Distinct cellular responses induced by saporin and a transferrin-saporin conjugate in two different human glioblastoma cell lines

Pathophysiological aspects of transferrin-A potential nano-based drug delivery signaling molecule in therapeutic target for varied diseases

Transferrin (Tf), widely known for its role as an iron-binding protein, exemplifies multitasking in biological processes. The role of Tf in iron metabolism involves both the uptake of iron from Tf by various cells, as well as the endocytosis mediated by the complex of Tf and the transferrin receptor (TfR). The direct conjugation of the therapeutic compound and immunotoxin studies using Tf peptide or anti-Tf receptor antibodies as targeting moieties aims to prolong drug circulation time and augment efficient cellular drug uptake, diminish systemic toxicity, traverse the blood-brain barrier, restrict systemic exposure, overcome multidrug resistance, and enhance therapeutic efficacy with disease specificity. This review primarily discusses the various biological actions of Tf, as well as the development of Tf-targeted nano-based drug delivery systems. The goal is to establish the use of Tf as a disease-targeting component, accentuating the potential therapeutic applications of this protein.

Cytotoxicity Effect of Quinoin, Type 1 Ribosome-Inactivating Protein from Quinoa Seeds, on Glioblastoma Cells

Ribosome-inactivating proteins (RIPs) are found in several edible plants and are well characterized. Many studies highlight their use in cancer therapy, alone or as immunoconjugates, linked to monoclonal antibodies directed against target cancer cells. In this context, we investigate the cytotoxicity of quinoin, a novel type 1 RIP from quinoa seeds, on human continuous and primary glioblastoma cell lines. The cytotoxic effect of quinoin was assayed on human continuous glioblastoma U87Mg cells. Moreover, considering that common conventional glioblastoma multiforme (GBM) cell lines are genetically different from the tumors from which they derive, the cytotoxicity of quinoin was subsequently tested towards primary cells NULU and ZAR (two cell lines established from patients’ gliomas), also in combination with the chemotherapeutic agent temozolomide (TMZ), currently used in glioblastoma treatment. The present study demonstrated that quinoin (2.5 and 5.0 nM) strongly reduced glioblastoma cells’ growth. The mechanisms responsible for the inhibitory action of quinoin are different in the tested primary cell lines, reproducing the heterogeneous response of glioblastoma cells. Interestingly, primary cells treated with quinoin in combination with TMZ were more sensitive to the treatment. Overall, our data highlight that quinoin could represent a novel tool for glioblastoma therapy and a possible adjuvant for the treatment of the disease in combination with TMZ, alone or as possible immunoconjugates/nanoconstructs.

Targeting Receptors on Cancer Cells with Protein Toxins

Cancer cells frequently upregulate surface receptors that promote growth and survival. These receptors constitute valid targets for intervention. One strategy involves the delivery of toxic payloads with the goal of killing those cancer cells with high receptor levels. Delivery can be accomplished by attaching a toxic payload to either a receptor-binding antibody or a receptor-binding ligand. Generally, the cell-binding domain of the toxin is replaced with a ligand or antibody that dictates a new binding specificity. The advantage of this “immunotoxin” approach lies in the potency of these chimeric molecules for killing cancer cells. However, receptor expression on normal tissue represents a significant obstacle to therapeutic intervention.

Antibody-Drug Conjugates: The New Frontier of Chemotherapy

In recent years, antibody-drug conjugates (ADCs) have become promising antitumor agents to be used as one of the tools in personalized cancer medicine. ADCs are comprised of a drug with cytotoxic activity cross-linked to a monoclonal antibody, targeting antigens expressed at higher levels on tumor cells than on normal cells. By providing a selective targeting mechanism for cytotoxic drugs, ADCs improve the therapeutic index in clinical practice. In this review, the chemistry of ADC linker conjugation together with strategies adopted to improve antibody tolerability (by reducing antigenicity) are examined, with particular attention to ADCs approved by the regulatory agencies (the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA)) for treating cancer patients. Recent developments in engineering Immunoglobulin (Ig) genes and antibody humanization have greatly reduced some of the problems of the first generation of ADCs, beset by problems, such as random coupling of the payload and immunogenicity of the antibody. ADC development and clinical use is a fast, evolving area, and will likely prove an important modality for the treatment of cancer in the near future.

Selective Cytotoxicity to HER2 Positive Breast Cancer Cells by Saporin-Loaded Nanobody-Targeted Polymeric Nanoparticles in Combination with Photochemical Internalization

In cancer treatment, polymeric nanoparticles (NPs) can serve as a vehicle for the delivery of cytotoxic proteins that have intracellular targets but that lack well-defined mechanisms for cellular internalization, such as saporin. In this work, we have prepared PEGylated poly(lactic acid- co-glycolic acid- co-hydroxymethyl glycolic acid) (PLGHMGA) NPs for the selective delivery of saporin in the cytosol of HER2 positive cancer cells. This selective uptake was achieved by decorating the surface of the NPs with the 11A4 nanobody that is specific for the HER2 receptor. Confocal microscopy observations showed rapid and extensive uptake of the targeted NPs (11A4-NPs) by HER2 positive cells (SkBr3) but not by HER2 negative cells (MDA-MB-231). This selective uptake was blocked upon preincubation of the cells with an excess of nanobody. Nontargeted NPs (Cys-NPs) were not taken up by either type of cells. Importantly, a dose-dependent cytotoxic effect was only observed on SkBr3 cells when these were treated with saporin-loaded 11A4-NPs in combination with photochemical internalization (PCI), a technique that uses a photosensitizer and local light exposure to facilitate endosomal escape of entrapped nanocarriers and biomolecules. The combined use of saporin-loaded 11A4-NPs and PCI strongly inhibited cell proliferation and decreased cell viability through induction of apoptosis. Also the cytotoxic effect could be reduced by an excess of nanobody, reinforcing the selectivity of this system. These results suggest that the combination of the targeting nanobody on the NPs with PCI are effective means to achieve selective uptake and cytotoxicity of saporin-loaded NPs.

Apolipoprotein E Peptide-Directed Chimeric Polymersomes Mediate an Ultrahigh-Efficiency Targeted Protein Therapy for Glioblastoma

The inability to cross the blood-brain barrier (BBB) prevents nearly all chemotherapeutics and biotherapeutics from the effective treatment of brain tumors, rendering few improvements in patient survival rates to date. Here, we report that apolipoprotein E peptide [ApoE, (LRKLRKRLL)₂C] specifically binds to low-density lipoprotein receptor members (LDLRs) and mediates superb BBB crossing and highly efficient glioblastoma (GBM)-targeted protein therapy in vivo. The in vitro BBB model studies reveal that ApoE induces 2.2-fold better penetration of the immortalized mouse brain endothelial cell line (bEnd.3) monolayer for chimeric polymersomes (CP) compared to Angiopep-2, the best-known BBB-crossing peptide used in clinical trials for GBM therapy. ApoE-installed CP (ApoE-CP) carrying saporin (SAP) displays a highly specific and potent antitumor effect toward U-87 MG cells with a low half-maximum inhibitory concentration of 14.2 nM SAP. Notably, ApoE-CP shows efficient BBB crossing as well as accumulation and penetration in orthotopic U-87 MG glioblastoma. The systemic administration of SAP-loaded ApoE-CP causes complete growth inhibition of orthotopic U-87 MG GBM without eliciting any observable adverse effects, affording markedly improved survival benefits. ApoE peptide provides an ultrahigh-efficiency targeting strategy for GBM therapy.

Protein Toxin Chaperoned by LRP-1-Targeted Virus-Mimicking Vesicles Induces High-Efficiency Glioblastoma Therapy In Vivo

Glioblastoma is a most intractable and high-mortality malignancy because of its extremely low drug accessibility resulting from the blood-brain barrier (BBB). Here, it is reported that angiopep-2-directed and redox-responsive virus-mimicking polymersomes (ANG-PS) (angiopep-2 is a peptide targeting to low-density lipoprotein receptor-related protein-1 (LRP-1)) can efficiently and selectively chaperone saporin (SAP), a highly potent natural protein toxin, to orthotopic human glioblastoma xenografts in nude mice. Unlike chemotherapeutics, free SAP has a low cytotoxicity. SAP-loaded ANG-PS displays, however, a striking antitumor activity (half-maximal inhibitory concentration, IC₅₀  = 30.2 × 10^-9 m) toward U-87 MG human glioblastoma cells in vitro as well as high BBB transcytosis and glioblastoma accumulation in vivo. The systemic administration of SAP-loaded ANG-PS to U-87 MG orthotopic human-glioblastoma-bearing mice brings about little side effects, effective tumor inhibition, and significantly improved survival rate. The protein toxins chaperoned by LRP-1-targeted virus-mimicking vesicles emerge as a novel and highly promising treatment modality for glioblastoma.

Targeted therapy of human glioblastoma via delivery of a toxin through a peptide directed to cell surface nucleolin

Targeted anticancer therapies demand discovery of new cellular targets to be exploited for the delivery of toxic molecules and drugs. In this perspective, in the last few years, nucleolin has been identified as an interesting surface marker to be used for the therapy of glioblastoma. In this study, we investigated whether a synthetic antagonist of cell-surface nucleolin known as N6L, previously reported to decrease both tumor growth and tumor angiogenesis in several cancer cell lines, including glioblastoma cells, as well as endothelial cells proliferation, could be exploited to deliver a protein toxin (saporin) to glioblastoma cells. The pseudopeptide N6L cross-linked to saporin-S6 induced internalization of the toxin inside glioblastoma cancer cells. Our results in vitro demonstrated the effectiveness of this conjugate in inducing cell death, with an ID50 four orders of magnitude lower than that observed for free N6L. Furthermore, the preliminary in vivo study demonstrated efficiency in reducing the tumor mass in an orthotopic mouse model of glioblastoma.

EV20-Sap, a novel anti-HER-3 antibody-drug conjugate, displays promising antitumor activity in melanoma

Melanoma is the most biologically aggressive skin cancer of well established constitutive and induced resistance to pharmacological treatment. Despite the recent progresses in immunotherapies, many advanced metastatic melanoma patients still face a significant mortality risk. The aggressive nature of this disease sustains an urgent need for more successful, effective drugs. HER-3 - one of the four member of the tyrosin kinase epidermal growth factor receptors (EGFRs) family- is frequently overexpressed in solid tumors, including melanoma. Moreover, up-regulation of HER-3 and its ligand NRGβ-1 are associated with poor prognosis, thus suggesting this receptor as a suitable target for cancer therapy. Several monoclonal antibodies targeting HER-3 are currently available, but preliminary results from clinical testing of these agents reveal a modest efficacy. Thus, a substantial improvement over this immunotherapeutic approach could be offered by an anti-HER-3 based Antibody-Drug Conjugate (ADC). In the present paper, we describe the generation of an ADC obtained by coupling the HER-3 targeting antibody EV20 linked to the plant toxin Saporin (Sap). In vitro, this ADC displays a powerful, specific and target-dependent cytotoxic activity which correlates with the degree of expression and internalization of HER-3 on tumor cells. Furthermore, in a murine melanoma model, EV20-Sap treatment leads to a significant reduction of the number of pulmonary metastasis.

The Glycoside Oleandrin Reduces Glioma Growth with Direct and Indirect Effects on Tumor Cells

Oleandrin is a glycoside that inhibits the ubiquitous enzyme Na⁺/K⁺-ATPase. In addition to its known effects on cardiac muscle, recent in vitro and in vivo evidence highlighted its potential for anticancer properties. Here, we evaluated for the first time the effect of oleandrin on brain tumors. To this aim, mice were transplanted with human or murine glioma and analyzed for tumor progression upon oleandrin treatment. In both systems, oleandrin impaired glioma development, reduced tumor size, and inhibited cell proliferation. We demonstrated that oleandrin does the following: (1) enhances the brain-derived neurotrophic factor (BDNF) level in the brain; (2) reduces both microglia/macrophage infiltration and CD68 immunoreactivity in the tumor mass; (3) decreases astrogliosis in peritumoral area; and (4) reduces glioma cell infiltration in healthy parenchyma. In BDNF-deficient mice (bdnftm1Jae/J) and in glioma cells silenced for TrkB receptor expression, oleandrin was not effective, indicating a crucial role for BDNF in oleandrin's protective and antitumor functions. In addition, we found that oleandrin increases survival of temozolomide-treated mice. These results encourage the development of oleandrin as possible coadjuvant agent in clinical trials of glioma treatment.SIGNIFICANCE STATEMENT In this work, we paved the road for a new therapeutic approach for the treatment of brain tumors, demonstrating the potential of using the cardioactive glycoside oleandrin as a coadjuvant drug to standard chemotherapeutics such as temozolomide. In murine models of glioma, we demonstrated that oleandrin significantly increased mouse survival and reduced tumor growth both directly on tumor cells and indirectly by promoting an antitumor brain microenvironment with a key protective role played by the neurotrophin brain-derived neurotrophic factor.

PDZ Domain in the Engineering and Production of a Saporin Chimeric Toxin as a Tool for targeting Cancer Cells

In this paper we have studied a PDZ protein domain as a possible tool for cellular targeting of the ribosome inactivating protein Saporin, exploiting the ability of PDZ domains to recognize and bind short peptide sequences located at the C-terminus of a cognate protein. We have focused our attention on the PDZ domain from hCASK (Human calcium/calmodulin-dependent serine protein kinase) that binds extracellular CD98 in epithelial cells, being this antigen recognized as a marker for several human tumors and particularly considered a negative prognostic marker for human glioblastoma. We produced recombinant fusions of one or two hCASK-PDZ domains with the ribosome inactivating protein Saporin and assayed them on two human glioblastoma cell lines (GL15 and U87). These constructs proved to be toxic, with increasing activity as a function of the number of PDZ domains, and induce cell death by apoptotic mechanisms in a dose-dependent and/or time dependent manner.

Astrocytes in the rat nucleus tractus solitarii are critical for cardiovascular reflex control

We have shown that an antibody to dopamine-β-hydroxylase conjugated with saporin (anti-DBH-SAP) damages catecholamine neurons in the nucleus tractus solitarii (NTS) of rat, attenuates arterial baroreflexes, and leads to lability of arterial blood pressure, damage to cardiac myocytes, and, in some animals, sudden death. However, others have shown that injection of 6-hydroxydopamine (6-OHDA), a toxin devoid of saporin, also damaged NTS catecholamine neurons but did not lead to these cardiovascular changes. We found similar cardiovascular changes after injecting a different SAP conjugate to target NTS neurons with neurokinin (NK1) receptors. Because ribosome-inactivating proteins may be toxic to glia, we hypothesized that SAP, a ribosome-inactivating protein, might target glia whose loss could account for physiological changes. We tested this hypothesis by assessing effects on select neurons and on glia in the NTS after exposure to SAP, targeted SAP conjugates, or 6-OHDA. SAP and all SAP conjugates led to loss of immunoreactivity for glial fibrillary acidic protein, a marker for astrocytes, in the NTS while 6-OHDA did not. As reported previously, anti-DBH-SAP selectively killed noradrenergic neurons in the NTS while SAP conjugated to stabilized substance P (SSP-SAP) selectively killed neurons with NK1 receptors. In contrast, SAP produced no demonstrable neuronal damage. All injections led to activation of microglia in the NTS; however, only SAP and its conjugates attenuated cardiovascular reflexes while also producing lability of arterial pressure, damage to cardiac myocytes, and in some animals, sudden death. Thus, NTS astrocytes may play a role in mediating cardiovascular reflex transmission through the NTS.

Saporin-S6: a useful tool in cancer therapy

Thirty years ago, the type 1 ribosome-inactivating protein (RIP) saporin-S6 (also known as saporin) was isolated from Saponaria officinalis L. seeds. Since then, the properties and mechanisms of action of saporin-S6 have been well characterized, and it has been widely employed in the construction of conjugates and immunotoxins for different purposes. These immunotoxins have shown many interesting results when used in cancer therapy, particularly in hematological tumors. The high enzymatic activity, stability and resistance to conjugation procedures and blood proteases make saporin-S6 a very useful tool in cancer therapy. High efficacy has been reported in clinical trials with saporin-S6-containing immunotoxins, at dosages that induced only mild and transient side effects, which were mainly fever, myalgias, hepatotoxicity, thrombocytopenia and vascular leak syndrome. Moreover, saporin-S6 triggers multiple cell death pathways, rendering impossible the selection of RIP-resistant mutants. In this review, some aspects of saporin-S6, such as the chemico-physical characteristics, the structural properties, its endocytosis, its intracellular routing and the pathogenetic mechanisms of the cell damage, are reported. In addition, the recent progress and developments of saporin-S6-containing immunotoxins in cancer immunotherapy are summarized, including in vitro and in vivo pre-clinical studies and clinical trials.

Imaging tumor burden in the brain with 89Zr-transferrin

A noninvasive technology that indiscriminately detects tumor tissue in the brain could substantially enhance the management of primary or metastatic brain tumors. Although the documented molecular heterogeneity of diseases that initiate or eventually deposit in the brain may preclude identifying a single smoking-gun molecular biomarker, many classes of brain tumors are generally avid for transferrin. Therefore, we reasoned that applying a radiolabeled derivative of transferrin ((89)Zr-labeled transferrin) may be an effective strategy to more thoroughly identify tumor tissue in the brain, regardless of the tumor's genetic background.

METHODS

Transferrin was radiolabeled with (89)Zr, and its properties with respect to human models of glioblastoma multiforme were studied in vivo.

RESULTS

In this report, we show proof of concept that (89)Zr-labeled transferrin ((89)Zr-transferrin) localizes to genetically diverse models of glioblastoma multiforme in vivo. Moreover, we demonstrate that (89)Zr-transferrin can detect an orthotopic lesion with exceptional contrast. Finally, the tumor-to-brain contrast conferred by (89)Zr-transferrin vastly exceeded that observed with (18)F-FDG, currently the most widely used radiotracer to assess tumor burden in the brain.

CONCLUSION

The results from this study suggest that (89)Zr-transferrin could be a broadly applicable tool for identifying and monitoring tumors in the brain, with realistic potential for near-term clinical translation.

Insights into the mechanism of cell death induced by saporin delivered into cancer cells by an antibody fusion protein targeting the transferrin receptor 1

We previously developed an antibody-avidin fusion protein (ch128.1Av) that targets the human transferrin receptor 1 (TfR1) and exhibits direct cytotoxicity against malignant B cells in an iron-dependent manner. ch128.1Av is also a delivery system and its conjugation with biotinylated saporin (b-SO6), a plant ribosome-inactivating toxin, results in a dramatic iron-independent cytotoxicity, both in malignant cells that are sensitive or resistant to ch128.1Av alone, in which the toxin effectively inhibits protein synthesis and triggers caspase activation. We have now found that the ch128.1Av/b-SO6 complex induces a transcriptional response consistent with oxidative stress and DNA damage, a response that is not observed with ch128.1Av alone. Furthermore, we show that the antioxidant N-acetylcysteine partially blocks saporin-induced apoptosis suggesting that oxidative stress contributes to DNA damage and ultimately saporin-induced cell death. Interestingly, the toxin was detected in nuclear extracts by immunoblotting, suggesting the possibility that saporin might induce direct DNA damage. However, confocal microscopy did not show a clear and consistent pattern of intranuclear localization. Finally, using the long-term culture-initiating cell assay we found that ch128.1Av/b-SO6 is not toxic to normal human hematopoietic stem cells suggesting that this critical cell population would be preserved in therapeutic interventions using this immunotoxin.

The transferrin receptor and the targeted delivery of therapeutic agents against cancer

BACKGROUND

Traditional cancer therapy can be successful in destroying tumors, but can also cause dangerous side effects. Therefore, many targeted therapies are in development. The transferrin receptor (TfR) functions in cellular iron uptake through its interaction with transferrin. This receptor is an attractive molecule for the targeted therapy of cancer since it is upregulated on the surface of many cancer types and is efficiently internalized. This receptor can be targeted in two ways: 1) for the delivery of therapeutic molecules into malignant cells or 2) to block the natural function of the receptor leading directly to cancer cell death.

SCOPE OF REVIEW

In the present article we discuss the strategies used to target the TfR for the delivery of therapeutic agents into cancer cells. We provide a summary of the vast types of anti-cancer drugs that have been delivered into cancer cells employing a variety of receptor binding molecules including Tf, anti-TfR antibodies, or TfR-binding peptides alone or in combination with carrier molecules including nanoparticles and viruses.

MAJOR CONCLUSIONS

Targeting the TfR has been shown to be effective in delivering many different therapeutic agents and causing cytotoxic effects in cancer cells in vitro and in vivo.

GENERAL SIGNIFICANCE

The extensive use of TfR for targeted therapy attests to the versatility of targeting this receptor for therapeutic purposes against malignant cells. More advances in this area are expected to further improve the therapeutic potential of targeting the TfR for cancer therapy leading to an increase in the number of clinical trials of molecules targeting this receptor. This article is part of a Special Issue entitled Transferrins: molecular mechanisms of iron transport and disorders.

Immunotoxins and other conjugates containing saporin-s6 for cancer therapy

Ribosome-inactivating proteins (RIPs) are a family of plant toxins that permanently damage ribosomes and possibly other cellular substrates, thus causing cell death. RIPs are mostly divided in two types: Type 1 RIPs that are single-chain enzymatic proteins, and type 2 RIPs that consist of an active A chain (similar to a type 1 RIP) linked to a B chain with lectin properties. RIP-containing conjugates have been used in many experimental strategies against cancer cells, often showing great efficacy in clinical trials. Saporin-S6, a type 1 RIP extracted from Saponaria officinalis L. seeds, has been extensively utilized to construct anti-cancer conjugates because of its high enzymatic activity, stability and resistance to conjugation procedures, resulting in the efficient killing of target cells. This review summarizes saporin-S6-containing conjugates and their application in cancer therapy, considering in-vitro and in-vivo studies both in animal models and in clinical trials. The review is structured on the basis of the targeting of hematological versus solid tumors and on the antigen recognized on the cell surface.