Targeting the Interplay of Independent Cellular Pathways and Immunity: A Challenge in Cancer Immunotherapy

Immunotherapy is a cancer treatment that exploits the capacity of the body's immune system to prevent, control, and remove cancer. Immunotherapy has revolutionized cancer treatment and significantly improved patient outcomes for several tumor types. However, most patients have not benefited from such therapies yet. Within the field of cancer immunotherapy, an expansion of the combination strategy that targets independent cellular pathways that can work synergistically is predicted. Here, we review some consequences of tumor cell death and increased immune system engagement in the modulation of oxidative stress and ubiquitin ligase pathways. We also indicate combinations of cancer immunotherapies and immunomodulatory targets. Additionally, we discuss imaging techniques, which are crucial for monitoring tumor responses during treatment and the immunotherapy side effects. Finally, the major outstanding questions are also presented, and directions for future research are described.

Dabrafenib-Panobinostat Salt: Improving the Dissolution Rate and Inhibition of BRAF Melanoma Cells

Cocrystallization of the drug-drug salt-cocrystal of the histone deacetylase inhibitor (HDACi) panobinostat (PAN) and b-rapidly accelerated fibrosarcoma (BRAF) inhibitor dabrafenib (DBF) afforded single crystals of a two-drug salt stabilized by N⁺-H···O and N⁺-H···N^- hydrogen bonds between the ionized panobinostat ammonium donor and dabrafenib sulfonamide anion acceptor in a 12-member ring motif. A faster dissolution rate for both drugs was achieved through the salt combination compared to the individual drugs in an aqueous acidic medium. The dissolution rate exhibited a peak concentration (C_max) of approximately 310 mg cm^-2 min^-1 for PAN and 240 mg cm^-2 min^-1 for DBF at a T_max of less than 20 min under gastric pH 1.2 (0.1 N HCl) compared to the pure drug dissolution values of 10 and 80 mg cm^-2 min^-1, respectively. The novel and fast-dissolving salt DBF^-·PAN⁺ was analyzed in BRAF^V600E melanoma cells Sk-Mel28. DBF^-·PAN⁺ reduced the dose-response from micromolar to nanomolar concentrations and lowered IC₅₀ (21.9 ± 7.2 nM) by half compared to PAN alone (45.3 ± 12.0 nM). The enhanced dissolution and lower survival rate of melanoma cells show the potential of novel DBF^-·PAN⁺ salt in clinical evaluation.

Serum Mass Spectrometry Proteomics and Protein Set Identification in Response to FOLFOX-4 in Drug-Resistant Ovarian Carcinoma

Ovarian cancer is a highly lethal gynecological malignancy. Drug resistance rapidly occurs, and different therapeutic approaches are needed. So far, no biomarkers have been discovered to predict early response to therapies in the case of multi-treated ovarian cancer patients. The aim of our investigation was to identify a protein panel and the molecular pathways involved in chemotherapy response through a combination of studying proteomics and network enrichment analysis by considering a subset of samples from a clinical setting. Differential mass spectrometry studies were performed on 14 serum samples from patients with heavily pretreated platinum-resistant ovarian cancer who received the FOLFOX-4 regimen as a salvage therapy. The serum was analyzed at baseline time (T0) before FOLFOX-4 treatment, and before the second cycle of treatment (T1), with the aim of understanding if it was possible, after a first treatment cycle, to detect significant proteome changes that could be associated with patients responses to therapy. A total of 291 shared expressed proteins was identified and 12 proteins were finally selected between patients who attained partial response or no-response to chemotherapy when both response to therapy and time dependence (T0, T1) were considered in the statistical analysis. The protein panel included APOL1, GSN, GFI1, LCATL, MNA, LYVE1, ROR1, SHBG, SOD3, TEC, VPS18, and ZNF573. Using a bioinformatics network enrichment approach and metanalysis study, relationships between serum and cellular proteins were identified. An analysis of protein networks was conducted and identified at least three biological processes with functional and therapeutic significance in ovarian cancer, including lipoproteins metabolic process, structural component modulation in relation to cellular apoptosis and autophagy, and cellular oxidative stress response. Five proteins were almost independent from the network (LYVE1, ROR1, TEC, GFI1, and ZNF573). All proteins were associated with response to drug-resistant ovarian cancer resistant and were mechanistically connected to the pathways associated with cancer arrest. These results can be the basis for extending a biomarker discovery process to a clinical trial, as an early predictive tool of chemo-response to FOLFOX-4 of heavily treated ovarian cancer patients and for supporting the oncologist to continue or to interrupt the therapy.

Telomere Dysfunction Is Associated with Altered DNA Organization in Trichoplein/Tchp/Mitostatin (TpMs) Depleted Cells

Recently, we highlighted a novel role for the protein Trichoplein/TCHP/Mitostatin (TpMs), both as mitotic checkpoint regulator and guardian of chromosomal stability. TpMs-depleted cells show numerical and structural chromosome alterations that lead to genomic instability. This condition is a major driving force in malignant transformation as it allows for the cells acquiring new functional capabilities to proliferate and disseminate. Here, the effect of TpMs depletion was investigated in different TpMs-depleted cell lines by means of 3D imaging and 3D Structured illumination Microscopy. We show that TpMs depletion causes alterations in the 3D architecture of telomeres in colon cancer HCT116 cells. These findings are consistent with chromosome alterations that lead to genomic instability. Furthermore, TpMs depletion changes the spatial arrangement of chromosomes and other nuclear components. Modified nuclear architecture and organization potentially induce variations that precede the onset of genomic instability and are considered as markers of malignant transformation. Our present observations connect the tumor suppression ability of TpMs with its novel functions in maintaining the proper chromosomal segregation as well as the proper telomere and nuclear architecture. Further investigations will investigate the connection between alterations in telomeres and nuclear architecture with the progression of human tumors with the aim of developing personalized therapeutic interventions.

Destabilizers of the thymidylate synthase homodimer accelerate its proteasomal degradation and inhibit cancer growth

Drugs that target human thymidylate synthase (hTS), a dimeric enzyme, are widely used in anticancer therapy. However, treatment with classical substrate-site-directed TS inhibitors induces over-expression of this protein and development of drug resistance. We thus pursued an alternative strategy that led us to the discovery of TS-dimer destabilizers. These compounds bind at the monomer-monomer interface and shift the dimerization equilibrium of both the recombinant and the intracellular protein toward the inactive monomers. A structural, spectroscopic, and kinetic investigation has provided evidence and quantitative information on the effects of the interaction of these small molecules with hTS. Focusing on the best among them, E7, we have shown that it inhibits hTS in cancer cells and accelerates its proteasomal degradation, thus causing a decrease in the enzyme intracellular level. E7 also showed a superior anticancer profile to fluorouracil in a mouse model of human pancreatic and ovarian cancer. Thus, over sixty years after the discovery of the first TS prodrug inhibitor, fluorouracil, E7 breaks the link between TS inhibition and enhanced expression in response, providing a strategy to fight drug-resistant cancers.

Folic Acid-Peptide Conjugates Combine Selective Cancer Cell Internalization with Thymidylate Synthase Dimer Interface Targeting

Drug-target interaction, cellular internalization, and target engagement should be addressed to design a lead with high chances of success in further optimization stages. Accordingly, we have designed conjugates of folic acid with anticancer peptides able to bind human thymidylate synthase (hTS) and enter cancer cells through folate receptor α (FRα) highly expressed by several cancer cells. Mechanistic analyses and molecular modeling simulations have shown that these conjugates bind the hTS monomer-monomer interface with affinities over 20 times larger than the enzyme active site. When tested on several cancer cell models, these conjugates exhibited FRα selectivity at nanomolar concentrations. A similar selectivity was observed when the conjugates were delivered in synergistic or additive combinations with anticancer agents. At variance with 5-fluorouracil and other anticancer drugs that target the hTS catalytic pocket, these conjugates do not induce overexpression of this protein and can thus help combating drug resistance associated with high hTS levels.

The 1,10-Phenanthroline Ligand Enhances the Antiproliferative Activity of DNA-Intercalating Thiourea-Pd(II) and -Pt(II) Complexes Against Cisplatin-Sensitive and -Resistant Human Ovarian Cancer Cell Lines

Ovarian cancer is the most lethal gynecological malignancy, often because of the frequent insurgence of chemoresistance to the drugs currently used. Thus, new therapeutical agents are needed. We tested the toxicity of 16 new DNA-intercalating agents to cisplatin (cDDP)-sensitive human ovarian carcinoma cell lines and their resistant counterparts. The compounds were the complexes of Pt(II) or Pd(II) with bipyridyl (bipy) and phenanthrolyl (phen) and with four different thiourea ancillary ligands. Within each of the four series of complexes characterized by the same thiourea ligand, the Pd(phen) drugs invariably showed the highest anti-proliferative efficacy. This paralleled both a higher intracellular drug accumulation and a more efficient DNA intercalation than all the other metal-bidentate ligand combinations. The consequent inhibition of topoisomerase II activity led to the greatest inhibition of DNA metabolism, evidenced by the inhibition of the expression of the folate cycle enzymes and a marked perturbation of cell-cycle distribution in both cell lines. These findings indicate that the particular interaction of Pd(II) with phenanthroline confers the best pharmacokinetic and pharmacodynamic properties that make this class of DNA intercalators remarkable inhibitors, even of the resistant cell growth.

Cyclic Peptides Acting as Allosteric Inhibitors of Human Thymidylate Synthase and Cancer Cell Growth

Thymidylate synthase (TS) is a prominent drug target for different cancer types. However, the prolonged use of its classical inhibitors, substrate analogs that bind at the active site, leads to TS overexpression and drug resistance in the clinic. In the effort to identify anti-TS drugs with new modes of action and able to overcome platinum drug resistance in ovarian cancer, octapeptides with a new allosteric inhibition mechanism were identified as cancer cell growth inhibitors that do not cause TS overexpression. To improve the biological properties, 10 cyclic peptides (cPs) were designed from the lead peptides and synthesized. The cPs were screened for the ability to inhibit recombinant human thymidylate synthase (hTS), and peptide 7 was found to act as an allosteric inhibitor more potent than its parent open-chain peptide [Pro³]LR. In cytotoxicity studies on three human ovarian cancer cell lines, IGROV-1, A2780, and A2780/CP, peptide 5 and two other cPs, including 7, showed IC₅₀ values comparable with those of the reference drug 5-fluorouracil, of the open-chain peptide [d-Gln⁴]LR, and of another seven prolyl derivatives of the lead peptide LR. These promising results indicate cP 7 as a possible lead compound to be chemically modified with the aim of improving both allosteric TS inhibitory activity and anticancer effectiveness.

Exploring the Biological Activity of a Library of 1,2,5-Oxadiazole Derivatives Endowed With Antiproliferative Activity

BACKGROUND/AIM

The identification of a series of oxadiazole-based compounds, as promising antiproliferative agents, has been previously reported. The aim of this study was to explore the SAR of newly-synthesized oxadiazole derivatives and identify their molecular targets.

MATERIALS AND METHODS

A small library of 1,2,5-oxadiazole derivatives was synthetized and their antiproliferative activity was tested by the MTT assay. Their interaction with topoisomerase I was evaluated and a molecular docking study was performed.

RESULTS

Several candidates showed cytotoxicity towards two human tumor cell lines, HCT-116 (colorectal carcinoma) and HeLa (cervix adenocarcinoma). Some derivatives exhibited inhibitory effects on the catalytic activity of topoisomerase I and this effect was supported by docking studies.

CONCLUSION

The enzyme inhibition results, although not directly related to cytotoxicity, suggest that a properly modified 1,2,5 oxadiazole scaffold could be considered for the development of new anti-topoisomerase agents.

Repurposing of Drugs Targeting YAP-TEAD Functions

Drug repurposing is a fast and consolidated approach for the research of new active compounds bypassing the long streamline of the drug discovery process. Several drugs in clinical practice have been reported for modulating the major Hippo pathway's terminal effectors, namely YAP (Yes1-associated protein), TAZ (transcriptional co-activator with PDZ-binding motif) and TEAD (transcriptional enhanced associate domains), which are directly involved in the regulation of cell growth and tissue homeostasis. Since this pathway is known to have many cross-talking phenomena with cell signaling pathways, many efforts have been made to understand its importance in oncology. Moreover, this could be relevant to obtain new molecular tools and potential therapeutic assets. In this review, we discuss the main mechanisms of action of the best-known compounds, clinically approved or investigational drugs, able to cross-talk and modulate the Hippo pathway, as an attractive strategy for the discovery of new potential lead compounds.

pH-Promoted Release of a Novel Anti-Tumour Peptide by "Stealth" Liposomes: Effect of Nanocarriers on the Drug Activity in Cis-Platinum Resistant Cancer Cells

PURPOSE

To evaluate the potential effects of PEGylated pH-sensitive liposomes on the intracellular activity of a new peptide recently characterized as a novel inhibitor of the human thymidylate synthase (hTS) over-expressed in many drug-resistant human cancer cell lines.

METHODS

Peptide-loaded pH-sensitive PEGylated (PpHL) and non-PEGylated liposomes (nPpHL) were carefully characterized and delivered to cis-platinum resistant ovarian cancer C13* cells; the influence of the PpHL on the drug intracellular activity was investigated by the Western Blot analysis of proteins involved in the pathway affected by hTS inhibition.

RESULTS

Although PpHL and nPpHL showed different sizes, surface hydrophilicities and serum stabilities, both carriers entrapped the drug efficiently and stably demonstrating a pH dependent release; moreover, the different behavior against J774 macrophage cells confirmed the ability of PEGylation in protecting liposomes from the reticuloendothelial system. Comparable effects were instead observed against C13* cells and biochemical data by immunoblot analysis indicated that PEGylated pH-sensitive liposomes do not modify the proteomic profile of the cells, fully preserving the activity of the biomolecule.

CONCLUSION

PpHL can be considered as efficient delivery systems for the new promising anti-cancer peptide.

Proteomic and Bioinformatic Studies for the Characterization of Response to Pemetrexed in Platinum Drug Resistant Ovarian Cancer

Proteomics and bioinformatics are a useful combined technology for the characterization of protein expression level and modulation associated with the response to a drug and with its mechanism of action. The folate pathway represents an important target in the anticancer drugs therapy. In the present study, a discovery proteomics approach was applied to tissue samples collected from ovarian cancer patients who relapsed after the first-line carboplatin-based chemotherapy and were treated with pemetrexed (PMX), a known folate pathway targeting drug. The aim of the work is to identify the proteomic profile that can be associated to the response to the PMX treatment in pre-treatement tissue. Statistical metrics of the experimental Mass Spectrometry (MS) data were combined with a knowledge-based approach that included bioinformatics and a literature review through ProteinQuest™ tool, to design a protein set of reference (PSR). The PSR provides feedback for the consistency of MS proteomic data because it includes known validated proteins. A panel of 24 proteins with levels that were significantly different in pre-treatment samples of patients who responded to the therapy vs. the non-responder ones, was identified. The differences of the identified proteins were explained for the patients with different outcomes and the known PMX targets were further validated. The protein panel herein identified is ready for further validation in retrospective clinical trials using a targeted proteomic approach. This study may have a general relevant impact on biomarker application for cancer patients therapy selection.

Human Thymidylate Synthase Inhibitors Halting Ovarian Cancer Growth

Human thymidylate synthase (hTS) has an important role in DNA biosynthesis, thus it is essential for cell survival. TS is involved in the folate pathways, specifically in the de novo pyrimidine biosynthesis. Structure and functions are intimately correlated, account for cellular activity and, in a broader view, with in vivo mechanisms. hTS is a target for anticancer agents, some of which are clinical drugs. The understanding of the detailed mechanism of TS inhibition by currently used drugs and of the interaction with the mechanism of action of other anticancer agents can suggest new perspective of TS inhibition able to improve the anticancer effect and to overcome drug resistance. TS-targeting drugs in therapy today are inhibitors that bind at the active site and that mostly resemble the substrates. Nonsubstrate analogs offer an opportunity for allosteric binding and novel mode of inhibition in the cancer cells. This chapter illustrates the relationship among the large number of hTS actions at molecular and clinical levels, its role as a target for ovarian cancer therapy, in particular in cases of overexpression of hTS and other folate proteins such as those induced by platinum drug treatments, and address the potential combination of TS inhibitors with other suitable anticancer agents.

Heat shock protein 90 and serine/threonine kinase B-Raf inhibitors have overlapping chemical space

With the aid of computational design, we show that Hsp90 and B-Raf inhibitors have overlapping chemical space and we disclose the first-in-class dual inhibitors.

Virtual Screening and X-ray Crystallography Identify Non-Substrate Analog Inhibitors of Flavin-Dependent Thymidylate Synthase

Thymidylate synthase X (ThyX) represents an attractive target for tuberculosis drug discovery. Herein, we selected 16 compounds through a virtual screening approach. We solved the first X-ray crystal structure of Thermatoga maritima (Tm) ThyX in complex with a nonsubstrate analog inhibitor. Given the active site similarities between Mycobacterium tuberculosis ThyX (Mtb-ThyX) and Tm-ThyX, our crystal structure paves the way for a structure-based design of novel antimycobacterial compounds. The 1H-imidazo[4,5-d]pyridazine was identified as scaffold for the development of Mtb-ThyX inhibitors.

Internalization and stability of a thymidylate synthase Peptide inhibitor in ovarian cancer cells

Information on the cellular internalization and stability of the ovarian cancer cell growth inhibitor peptide, LSCQLYQR (LR), is vital for lead optimization. Ad-hoc-synthesized LR/fluorescent-probe conjugates were used to monitor the internalization of the peptide. Mass spectrometry was used to identify adducts resulting from the thiol reactivity of the cysteine residue in LR. A mechanistic model is proposed to explain the observed change in intracellular peptide amount over time. Structural modifications can be foreseen to improve the peptide stability.

Translational repression of thymidylate synthase by targeting its mRNA

Resistance to drugs targeting human thymidylate synthase (TS) poses a major challenge in the field of anti-cancer therapeutics. Overexpression of the TS protein has been implicated as one of the factors leading to the development of resistance. Therefore, repressing translation by targeting the TS mRNA could help to overcome this problem. In this study, we report that the compound Hoechst 33258 (HT) can reduce cellular TS protein levels without altering TS mRNA levels, suggesting that it modulates TS expression at the translation level. We have combined nuclear magnetic resonance, UV-visible and fluorescence spectroscopy methods with docking and molecular dynamics simulations to study the interaction of HT with a region in the TS mRNA. The interaction predominantly involves intercalation of HT at a CC mismatch in the region near the translational initiation site. Our results support the use of HT-like compounds to guide the design of therapeutic agents targeting TS mRNA.

Transcriptional activation and cell cycle block are the keys for 5-fluorouracil induced up-regulation of human thymidylate synthase expression

Background

5-fluorouracil, a commonly used chemotherapeutic agent, up-regulates expression of human thymidylate synthase (hTS). Several different regulatory mechanisms have been proposed to mediate this up-regulation in distinct cell lines, but their specific contributions in a single cell line have not been investigated to date. We have established the relative contributions of these previously proposed regulatory mechanisms in the ovarian cancer cell line 2008 and the corresponding cisplatin-resistant and 5-FU cross-resistant-subline C13*.

Methodology/Principal Findings

Using RNA polymerase II inhibitor DRB treated cell cultures, we showed that 70–80% of up-regulation of hTS results from transcriptional activation of TYMS mRNA. Moreover, we report that 5-FU compromises the cell cycle by blocking the 2008 and C13* cell lines in the S phase. As previous work has established that TYMS mRNA is synthesized in the S and G₁ phase and hTS is localized in the nuclei during S and G₂-M phase, the observed cell cycle changes are also expected to affect the intracellular regulation of hTS. Our data also suggest that the inhibition of the catalytic activity of hTS and the up-regulation of the hTS protein level are not causally linked, as the inactivated ternary complex, formed by hTS, deoxyuridine monophosphate and methylenetetrahydrofolate, was detected already 3 hours after 5-FU exposure, whereas substantial increase in global TS levels was detected only after 24 hours.

Conclusions/Significance

Altogether, our data indicate that constitutive TYMS mRNA transcription, cell cycle-induced hTS regulation and hTS enzyme stability are the three key mechanisms responsible for 5-fluorouracil induced up-regulation of human thymidylate synthase expression in the two ovarian cancer cell lines studied. As these three independent regulatory phenomena occur in a precise order, our work provides a feasible rationale for earlier observed synergistic combinations of 5-FU with other drugs and may suggest novel therapeutic strategies.

Newly synthesized curcumin derivatives: crosstalk between chemico-physical properties and biological activity

New curcumin analogues (ester and acid series) were synthesized with the aim to improve the chemical stability in physiological conditions and potential anticancer activity. Cytotoxicity against different tumorigenic cell lines (human ovarian carcinoma cells -2008, A2780, C13*, and A2780/CP, and human colon carcinoma cells HCT116 and LoVo) was tested to evaluate cellular specificity and activity. Physico-chemical properties such as acidity, lipophilicity, kinetic stability, and free radical scavenging activity were investigated to shed light on the structure-activity relationship and provide new attractive candidates for drug development. Most of ester derivatives show IC(50) values lower than curcumin and exhibit selectivity against colon carcinoma cells. Especially they are extremely active after 24 h exposure showing enhanced inhibitory effect on cell viability. The best performances of ester curcuminoids could be ascribed to their high lipophilicity that favors a greater and faster cellular uptake overcoming their apparently higher instability in physiological condition.

Distamycin A and derivatives as synergic drugs in cisplatin-sensitive and -resistant ovarian cancer cells

Acquired resistance to cisplatin (cDDP) is a multifactorial process that represents one of the main problems in ovarian cancer therapy. Distamycin A is a minor groove DNA binder whose toxicity has limited its use and prompted the synthesis of derivatives such as NAX001 and NAX002, which have a carbamoyl moiety and different numbers of pyrrolamidine groups. Their interaction with a B-DNA model and with an extended-TATA box model, [Polyd(AT)], was investigated using isothermal titration calorimetry (ITC) to better understand their mechanism of interaction with DNA and therefore better explain their cellular effects. Distamycin A interactions with Dickerson and Poly[d(AT)(6)] oligonucleotides show a different thermodynamic with respect to NAX002. The bulkier distamycin A analogue shows a non optimal binding to DNA due to its additional pyrrolamidine group. Cellular assays performed on cDDP-sensitive and -resistant cells showed that these compounds, distamycin A in particular, affect the expression of folate cycle enzymes even at cellular level. The optimal interaction of distamycin A with DNA may account for the down-regulation of both dihydrofolate reductase (DHFR) and thymidylate synthase (TS) and the up-regulation of spermidine/spermine N1-acetyltransferase (SSAT) caused by this compound. These effects seem differently modulated by the cDDP-resistance phenotype. NAX002 which presents a lower affinity to DNA and slightly affected these enzymes, showed a synergic inhibition profile in combination with cDDP. In addition, their combination with cDDP or polyamine analogues increased cell sensitivity to the drugs suggesting that these interactions may have potential for development in the treatment of ovarian carcinoma.

Spermidine/spermine N 1 -acetyltransferase transient overexpression restores sensitivity of resistant human ovarian cancer cells to N 1 ,N 12 -bis(ethyl)spermine and to cisplatin

The limited induction of spermidine/spermine N1-acetyltransferase (SSAT) activity has been implicated as an important determinant of the reduced response to the spermine analogue N1,N12-bis(ethyl)spermine (BESpm) by the cisplatin or cis-diamminedichloroplatinum(II) (cDDP)-resistant human ovarian carcinoma cell line (C13*). We checked whether or not under conditions of SSAT overexpression, enzyme induction and cell sensitivity to both, BESpm and cDDP, were restored to levels comparable with those of more responsive cDDP-sensitive 2008 cells. We transiently transfected the SSAT repressed C13* cells with two expression vectors driving human SSAT overexpression by diverse promoters. We then analysed their responses in the absence and in the presence of BESpm. SSAT activity was promptly, but briefly, expressed by transfection with both pOP/SSAT and pCMV-SSAT plasmids. However, only in the presence of BESpm, did SSAT activity reach the highest levels of induction for longer duration, with different time-courses for the two vectors, that paralleled the effect on cell growth. Under these conditions, growth sensitivity to BESpm of the less-responsive C13* cells was 25% reverted to cell growth inhibition displayed by 2008 cells. More interestingly, the sensitivity to cDDP cytotoxicity also increased in parallel to SSAT overexpression. BESpm induction of pCMV-SSAT-transfected cells caused a further 20-30% reduction of cell survival induced by cDDP, almost recovering the sensitivity of 2008 cells. The enhanced effectiveness of cDDP was also confirmed by the comet assay, showing an increase in the number and length of tails of damaged DNA. These findings confirm that SSAT overexpression inhibits cell growth and enhances growth sensitivity to BESpm in C13* cells, showing for the first time that restoring high inducibility of SSAT activity subverts the reduced sensitivity to cDDP of SSAT-deficient cells, making them almost indistinguishable from the responsive parental 2008 cells.

Polyamine depletion switches the form of 2-deoxy-D-ribose-induced cell death from apoptosis to necrosis in HL-60 cells

Our previous studies demonstrated that intracellular polyamine depletion blocked HL-60 cell apoptosis triggered by exposure to 2-deoxy-d-ribose (dRib). Here, we have characterized the intracellular events underlying the apoptotic effects of dRib and the involvement of polyamines in these effects. Treatment of HL-60 cells with dRib induces loss of mitochondrial transmembrane potential, radical oxygen species production, intracellular glutathione depletion and translocation of Bax from cytosol to membranes. These effects are followed by cell death. However, the mode of cell death caused by dRib depends on intracellular levels of polyamines. d-Rib-treated cells with normal polyamine levels, progressing through the G(1) into the S and G(2)/M phases, undergo apoptosis, while in polyamine-depleted cells, being blocked at the G(1) phase, cell death mechanisms are switched to necrosis. The present study points to a relationship between the cell cycle distribution and the mode of cell death, and suggests that the level of intracellular spermidine, essential to cell cycle progression, may determine whether a cell dies by apoptosis or necrosis in response to a death stimulus.

Cisplatin-resistance modulates the effect of protein synthesis inhibitors on spermidine/spermine N(1)-acetyltransferase expression

Cisplatin (DDP)-resistance confers a deficient expression of spermidine/spermine N(1)-acetyltransferase (SSAT) gene in response to the spermine analog N(1),N(12)-bis(ethyl)spermine (BESpm) in the DDP-resistant human ovarian carcinoma cell line (C13*), compared with their parental DDP-sensitive 2008 cells. This SSAT gene deficiency is correlated with a reduced growth sensitivity to spermine analogs. This study was performed to determine whether SSAT gene expression of resistant cells was kept suppressed by labile repressor proteins developed during resistance selection. We show here that inhibitory concentrations of cycloheximide (CHX) and anisomycin (ANISO) differentially affect BESpm-induced SSAT activity in 2008 and in C13* cells in a concentration-dependent manner and allow resistant cells to reach activation levels comparable to those of the sensitive cells. Northern blot analysis revealed that both CHX and ANISO in combination with BESpm caused a synergistic BESpm-mediated accumulation of SSAT mRNA in C13* cells, with respect to each drug alone, while in 2008 cells only a slight increase was observed. The more pronounced effect of inhibitors on the SSAT activity induced by BESpm in the resistant cells was also the result of a more prolonged stabilization of SSAT mRNA and enzyme protein. By contrast, sub-inhibitory concentrations of CHX and ANISO did not significantly stimulate BESpm-induced SSAT transcription and activity. These results suggest that labile repressor proteins, related to DDP-resistance phenotype, play a regulatory role in SSAT gene expression, and further indicate that by overcoming this inhibitory control it is possible to recover BESpm response.

Differential induction of spermidine/spermine N1-acetyltransferase activity in cisplatin-sensitive and -resistant ovarian cancer cells in response to N1,N12-bis(ethyl)spermine involves transcriptional and post-transcriptional regulation

The growth inhibition that occurs in cisplatin-sensitive 2008 human ovarian cancer cells in response to the spermine analogue, N1,N12-bis(ethyl)spermine (BESpm), is associated with a potent induction of spermidine/spermine N1-acetyltransferase (SSAT), the rate-limiting enzyme in polyamine catabolism. Conversely, in cisplatin-resistant C13* cells, which are less responsive to BESpm, enzyme induction does not occur at comparable levels after exposure to the bis(ethyl)-derivative. In this study, we investigated the molecular mechanisms underlying the differential induction of SSAT activity in cisplatin-sensitive and -resistant cells. Northern blot analysis revealed a difference in the level of SSAT mRNA expression in the two cell lines; in particular, 2008 cells treated with 10 microM BESpm for progressively increasing periods of time accumulated more heteronuclear (3.5 kb) and mature (1.3/1.5 kb) SSAT mRNAs than its resistant variant. SSAT mRNA accumulation paralleled enzyme activity and both were almost completely prevented in the two lines by co-treatment with 5 microg/ml actinomycin-D (Act-D), suggesting that transcription plays a major role in the analogue-mediated induction of SSAT. Moreover, when Act-D was added 48 h after BESpm exposure, SSAT mRNA and enzyme activity were stabilised in both cell lines. Therefore, the marked difference in the induction of SSAT activity seems to be related to increased enzyme synthesis, particularly in sensitive cells, whose SSAT protein turnover was also greatly reduced (half-life >12 h in 2008 cells versus 5 h in C13* cells) in the presence of BESpm. These findings suggest that cisplatin-resistance modulates the SSAT response to BESpm at transcriptional and post-transcriptional levels.

2-deoxy-d-ribose-induced apoptosis in HL-60 cells is associated with the cell cycle progression by spermidine

The presence of polyamines is required for the apoptotic program triggered by 2-deoxy-D-ribose (dRib) in HL-60 cells, but their oxidative metabolites does not appear to be involved in the oxidative stress caused by the sugar. The present study points to a relationship between spermidine-induced G1 to S phase transition and the onset of dRib-induced apoptosis. Conversely, the G1 block induced by alpha-difluoromethylornithine (DFMO) is associated with a protective effect against dRib-induced cell suicide. Replenishment of the intracellular spermidine pool by exogenous putrescine and spermidine induces cell cycle progression and restores apoptotic levels. The present data indicate that the induction of cell cycle progression by spermidine is a condition facilitating the activation of the apoptotic process by dRib.

N1,N12-bis(ethyl)spermine effect on growth of cis-diamminedichloroplatinum(II)-sensitive and -resistant human ovarian-carcinoma cell lines

The results presented here demonstrate that a cis-diamminedichloroplatinum(II) (DDP)-resistant human ovarian-carcinoma cell line is also cross-resistant to the spermine analogue N1,N12-bis(ethyl)spermine (BESPM). We report that C13* cells, which are approximately 20-fold resistant to DDP, similarly showed 7-fold resistance to BESPM by colony-forming assay with an IC50 value of 24.6 +/- 2 microM vs. 3.4 +/- 0.8 microM of 2008 cells. Resistance appears to be the result of many effects, such as different morphological and functional modifications of mitochondria. Furthermore, although BESPM accumulation was almost identical in sensitive and resistant cells, the intracellular polyamine pool of the 2 cell lines was differentially affected by this polyamine analogue. In fact, when spermidine (SPD) was still detectable in C13* cells, in 2008 cells it was not, and the spermine (SPM) content was always more markedly reduced in sensitive cells than in the resistant variant. The lower polyamine content of 2008 cells could be related to a higher degree of induction of spermidine/ spermine N1-acetyltransferase (SSAT) activity by BESPM in sensitive cells than in their resistant counterpart. Despite the observed cross-resistance, the combination of the 2 drugs resulted in supra-additive and synergistic effects in both cell lines, depending on concentration, as assessed by median-effect analysis of the survival data. The effectiveness of this combination was also confirmed by the increased accumulation of cells in the G2/M phase of the cell cycle in both cell lines. Taken together, these data suggest that BESPM effect on cell growth of DDP-sensitive and DDP-resistant cells involves multiple mechanisms that are differently modulated by the DDP-resistant phenotype.

Polyamine depletion protects HL-60 cells from 2-deoxy-D-ribose-induced apoptosis

We investigated the involvement of natural polyamines in HL-60 cell death triggered by exposure to 2-deoxy-D-ribose (dRib). In contrast to previous studies, exogenous polyamines failed to protect HL-60 cells against apoptosis caused by dRib. Moreover, in our experimental conditions, depletion of intracellular levels of putrescine and spermidine by alpha-difluoromethylornithine (DFMO) delayed the onset of apoptosis by at least a day or so. Exogenous polyamines reversed the beneficial effect of DFMO and restored the apoptotic levels observed in dRib-treated cells. We suggested that polyamines, especially putrescine and spermidine, act as facilitating factors in the induction of apoptosis triggered by dRib in HL-60 cells.

Modulation of cis-diamminedichloroplatinum (II) accumulation and cytotoxicity by spermine in sensitive and resistant human ovarian carcinoma cells

The effect of spermine (Sp), a natural polycationic amine, on cisplatin (CDDP) sensitivity and accumulation of a human ovarian CDDP-sensitive cell line (2008) and its resistant variant (C13*) was investigated. Survival was also studied. The C13* cells were approximately 20-fold resistant to CDDP, yet were found to be just as sensitive to Sp as 2008 cells. When Sp was concurrently added with CDDP to the colony-forming assay, the IC50 dose was approximately 3-fold lower than that of CDDP alone. This decrease was the result of a synergistic interaction, as assessed by median effect analysis. The incubation of cells with the approximate IC50 dose of Sp for 1-8 h indicated that this synergism could be due to stimulation of CDDP accumulation, showing maximal uptake after 4 h of Sp exposure. This stimulation may be the result of a modulation of cellular membrane permeability by Sp, as assessed by the accumulation of [3H]mannitol. Exposure to Sp concentrations active on CDDP uptake also significantly increased [3H]mannitol accumulation in both cell lines. The triamine spermidine (Spd) did not significantly affect either the sensitivity of the two cell lines or CDDP and [3H]mannitol accumulation. These results suggest that Sp is a positive modulator of CDDP uptake, and thus of its cytotoxicity, even in resistant cells, where the phenotype is partly due to a CDDP accumulation defect.

Stimulation of cis-diamminedichloroplatinum(II) accumulation by modulation of passive permeability with genistein: an altered response in accumulation-defective resistant cells

The effect of the tyrosine kinase inhibitor genistein on the accumulation of cisplatin (DDP) was investigated in DDP-sensitive and -resistant human 2008 ovarian carcinoma cell lines. DDP accumulation after a 1-h exposure was maximally increased by concurrent 40 micrometer genistein. The maximal stimulation of accumulation was observed after 2 h of total genistein exposure and was 83 +/- 13% (n = 5) higher than controls. With resistant C13(*) cells, however, the stimulation of accumulation was delayed until 4 h and was increased only 46 +/- 18% compared to controls. Revertant RH4 cells that retained the accumulation defect behaved like the C13(*) cells. Genistein stimulated [3H]mannitol accumulation (a marker of passive permeability) by 43 +/- 9% (n = 3) in 2008 cells, and the effect was maximal after 2 h of total genistein exposure. Changes in [3H]mannitol accumulation in 2008 parent cells were highly correlated with DDP accumulation (r = 0.9010). These experiments also revealed that [3H]mannitol accumulation after 2 h in C13(*) cells was reduced 38% compared to 2008 cells, a decrease that reflected the DDP accumulation defect. Fluid-phase pinocytosis determined with lucifer yellow CH as a marker showed no difference between 2008 and C13(*) cells and no effect of genistein. Genistein was demonstrated to clearly inhibit protein-tyrosine phosphorylation initiated by the epidermal growth factor receptor kinase. Differences were noted in the phosphotyrosine pattern between the 2008 and C13(*) cells. Under the conditions that had the maximal effect on DDP accumulation in 2008 cells, genistein decreased the IC50 of DDP 8.2-fold in 2008 cells and 4.7-fold in C13(*) cells. We conclude that: (a) genistein stimulates DDP accumulation by modulating the passive permeability of the plasma membrane; (b) C13(*) cells are less permeable to passively diffusing small molecules, which offers a mechanism for the DDP accumulation defect without invoking carrier proteins; (c) the effect of tyrosine kinase inhibition on passive permeability is altered in C13(*) cells; and (d) pinocytosis contributes insignificantly to DDP accumulation. Genistein, a dietary isoflavone, thus seems to be a promising clinical candidate for combination with DDP.

Inhibition of cell growth by accumulated spermine is associated with a transient alteration of cell cycle progression

Exposure of HL-60 cells to millimolar levels of spermine resulted in the inhibition of cell growth. Flow cytometry revealed that the addition of exogenous spermine prevented the accumulation of cells in the S and G2/M phases of the cell cycle as observed in the control cells. High intracellular levels of spermine completely suppressed the early onset of ornithine decarboxylase activity and, consequently, the intracellular increase in spermidine and putrescine. On the other hand, the addition of exogenous spermidine or putrescine also abolished ornithine decarboxylase activity, but in this case neither the growth of spermidine- or putrescine-treated cells nor the cell cycle phase distribution was affected. In the latter cells, intracellular levels of spermidine were not significantly different from control ones. These results suggest that the addition of exogenous spermine inhibits cell proliferation by hindering the increase in cellular spermidine needed to accelerate the G1 to S phase transition.

The effect of spermine on calcium requirement for protein kinase C association with phospholipid vesicles

We have previously reported that polyamines interfere with protein kinase C-membrane interactions. With the aim of clarifying the influence of the relationship between calcium and polyamines on this process we have investigated the effect of spermine on the formation of active protein kinase C-membrane complexes as a function of Ca++ concentrations. Protein kinase C, purified from rat brain, was allowed to interact with phospholipid vesicles of defined composition. The active complex protein kinase C-liposomes was determined by its ability to bind radioactive phorbol ester with an exact 1:1 stoichiometry. The results show that, at Ca++ levels below 0.1 microM, spermine inhibits the formation of complexes between protein kinase C and membranes. At higher Ca++ concentrations, spermine does not prevent the association process but does influence the ratio between the enzyme molecules irreversibly inserted into the membrane and those reversibly associated with it. We have also demonstrated that spermine, by reducing the density of acidic component of liposomes, influences the calcium requirement for protein kinase C-membrane binding. This study indicates that spermine may regulate the activation of protein kinase C and affects the calcium requirement for the association of this enzyme with the phospholipid bilayer. The results suggest a possible role for polyamines in signal transduction when protein kinase C is involved.

Spermine protects protein kinase C from phospholipid-induced inactivation

Phosphatidylserine (PS), an activator of protein kinase C (PKC) in the assay of protein phosphorylation, inhibited this enzyme in a time-dependent manner following preincubation in the absence of Ca2+. The phospholipid-induced inactivation of kinase activity was dependent on the PS content and on the charge density of liposomes. This inactivation of PKC could be reduced, but not completely eliminated, by addition of Ca2+. In the present work the effect of a naturally occurring polyamine (spermine) on the PS-induced inactivation of PKC was investigated. The presence of spermine during preincubation without Ca2+ was effective in suppressing the PS-induced inactivation of PKC over the period (20 min) required for PS to inhibit the enzyme by 95%. PKC exists in two membrane-bound states: a reversible one which can be dissociated by Ca2+ chelators (membrane-associated form) and an irreversible one which is chelator-stable (membrane-inserted form). Gel filtration experiments on the PKC-PS complex formed in the presence of Ca2+ indicated that less insertion of enzyme into liposomes occurred in the presence of spermine and that the kinase activity of the reversibly membrane-associated PKC was protected from PS inactivation.

Effect of spermine on membrane-associated and membrane-inserted forms of protein kinase C

Protein kinase C is reported to exist in two membrane-bound states: a reversible one which can be dissociated by calcium chelators (membrane-associated form) and an irreversible one which is chelator stable (membrane-inserted form). In the present work the effects of a naturally occurring polyamine (spermine) on the membrane-associated and membrane-inserted forms of protein kinase C were investigated using a reconstituted system consisting of partially purified protein kinase C from rat brain and phospholipid vesicles of defined composition. The active membrane-bound complex was conveniently determined by its ability to bind radioactive phorbol ester with an exact 1:1 stoichiometry. Our experimental data show that, in the absence of calcium ions, the amount of enzyme bound to phospholipids vesicles was dramatically reduced by the presence of spermine whereas the PDBu binding affinity was not significantly affected. The addition of the divalent cation increased the affinity of phorbol ester for the active complex but had no effect on Nmax; spermine added in this experimental conditions was no longer able to decrease the total number of enzyme molecules bound to liposomes. Moreover gel filtration experiments of the protein kinase C-phospholipids complex formed in the presence of calcium, indicated that polyamine added during the association process was able to reduce the extent of enzyme insertion into liposomes. Since the increase in phospholipid concentration resulted in a higher level of non-dissociable protein kinase C-liposomes complex we propose that spermine, complexing to membrane binding sites both in the absence and in the presence of Ca++, could promote binding conditions that oppose to the formation of the inserted form of the enzyme. As a consequence the distribution between the reversible and the irreversible membrane-bound forms of protein kinase C is affected.