Pegylated arginine deiminase synergistically increases the cytotoxicity of gemcitabine in human pancreatic cancer.

BACKGROUND: Pancreatic ductal adenocarcinoma has proven to be one of the most chemo-resistant among all solid organ malignancies. Several mechanisms of resistance have been described, though few reports of strategies to overcome this chemo-resistance have been successful in restoring sensitivity to the primary chemotherapy (gemcitabine) and enter the clinical treatment arena. METHODS: We examined the ability of cellular arginine depletion through treatment with PEG-ADI to alter in vitro and in vivo cytotoxicity of gemcitabine. The effect on levels of key regulators of gemcitabine efficacy (e.g. RRM2, hENT1, and dCK) were examined. RESULTS: Combination of PEG-ADI and gemcitabine substantially increases growth arrest, leading to increased tumor response in vivo. PEG-ADI is a strong inhibitor of the gemcitabine-induced overexpression of ribonucleotide reductase subunit M2 (RRM2) levels both in vivo and in vitro, which is associated with gemcitabine resistance. This mechanism is through the abrogation of the gemcitabine-mediated inhibitory effect on E2F-1 function, a transcriptional repressor of RRM2. CONCLUSION: The ability to alter gemcitabine resistance in a targeted manner by inducing metabolic stress holds great promise in the treatment of advanced pancreatic cancer.

Role of autophagy in apoptotic regulation by Akt in pancreatic cancer.

BACKGROUND/AIM: The Akt signaling pathway mediates a potent anti-apoptotic signal in pancreatic cancer and inhibition of this pathway has become an attractive mechanism to increase the efficacy of traditional chemotherapies. Autophagy is a lysosomal catabolic pathway by which eukaryotic cells recycle macromolecules and organelles. Although autophagy may function as a survival mechanism under metabolic stress conditions, it also serves as an alternate route to programmed cell death distinct from apoptosis. In the present study, we examined the role of autophagy in Akt-mediated regulation of cell death in pancreatic cancer. MATERIALS AND METHODS: Mia-PaCa-2 and PANC-1 human pancreatic cancer cell lines were used in our experiments. The small-molecule inhibitor A-443654 was used to inhibit Akt, and rapamycin was used to inhibit mTOR. Autophagy was inhibited with Chloroquine and 3-methyladenine. Autophagy was assessed by immunoblotting for light chain-3 (LC-3) processing as well as fluorescence microscopy for autophagosome formation following transfection with a LC-3/GFP construct. Cell death was determined by fluorescence-activated cell sorting (FACS) with quantitation of the sub-G0 content. RESULTS: Inhibition of either Akt or mTOR induced autophagy; inhibition of Akt but not of mTOR led to traditional caspase-mediated apoptosis. When autophagy was inhibited, cell death was abrogated following Akt, but not mTOR, inhibition. CONCLUSION: The Akt signaling pathway regulates both autophagy and apoptosis through divergent pathways; mTOR mediates autophagy signaling but appears to be un-involved in cell death. Autophagy appears to play a role in the regulation of cell survival by Akt, but only when proximal signaling pathways not involving mTOR are simultaneously activated.

The role of Akt activation in the response to chemotherapy in pancreatic cancer.

UNLABELLED: The PI3K/Akt signaling pathway is constitutively activated in some pancreatic cancers; when activated, it inhibits chemotherapy-mediated apoptosis. We examined whether Akt activity correlates with apoptotic resistance to chemotherapy in pancreatic cancer. MATERIALS AND METHODS: A panel of human pancreatic cancer cells was evaluated for basal Akt activity as well as response to three chemotherapies. Chemotherapy-induced cell death was evaluated following either up- or down-regulation of Akt activity. Evaluation of phosphorylation of p21Cip/Waf1, a downstream target of Akt, was also evaluated. RESULTS: There was a broad distribution among pancreatic cancer cell lines by Akt activity, as well as sensitivity to the three chemotherapeutic agents with no apparent correlation. Phosphorylation of p21Cip/Waf1, but not change in total levels, correlated with the chemosensitizing effect of Akt inhibition to paclitaxel. CONCLUSIONS: Basal Akt activity does not appear to be a useful predictor for selection of pancreatic cancers in targeting Akt to broadly induce chemosensitivity.

Targeting Bcl-2-mediated cell death as a novel therapy in pancreatic cancer.

BACKGROUND: Bcl-2 is an essential regulator of programmed cell death (PCD). Overexpression of Bcl-2 is common in pancreatic cancer; the high levels have been shown to correlate with resistance to PCD. This resistance is mediated by binding of Bcl-2 via its BH-3 domain to diverse proteins, including the Bax/Bak family members, various protein kinases, and beclin 1, which are involved in regulation of autophagy (type II PCD). Small molecule inhibitors of BH-3-mediated binding of Bcl-2 have been recently developed, although no investigation has been conducted in pancreatic cancer, a malignancy characterized by extreme resistance to PCD. METHODS: The effect of the Bcl-2 binding inhibitor A-779024 on PCD was assessed by fluorescence activated cell sorting; the effect on Bcl-2 and other PCD-related proteins was analyzed by immunoblotting. Induction of autophagy was determined by fluorescence microscopy using a stably transfected GFP-LC3 construct to visualize autophagosome formation. Co-localization of Bcl-2 with binding partners regulating PCD was examined by immunoprecipitation and confocal immunofluorescent microscopy. RESULTS: A-779024 induced PCD in a dose- and time-dependent fashion. No change was seen in the protein levels of Bcl-2, Bax, Bcl-XL, or Mcl-1. Contrary to prediction, A-779024 was ineffective at inducing autophagy in these cells. Co-localization studies demonstrated that Bcl-2 was not bound to beclin 1 and, therefore, treatment with A-779024 could not induce release of beclin 1 and initiation of autophagy. CONCLUSIONS: Disruption of Bcl-2 activity using the small molecule inhibitor A-779024 induces apoptotic but not autophagic PCD. This approach may be a novel therapy, either alone or in combination with other treatments such as chemotherapy or autophagy modulating agents in pancreatic cancer.

ERK/BCL-2 pathway in the resistance of pancreatic cancer to anoikis.

BACKGROUND: Anoikis is a special type of programmed cell death after loss of cell-cell and cell-extracellular matrix interactions. Resistance to anoikis is likely involved in the process of metastasis, specifically during the tumor cell migration through lymph or vascular channels. We have previously shown that BCL-2 confers resistance to other forms of programmed cell death (i.e., apoptosis); furthermore, the extracellular signaling-regulated kinase (ERK) signaling pathway regulates BCL-2 expression. We therefore tested the hypothesis that pancreatic cancer cell lines are resistant to anoikis and this resistance is due to activation of ERK1/2 and subsequent overexpression of BCL-2. MATERIALS AND METHODS: Pancreatic cancer cell lines (MIA-PaCa-2 and BxPC-3) were examined for cell death following loss of adherence to extracellular matrix. Subclones of the MIA-PaCa-2 cell line (either selected in vivo for increased metastatic potential [MIA-LM2] or overexpressing BCL-2 [MIA-BCL2]) were also examined for induction of anoikis following loss of extracellular matrix adherence. Finally, the effect of the ERK inhibitor (PD98059) on BCL-2 expression and induction of anoikis was examined. RESULTS: Under conditions of loss of cell-extracellular matrix interaction, pancreatic cancer cells undergo varying amounts of anoikis. Basal levels of activated ERK and BCL-2 paralleled the sensitivity to induction of anoikis. The highly metastatic cell line, MIA-LM2, was more resistant to anoikis than the parental cell line. Inhibition of ERK down-regulated BCL-2 and was associated with restoration of sensitivity to anoikis. CONCLUSIONS: Activation of a signaling pathway from ERK to overexpression of BCL-2 may confer resistance to anoikis, a critical step in the development of metastasis. Targeting the ERK/BCL-2 pathway may lead to sensitization of pancreatic cancer to anoikis, thereby decreasing the ability of these cells to metastasize.

Pancreatic cancer cell lines deficient in argininosuccinate synthetase are sensitive to arginine deprivation by arginine deiminase.

Eukaryotic cells can synthesize the non-essential amino acid arginine from aspartate and citrulline using the enzyme argininosuccinate synthetase (ASS). It has been observed that ASS is underexpressed in various types of cancers ASS, for which arginine become auxotrophic. Arginine deiminase (ADI) is a prokaryotic enzyme that metabolizes arginine to citrulline and has been found to inhibit melanoma and hepatoma cancer cells deficient of ASS. We tested the hypothesis that pancreatic cancers have low ASS expression and therefore arginine deprivation by ADI will inhibit cell growth. ASS expression was examined in 47 malignant and 20 non-neoplastic pancreatic tissues as well as a panel of human pancreatic cancer cell lines. Arginine deprivation was achieved by treatment with a recombinant form of ADI formulated with polyethylene glycol (PEG-ADI). Effects on caspase activation, cell growth and cell death were examined. Furthermore, the effect of PEG-ADI on the in vivo growth of pancreatic xenografts was examined. Eighty-seven percent of the tumors lacked ASS expression; 5 of 7 cell lines similarly lacked ASS expression. PEG-ADI specifically inhibited growth of those cell lines lacking ASS. PEG-ADI treatment induced caspase activation and induction of apoptosis. PEG-ADI was well tolerated in mice despite complete elimination of plasma arginine; tumor growth was inhibited by approximately 50%. Reduced expression of ASS occurs in pancreatic cancer and predicts sensitivity to arginine deprivation achieved by PEG-ADI treatment. Therefore, these findings suggest that arginine deprivation by ADI could provide a beneficial strategy for the treatment of pancreatic cancer, a malignancy in which new therapy is desperately needed.

BCL-2 functions as an activator of the AKT signaling pathway in pancreatic cancer.

BCL-2 is the prototypic anti-apoptotic protein involved in the regulation of apoptosis. Overexpression of BCL-2 is common in pancreatic cancer and confers resistance to the apoptotic effect of chemo- and radiotherapy. Although these cellular effects of BCL-2 are traditionally related to pathways involving the mitochondrial membrane, we sought to investigate whether BCL-2 is involved in other signaling pathways regulating cell survival and focused on AKT. We examined the effect of overexpression of BCL-2 in the MIA-PaCa-2 human pancreatic cancer cell line on the function and subcellular location of AKT. We observed that the stable subclones of MIA-PaCa-2 overexpressing BCL-2 demonstrated increased activity of AKT as well as IKK (a downstream target of AKT), increasing the transcriptional activity of NF-kappaB. Using immunoprecipitation techniques, we observed co-immunoprecipitation of AKT and BCL-2. Immunocytochemistry demonstrated co-localization of BCL-2 and AKT, which was abrogated by treatment with HA14-1, a small molecule inhibitor of BH-3-mediated protein interaction by BCL-2. Furthermore, treatment with HA14-1 decreased phosphorylation of AKT and increased sensitivity to the apoptotic effect of the chemotherapeutic agent, paclitaxel. These results demonstrate an additional mechanism of regulation of cell survival mediated by BCL-2, namely through AKT activation, in the MIA-PaCa-2 pancreatic cancer cell line. Therefore, directed inhibition of BCL-2 may alter diverse pathways controlling cell survival and overcome the apoptotic resistance that is the hallmark of pancreatic cancer.

Reduction in BCL-2 levels by 26S proteasome inhibition with bortezomib is associated with induction of apoptosis in small cell lung cancer.

Overexpression of the anti-apoptotic protein BCL-2 is frequently observed in small cell lung cancers (SCLC) and is associated with chemoresistance. We examined the signaling pathways involved in upregulation of BCL-2 in SCLC, and whether inhibition of NF-kappaB using the 26S proteasome inhibitor bortezomib had any effect on BCL-2 levels or apoptosis. Mutation of a NF-kappaB site in the BCL-2 promoter reduced promoter activity to less than 20% of the wild-type promoter. Treatment with bortezomib resulted in decreased transcription of the BCL-2 promoter, decreased BCL-2 levels, and induced apoptosis. These data provide the necessary laboratory background for further investigation of bortezomib in the treatment of SCLC.

Targeting BCL-2 overexpression in various human malignancies through NF-kappaB inhibition by the proteasome inhibitor bortezomib.

BACKGROUND: BCL-2 overexpression occurs in many cancer types and is associated with chemoresistance and radioresistance. The mechanisms responsible for its aberrant expression are thought to be transcriptionally mediated but remain unclear. We examined the cell type-specific mechanism of BCL-2 gene transcription in various solid organ malignancies. METHODS: Regulation of BCL-2 gene transcription was examined in seven different human cancer cell lines including two pancreatic (MIA-PaCa-2, PANC-1), two prostate (LNCaP, PC-3), two lung (Calu-1, A549) and one breast (MCF-7) cancer cell line. Cells were treated with inhibitors of phosphatidylinositol-3 kinase (PI3K), MEK/ERK, and p38MAPK. The effect of mutation of a NF-kappaB site in the BCL-2 promoter was determined, as was the effect of inhibition of NF-kappaB function using a 26S proteasome inhibitor (bortezomib) on both BCL-2 transcription and induction of apoptosis. RESULTS: BCL-2 expression varied both between and within tumor types; four of seven cell lines demonstrated high BCL-2 levels (MIA-PaCa-2, PC-3, Calu-1 and MCF-7). No signaling pathway was uniformly responsible for overexpression of BCL-2; however, mutation of the NF-kappaB site decreased BCL-2 promoter activity in all cell lines. Inhibition of NF-kappaB activity decreased BCL-2 protein levels independently of the signaling pathway involved in transcriptional activation of the BCL-2 gene. CONCLUSIONS: Diverse signaling pathways variably regulate BCL-2 gene expression in a cell type-specific fashion. Therapy to decrease BCL-2 levels in various human cancers would be more broadly applicable if targeted to transcriptional activation rather than signal transduction cascades. Finally, the apoptotic efficacy of proteasome inhibition with bortezomib paralleled the ability to inhibit NF-kappaB activity and decrease BCL-2 levels.

Effects of the proteasome inhibitor bortezomib alone and in combination with chemotherapy in the A549 non-small-cell lung cancer cell line.

BACKGROUND: Non-small-cell lung cancer (NSCLC) has a poor prognosis. Despite advances in therapy, survival has improved only slightly. The 26S proteasome regulates multiple cellular processes through degradation of ubiquitin-tagged proteins. The proteasome inhibitor, bortezomib (Velcade, formerly PS-341), has been shown to be an active anticancer agent both in vitro and in vivo in multiple tumor types. PURPOSE: To determine the molecular and cellular effects of the proteasome inhibitor in NSCLC as well as to evaluate the effectiveness of sequential treatment with bortezomib and gemcitabine/carboplatin (G/C) chemotherapy both in vitro and in vivo. METHODS: All experiments were performed in the A549 NSCLC cell line. MTT assays were used to evaluate cytotoxicity. Western blotting evaluated protein levels. Measures of apoptosis included FACS analysis, DAPI staining and caspase-3 cleavage. Long-term cell viability was determined using an anchorage-dependent clonogenic assay. Sequential studies were performed in vitro and in vivo. RESULTS: Bortezomib increased p21(waf1/cip1), induced G(2)/M arrest, and triggered a small amount of apoptosis. The apoptotic effect of G/C chemotherapy was eliminated when bortezomib was administered prior to the chemotherapy; however, it was accentuated when the bortezomib was given simultaneously or after the chemotherapy. CONCLUSIONS: Bortezomib improves efficacy in combination with gemcitabine and carboplatin in NSCLC, but sequential effects are important and must be considered when developing therapeutic regimens.

Inhibition of AKT abrogates chemotherapy-induced NF-kappaB survival mechanisms: implications for therapy in pancreatic cancer.

BACKGROUND: When activated, the nuclear factor (NF)-kappaB pathway is a potent cellular signal that inhibits apoptotic cell death. Pancreatic cancer is resistant to the apoptotic effect of chemotherapy, though it is unclear whether this is an inherent feature or a survival signal engaged in response to chemotherapy. We investigated whether pancreatic cancer cells activate the NF-kappaB pathway in response to chemotherapy and whether inhibition of this response altered the apoptotic efficacy of chemotherapy. STUDY DESIGN: We determined NF-kappaB activity after chemotherapy treatment of the MIA-PaCa-2 human pancreatic cancer cell line using both physical (electrophoretic mobility shift assay) and functional (luciferase) techniques. The effect of chemotherapy on transcription of the antiapoptotic gene BCL-2, a target of NF-kappaB, was determined. We examined the effect of inhibition of Akt, an upstream activator of NF-kappaB, on the molecular (NF-kappaB function and BCL-2 transcription) and cellular (apoptosis) effect of chemotherapy. RESULTS: Both the chemotherapeutic agents gemcitabine and paclitaxel activated NF-kappaB and stimulated BCL-2 gene promoter activity. The stimulation of BCL-2 promoter function was directly regulated by NF-kappaB. These cellular responses were blocked by inhibition of Akt. The apoptotic effect of gemcitabine and paclitaxel also was enhanced after Akt inhibition. CONCLUSIONS: Part of the apoptotic resistance of pancreatic cancer may be mediated by activation of the NF-kappaB survival pathway in response to chemotherapy. Inhibition of this response may be an important adjunct to increase the efficacy of chemotherapy.