Topical photodynamic therapy induces systemic immunosuppression via generation of platelet-activating factor receptor ligands

Topical Photodynamic Therapy Generates Bioactive Microvesicle Particles: Evidence for a Pathway Involved in Immunosuppressive Effects

Although effective in treating actinic damage, topical photodynamic therapy (PDT) has been shown to be immunosuppressive through unknown mechanisms, which could potentially limit its effectiveness. Multiple types of environmental stressors, including PDT, can produce the immunosuppressive lipid mediator platelet-activating factor (PAF). Because PAF can produce subcellular microvesicle particles (MVPs), these studies tested whether PDT can generate PAF and MVP release and whether these are involved in PDT-induced immunosuppression. Previously, topical PDT using blue light and 5-aminolevulinic acid was found to be a potent stimulus for PAF production in mice and human skin explants and human patients, and we show that experimental PDT also generates high levels of MVP. PDT-generated MVPs were independent of the PAF receptor but were dependent on the MVP-generating enzyme acid sphingomyelinase. Patients undergoing topical PDT treatment to at least 10% of body surface area showed local and systemic immunosuppression as measured by inhibition of delayed-type hypersensitivity reactions. Finally, using a murine model of contact hypersensitivity, PDT immunosuppression was blocked by genetic and pharmacologic inhibition of acid sphingomyelinase and genetic inhibition of PAF receptor signaling. These studies describe a mechanism involving MVP through which PDT exerts immunomodulatory effects, providing a potential target to improve its effectiveness.

5-FU mediated depletion of myeloid suppressor cells enhances T-cell infiltration and anti-tumor response in immunotherapy-resistant lung tumor

Tumor microenvironment (TME) is a heterogeneous system consisting of both cellular and acellular components. The growth and progression of tumors rely greatly on the nature of TME, marking it as an important target in cancer immunotherapy. Lewis Lung Carcinoma (LLC) is an established murine lung cancer model representing immunologically 'cold' tumors characterized by very few infiltrated cytotoxic T-cells, high levels of Myeloid-Derived Suppressor Cells (MDSCs) and Tumor-Associated Macrophages (TAMs). Here, we report various strategies we applied to reverse the non-immunogenic character of this cold tumor by imparting: a) immunogenic cell death using Hypericin nanoparticle-based photodynamic therapy (PDT), b) repolarising TAM using a TLR7/8 agonist, resiquimod, c) immune checkpoint inhibition using anti-PD-L1 and d) depleting MDSCs using low-dose 5-fluorouracil (5-FU) chemotherapy. Interestingly, the nano-PDT, resiquimod or anti-PD-L1 treatment had no major impact on tumor growth, whereas low-dose 5-FU-mediated depletion of MDSCs showed significant anti-tumor effect, primarily caused by the increased infiltration of CD8⁺ cytotoxic T-cells (∼96%). Though we have tested combining PDT with resiquimod or 5-FU for any synergistic effect, low-dose 5-FU alone showed better response than combinations. In effect, we show that depletion of MDSCs using low-dose 5-FU was one of the best methods to augment infiltration of CD8⁺ cytotoxic T-cells into a cold tumor, which is resistant to conventional therapies including immune checkpoint inhibitors.

Inhibition of photodynamic therapy induced-immunosuppression with aminolevulinic acid leads to enhanced outcomes of tumors and pre-cancerous lesions

Photodynamic therapy (PDT) is a treatment option for tumors and pre-cancerous lesions, but it has immunosuppressive side effects that limit its effectiveness. Recent studies suggest that PDT-mediated immunosuppression occurs through a cyclooxygenase type 2 (COX-2) mediated pathway that leads to increases in regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs), which act as negative regulators of immune responses. Given this pathway, there are three main methods to block immunosuppression: i) Inhibiting the proliferation of Tregs, which can be achieved with the administration of cyclophosphamide or inhibitors of indoleamine 2,3-dioxygenase 1, an activator of Tregs; ii) inhibiting MDSCs by reducing hypoxia around the tumor to create an unfavorable environment or administering all-trans-retinoic acid, which converts MDSCs to a non-immunosuppressive state; and iii) inhibiting COX-2 through selective or non-selective COX-inhibitors. In the present review article, strategies that have shown increased efficacy of PDT in treating tumors and pre-cancerous lesions by blocking the immunosuppressive side effects are outlined and discussed.

CAR-T therapy alters synthesis of platelet-activating factor in multiple myeloma patients

The chimera antigen receptor (CAR) T cell therapy is a novel and potential targeted therapy and has achieved satisfactory efficacy in patients with relapsed or refractory multiple myeloma (MM) in recent years. However, cytokine release syndrome (CRS) and clinical efficacy have become the major obstacles which limit the application of CAR-T in clinics. To explore the potential biomarkers in plasma for evaluating CRS and clinical efficacy, we performed metabolomic and lipidomic profiling of plasma samples from 17 relapsed or refractory MM patients received CAR-T therapy. Our study showed that glycerophosphocholine (GPC), an intermediate of platelet-activating factor (PAF)-like molecule, was significantly decreased when the participants underwent CRS, and the remarkable elevation of lysophosphatidylcholines (lysoPCs), which were catalyzed by lysoPC acyltransferase (LPCAT) was a distinct metabolism signature of relapsed or refractory MM patients with prognostic value post-CAR-T therapy. Both GPC and lysoPC are involved in platelet-activating factor (PAF) remodeling pathway. Besides, these findings were validated by LPCAT1 expression, a key factor in the PAF pathway, associated with poor outcome in three MM GEP datasets of MM. In conclusion, CAR-T therapy alters PAF synthesis in MM patients, and targeting PAF remodeling may be a promising strategy to enhance MM CAR-T therapy.

The role of oxidized phospholipids in the development of disease

Oxidized phospholipids (OxPLs), complex mixtures of phospholipid oxidation products generated during normal or pathological processes, are increasingly recognized to show bioactive effects on many cellular signalling pathways. There is a growing body of evidence showing that OxPLs play an important role in many diseases, so it is essential to define the specific role of OxPLs in different diseases for the design of disease therapies. In vastly diverse pathological processes, OxPLs act as pro-inflammatory agents and contribute to the progression of many diseases; in addition, they play a role in anti-inflammatory processes, promoting the dissipation of inflammation and inhibiting the progression of some diseases. In addition to participating in the regulation of inflammatory responses, OxPLs affect the occurrence and development of diseases through other pathways, such as apoptosis promotion. In this review, the different and even opposite effects of different OxPL molecular species are discussed. Furthermore, the specific effects of OxPLs in various diseases, as well as the receptor and cellular mechanisms involved, are summarized.

Lipids in ultraviolet radiation-induced immune modulation

Ultraviolet (UV) radiation modulates cutaneous lipids which in turn mediates immune suppression – a key mechanism conferring both detrimental and beneficial impacts of sun exposure on human health.

Platelet-Activating Factor as an Effector for Environmental Stressors

Environmental stressors exert a profound effect on humans. Many environmental stressors have in common the ability to induce reactive oxygen species. The goal of this chapter is to present evidence that the potent lipid mediator platelet-activating factor (PAF) is involved in the effects of many stressors ranging from cigarette smoke to ultraviolet B radiation. These environmental stressors can generate PAF enzymatically as well as PAF-like lipids produced by free radical-mediated attack of glycerophosphocholines. Inasmuch as PAF exerts both acute inflammation and delayed immunosuppressive effects, involvement of the PAF system can provide an explanation for many consequences of environmental stressor exposures.

Gemcitabine Induces Microvesicle Particle Release in a Platelet-Activating Factor-Receptor-Dependent Manner via Modulation of the MAPK Pathway in Pancreatic Cancer Cells

Studies, including ours, have shown that pro-oxidative stressors, such as chemotherapeutic agents, generate oxidized lipids with agonistic platelet-activating factor (PAF) activity. Importantly, recent reports have implicated that these PAF-agonists are transported extracellularly via microvesicle particles (MVPs). While the role of PAF-receptor (PAF-R) has been implicated in mediating chemotherapy effects, its significance in chemotherapy-mediated MVP release in pancreatic cancer has not been studied. The current studies determined the functional significance of PAF-R in gemcitabine chemotherapy-mediated MVP release in human pancreatic cancer cells. Using PAF-R-expressing (PANC-1) and PAF-R-deficient (Hs766T) cells, we demonstrate that gemcitabine induces MVP release in a PAF-R-dependent manner. Blocking of PAF-R via PAF-R antagonist or inhibition of MVP generation via inhibitor of acid sphingomyelinase (aSMase) enzyme, significantly attenuated gemcitabine-mediated MVP release from PANC-1 cells, however, exerted no effects in Hs766T cells. Notably, MVPs from gemcitabine-treated PANC-1 cells, contained a measurable amount of PAF-agonists. Mechanistically, pretreatment with ERK1/2 or p38 inhibitors significantly abrogated gemcitabine-mediated MVP release, indicating the involvement of mitogen-activated protein kinase (MAPK) pathway in PAF-R-dependent gemcitabine-mediated MVP release. These findings demonstrate the significance of PAF-R in gemcitabine-mediated MVP release, as well as the rationale of evaluating PAF-R targeting agents with gemcitabine against pancreatic cancer.

Platelet activating factor receptor antagonists improve the efficacy of experimental chemo- and radiotherapy

Platelet activating factor is a lipid mediator of inflammation, and in recent decades, it has emerged as an important factor in tumor outcomes. Platelet activating factor acts by specific binding to its receptor, which is present in both tumor cells and cells that infiltrate tumors. Pro-tumorigenic effects of platelet activating factor receptor in tumors includes promotion of tumor cell proliferation, production of survival signals, migration of vascular cells and formation of new vessels and stimulation of dendritic cells and macrophages suppressor phenotype. In experimental models, blocking of platelet activating factor receptor reduced tumor growth and increased animal survival. During chemotherapy and radiotherapy, tumor cells that survive treatment undergo accelerated proliferation, a phenomenon known as tumor cell repopulation. Work from our group and others showed that these treatments induce overproduction of platelet activating factor-like molecules and increase expression of its receptor in tumor cells. In this scenario, antagonists of platelet activating factor markedly reduced tumor repopulation. Here, we note that combining chemo- and radiotherapy with platelet activating factor antagonists could be a promising strategy for cancer treatment.

Enhanced Platelet-Activating Factor Synthesis Facilitates Acute and Delayed Effects of Ethanol-Intoxicated Thermal Burn Injury

Thermal burn injuries in patients who are alcohol-intoxicated result in greater morbidity and mortality. Murine models combining ethanol and localized thermal burn injury reproduce the systemic toxicity seen in human subjects, which consists of both acute systemic cytokine production with multiple organ dysfunction, as well as a delayed systemic immunosuppression. However, the exact mechanisms for these acute and delayed effects are unclear. These studies sought to define the role of the lipid mediator platelet-activating factor in the acute and delayed effects of intoxicated burn injury. Combining ethanol and thermal burn injury resulted in increased enzymatic platelet-activating factor generation in a keratinocyte cell line in vitro, human skin explants ex vivo, as well as in murine skin in vivo. Further, the acute increase in inflammatory cytokines, such as IL-6, and the systemic immunosuppressive effects of intoxicated thermal burn injury were suppressed in mice lacking platelet-activating factor receptors. Together, these studies provide a potential mechanism and treatment strategies for the augmented toxicity and immunosuppressive effects of thermal burn injury in the setting of acute ethanol exposure, which involves the pleotropic lipid mediator platelet-activating factor.

Acetylsalicylic acid inhibits the growth of melanoma tumors via SOX2-dependent-PAF-R-independent signaling pathway

Acquired resistance to standard therapies remains a serious challenge, requiring novel therapeutic approaches that incorporate potential factors involved in tumor resistance. As cancers including melanoma express inflammatory cyclooxygenases generating prostaglandins implicated in tumor growth, we investigated mechanism of anti-inflammatory drug, acetylsalicylic acid (ASA) which has been shown to inhibit various tumor types, however, its effects against highly aggressive melanoma model are unclear. Given our reports that an activation of platelet-activating factor-receptor (PAF-R) augments the growth and impede efficacies of therapeutic agents in experimental melanoma, we also sought to determine if PAF-R mediates anti-melanoma activity of ASA. The current studies using stably PAF-R-positive (B16-PAFR) and negative (B16-MSCV) murine melanoma cells and PAF-R-expressing and deficient mice, demonstrate that ASA inhibits the in-vitro and in-vivo growth of highly aggressive B16F10 melanoma via bypassing tumoral or stromal PAF-R signaling. Similar ASA-induced effects in-vitro were seen in human melanoma and nasopharyngeal carcinoma cells positive or negative in PAF-R. Mechanistically, the ASA-induced decrease in cell survival and increase in apoptosis were significantly blocked by prostaglandin F2 alpha (PGF2α) agonists. Importantly, PCR array and qRT-PCR analysis of B16-tumors revealed significant downregulation of sry-related high-mobility-box-2 (SOX2) oncogene by ASA treatment. Interestingly, modulation of SOX2 expression by PGF2α agonists and upregulation by fibroblast growth factor 1 (FGF-1) rescued melanoma cells from ASA-induced decreased survival and increased apoptosis. Moreover, PGF2α-receptor antagonist, AL8810 mimics ASA-induced decreased melanoma cells survival which was significantly blocked by PGF2α and FGF-1. These findings indicate that ASA inhibits the growth of aggressive melanoma via SOX2-dependent-PAF-R-indepedent pathway.

Antiplatelet agents for cancer treatment: a real perspective or just an echo from the past?

The association between coagulation and cancer development has been observed for centuries. However, the connection between inflammation and malignancy is also well-recognized. The plethora of evidence indicates that among multiple hemostasis components, platelets play major roles in cancer progression by providing surface and granular contents for several interactions as well as behaving like immune cells. Therefore, the anticancer potential of anti-platelet therapy has been intensively investigated for many years. Anti-platelet agents may prevent cancer, decrease tumor growth, and metastatic potential, as well as improve survival of cancer patients. On the other hand, there are suggestions that antiplatelet treatment may promote solid tumor development in a phenomenon described as "cancers follow bleeding." The controversies around antiplatelet agents justify insight into the subject to establish what, if any, role platelet-directed therapy has in the continuum of anticancer management.

Platelet-Activating Factor Receptor Ligands Protect Tumor Cells from Radiation-Induced Cell Death

Irradiation generates oxidized phospholipids that activate platelet-activating factor receptor (PAFR) associated with pro-tumorigenic effects. Here, we investigated the involvement of PAFR in tumor cell survival after irradiation. Cervical cancer samples presented higher levels of PAF-receptor gene (PTAFR) when compared with normal cervical tissue. In cervical cancer patients submitted to radiotherapy (RT), the expression of PTAFR was significantly increased. Cervical cancer-derived cell lines (C33, SiHa, and HeLa) and squamous carcinoma cell lines (SCC90 and SCC78) express higher levels of PAFR mRNA and protein than immortalized keratinocytes. Gamma radiation increased PAFR expression and induced PAFR ligands and prostaglandin E2 (PGE2) in these tumor cells. The blocking of PAFR with the antagonist CV3938 before irradiation inhibited PGE2 and increased tumor cells death. Similarly, human carcinoma cells transfected with PAFR (KBP) were more resistant to radiation compared to those lacking the receptor (KBM). PGE2 production by irradiated KBP cells was also inhibited by CV3988. These results show that irradiation of carcinoma cells generates PAFR ligands that protect tumor cells from death and suggests that the combination of RT with a PAFR antagonist could be a promising strategy for cancer treatment.

Radiation therapy generates platelet-activating factor agonists

Pro-oxidative stressors can suppress host immunity due to their ability to generate oxidized lipid agonists of the platelet-activating factor-receptor (PAF-R). As radiation therapy also induces reactive oxygen species, the present studies were designed to define whether ionizing radiation could generate PAF-R agonists and if these lipids could subvert host immunity. We demonstrate that radiation exposure of multiple tumor cell lines in-vitro, tumors in-vivo, and human subjects undergoing radiation therapy for skin tumors all generate PAF-R agonists. Structural characterization of radiation-induced PAF-R agonistic activity revealed PAF and multiple oxidized glycerophosphocholines that are produced non-enzymatically. In a murine melanoma tumor model, irradiation of one tumor augmented the growth of the other (non-treated) tumor in a PAF-R-dependent process blocked by a cyclooxygenase-2 inhibitor. These results indicate a novel pathway by which PAF-R agonists produced as a byproduct of radiation therapy could result in tumor treatment failure, and offer important insights into potential therapeutic strategies that could improve the overall antitumor effectiveness of radiation therapy regimens.

Pleiotropic effects of oxidized phospholipids

Oxidized phospholipids (OxPLs) are increasingly recognized to play a role in a variety of normal and pathological states. OxPLs were implicated in regulation of inflammation, thrombosis, angiogenesis, endothelial barrier function, immune tolerance and other important processes. Rapidly accumulating evidence suggests that OxPLs are biomarkers of atherosclerosis and other pathologies. In addition, successful application of experimental drugs based on structural scaffold of OxPLs in animal models of inflammation was recently reported. This review briefly summarizes current knowledge on generation, methods of quantification and biological activities of OxPLs. Furthermore, receptor and cellular mechanisms of these effects are discussed. The goal of the review is to give a broad overview of this class of lipid mediators inducing pleiotropic biological effects.

Epidermal PPARγ influences subcutaneous tumor growth and acts through TNF-α to regulate contact hypersensitivity and the acute photoresponse

It is known that ultraviolet B (UVB) induces PPARγ ligand formation while loss of murine epidermal PPARγ (Pparg-/-^epi) promotes UVB-induced apoptosis, inflammation, and carcinogenesis. PPARγ is known to suppress tumor necrosis factor-α (TNF-α) production. TNF-α is also known to promote UVB-induced inflammation, apoptosis, and immunosuppression. We show that Pparg-/-^epi mice exhibit increased baseline TNF-α expression. Neutralizing Abs to TNF-α block the increased photo-inflammation and photo-toxicity that is observed in Pparg-/-^epi mouse skin. Interestingly, the increase in UVB-induced apoptosis in Pparg-/-^epi mice is not accompanied by a change in cyclobutane pyrimidine dimer clearance or in mutation burden. This suggests that loss of epidermal PPARγ does not result in a significant alteration in DNA repair capacity. However, loss of epidermal PPARγ results in marked immunosuppression using a contact hypersensitivity (CHS) model. This impaired CHS response was significantly alleviated using neutralizing TNF-α antibodies or loss of germline Tnf. In addition, the PPARγ agonist rosiglitazone reversed UVB-induced systemic immunosuppression (UV-IS) as well as UV-induced growth of B16F10 melanoma tumor cells in syngeneic mice. Finally, increased B16F10 tumor growth was observed when injected subcutaneously into Pparg-/-^epi mice. Thus, we provide novel evidence that epidermal PPARγ is important for cutaneous immune function and the acute photoresponse.

Platelet-activating factor (PAF) receptor as a promising target for cancer cell repopulation after radiotherapy

A major drawback of radiotherapy is the accelerated growth of the surviving tumor cells. Radiotherapy generates a variety of lipids that bind to the receptor for platelet-activating factor, expressed by cells in the tumor microenvironment. In the present study, using the TC-1 tumor cell line, we found that irradiation induced a twofold increase in receptor expression and generated agonists of receptor. Irradiated cells induced a 20-fold increase in live TC-1 proliferation in vitro. Furthermore, subcutaneous co-injection of irradiated TC-1 cells with TC-1 expressing luciferase (TC-1 fluc⁺) markedly increased TC-1 fluc⁺ proliferation in a receptor-dependent way. Moreover we used a human carcinoma cell line not expressing the PAF receptor (KBM) and the same cell transfected with the receptor gene (KBP). Following co-injection of live KBP cells with irradiated KBM in RAG mice, the tumor growth was significantly increased compared with tumor formed following co-injection of live KBM with irradiated KBM. This tumor cell repopulation correlated with increased infiltration of tumor-promoting macrophages (CD206+). We propose that receptor represents a possible target for improving the efficacy of radiotherapy through inhibition of tumor repopulation.

Implications of Aspirin for Melanoma Treatment: A Short Perspective

Several human cancers including melanoma exhibit increased expression of inflammatory cyclooxygenases (COX) enzymes that catalyze the conversion of arachidonic acid to prostaglandins (PGs) implicated in tumor growth. As aspirin has been used in the treatment of various ailments including inflammatory diseases, and cancers due to its anti-inflammatory property via inhibiting COX enzymes its significance particularly in reducing the risk of advanced stage or metastatic melanoma has yielded mixed responses. This mini review addresses some of the discrepancies of implications of aspirin from preclinical and clinical studies, and recent updates into its mechanisms of actions in melanoma treatment.

Understanding the connection between platelet-activating factor, a UV-induced lipid mediator of inflammation, immune suppression and skin cancer

Lipid mediators of inflammation play important roles in several diseases including skin cancer, the most prevalent type of cancer found in the industrialized world. Ultraviolet (UV) radiation is a complete carcinogen and is the primary cause of skin cancer. UV radiation is also a potent immunosuppressive agent, and UV-induced immunosuppression is a well-known risk factor for skin cancer induction. An essential mediator in this process is the glyercophosphocholine 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine commonly referred to as platelet-activating factor (PAF). PAF is produced by keratinocytes in response to diverse stimuli and exerts its biological effects by binding to a single specific G-protein-coupled receptor (PAF-R) expressed on a variety of cells. This review will attempt to describe how this lipid mediator is involved in transmitting the immunosuppressive signal from the skin to the immune system, starting from its production by keratinocytes, to its role in activating mast cell migration in vivo, and to the mechanisms involved that ultimately lead to immune suppression. Recent findings related to its role in regulating DNA repair and activating epigenetic mechanisms, further pinpoint the importance of this bioactive lipid, which may serve as a critical molecular mediator that links the environment (UVB radiation) to the immune system and the epigenome.

PAFR in adipose tissue macrophages is associated with anti-inflammatory phenotype and metabolic homoeostasis

Metabolic dysfunction is associated with adipose tissue inflammation and macrophage infiltration. PAFR (platelet-activating factor receptor) is expressed in several cell types and binds to PAF (platelet-activating factor) and oxidized phospholipids. Engagement of PAFR in macrophages drives them towards the anti-inflammatory phenotype. In the present study, we investigated whether genetic deficiency of PAFR affects the phenotype of ATMs (adipose tissue macrophages) and its effect on glucose and insulin metabolism. PARFKO (PAFR-knockout) and WT (wild-type) mice were fed on an SD (standard diet) or an HFD (high-fat diet). Glucose and insulin tolerance tests were performed by blood monitoring. ATMs were evaluated by FACS for phenotypic markers. Gene and protein expression was investigated by real-time reverse transcription-quantitative PCR and Western blotting respectively. Results showed that the epididymal adipose tissue of PAFRKO mice had increased gene expression of Ccr7, Nos2, Il6 and Il12, associated with pro-inflammatory mediators, and reduced expression of the anti-inflammatory Il10. Moreover, the adipose tissue of PAFRKO mice presented more pro-inflammatory macrophages, characterized by an increased frequency of F4/80(+)CD11c(+) cells. Blood monocytes of PAFRKO mice also exhibited a pro-inflammatory phenotype (increased frequency of Ly6C(+) cells) and PAFR ligands were detected in the serum of both PAFRKO and WT mice. Regarding metabolic parameters, compared with WT, PAFRKO mice had: (i) higher weight gain and serum glucose concentration levels; (ii) decreased insulin-stimulated glucose disappearance; (iii) insulin resistance in the liver; (iv) increased expression of Ldlr in the liver. In mice fed on an HFD, some of these changes were potentiated, particularly in the liver. Thus it seems that endogenous ligands of PAFR are responsible for maintaining the anti-inflammatory profile of blood monocytes and ATMs under physiological conditions. In the absence of PAFR signalling, monocytes and macrophages acquire a pro-inflammatory phenotype, resulting in adipose tissue inflammation and metabolic dysfunction.

Systemic chemotherapy is modulated by platelet-activating factor-receptor agonists

Chemotherapy is used to treat numerous cancers including melanoma. However, its effectiveness in clinical settings is often hampered by various mechanisms. Previous studies have demonstrated that prooxidative stressor-mediated generation of oxidized lipids with platelet-activating factor-receptor (PAF-R) agonistic activity induces systemic immunosuppression that augments the growth of experimental melanoma tumors. We have recently shown that treatment of murine B16F10 melanoma cells in vitro or tumors implanted into syngeneic mice and treated intratumorally with various chemotherapeutic agents generated PAF-R agonists in a process blocked by antioxidants. Notably, these intratumoral chemotherapy-generated PAF-R agonists augmented the growth of secondary (untreated) tumors in a PAF-R dependent manner. As both localized and systemic chemotherapies are used based on tumor localization/stage and metastases, the current studies were sought to determine effects of PAF-R agonists on systemic chemotherapy against experimental melanoma. Here, we show that systemic chemotherapy with etoposide (ETOP) attenuates the growth of melanoma tumors when given subsequent to the tumor cell implantation. Importantly, this ETOP-mediated suppression of melanoma tumor growth was blocked by exogenous administration of a PAF-R agonist, CPAF. These findings indicate that PAF-R agonists not only negatively affect the ability of localized chemotherapy but also compromise the efficacy of systemic chemotherapy against murine melanoma.