Background
Arginine, a semi-essential amino acid in humans, is critical for the growth of human cancers [
1]. Tumoral down-regulation of the enzyme argininosuccinate synthetase (ASS1), the rate-limiting step in arginine synthesis, results in a critical dependence on extracellular arginine due to an inability to synthesize this amino acid from citrulline. Such a dependence on extracellular arginine is known as arginine auxotrophy. Many advanced human tumors more commonly associated with chemoresistance and poor clinical outcome, including hepatocellular carcinoma (HCC), melanoma, mesothelioma, pancreatic cancer, prostate cancer, renal cell carcinoma, sarcoma and small cell lung cancer, exhibit loss of ASS1 expression and are thus arginine auxotrophs [
2‐
9]. Conversely, other tumor types such as colorectal, gastric and ovarian cancer tend to have higher expression of ASS1 [
10,
11]. The mycoplasma-derived enzyme, arginine deiminase (ADI-PEG 20), PEGylated to enhance bioavailability and reduce immunogenicity, selectively degrades arginine, resulting in cell death in tumors lacking ASS1 [
12]. Several phase I/II clinical trials of ADI-PEG 20 in patients with HCC and metastatic melanoma have shown promising indication of clinical benefit and low toxicity in patients with ASS1-deficient tumors [
13‐
18]. A recently completed phase II trial of single-agent ADI-PEG 20 in ASS1-negative patients with mesothelioma also revealed encouraging efficacy results [
19,
20].
The significance for ASS1 loss in cancer is currently unclear; however, several groups have revealed that reduced expression of ASS1 is a predictive biomarker for the development of metastasis and is associated with a worse clinical outcome [
21‐
25]. Epigenetic silencing via methylation of the CpG islands within the ASS1 promoter accounts for loss of ASS1 expression in many solid tumors studied to date, including ovarian, malignant pleural mesothelioma, glioblastoma, myxofibrosarcoma and bladder, as well as in some lymphoid malignancies [
4,
22‐
24,
26,
27]. Interestingly, the methylation status of ASS1 has been linked to platinum resistance in ovarian cancer [
22]. Furthermore, it was found that patients treated with first line platinum/paclitaxel for ovarian cancer had a poor overall and disease-free survival if the tumor exhibited methylated ASS1 compared to unmethylated ASS1 [
22,
28]. In addition, methylated ASS1 has been linked to increased proliferation and invasion of bladder cancer cells [
24].
HCC is one of the most common cancers in the world, especially in Asia and Africa [
29]. Cisplatin has been commonly used as a chemotherapeutic agent for HCC, but it has not satisfactorily improved the survival rate for patients with advanced HCC, either as a single agent or in combination, due to acquired or intrinsic drug resistance [
30]. Intriguingly, drug resistance is an important contributor for treatment failure of ASS1-negative tumors by ADI-PEG 20, possibly due to re-expression of the once-silenced ASS1 that has been observed in melanoma cell lines [
31‐
33]. To overcome this type of resistance, a second drug must be added to drive cell death. For example, it has been observed that the combination of ADI-PEG 20 and cisplatin can increase apoptosis in melanoma cell lines [
34]. In addition, combined treatment of oxaliplatin and human arginase in HCC exhibited synergistic inhibiting effect on tumor growth [
35], providing further support that a platinum and an arginine-deprivation agent would be a good combination in this cancer. ADI-PEG 20 is currently being utilized in several clinical trials, including a global phase III trial for HCC as a monotherapy, as well as in combination with cytotoxics such as cisplatin for the treatment of melanoma and ovarian cancer.
Previous work has shown that the sensitivity of HCC cell lines to ADI-PEG 20 is due to the absence of ASS1 [
3]. However, the mechanism of ASS1 silencing, as well as the correlation with platinum resistance has not been explored in HCC. In addition, although ASS1 loss has been identified as a potential indicator of arginine auxotrophy in cancer, its regulation is complex and its use as a biomarker in combination therapy is unfamiliar. The current investigation was initiated to elucidate the relationship between ASS1 protein expression, ADI-PEG 20 sensitivity and cisplatin resistance, as well as to assess ASS1 regulation in response to cisplatin and in combination with ADI-PEG 20 in HCC. Utilizing several human HCC cell lines with varying amounts of ASS1, we report that ASS1 silencing confers sensitivity to ADI-PEG 20 and resistance to cisplatin. A good correlation is also observed between the methylation status of the ASS1 promoter, sensitivity to ADI-PEG 20 and resistance to cisplatin. In addition, cisplatin treatment down-regulates ASS1 protein expression in select HCC cell lines. Finally, the expression level of ASS1 during combination drug treatments with ADI-PEG 20 and cisplatin is cell line and concentration-dependent, but is predominantly dictated by ADI-PEG 20 at more clinically relevant concentrations. Taken together, our data indicate that ADI-PEG 20 and cisplatin will complement each other in a clinically relevant heterogeneous tumor, thus providing a rationale for combining these two drugs for the treatment of HCC.
Methods
Cell culture
The following human HCC cell lines were obtained from Dr. Yuh-Shan Jou at the Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan: Sk-Hep1, Huh7, Tong, HCC36, Hep3B, Malhavu, PLC5 and Huh6. The human HCC cell lines HepG2, SNU398 and SNU182 were from American Type Culture Collection (ATCC, Manassas, VA). A2780 is an ovarian cancer cell line (cisplatin sensitive) derived from a patient prior to treatment and A2780CR is a cisplatin-resistant cell line that was developed by exposure of the parent A2780 cell line to increasing concentrations of cisplatin. Both A2780 and A2780CR cell lines were obtained from Sigma-Aldrich (St. Louis, MO). The following cells were grown in Dulbecco’s Modified Eagle Medium (DMEM) (Lonza, Allendale, NJ) containing 10% heat-inactivated fetal bovine serum (FBS; Life Technologies, Carlsbad, CA), 1% L-glutamine (Life Technologies) and 1% non-essential amino acids (NEAA, Life Technologies): Sk-Hep1, Huh7, Tong, HCC36, Hep3B, Malhavu, PLC5, Huh6 and HepG2. SNU398, SNU182 and the ovarian cell lines were maintained in RPMI 1640 (Lonza) with 10% heat-inactivated FBS and 1% L-glutamine. All cells were sub-cultured two times a week using trypsin/EDTA (Life Technologies) and were grown at 37°C in 5% CO2.
Cell viability assay
Cell viability (IC50) values for ADI-PEG 20 and cisplatin (Sigma-Aldrich) were determined using the CellTiter-Glo (CTG) luminescent cell viability assay (Promega, Madison, WI). Cells (3,000-6,000 cells/well) were plated in 100 μL medium/well in 96-well black micro-clear plates (Greiner bio-one, Monroe, NC). Following overnight incubation at 37°C and 5% CO2, cells were exposed to a range of drug concentrations from a 50X plate (2 μL/well). Each concentration of drug was added to duplicate wells. After 72 h incubation, 25 μL/well of CTG reagent was added directly to the medium and the plates were shaken for 5 min, resulting in cell lysis and the generation of a luminescent signal proportional to the amount of ATP present. The luminescence values were read on a SpectraMax M3 microplate reader (Molecular Devices, Sunnyvale, CA) and converted to a percent cell viability that was calculated relative to the viability in corresponding matched DMSO-treated cells, which was designated as 100% viable. IC50 values (concentration of drug that results in 50% of luminescence signal compared with the DMSO-treated control) were obtained from nonlinear regression analysis of concentration-effect curves using GraphPad Prism version 6.0 software (San Diego, CA).
Immunoblot analysis
Whole-cell extracts were made from 90% confluent cultures of all human cells. Cells were lysed in RIPA buffer (Sigma-Aldrich), with added protease inhibitor cocktail (Roche Molecular Systems, Pleasanton, CA) and PMSF (Sigma-Aldrich). Total lysate protein was quantified using a Coomassie Plus (Bradford) Protein Assay Reagent (Pierce, Rockford, IL). Cell extracts (20 μg) were run on NuPage 4-12% Bis-Tris Gels (Life Technologies) and then transferred to PVDF membranes (Life Technologies). The membranes were blocked in TBST buffer (Tris-HCL, 0.1% Tween) containing 5% Blotting-Grade Blocker (Bio-Rad, Hercules, CA) for 2 h at room temperature and then probed using a mouse monoclonal antibody to ASS1 (Polaris Pharmaceuticals, in-house) at a dilution of 1:500. GAPDH was used as a loading control for each western blot, so the membranes were cut and also probed with a rabbit polyclonal antibody to GAPDH (Millipore, Billerica, MA) at a dilution of 1:10,000. The blots were incubated with both primary antibodies overnight at 4°C on a rocker. After washing with TBST buffer, the membranes were incubated with secondary antibodies: goat anti-mouse for ASS1 (Santa Cruz Biotechnology, Dallas, TX) (1:10,000) and goat anti-rabbit for GAPDH (Santa Cruz Biotechnology) (1:60,000) and incubated at room temperature for 1 h. The secondary antibodies were detected using either the SuperSignal West Pico (GAPDH) or Femto (ASS1) Chemiluminescent Substrate (Pierce) and blots were read on a Bio-Rad ChemiDox XRS + System. ASS1 and GAPDH levels were quantified using Image Lab Software (Bio-Rad, Hercules, CA).
For ASS1 protein determination after cisplatin treatments or for ADI-PEG 20 and cisplatin combination analysis, the same procedure was used with the following modifications. Cells were plated in two identical 96-well plates: one for cell viability and/or normalization for cell numbers between wells (luminescence assay; see Methods above) and one for lysis (ASS1 detection). After 72 h drug treatments, lysates were made and probed for protein analysis. For ASS1 and GAPDH detection, media was removed and each well of the microplate was washed with 100 μL of PBS buffer (Gibco by Life Technologies). NuPage LDS sample buffer (30 μL of 1x sample buffer, Life Technologies) containing 50 mM DTT was then added to each well and the plate was wrapped in parafilm and frozen at -80°C for at least one hour to ensure lysis. After lysis, the samples in each well were spun and then used for immunoblot analysis. To account for the different number of viable cells in each well of the microplate, samples were normalized using the relative luminescence values for each corresponding well of the identical microplate.
Bisulfite modification and methylation-specific PCR
The EZ DNA Methylation-Direct Kit (Zymo Research Corporation, Irvine, CA) was used to perform complete DNA bisulfite conversion directly from the human cell lines. This process converts unmethylated cytosine residues to uracil while methylated cytosine residues remain unchanged. In general, 10,000-40,000 cells (~60-250 ng genomic DNA) were used as starting material for each cell line. Methylation-specific PCR (MSP) of a 188 bp fragment located between 300 and 500 bp downstream of the transcription start site (TSS) was then performed to determine the methylation status of the ASS1 promoter. Bisulfite-modified DNA (150 ng) was used as a template for PCR reactions with primers specific for methylated (M) or unmethylated (UM) sequences. Primer sequences are: (1) M forward: 5′-TTTTTTTCGTTGTTTATTTTTTAGTC-3′; (2) M reverse: 5′-CTAAAATCCGATACCAAACGTT-3′; (3) UM forward: 5′-TTTT TGTTGTTTATTTTTTAGTTGA-3′ and (4) UM reverse: 5′-AACCTAAAATCCAATACCAAACATT-3′. Primers were purchased from IDT Technologies (San Diego, CA). PCR conditions for the methylated primers were as follows: 8 cycles of 95°C for 2 min, 54.8°C for 30 sec and 72°C for 30 sec were followed by 32 cycles of 95°C for 30 sec, 54.8°C for 30 sec and 72°C for 30 sec, then a final extension at 72°C for 10 min. The PCR conditions for the unmethylated primers were identical, except the annealing temperature was 48°C instead of 54.8°C. The HotStarTaq d-Tect polymerase (EpiTect MSP kit; Qiagen, Valencia, CA) was used for the PCR reactions. The PCR samples were run on 2% pre-cast agarose E-gels (Life Technologies) containing a fluorescent stain for visualization. The human methylated and non-methylated DNA set (Zymo Research) were used as negative and positive controls for bisulfite conversion efficiency, and MSP was similarly performed using a set of control primers designed to amplify non-methylated, methylated and mixed methylation copies of the death-associated protein kinase 1 gene (DAPK1), with an expected size of 274 bp.
Statistical analysis
GraphPad Prism version 6.0 was used to test results for statistical significance. Differences in ASS1 levels between groups were analyzed using an unpaired two-tailed t-test. A p value < 0.05 was set as a level of statistical significance. In determining statistical significance for ASS1 protein levels after cisplatin treatments or for ADI-PEG 20 and cisplatin combination analysis, each drug concentration was compared to the untreated, or zero drug, sample to attain a p value for that particular drug concentration.
Discussion
The future for the treatment of arginine auxotrophic cancers lies in combination therapies. Several ADI-PEG 20 and cisplatin combination trials are planned. Therefore, understanding the correlation between ASS1 expression and cisplatin and ADI-PEG 20 sensitivities, as well as how ASS1 is regulated by both drugs could provide valuable information for trial design. For the first time, we have shown that there is a reciprocal relationship between ASS1 expression and cisplatin resistance in several human HCC cell lines. We have observed that resistance is specific to cisplatin, as sensitivity to other platinums and chemotherapeutic agents are unaffected by ASS1 expression. In addition, methylation of the ASS1 promoter does associate with sensitivity to ADI-PEG 20, and in HCC, also corresponds with cisplatin resistance, as previously demonstrated in ovarian cancer [
22]. These findings suggest that the methylation status of the ASS1 promoter in tumors may predict sensitivity to arginine deprivation with ADI-PEG 20 and also support the future prospect of using methylation profiling to identify which HCC patients may benefit from either cisplatin or ADI-PEG 20.
Our novel data also indicate that cisplatin down-regulates ASS1 protein expression in three HCC cell lines. How exactly cisplatin is affecting ASS1 levels during these acute treatments is currently unknown. Previously published studies indicate that ASS1 regulation occurs at the transcriptional level [
37‐
40]. For example, it has been demonstrated that glutamine stimulated ASS1 expression in Caco-2 cells through O-glycosylation of the transcription factor Sp1 [
40], while expression of the ASS1 gene has been shown to be stimulated by interleukin-1β in Caco-2 cells through activation of the transcription factor nuclear factor-ĸβ [
38]. In melanoma cells, hypoxia-inducible factor (HIF-1α)-mediated transcriptional repression of ASS1 has been observed [
31,
33]. Other factors have been shown to positively or negatively regulate ASS1 expression. For example, cAMP increases ASS1 expression, while fatty acids cause suppression of this protein [
41,
42], and factors such as hormones and pro-inflammatory stimuli are also known to regulate ASS1 expression [
39,
43].
Interestingly, there is suggestion that acquired resistance to cytotoxic agents occurs predominantly via epigenetic events [
44,
45]. A significant function for ASS1 in regulating platinum sensitivity via methylation of the ASS1 promoter has been observed in ovarian cancer utilizing the A2780 and A2780CR cell lines [
22]. The A2780CR cell line was established by intermittent exposure of the parental A2780 cell line to stepwise, increasing concentrations of cisplatin up to a concentration of 8 μM over a period of approximately 9 months [
46]. This cell line was found to be 7.3-fold more resistant than the parental line, and it was indicated that this degree of resistance in the A2780CR cell line was stable for at least nine months during subculture in drug-free medium. Our experience with a commercially available A2780CR cell line is similar. We have observed that A2780CR does not express ASS1, is 12-fold more resistant to cisplatin than the parental cell line, and is completely methylated at the ASS1 promoter after being subcultured in cisplatin-free medium for 2 months. Given the similarities to ovarian cancer that we have observed in our HCC cell lines regarding ASS1 expression, methylation status of the ASS1 promoter and cisplatin resistance, we are currently establishing a HepG2 cisplatin-resistant (HepG2CR) cell line by progressively exposing HepG2 cells to increasing cisplatin. Preliminary data indicate a three-fold IC
50 value increase for cisplatin in HepG2CR over the parental cell line after only one month of drug exposure. Once we acquire a more permanent resistant phenotype, we will determine the methylation status of HepG2CR and perform other analyses to understand the mechanisms of acquired cisplatin resistance in HCC.
Several ADI-PEG 20 combination trials are ongoing or planned, including a combination with cisplatin for metastatic melanoma, ovarian cancer and other solid tumors, docetaxel for prostate and non-small cell lung cancer (NSCLC), doxorubicin for breast cancer, and cisplatin and pemetrexed for NSCLC and malignant pleural mesothelioma [
20]. We have determined that ASS1 loss is a biomarker of cisplatin resistance and ADI-PEG 20 sensitivity, whereas ASS1 positivity is an indicator of cisplatin sensitivity and ADI-PEG 20 resistance in HCC cell lines. This observation suggests that a cisplatin and ADI-PEG 20 regimen should be superior to either drug alone for the treatment of HCC patients. To examine the potential for the use of ASS1 as a predictor in combination therapy, we sought to determine the ASS1 levels in HCC cells with both drugs present. Predictably, we found that the ASS1 protein levels will be dictated by one of the two drugs and is concentration-and cell-line dependent. In two of the three cell lines tested, the ASS1 levels seemed to be controlled by ADI-PEG 20, while cisplatin was able to maintain low ASS1 levels in the remaining cell line. Obviously, it is hard to predict clinical behavior from cell-based assays. We believe that ADI-PEG 20 will influence the ASS1 level of this two-drug regimen at more clinically relevant concentrations, resulting in higher ASS1 levels. This observation suggests that long term treatment with this combination could result in cisplatin resistant cells becoming cisplatin sensitive. Furthermore, several groups have observed that reduced expression of ASS1 is significantly associated with advanced tumor stage and an association with a worse clinical outcome [
21‐
25]. These observations imply that the higher ASS1 levels present with the addition of ADI-PEG 20 to cisplatin may elicit better clinical outcomes for HCC patients. Extending these observations further to the clinic, our results suggest that while ASS1 may be a predictive biomarker for either ADI-PEG 20 or cisplatin as a single agent or pre-therapy, the status of this indicator may change by addition of the second drug and possibly evolve during tumor progression or metastasis. This concept of intratumoral heterogeneity within the same patient is growing in recognition and discordance of predictive or prognostic biomarker testing results between primary tumor and metastases or resistance acquisition has been reported in several tumor types [
47].
Conclusion
Our data support the rationale of combining cisplatin and ADI-PEG 20 in the clinical treatment of HCC. We believe these two drugs will be complementary in a clinically relevant heterogeneous tumor. Furthermore, in HCC, sensitivity to ADI-PEG 20 may be superior in cases that have relapsed after cisplatin-based chemotherapy. Extending beyond HCC to other cancers, our results suggest that in the combination setting, a patient does not necessarily need to have an ASS1-deficient tumor to reap benefit from an ADI-PEG 20 and cisplatin drug treatment.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
JAM conceived and designed the study, performed experimental supervision and coordination, conducted data analysis and interpretation and wrote the manuscript. HTL and KCW acquired data and performed analysis. SKK assisted in methodology development, helped performed data analysis and interpretation and provided experimental supervision and technical assistance with tissue culture. JAT critically reviewed the scientific content of the manuscript and assisted with drafts. All authors read and approved the final manuscript.