Potent synergistic interaction between the Nampt inhibitor APO866 and the apoptosis activator TRAIL in human leukemia cells

Review of various NAMPT inhibitors for the treatment of cancer

Nicotinamide phosphoribosyltransferase (NAMPT) is a rate-limiting enzyme in the NAD salvage pathway of mammalian cells and is overexpressed in numerous types of cancers. These include breast cancer, ovarian cancer, prostate cancer, gastric cancer, colorectal cancer, glioma, and b-cell lymphoma. NAMPT is also known to impact the NAD and NADPH pool. Research has demonstrated that NAMPT can be inhibited. NAMPT inhibitors are diverse anticancer medicines with significant anti-tumor efficacy in ex vivo tumor models. A few notable NAMPT specific inhibitors which have been produced include FK866, CHS828, and OT-82. Despite encouraging preclinical evidence of the potential utility of NAMPT inhibitors in cancer models, early clinical trials have yielded only modest results, necessitating the adaptation of additional tactics to boost efficacy. This paper examines a number of cancer treatment methods which target NAMPT, including the usage of individual inhibitors, pharmacological combinations, dual inhibitors, and ADCs, all of which have demonstrated promising experimental or clinical results. We intend to contribute further ideas regarding the usage and development of NAMPT inhibitors in clinical therapy to advance the field of research on this intriguing target.

Advances in NAD-Lowering Agents for Cancer Treatment

Nicotinamide adenine dinucleotide (NAD) is an essential redox cofactor, but it also acts as a substrate for NAD-consuming enzymes, regulating cellular events such as DNA repair and gene expression. Since such processes are fundamental to support cancer cell survival and proliferation, sustained NAD production is a hallmark of many types of neoplasms. Depleting intratumor NAD levels, mainly through interference with the NAD-biosynthetic machinery, has emerged as a promising anti-cancer strategy. NAD can be generated from tryptophan or nicotinic acid. In addition, the "salvage pathway" of NAD production, which uses nicotinamide, a byproduct of NAD degradation, as a substrate, is also widely active in mammalian cells and appears to be highly exploited by a subset of human cancers. In fact, research has mainly focused on inhibiting the key enzyme of the latter NAD production route, nicotinamide phosphoribosyltransferase (NAMPT), leading to the identification of numerous inhibitors, including FK866 and CHS-828. Unfortunately, the clinical activity of these agents proved limited, suggesting that the approaches for targeting NAD production in tumors need to be refined. In this contribution, we highlight the recent advancements in this field, including an overview of the NAD-lowering compounds that have been reported so far and the related in vitro and in vivo studies. We also describe the key NAD-producing pathways and their regulation in cancer cells. Finally, we summarize the approaches that have been explored to optimize the therapeutic response to NAMPT inhibitors in cancer.

In cancer, all roads lead to NADPH

Cancer cells require increased levels of NADPH for increased nucleotide synthesis and for protection from ROS. Recent studies show that increased NADPH is generated in several ways. Activated AKT phosphorylates NAD kinase (NADK), increasing its activity. NADP formed, is rapidly converted to NADPH by glucose 6-phosphate dehydrogenase and malic enzymes, overexpressed in tumor cells with mutant p53. Calmodulin, overexpressed in some cancers, also increases NADK activity. Also, in IDH1/2 mutant cancer, NADPH serves as the cofactor to generate D-2 hydroxyglutarate, an oncometabolite. The requirement of cancer cells for elevated levels of NADPH provides an opportunity to target its synthesis for cancer treatment.

CD38 downregulation modulates NAD+ and NADP(H) levels in thermogenic adipose tissues

Different strategies to boost NAD⁺ levels are considered promising means to promote healthy aging and ameliorate dysfunctional metabolism. CD38 is a NAD⁺-dependent enzyme involved in the regulation of different cell functions. In the context of systemic energy metabolism, it has been demonstrated that brown adipocytes, the parenchymal cells of brown adipose tissue (BAT) as well as beige adipocytes that emerge in white adipose tissue (WAT) depots in response to catabolic conditions, are important to maintain metabolic homeostasis. In this study we aim to understand the functional relevance of CD38 for NAD⁺ and energy metabolism in BAT and WAT, also using a CD38^-/- mouse model. During cold exposure, an increase in NAD⁺ levels occurred in BAT of wild type mice, together with a marked downregulation of CD38, as detected at the mRNA and protein level. CD38 downregulation was observed also in WAT of cold-exposed mice, where it was accompanied by a strong increase in NADP(H) levels. Accordingly, NAD kinase and glucose-6-phosphate dehydrogenase activities were enhanced in WAT (but not in BAT). Increased NAD⁺ levels were observed in BAT/WAT from CD38^-/- compared with wild type mice, in line with CD38 being a major NAD⁺-consumer in AT. CD38^-/- mice kept at 6 °C had higher levels of Ucp1 and Pgc-1α in BAT and WAT, and increased levels of phosphorylated hormone-sensitive lipase in BAT, compared with wild type mice. These results demonstrate that CD38, by modulating cellular NAD(P)⁺ levels, is involved in the regulation of thermogenic responses in cold-activated BAT and WAT.

Recent Advances in NAMPT Inhibitors: A Novel Immunotherapic Strategy

Nicotinamide adenine dinucleotide (NAD) is a cofactor of many enzymatic reactions as well as being a substrate for a number of NAD-consuming enzymes (e.g., PARPS, sirtuins, etc). NAD can be synthesized de novo starting from tryptophan, nicotinamide, nicotinic acid, or nicotinamide riboside from the diet. On the other hand, the nicotinamide that is liberated by NAD-consuming enzymes can be salvaged to re-form NAD. In this former instance, nicotinamide phosphoribosyltransferase (NAMPT) is the bottleneck enzyme. In the many cells in which the salvage pathway is predominant, NAMPT, therefore, represents an important controller of intracellular NAD concentrations, and as a consequence of energy metabolism. It is, therefore, not surprising that NAMPT is over expressed by tumoral cells, which take advantage from this to sustain growth rate and tumor progression. This has led to the initiation of numerous medicinal chemistry programs to develop NAMPT inhibitors in the context of oncology. More recently, however, it has been shown that NAMPT inhibitors do not solely target the tumor but also have an effect on the immune system. To add complexity, this enzyme can also be secreted by cells, and in the extracellular space it acts as a cytokine mainly through the activation of Toll like Receptor 4 (TLR4), although it has not been clarified yet if this is the only receptor responsible for its actions. While specific small molecules have been developed only against the intracellular form of NAMPT, growing evidences sustain the possibility to target the extracellular form. In this contribution, the most recent evidences on the medicinal chemistry of NAMPT will be reviewed, together with the key elements that sustain the hypothesis of NAMPT targeting and the drawbacks so far encountered.

NAD- and NADPH-Contributing Enzymes as Therapeutic Targets in Cancer: An Overview

Actively proliferating cancer cells require sufficient amount of NADH and NADPH for biogenesis and to protect cells from the detrimental effect of reactive oxygen species. As both normal and cancer cells share the same NAD biosynthetic and metabolic pathways, selectively lowering levels of NAD(H) and NADPH would be a promising strategy for cancer treatment. Targeting nicotinamide phosphoribosyltransferase (NAMPT), a rate limiting enzyme of the NAD salvage pathway, affects the NAD and NADPH pool. Similarly, lowering NADPH by mutant isocitrate dehydrogenase 1/2 (IDH1/2) which produces D-2-hydroxyglutarate (D-2HG), an oncometabolite that downregulates nicotinate phosphoribosyltransferase (NAPRT) via hypermethylation on the promoter region, results in epigenetic regulation. NADPH is used to generate D-2HG, and is also needed to protect dihydrofolate reductase, the target for methotrexate, from degradation. NAD and NADPH pools in various cancer types are regulated by several metabolic enzymes, including methylenetetrahydrofolate dehydrogenase, serine hydroxymethyltransferase, and aldehyde dehydrogenase. Thus, targeting NAD and NADPH synthesis under special circumstances is a novel approach to treat some cancers. This article provides the rationale for targeting the key enzymes that maintain the NAD/NADPH pool, and reviews preclinical studies of targeting these enzymes in cancers.

Beyond Energy Metabolism: Exploiting the Additional Roles of NAMPT for Cancer Therapy

Tumor cells have increased requirements for NAD⁺. Thus, many cancers exhibit an increased reliance on NAD⁺ production pathways. This dependence may be exploited therapeutically through pharmacological targeting of NAMPT, the rate-limiting enzyme in the NAD⁺ salvage pathway. Despite promising preclinical data using NAMPT inhibitors in cancer models, early NAMPT inhibitors showed limited efficacy in several early phase clinical trials, necessitating the identification of strategies, such as drug combinations, to enhance their efficacy. While the effect of NAMPT inhibitors on impairment of energy metabolism in cancer cells has been well-described, more recent insights have uncovered a number of additional targetable cellular processes that are impacted by inhibition of NAMPT. These include sirtuin function, DNA repair machinery, redox homeostasis, molecular signaling, cellular stemness, and immune processes. This review highlights the recent findings describing the effects of NAMPT inhibitors on the non-metabolic functions of malignant cells, with a focus on how this information can be leveraged clinically. Combining NAMPT inhibitors with other therapies that target NAD⁺-dependent processes or selecting tumors with specific vulnerabilities that can be co-targeted with NAMPT inhibitors may represent opportunities to exploit the multiple functions of this enzyme for greater therapeutic benefit.

Effective targeting of NAMPT in patient-derived xenograft models of high-risk pediatric acute lymphoblastic leukemia

The prognosis for children diagnosed with high-risk acute lymphoblastic leukemia (ALL) remains suboptimal, and more potent and less toxic treatments are urgently needed. We investigated the efficacy of a novel nicotinamide phosphoribosyltransferase inhibitor, OT-82, against a panel of patient-derived xenografts (PDXs) established from high-risk and poor outcome pediatric ALL cases. OT-82 was well-tolerated and demonstrated impressive single agent in vivo efficacy, achieving significant leukemia growth delay in 95% (20/21) and disease regression in 86% (18/21) of PDXs. In addition, OT-82 enhanced the efficacy of the established drugs cytarabine and dasatinib and, as a single agent, showed similar efficacy as an induction-type regimen combining three drugs used to treat pediatric ALL. OT-82 exerted its antileukemic action by depleting NAD⁺ and ATP, inhibiting the NAD⁺-requiring DNA damage repair enzyme PARP-1, increasing mitochondrial ROS levels and inducing DNA damage, culminating in apoptosis induction. OT-82 sensitivity was associated with the occurrence of mutations in major DNA damage response genes, while OT-82 resistance was characterized by high expression levels of CD38. In conclusion, our study provides evidence that OT-82, as a single agent, and in combination with established drugs, is a promising new therapeutic strategy for a broad spectrum of high-risk pediatric ALL for which improved therapies are urgently needed.

Reactive oxygen/nitrogen species contribute substantially to the antileukemia effect of APO866, a NAD lowering agent

APO866 is a small molecule drug that specifically inhibits nicotinamide phosphoribosyltransferase (NAMPT), a key enzyme involved in nicotinamide adenine dinucleotide (NAD) biosynthesis from the natural precursor nicotinamide. Although, the antitumor activity of APO866 on various types of cancer models has been reported, information regarding mechanisms by which APO866 exerts its cytotoxic effects is not well defined. Here we show that APO866 induces a strong, time-dependent increase in highly reactive ROS, nitric oxide, cytosolic/mitochondrial superoxide anions and hydrogen peroxide. We provide evidence that APO866-mediated ROS production is modulated by PARP1 and triggers cell death through mitochondria depolarization and ATP loss. Genetic or pharmacologic inhibition of PARP1 prevented hydrogen peroxide accumulation, caspase activation, mitochondria depolarization, ATP loss and abrogates APO866-induced cell death, suggesting that the integrity of PARP1 status is required for cell death. Conversely, PARP1 activating drugs enhanced the anti-leukemia activity of APO866 Collectively, our studies show that APO866 induces ROS/RNS productions, which mediate its anti-leukemia effect. These results support testing new combinatorial strategies to enhance the antitumor activities of APO866.

Autophagy regulation and its role in normal and malignant hematopoiesis

Autophagy, the molecular machinery of self-eating, plays a dual role of a tumor promoter and tumor suppressor. This mechanism affects different clinical responses in cancer cells. Autophagy is targeted for treating patients resistant to chemotherapy or radiation. Limited reports investigate the significance of autophagy in cancer therapy, the regulation of hematopoietic and leukemic stem cells and leukemia formation. In the current review, the role of autophagy is discussed in various stages of hematopoiesis including quiescence, self-renewal, and differentiation.

SIRT6 deacetylase activity regulates NAMPT activity and NAD(P)(H) pools in cancer cells

Nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme in the NAD⁺ salvage pathway from nicotinamide. By controlling the biosynthesis of NAD⁺, NAMPT regulates the activity of NAD⁺-converting enzymes, such as CD38, poly-ADP-ribose polymerases, and sirtuins (SIRTs). SIRT6 is involved in the regulation of a wide number of metabolic processes. In this study, we investigated the ability of SIRT6 to regulate intracellular NAMPT activity and NAD(P)(H) levels. BxPC-3 cells and MCF-7 cells were engineered to overexpress a catalytically active or a catalytically inactive SIRT6 form or were engineered to silence endogenous SIRT6 expression. In SIRT6-overexpressing cells, NAD(H) levels were up-regulated, as a consequence of NAMPT activation. By immunopurification and incubation with recombinant SIRT6, NAMPT was found to be a direct substrate of SIRT6 deacetylation, with a mechanism that up-regulates NAMPT enzymatic activity. Extracellular NAMPT release was enhanced in SIRT6-silenced cells. Also glucose-6-phosphate dehydrogenase activity and NADPH levels were increased in SIRT6-overexpressing cells. Accordingly, increased SIRT6 levels reduced cancer cell susceptibility to H₂O₂-induced oxidative stress and to doxorubicin. Our data demonstrate that SIRT6 affects intracellular NAMPT activity, boosts NAD(P)(H) levels, and protects against oxidative stress. The use of SIRT6 inhibitors, together with agents inducing oxidative stress, may represent a promising treatment strategy in cancer.-Sociali, G., Grozio, A., Caffa, I., Schuster, S., Becherini, P., Damonte, P., Sturla, L., Fresia, C., Passalacqua, M., Mazzola, F., Raffaelli, N., Garten, A., Kiess, W., Cea, M., Nencioni, A., Bruzzone, S. SIRT6 deacetylase activity regulates NAMPT activity and NAD(P)(H) pools in cancer cells.

Cancer cell metabolic plasticity allows resistance to NAMPT inhibition but invariably induces dependence on LDHA

Background

Inhibitors of nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme in NAD⁺ biosynthesis from nicotinamide, exhibit anticancer effects in preclinical models. However, continuous exposure to NAMPT inhibitors, such as FK866, can induce acquired resistance.

Methods

We developed FK866-resistant CCRF-CEM (T cell acute lymphoblastic leukemia) and MDA MB231 (breast cancer) models, and by exploiting an integrated approach based on genetic, biochemical, and genome wide analyses, we annotated the drug resistance mechanisms.

Results

Acquired resistance to FK866 was independent of NAMPT mutations but rather was based on a shift towards a glycolytic metabolism and on lactate dehydrogenase A (LDHA) activity. In addition, resistant CCRF-CEM cells, which exhibit high quinolinate phosphoribosyltransferase (QPRT) activity, also exploited amino acid catabolism as an alternative source for NAD⁺ production, becoming addicted to tryptophan and glutamine and sensitive to treatment with the amino acid transport inhibitor JPH203 and with l-asparaginase, which affects glutamine exploitation. Vice versa, in line with their low QPRT expression, FK866-resistant MDA MB231 did not rely on amino acids for their resistance phenotype.

Conclusions

Our study identifies novel mechanisms of resistance to NAMPT inhibition, which may be useful to design more rational strategies for targeting cancer metabolism.

Inhibitor of Nicotinamide Phosphoribosyltransferase Sensitizes Glioblastoma Cells to Temozolomide via Activating ROS/JNK Signaling Pathway

Overcoming temozolomide (TMZ) resistance is a great challenge in glioblastoma (GBM) treatment. Nicotinamide phosphoribosyltransferase (NAMPT) is a rate-limiting enzyme in the biosynthesis of nicotinamide adenine dinucleotide and has a crucial role in cancer cell metabolism. In this study, we investigated whether FK866 and CHS828, two specific NAMPT inhibitors, could sensitize GBM cells to TMZ. Low doses of FK866 and CHS828 (5 nM and 10 nM, resp.) alone did not significantly decrease cell viability in U251-MG and T98 GBM cells. However, they significantly increased the antitumor action of TMZ in these cells. In U251-MG cells, administration of NAMPT inhibitors increased the TMZ (100 μM)-induced apoptosis and LDH release from GBM cells. NAMPT inhibitors remarkably enhanced the activities of caspase-1, caspase-3, and caspase-9. Moreover, NAMPT inhibitors increased reactive oxygen species (ROS) production and superoxide anion level but reduced the SOD activity and total antioxidative capacity in GBM cells. Treatment of NAMPT inhibitors increased phosphorylation of c-Jun and JNK. Administration of JNK inhibitor SP600125 or ROS scavenger tocopherol with TMZ and NAMPT inhibitors substantially attenuated the sensitization of NAMPT inhibitor on TMZ antitumor action. Our data indicate a potential value of NAMPT inhibitors in combined use with TMZ for GBM treatment.

Antitumor effect of combined NAMPT and CD73 inhibition in an ovarian cancer model

Nicotinamide phosphoribosyltransferase (NAMPT) is a crucial enzyme in the biosynthesis of intracellular NAD⁺. NAMPT inhibitors have potent anticancer activity in several preclinical models by depleting NAD⁺ and ATP levels. Recently, we demonstrated that CD73 enables the utilization of extracellular NAD⁺/nicotinamide mononucleotide (NMN) by converting them to Nicotinamide riboside (NR), which can cross the plasmamembrane and fuel intracellular NAD⁺ biosynthesis in human cells. These processes are herein confirmed to also occur in a human ovarian carcinoma cell line (OVCAR-3), by means of CD73 or NRK1 specific silencing. Next, we investigated the anti-tumor activity of the simultaneous inhibition of NAMPT (with FK866) and CD73 (with α, β-methylene adenosine 5′-diphosphate, APCP), in an in vivo human ovarian carcinoma model. Interestingly, the combined therapy was found to significantly decrease intratumor NAD⁺, NMN and ATP levels, compared with single treatments. In addition, the concentration of these nucleotides in ascitic exudates was more remarkably reduced in animals treated with both FK866 and APCP compared with single treatments. Importantly, tumors treated with FK866 in combination with APCP contained a statistically significant lower proportion of Ki67 positive proliferating cells and a higher percentage of necrotic area. Finally, a slight but significant increase in animal survival in response to the combined therapy, compared to the single agents, could be demonstrated. Our results indicate that the pharmacological inhibition of CD73 enzymatic activity could be considered as a means to potentiate the anti-cancer effects of NAMPT inhibitors.

NAD+ salvage pathway in cancer metabolism and therapy

Nicotinamide adenine dinucleotide (NAD⁺) is an essential coenzyme for various physiological processes including energy metabolism, DNA repair, cell growth, and cell death. Many of these pathways are typically dysregulated in cancer cells, making NAD⁺ an intriguing target for cancer therapeutics. NAD⁺ is mainly synthesized by the NAD⁺ salvage pathway in cancer cells, and not surprisingly, the pharmacological targeting of the NAD⁺ salvage pathway causes cancer cell cytotoxicity in vitro and in vivo. Several studies have described the precise consequences of NAD⁺ depletion on cancer biology, and have demonstrated that NAD+ depletion results in depletion of energy levels through lowered rates of glycolysis, reduced citric acid cycle activity, and decreased oxidative phosphorylation. Additionally, depletion of NAD⁺ causes sensitization of cancer cells to oxidative damage by disruption of the anti-oxidant defense system, decreased cell proliferation, and initiation of cell death through manipulation of cell signaling pathways (e.g., SIRT1 and p53). Recently, studies have explored the effect of well-known cancer therapeutics in combination with pharmacological depletion of NAD⁺ levels, and found in many cases a synergistic effect on cancer cell cytotoxicity. In this context, we will discuss the effects of NAD⁺ salvage pathway inhibition on cancer cell biology and provide insight on this pathway as a novel anti-cancer therapeutic target.

Cytoplasmic proliferating cell nuclear antigen connects glycolysis and cell survival in acute myeloid leukemia

Cytosolic proliferating cell nuclear antigen (PCNA), a scaffolding protein involved in DNA replication, has been described as a key element in survival of mature neutrophil granulocytes, which are non-proliferating cells. Herein, we demonstrated an active export of PCNA involved in cell survival and chemotherapy resistance. Notably, daunorubicin-resistant HL-60 cells (HL-60R) have a prominent cytosolic PCNA localization due to increased nuclear export compared to daunorubicin-sensitive HL-60 cells (HL-60S). By interacting with nicotinamide phosphoribosyltransferase (NAMPT), a protein involved in NAD biosynthesis, PCNA coordinates glycolysis and survival, especially in HL-60R cells. These cells showed a dramatic increase in intracellular NAD+ concentration as well as glycolysis including increased expression and activity of hexokinase 1 and increased lactate production. Furthermore, this functional activity of cytoplasmic PCNA was also demonstrated in patients with acute myeloid leukemia (AML). Our data uncover a novel pathway of nuclear export of PCNA that drives cell survival by increasing metabolism flux.

Nicotinamide phosphoribosyltransferase (Nampt) in carcinogenesis: new clinical opportunities

INTRODUCTION

Nicotinamide phosphoribosyltransferase (Nampt) is the rate-limiting enzyme that catalyzes the first step in the mammalian nicotinamide adenine dinucleotide (NAD) salvage pathway. Aberrant NAD metabolism was associated with oncogenic signal transduction, suggesting the critical roles of Nampt in tumorigenesis and metastasis. Additionally, Nampt can be secreted out of the cell, and this extracellular form of Nampt (eNampt) was shown to induce inflammation and angiogenesis due to its cytokine activity, which may also be involved in carcinogenesis.

AREAS COVERED

This article reviews recent advances in the studies of Nampt in carcinogenesis, with a special highlight on Nampt inhibitors and future clinical application, including cancer diagnosis, prognosis and therapy. Expert commentary: Nampt not only maintains the balance of cellular metabolism, but also has a profound influence on multiple aspects of carcinogenesis. Therefore, elucidation of these mechanisms opens the door for future clinical applications targeting this protein. Additional studies are needed to address important questions including the relationship between extracellular Nampt and carcinogenesis.

Dual NAMPT and BTK Targeting Leads to Synergistic Killing of Waldenström Macroglobulinemia Cells Regardless of MYD88 and CXCR4 Somatic Mutation Status

PURPOSE

Nicotinamide phosphoribosyltransferase (Nampt) regulates intracellular NAD⁺ pool and is highly expressed in a number of malignancies. FK866, a selective inhibitor of Nampt, depletes intracellular NAD⁺ levels, thereby blocking cellular metabolism and triggering sensitization to other drugs and cell death. Here we characterized the antitumor effects of Nampt inhibition in Waldenström macroglobulinemia.

EXPERIMENTAL DESIGN

We investigated Nampt role in MW cells using both mRNA and protein expression analyses. We have also used loss-of-function approaches to investigate the growth and survival effects of Nampt on MW cells and further tested the anti-MW activity of dual Nampt and BTK inhibition in vitro and in vivo RESULTS: We found that Waldenström macroglobulinemia cells exhibit high levels of Nampt compared with normal B cells. Loss of function studies suggested a potential oncogenic role of Nampt in Waldenström macroglobulinemia cells, and BTK-inhibitor ibrutinib and FK866 resulted in a significant and synergistic anti-Waldenström macroglobulinemia cell death, regardless of MYD88 and CXCR4 mutational status. Cell death was associated with: (i) activation of caspase-3, PARP and downregulation of Mcl-1, (ii) enhanced intracellular ATP and NAD⁺ depletion, (iii) inhibition of NF-κB signaling, and (iv) inhibition of multiple prosurvival signaling pathways. In a murine xenograft Waldenström macroglobulinemia model, low-dose combination FK866 and ibrutinib is well tolerated, significantly inhibits tumor growth, and prolongs host survival.

CONCLUSIONS

Our results show intracellular NAD⁺ level as crucial for proliferation and survival of Waldenström macroglobulinemia cells, and provides the mechanistic preclinical rationale for targeting Nampt, either alone or with Ibrutinib, to overcome drug resistance and improve patient outcome in Waldenström macroglobulinemia. Clin Cancer Res; 22(24); 6099-109. ©2016 AACR.

The Role of Nicotinamide Phosphoribosyltransferase in Cerebral Ischemia

As recombinant tissue plasminogen activator is the only drug approved for the clinical treatment of acute ischemic stroke, there is an urgent unmet need for novel stroke treatments. Endogenous defense mechanisms against stroke may hold the key to new therapies for stroke. A large number of studies suggest that nicotinamide phosphoribosyl-transferase (NAMPT is an attractive candidate to improve post-stroke recovery. NAMPT is a multifunctional protein and plays important roles in immunity, metabolism, aging, inflammation, and stress responses. NAMPT exists in both the intracellular and extracellular space. As a rate-limiting enzyme, the intracellular form (iNAMPT catalyzes the first step in the biosynthesis of nicotinamide adenine dinucleotide (NAD from nicotinamide. iNAMPT closely regulates energy metabolism, enhancing the proliferation of endothelial cells, inhibiting apoptosis, regulating vascular tone, and stimulating autophagy in disease conditions such as stroke. Extracellular NAMPT (eNAMPT is also known as visfatin (visceral fat-derived adipokine and has pleotropic effects. It is widely believed that the diverse biological functions of eNAMPT are attributed to its NAMPT enzymatic activity. However, the effects of eNAMPT on ischemic injury are still controversial. Some authors have argued that eNAMPT exacerbates ischemic neuronal injury non-enzymatically by triggering the release of TNF-α from glial cells. In addition, NAMPT also participates in several pathophysiological processes such as hypertension, atherosclerosis, and ischemic heart disease. Thus, it remains unclear under what conditions NAMPT is beneficial or destructive. Recent work using in vitro and in vivo genetic/ pharmacologic manipulations, including our own studies, has greatly improved our understanding of NAMPT. This review focuses on the multifaceted and complex roles of NAMPT under both normal and ischemic conditions.

EIF2A-dependent translational arrest protects leukemia cells from the energetic stress induced by NAMPT inhibition

Background

Nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme in NAD⁺ biosynthesis from nicotinamide, is one of the major factors regulating cancer cells metabolism and is considered a promising target for treating cancer. The prototypical NAMPT inhibitor FK866 effectively lowers NAD⁺ levels in cancer cells, reducing the activity of NAD⁺-dependent enzymes, lowering intracellular ATP, and promoting cell death.

Results

We show that FK866 induces a translational arrest in leukemia cells through inhibition of MTOR/4EBP1 signaling and of the initiation factors EIF4E and EIF2A. Specifically, treatment with FK866 is shown to induce 5′AMP-activated protein kinase (AMPK) activation, which, together with EIF2A phosphorylation, is responsible for the inhibition of protein synthesis. Notably, such an effect was also observed in patients’ derived primary leukemia cells including T-cell Acute Lymphoblastic Leukemia. Jurkat cells in which AMPK or LKB1 expression was silenced or in which a non-phosphorylatable EIF2A mutant was ectopically expressed showed enhanced sensitivity to the NAMPT inhibitor, confirming a key role for the LKB1-AMPK-EIF2A axis in cell fate determination in response to energetic stress via NAD⁺ depletion.

Conclusions

We identified EIF2A phosphorylation as a novel early molecular event occurring in response to NAMPT inhibition and mediating protein synthesis arrest. In addition, our data suggest that tumors exhibiting an impaired LBK1- AMPK- EIF2A response may be especially susceptible to NAMPT inhibitors and thus become an elective indication for this type of agents.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1845-1) contains supplementary material, which is available to authorized users.

New strategies to maximize therapeutic opportunities for NAMPT inhibitors in oncology

Nicotinamide phosphoribosyltransferase (NAMPT) is crucial for nicotinamide adenine dinucleotide (NAD(+)) biosynthesis in mammalian cells. NAMPT inhibitors represent multifunctional anticancer agents that act on NAD(+) metabolism to shut down glycolysis, nucleotide biosynthesis, and ATP generation and act indirectly as PARP and sirtuin inhibitors. The selectivity of NAMPT inhibitors preys on the increased metabolic requirements to replenish NAD(+) in cancer cells. Although initial clinical studies with NAMPT inhibitors did not achieve single-agent therapeutic levels before dose-limiting toxicities were reached, a new understanding of alternative rescue pathways and a biomarker that can be used to select patients provides new opportunities to widen the therapeutic window and achieve efficacious doses in the clinic. Recent work has also illustrated the potential for drug combination strategies to further enhance the therapeutic opportunities. This review summarizes recent discoveries in NAD(+)/NAMPT inhibitor biology in the context of exploiting this new knowledge to optimize the clinical outcomes for this promising new class of agents.

APO866 Increases Antitumor Activity of Cyclosporin-A by Inducing Mitochondrial and Endoplasmic Reticulum Stress in Leukemia Cells

PURPOSE

The nicotinamide phosphoribosyltransferase (NAMPT) inhibitor, APO866, has been previously shown to have antileukemic activity in preclinical models, but its cytotoxicity in primary leukemia cells is frequently limited. The success of current antileukemic treatments is reduced by the occurrence of multidrug resistance, which, in turn, is mediated by membrane transport proteins, such as P-glycoprotein-1 (Pgp). Here, we evaluated the antileukemic effects of APO866 in combination with Pgp inhibitors and studied the mechanisms underlying the interaction between these two types of agents.

EXPERIMENTAL DESIGN

The effects of APO866 with or without Pgp inhibitors were tested on the viability of leukemia cell lines, primary leukemia cells (AML, n = 6; B-CLL, n = 19), and healthy leukocytes. Intracellular nicotinamide adenine dinucleotide (NAD(+)) and ATP levels, mitochondrial transmembrane potential (ΔΨ(m)), markers of apoptosis and of endoplasmic reticulum (ER) stress were evaluated.

RESULTS

The combination of APO866 with Pgp inhibitors resulted in a synergistic cytotoxic effect in leukemia cells, while sparing normal CD34(+) progenitor cells and peripheral blood mononuclear cells. Combining Pgp inhibitors with APO866 led to increased intracellular APO866 levels, compounded NAD(+) and ATP shortage, and induced ΔΨ(m) dissipation. Notably, APO866, Pgp inhibitors and, to a much higher extent, their combination induced ER stress and ER stress inhibition strongly reduced the activity of these treatments.

CONCLUSIONS

APO866 and Pgp inhibitors show a strong synergistic cooperation in leukemia cells, including acute myelogenous leukemia (AML) and B-cell chronic lymphocytic leukemia (B-CLL) samples. Further evaluations of the combination of these agents in clinical setting should be considered.

High expression of PI3K core complex genes is associated with poor prognosis in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is the most common leukemia among adults in the Western world. Autophagy is a highly conserved process in eukaryotic cells. In CLL autophagy is involved in mediating the effect of chemotherapy but the role of autophagy in CLL pathogenesis remains unknown. In the present study, we used real-time RT-PCR to analyze expression of the PIK3C3, PIK3R4, and BECN1 genes. These genes encode the components of the PI3K core complex, which is central to initiation of autophagy. A consecutive series of 149 well-characterized CLL cases from Region of Southern Denmark were included in the study. All three genes were observed to be independent markers of prognosis in CLL with high expression being associated with more aggressive disease. With this clear association with outcome in CLL, these genes thereby represent promising candidates for future functional studies on the role of autophagy in CLL, and they may further represent targets of treatment.

Autophagy in blood cancers: biological role and therapeutic implications

Autophagy is a cell recycling process the molecular apparatus of which has been identified over the past decade. Autophagy allows cells to survive starvation and inhospitable conditions and plays a key role in numerous physiological functions, including hematopoiesis and immune responses. In hematologic malignancies, autophagy can either act as a chemo-resistance mechanism or have tumor suppressive functions, depending on the context. In addition, autophagy is involved in other important aspects of blood cancers as it promotes immune competence and anti-cancer immunity, and may even help enhance patient tolerance to standard treatments. Approaches exploiting autophagy, either to activate or inhibit it, could find broad application in hematologic malignancies and contribute to improved clinical outcomes. These aspects are discussed here together with a brief introduction to the molecular machinery of autophagy and to its role in blood cell physiology.

Nicotinamide phosphoribosyltransferase in malignancy: a review

Nicotinamide phosphoribosyltransferase (Nampt) catalyzes the rate-limiting step of nicotinamide adenine dinucleotide (NAD) synthesis. Both intracellular and extracellular Nampt (iNampt and eNampt) levels are increased in several human malignancies and some studies demonstrate increased iNampt in more aggressive/invasive tumors and in tumor metastases. Several different molecular targets have been identified that promote carcinogenesis following iNampt overexpression, including SirT1, CtBP, and PARP-1. Additionally, eNampt is elevated in several human cancers and is often associated with a higher tumor stage and worse prognoses. Here we review the roles of Nampt in malignancy, some of the known mechanisms by which it promotes carcinogenesis, and discuss the possibility of employing Nampt inhibitors in cancer treatment.

The anti-lymphoma activity of APO866, an inhibitor of nicotinamide adenine dinucleotide biosynthesis, is potentialized when used in combination with anti-CD20 antibody

APO866 is an inhibitor of nicotinamide adenine dinucleotide (NAD) biosynthesis that exhibits potent anti-lymphoma activity. Rituximab (RTX), an anti-CD20 antibody, kills lymphoma cells by direct apoptosis and antibody- and complement-dependent cell-mediated cytotoxicities, and has clinical efficacy in non-Hodgkin cell lymphomas. In the present study, we evaluated whether RTX could potentiate APO866-induced human B-lymphoma cell death and shed light on death-mediated mechanisms associated with this drug combination. We found that RTX significantly increases APO866-induced death in lymphoma cells from patients and lines. Mechanisms include enhancement of autophagy-mediated cell death, activation of caspase 3 and exacerbation of mitochondrial depolarization, but not increase of reactive oxygen species (ROS) production, when compared with those induced by each drug alone. In vivo, combined administration of APO866 with RTX in a laboratory model of human aggressive lymphoma significantly decreased tumor burden and prolonged survival over single-agent treatment. Our study demonstrates that the combination of RTX and APO866 optimizes B-cell lymphoma apoptosis and therapeutic efficacy over both compounds administered separately.

A critical role of autophagy in antileukemia/lymphoma effects of APO866, an inhibitor of NAD biosynthesis

APO866, an inhibitor of NAD biosynthesis, exhibits potent antitumor properties in various malignancies. Recently, it has been shown that APO866 induces apoptosis and autophagy in human hematological cancer cells, but the role of autophagy in APO866-induced cell death remains unclear. Here, we report studies on the molecular mechanisms underlying APO866-induced cell death with emphasis on autophagy. Treatment of leukemia and lymphoma cells with APO866 induced both autophagy, as evidenced by an increase in autophagosome formation and in SQSTM1/p62 degradation, but also increased caspase activation as revealed by CASP3/caspase 3 cleavage. As an underlying mechanism, APO866-mediated autophagy was found to deplete CAT/catalase, a reactive oxygen species (ROS) scavenger, thus promoting ROS production and cell death. Inhibition of autophagy by ATG5 or ATG7 silencing prevented CAT degradation, ROS production, caspase activation, and APO866-induced cell death. Finally, supplementation with exogenous CAT also abolished APO866 cytotoxic activity. Altogether, our results indicated that autophagy is essential for APO866 cytotoxic activity on cells from hematological malignancies and also indicate an autophagy-dependent CAT degradation, a novel mechanism for APO866-mediated cell killing. Autophagy-modulating approaches could be a new way to enhance the antitumor activity of APO866 and related agents.

Interplay between apoptosis and autophagy, a challenging puzzle: new perspectives on antitumor chemotherapies

Autophagy is a mechanism of protection against various forms of human diseases, such as cancer, in which autophagy seems to have an extremely complex role. In cancer, there is evidence that autophagy may be oncogenic in some contexts, whereas in others it clearly contributes to tumor suppression. In addition, studies have demonstrated the existence of a complex relationship between autophagy and cell death, determining whether a cell will live or die in response to anticancer therapies. Nevertheless, we still need to complete the autophagy-apoptosis puzzle in the tumor context to better address appropriate chemotherapy protocols with autophagy modulators. Generally, tumor cell resistance to anticancer induced-apoptosis can be overcome by autophagy inhibition. However, when an extensive autophagic stimulus is activated, autophagic cell death is observed. In this review, we discuss some details of autophagy and its relationship with tumor progression or suppression, as well as role of autophagy-apoptosis in cancer treatments.

CD73 protein as a source of extracellular precursors for sustained NAD+ biosynthesis in FK866-treated tumor cells

NAD(+) is mainly synthesized in human cells via the "salvage" pathways starting from nicotinamide, nicotinic acid, or nicotinamide riboside (NR). The inhibition with FK866 of the enzyme nicotinamide phosphoribosyltransferase (NAMPT), catalyzing the first reaction in the "salvage" pathway from nicotinamide, showed potent antitumor activity in several preclinical models of solid and hematologic cancers. In the clinical studies performed with FK866, however, no tumor remission was observed. Here we demonstrate that low micromolar concentrations of extracellular NAD(+) or NAD(+) precursors, nicotinamide mononucleotide (NMN) and NR, can reverse the FK866-induced cell death, this representing a plausible explanation for the failure of NAMPT inhibition as an anti-cancer therapy. NMN is a substrate of both ectoenzymes CD38 and CD73, with generation of NAM and NR, respectively. In this study, we investigated the roles of CD38 and CD73 in providing ectocellular NAD(+) precursors for NAD(+) biosynthesis and in modulating cell susceptibility to FK866. By specifically silencing or overexpressing CD38 and CD73, we demonstrated that endogenous CD73 enables, whereas CD38 impairs, the conversion of extracellular NMN to NR as a precursor for intracellular NAD(+) biosynthesis in human cells. Moreover, cell viability in FK866-treated cells supplemented with extracellular NMN was strongly reduced in tumor cells, upon pharmacological inhibition or specific down-regulation of CD73. Thus, our study suggests that genetic or pharmacologic interventions interfering with CD73 activity may prove useful to increase cancer cell sensitivity to NAMPT inhibitors.

Intracellular NAD⁺ depletion enhances bortezomib-induced anti-myeloma activity

We recently demonstrated that Nicotinamide phosphoribosyltransferase (Nampt) inhibition depletes intracellular NAD⁺ content leading, to autophagic multiple myeloma (MM) cell death. Bortezomib has remarkably improved MM patient outcome, but dose-limiting toxicities and development of resistance limit its long-term utility. Here we observed higher Nampt messenger RNA levels in bortezomib-resistant patient MM cells, which correlated with decreased overall survival. We demonstrated that combining the NAD⁺ depleting agent FK866 with bortezomib induces synergistic anti-MM cell death and overcomes bortezomib resistance. This effect is associated with (1) activation of caspase-8, caspase-9, caspase-3, poly (ADP-ribose) polymerase, and downregulation of Mcl-1; (2) enhanced intracellular NAD⁺ depletion; (3) inhibition of chymotrypsin-like, caspase-like, and trypsin-like proteasome activities; (4) inhibition of nuclear factor κB signaling; and (5) inhibition of angiogenesis. Furthermore, Nampt knockdown significantly enhances the anti-MM effect of bortezomib, which can be rescued by ectopically overexpressing Nampt. In a murine xenograft MM model, low-dose combination FK866 and Bortezomib is well tolerated, significantly inhibits tumor growth, and prolongs host survival. Taken together, these findings indicate that intracellular NAD⁺ level represents a major determinant in the ability of bortezomib to induce apoptosis in MM cells and provide proof of concept for the combination with FK866 as a new strategy to enhance sensitivity or overcome resistance to bortezomib.

Medicinal chemistry of nicotinamide phosphoribosyltransferase (NAMPT) inhibitors

Nicotinamide phoshophoribosyltransferase (NAMPT) plays a key role in the replenishment of the NAD pool in cells. This in turn makes this enzyme an important player in bioenergetics and in the regulation of NAD-using enzymes, such as PARPs and sirtuins. Furthermore, there is now ample evidence that NAMPT is secreted and has a role as a cytokine. An important role of either the intracellular or extracellular form of NAMPT has been shown in cancer, inflammation, and metabolic diseases. The first NAMPT inhibitors (FK866 and CHS828) have already entered clinical trials, and a surge in interest in the synthesis of novel molecules has occurred. The present review summarizes the recent progress in this field.

Involvement of p53 in the cytotoxic activity of the NAMPT inhibitor FK866 in myeloid leukemic cells

FK866 is a specific inhibitor of NAMPT and induces apoptosis of leukemic cells by depletion of intracellular NAD⁺. Since up-regulation of NAMPT is associated with several cases of cancers, including leukemias, we asked whether in leukemic cells inhibition of NAMPT involves p53 pathway. We observed that FK866 induced apoptosis and reduced cell proliferation in NB-4, OCI-AML3 and MOLM-13 cell lines. In contrast, the leukemia cell lines, K-562 and Kasumi, containing nonfunctional p53 were relatively unaffected by FK866 treatment. Importantly, direct inhibition of sirtuins significantly reduced the viability of NB-4, OCI-AML3 and MOLM-13 cell lines. Activation of p53 by FK866 involved increased acetylation of p53 at lysine 382 with subsequent increase in the expression of p21 and BAX. Further, knockdown of p53 attenuated the effects of FK866 on apoptosis and cell cycle arrest, which was partly associated with decreased expression of p21 and BAX. Our results suggest the role of p53 acetylation pathway in the anti-leukemic effect of FK866.

Targeting cancer metabolism--aiming at a tumour's sweet-spot

Targeting cancer metabolism has emerged as a hot topic for drug discovery. Most cancers have a high demand for metabolic inputs (i.e. glucose/glutamine), which aid proliferation and survival. Interest in targeting cancer metabolism has been renewed in recent years with the discovery that many cancer-related (e.g. oncogenic and tumour suppressor) pathways have a profound effect on metabolism and that many tumours become dependent on specific metabolic processes. Considering the recent increase in our understanding of cancer metabolism and the increasing knowledge of the enzymes and pathways involved, the question arises: could metabolism be cancer's Achilles heel? During recent years, interest into the possible therapeutic benefit of targeting metabolic pathways in cancer has increased dramatically with academic and pharmaceutical groups actively pursuing this aspect of tumour physiology. Therefore, what has fuelled this revived interest in targeting cancer metabolism and what are the major advances and potential challenges faced in the race to develop new therapeutics in this area? This review will attempt to answer these questions by summarising recent developments in this field. We aim to illustrate why we, and others, believe that targeting metabolism in cancer presents such a promising therapeutic rationale.

Reciprocal potentiation of the antitumoral activities of FK866, an inhibitor of nicotinamide phosphoribosyltransferase, and etoposide or cisplatin in neuroblastoma cells

NAD is an essential coenzyme involved in numerous metabolic pathways. Its principal role is in redox reactions, and as such it is not heavily "consumed" by cells. Yet a number of signaling pathways that bring about its consumption have recently emerged. This has brought about the hypothesis that the enzymes that lead to its biosynthesis may be targets for anticancer therapy. In particular, inhibition of the enzyme nicotinamide phosphoribosyl transferase has been shown to be an effective treatment in a number of preclinical studies, and two lead molecules [N-[4-(1-benzoyl-4-piperidinyl)butyl]-3-(3-pyridinyl)-2E-propenamide (FK866) and (E)-1-[6-(4-chlorophenoxy)hexyl]-2-cyano-3-(pyridin-4-yl)guanidine (CHS 828)] have now entered preclinical trials. Yet, the full potential of these drugs is still unclear. In the present study we have investigated the role of FK866 in neuroblastoma cell lines. We now confirm that FK866 alone in neuroblastoma cells induces autophagy, and its effects are potentiated by chloroquine and antagonized by 3-methyladenine or by down-regulating autophagy-related protein 7. Autophagy, in this model, seems to be crucial for FK866-induced cell death. On the other hand, a striking potentiation of the effects of cisplatin and etoposide is given by cotreatment of cells with ineffective concentrations of FK866 (1 nM). The effect of etoposide on DNA damage is potentiated by FK866 treatment, whereas the effect of FK866 on cytosolic NAD depletion is potentiated by etoposide. Even more strikingly, cotreatment with etoposide/cisplatin and FK866 unmasks an effect on mitochondrial NAD depletion.

High visfatin expression in breast cancer tissue is associated with poor survival

BACKGROUND

Adipocytokines, adipocyte-secreted hormones, play a critical role in breast cancer development. The expression of visfatin, a newly discovered adipocytokine, in breast cancer tissues was determined and correlated with patient clinicopathologic variables.

METHODS

Visfatin expression in breast cancer tissues was analyzed by immunohistochemistry. Visfatin expression was correlated with clinicopathologic variables as well as recurrence rates, using the χ(2) test. The prognostic value of visfatin for disease-free and overall survival was evaluated by Kaplan-Meier estimates, and the significance of differences between curves was evaluated by the log-rank test.

RESULTS

High visfatin expression in breast cancer tissues was significantly correlated with tumor size, estrogen receptor (ER) negativity, and progesterone receptor (PR) negativity. Hormone therapy, but not radiotherapy or chemotherapy, decreased the recurrence rate in patients with high visfatin expression. Whereas high visfatin expression alone was associated with poor disease-free and overall survival, worse disease-free and overall survival was observed when high visfatin expression was combined with ER- and PR-negative status. Cox regression analysis also revealed that visfatin is an independent predictor of disease-free and overall survival.

CONCLUSION

High visfatin expression in breast cancer tissue is associated with more malignant cancer behavior as well as poor patient survival.

IMPACT

Visfatin is an independent prognosis predictor for breast cancer.