGACCA
CGGCAAGTCACACATTGG-3’. The PCR cycling parameters were 30 s at 95°C, followed by 18 cycles of 30 s at 95°C, 1 min at 55°C and 10 min at 72°C. The reaction was placed on ice for 2 minutes, 1 μl
Dpn I (10 U/μl, New England Biolabs, Ipswich, Massachusetts, USA) was added, incubated at 37°C overnight to digest the parental (i.e., the non-mutated) plasmid template DNA [
17] and the recircularized vector DNA incorporating the desired mutations was transformed into competent DH5α E. coli.
Western blotting
Total protein was extracted using a lysis buffer containing 50 mM Tris–HCl (pH 7.4), 150 mM NaCl, 1% Triton X-100, 0.1% SDS, 1 mM EDTA and supplemented with protease inhibitors [10 mg/ml leupeptin, 10 mg/ml aprotinin, 10 mg/mL pepstatin A, and 1 mM 4-(2-aminoethyl) benzenesulfonyl fluoride]. The protein extract was loaded, size-fractionated by SDS–polyacrylamide gel electrophoresis and transferred to PVDF membranes (Bio-Rad Laboratories, Hercules, CA, USA). After blocking, the membranes were incubated with primary antibodies at 4°C overnight and protein expression was visualized using horseradish peroxidase-conjugated antibodies and enhanced chemiluminescence (ECL) (Amersham Pharmacia Biotech, Buckinghamshire, UK). β-actin was used as an internal control.
Luciferase reporter gene assay
The
pgk promoter [
18] was cloned into pGL3-Basic (Promega, Madison, WI, USA) using the
NheI and
BglII restriction enzymes (New England Biolabs, Beverly, MA, USA) to generate the reporter plasmid, pGL3-Basic-pgk. The reporter plasmid, pGL3-Basic-AFpg, containing the
AFP enhancer and
pgk promoter was constructed as previously described [
12]. In brief, the
AFP enhancer, including the A and B domains [
19], was cloned into pGL3-Basic using the
KpnI and
NheI restriction enzymes, then the
pgk promoter [
18] was cloned into the
NheI and
BglII restriction sites. The positive control reporter plasmid, pGL3-Control, which contained the
SV40 promoter and enhancer sequences, and the internal control plasmid, pRL-SV40, containing the Renilla luciferase gene, were obtained from Promega. Cells were seeded in 24-well plates and transiently co-transfected with the reporter plasmids (500 ng/well) and the pRL-SV40 plasmid (100 ng/well) using X-tremeGENE HP DNA Transfection Reagent (Roche, Indianapolis, USA) according to the manufacturer's protocol, and the media was renewed after 8 h. After 24 h, the cells were lysed and luciferase activity was measured using the Dual-Luciferase Reporter Assay System (Promega) according to the manufacturer's recommendations using the TD-20/20 luminometer (Turner Designs, Sunnyvale, CA, USA). The results were expressed as relative luciferase activity (the ratio of firefly luciferase activity to Renilla luciferase activity).
Preparation of recombinant adenoviruses
The shuttle plasmids were respectively recombined with the backbone vector pAdEasy-1 in BJ5183 bacteria. Adenovirus generation, amplification, and titration were performed as previously described [
20] and viral particles were purified using the Virabind adenovirus purification kit (Cell Biolabs, Inc., San Diego, CA, USA).
Apoptosis and cell cycle distribution analysis
Apoptosis was quantified and the cell cycle was analyzed as described by Nicoletti et al. [
21]. Briefly, the cells were fixed in 80% chilled ethanol 48 h after treatment, and then incubated with 0.5% Triton X-100 solution containing 1 mg/ml RNase A at 37°C for 30 min. Propidium iodide (PI; Sigma) was added at a final concentration of 50 μg/ml, incubated for 30 min in the dark, and the cellular DNA content was analyzed using a fluorescence-activated cell sorter (FACS; Becton Dickinson, San Jose, CA, USA) and the data was processed using WinMDI29 software (Becton Dickinson).
Cells were seeded at a density of 1,000 cells/well in 6-well plates, and treated 12 h later. After 10 days, the cells were stained with 1% methylrosanilinium chloride and the numbers of visible colonies were counted. The relative clone formation ability was calculated as: relative clone formation ability = (mean experimental clone number/mean control clone number) × 100 (%).
Tumor xenograft model and adenovirus treatment
Six- to eight-week old male BALB/c athymic nude mice were purchased from the Shanghai Experimental Animal Center (Shanghai, China) and inoculated on the flank with 5 × 106 HepG2 or SK-Hep-1 cells. Tumors were allowed to grow to a volume of 100 mm3, and the animals were divided into four treatment groups: control vehicle injection (n = 6); Ad-CMV-DN-PP2Acα injection (n = 6); Ad-AFpg-luciferase injection (n = 6) and Ad-AFpg-DN-PP2Acα injection (n = 6). Adenovirus vectors (1 × 108 plaque forming units/100 μl) were injected directly into the tumor foci center on days 0, 2 and 4 of treatment. Tumor length and width were measured with calipers over a period of five weeks. Tumor volume was calculated as (length × width2)/2. All animals received humane care according to the Institutional Animal Care and Treatment Committee of Soochow University.
Statistical analysis
Results were expressed as the mean value ± standard deviation (S.D.). Statistical analysis was performed using unpaired Student’s t-tests; P values less than 0.05 were considered significant.
Discussion
Gene therapy is a promising approach for the treatment of cancer, and enables the transfer of genetic material to cells to produce a therapeutic effect. A successful gene therapy strategy requires both an effective target gene and a promoter which exhibits high levels of cancer-specific expression.
PP2A (protein phosphatase 2A) is a multimeric serine/threonine phosphatase [
24]. In our previous studies, we found that inhibition of PP2A exerted a cytotoxic effect in cancer cells [
7,
10,
11]. Moreover, cantharidin, a potent and selective inhibitor of PP2A, demonstrated promising therapeutic effects against HCC in clinical trials [
6,
7], suggesting PP2A is a promising target for the treatment of HCC. Unfortunately, the extensive constitutive expression of PP2A in normal tissues, and its complex physiological function obstruct the application of PP2A as a therapeutic target for the treatment of cancer. In clinical trials, cantharidin exerted cytotoxic effects against normal hepatic tissue and the urinary system [
6], indicating that the therapeutic inhibition of PP2A must be mediated using a cancer tissue-specific gene delivery system.
To develop a gene therapy system targeting PP2A, we firstly constructed a DN-PP2Acα expression vector driven by the cytomegalovirus (
CMV) promoter. The
CMV promoter has been widely used, as it is one of the strongest promoters in mammalian cells. The expression of DN-PP2Acα driven by the
CMV promoter induced cytotoxicity in HCC cells. The mechanism of DN-PP2Acα induced-cytotoxicity was linked to increased levels of apoptosis and triggering of G2/M cell cycle arrest, as previously described [
7,
10,
11], suggesting that PP2A is a promising target for the treatment of HCC. However, the
CMV promoter induces target gene expression in both normal cells and cancer cells. As
CMV promoter-driven expression of DN-PP2Acα induced cytotoxicity in both HCC cells and normal liver cells, cancer-specific delivery and/or gene expression are critical for the safety of gene therapy approaches which aim to inhibit PP2A. To solve this problem, one important approach is to use tumor-specific promoters.
Many cancers often re-express fetal or embryonic genes, and
AFP gene expression is reactivated in HCC cells. Although the
AFP promoter is a promising candidate for achieving selective transgene expression in HCC, the weak activity of the
AFP promoter may limit its utility for gene therapy strategies targeting HCC. It has been proven that the
AFP enhancer fragment can provide HCC-selective activity to the promoter of the non-tissue-specific, housekeeping gene
pgk. The
pgk promoter is recognized as a general, strong promoter and has been used for various gene transfer experiments [
25‐
27]. In this study, addition of the human
AFP enhancer fragment to the
pgk promoter provided selectivity to the non-tissue-specific
pgk promoter in AFP-expressing HCC cells, as previously described [
12]. The
AFpg promoter induced selective cytotoxic effects of DN-PP2Acα in AFP-positive cells. As the
AFpg promoter has not been evaluated in vivo, we examined the cytotoxic effect of specific expression of DN-PP2Acα, driven by the
AFpg promoter, in AFP-positive cells using a tumor xenograft model. Ad-AFpg-DN-PP2Acα restrained the tumor growth of AFP-positive xenografts in vivo, but did not affect AFP-negative xenografts.
Conclusions
In this study, we developed a hepatocellular carcinoma (HCC)-specific gene therapy system by expressing a dominant negative mutant form of the PP2A catalytic subunit under direct transcriptional control of the AFP enhancer/pgk promoter, and investigated the therapeutic effects of this system in HCC in vitro and in vivo. The data presented indicates that the use of a vector construct targeting PP2A, under the transcriptional control of the AFP enhancer fragment and the pgk promoter, is a practical and promising strategy to deliver HCC-specific gene therapy.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
WL and DL designed, performed experiments, and participated in drafting the manuscript; KC and ZC participated in plasmids construction; YZ and HY performed flow cytometry assays; ZX and YZ participated in design experiments and discussion of the results; FG and MT conceived of the study and participated in design experiments and coordination, and critically revised the manuscript. The authors read and approved the final manuscript.