Apolipoprotein A-I anti-tumor activity targets cancer cell metabolism.

Previously, we reported apolipoprotein A-I (apoA-I), the major protein component of high-density lipoprotein (HDL), has potent anti-melanoma activity. We used DNA microarray and bioinformatics to interrogate gene expression profiles of tumors from apoA-I expressing (A-I Tg+/-) versus apoA-I-null (A-I KO) animals to gain insights into mechanisms of apoA-I tumor protection. Differential expression analyses of 11 distinct tumors per group with > 1.2-fold cut-off and a false discovery rate adjusted p < 0.05, identified 176 significant transcripts (71 upregulated and 105 downregulated in A-I Tg+/- versus A-I KO group). Bioinformatic analyses identified the mevalonate and de novo serine/glycine synthesis pathways as potential targets for apoA-I anti-tumor activity. Relative to A-I KO, day 7 B16F10L melanoma tumor homografts from A-I Tg+/- exhibited reduced expression of mevalonate-5-pyrophosphate decarboxylase (Mvd), a key enzyme targeted in cancer therapy, along with a number of key genes in the sterol synthesis arm of the mevalonate pathway. Phosphoglycerate dehydrogenase (Phgdh), the first enzyme branching off glycolysis into the de novo serine synthesis pathway, was the most repressed transcript in tumors from A-I Tg+/-. We validated our mouse tumor studies by comparing the significant transcripts with adverse tumor markers previously identified in human melanoma and found 45% concordance. Our findings suggest apoA-I targets the mevalonate and serine synthesis pathways in melanoma cells in vivo, thus providing anti-tumor metabolic effects by inhibiting the flux of biomolecular building blocks for macromolecule synthesis that drive rapid tumor growth.

Site-specific 5-hydroxytryptophan incorporation into apolipoprotein A-I impairs cholesterol efflux activity and high-density lipoprotein biogenesis.

Apolipoprotein A-I (apoA-I) is the major protein constituent of high-density lipoprotein (HDL) and a target of myeloperoxidase-dependent oxidation in the artery wall. In atherosclerotic lesions, apoA-I exhibits marked oxidative modifications at multiple sites, including Trp72 Site-specific mutagenesis studies have suggested, but have not conclusively shown, that oxidative modification of Trp72 of apoA-I impairs many atheroprotective properties of this lipoprotein. Herein, we used genetic code expansion technology with an engineered Saccharomyces cerevisiae tryptophanyl tRNA-synthetase (Trp-RS):suppressor tRNA pair to insert the noncanonical amino acid 5-hydroxytryptophan (5-OHTrp) at position 72 in recombinant human apoA-I and confirmed site-specific incorporation utilizing MS. In functional characterization studies, 5-OHTrp72 apoA-I (compared with WT apoA-I) exhibited reduced ABC subfamily A member 1 (ABCA1)-dependent cholesterol acceptor activity in vitro (41.73 ± 6.57% inhibition; p < 0.01). Additionally, 5-OHTrp72 apoA-I displayed increased activation and stabilization of paraoxonase 1 (PON1) activity (µmol/min/mg) when compared with WT apoA-I and comparable PON1 activation/stabilization compared with reconstituted HDL (WT apoA-I, 1.92 ± 0.04; 5-OHTrp72 apoA-I, 2.35 ± 0.0; and HDL, 2.33 ± 0.1; p < 0.001, p < 0.001, and p < 0.001, respectively). Following injection into apoA-I-deficient mice, 5-OHTrp72 apoA-I reached plasma levels comparable with those of native apoA-I yet exhibited significantly reduced (48%; p < 0.01) lipidation and evidence of HDL biogenesis. Collectively, these findings unequivocally reveal that site-specific oxidative modification of apoA-I via 5-OHTrp at Trp72 impairs cholesterol efflux and the rate-limiting step of HDL biogenesis both in vitro and in vivo.

Myeloid-specific genetic ablation of ATP-binding cassette transporter ABCA1 is protective against cancer.

Increased circulating levels of apolipoprotein A-I (apoA-I), the major protein of high-density lipoprotein (HDL), by genetic manipulation or infusion, protects against melanoma growth and metastasis. Herein, we explored potential roles in melanoma tumorigenesis for host scavenger receptor class B, type 1 (SR-B1), and ATP-binding cassette transporters A1 (ABCA1) and G1 (ABCG1), all mediators of apoA-I and HDL sterol and lipid transport function. In a syngeneic murine melanoma tumor model, B16F10, mice with global deletion of SR-B1 expression exhibited increased plasma HDL cholesterol (HDLc) levels and decreased tumor volume, indicating host SR-B1 does not directly contribute to HDL-associated anti-tumor activity. In mice with myeloid-specific loss of ABCA1 (Abca1-M/-M ; A1-M/-M), tumor growth was inhibited by ∼4.8-fold relative to wild type (WT) animals. Abcg1-M/-M (G1-M/-M) animals were also protected by 2.5-fold relative to WT, with no further inhibition of tumor growth in Abca1/Abcg1 myeloid-specific double knockout animals (DKO). Analyses of tumor-infiltrating immune cells revealed a correlation between tumor protection and decreased presence of the immune suppressive myeloid-derived suppressor cell (MDSC) subsets, Ly-6G+Ly-6CLo and Ly-6GnegLy-6CHi cells. The growth of the syngeneic MB49 murine bladder cancer cells was also inhibited in A1-M/-M mice. Collectively, our studies provide further evidence for an immune modulatory role for cholesterol homeostasis pathways in cancer.

Acute exposure to apolipoprotein A1 inhibits macrophage chemotaxis in vitro and monocyte recruitment in vivo.

Apolipoprotein A1 (apoA1) is the major protein component of high-density lipoprotein (HDL) and has well documented anti-inflammatory properties. To better understand the cellular and molecular basis of the anti-inflammatory actions of apoA1, we explored the effect of acute human apoA1 exposure on the migratory capacity of monocyte-derived cells in vitro and in vivo. Acute (20-60 min) apoA1 treatment induced a substantial (50-90%) reduction in macrophage chemotaxis to a range of chemoattractants. This acute treatment was anti-inflammatory in vivo as shown by pre-treatment of monocytes prior to adoptive transfer into an on-going murine peritonitis model. We find that apoA1 rapidly disrupts membrane lipid rafts, and as a consequence, dampens the PI3K/Akt signalling pathway that coordinates reorganization of the actin cytoskeleton and cell migration. Our data strengthen the evidence base for therapeutic apoA1 infusions in situations where reduced monocyte recruitment to sites of inflammation could have beneficial outcomes.

MEK inhibition abrogates sunitinib resistance in a renal cell carcinoma patient-derived xenograft model.

BACKGROUND: Renal cell carcinoma (RCC) patients treated with tyrosine kinase inhibitors (TKI) typically respond initially, but usually develop resistance to therapy. We utilised transcriptome analysis to identify gene expression changes during development of sunitinib resistance in a RCC patient-derived xenograft (PDX) model. METHODS: RCC tumours were harvested during pre-treatment, response and escape phases. Direct anti-proliferative effects of sunitinib plus MEK inhibitor were assessed. Activation status (phosphorylation) of MEK1/2 and ERK1/2 was determined, myeloid-derived suppressor cells (MDSC) sub-fractions were quantitated and G-CSF was measured by ELISA. RESULTS: During the response phase, tumours exhibited 91% reduction in volume, characterised by decreased expression of cell survival genes. After 4-week treatment, tumours developed resistance to sunitinib, associated with increased expression of pro-angiogenic and cell survival genes. During tumour escape, cellular movement, inflammatory response and immune cell trafficking genes were induced, along with intra-tumoural accumulation of MDSC. In this PDX model, either continuous treatment with sunitinib plus MEK inhibitor PD-0325901, or switching from sunitinib to PD-0325901 was effective. The combination of PD-0325901 with TKI suppressed intra-tumoural phospho-MEK1/2, phospho-ERK1/2 and MDSC. CONCLUSIONS: Continuous treatment with sunitinib alone did not maintain anti-tumour response; addition of MEK inhibitor abrogated resistance, leading to improved anti-tumour efficacy.

Apolipoprotein A-I and Cancer.

High-density lipoprotein (HDL) and apolipoprotein A-I (apoA-I), the predominant protein in plasma HDL, have long been the focus of intense studies in the field of atherosclerosis and cardiovascular disease. ApoA-I, in large part, is responsible for HDL assembly and its main atheroprotective function, that of shuttling excess cholesterol from peripheral tissues to the liver for excretion (reverse cholesterol transport). Recently, a protective role for HDL in cancer was suggested from several large clinical studies where an inverse relationship between plasma HDL-cholesterol (HDL-C) levels and risk of developing cancer was noted. This notion has now been tested and found to be supported in mouse tumor studies, where increasing levels of apoA-I/HDL were discovered to protect against tumor development and provision of human apoA-I was therapeutic against established tumors. This mini-review discusses the emerging role of apoA-I in tumor biology and its potential as cancer therapeutic.

Non-lethal Inhibition of Gut Microbial Trimethylamine Production for the Treatment of Atherosclerosis.

Trimethylamine (TMA) N-oxide (TMAO), a gut-microbiota-dependent metabolite, both enhances atherosclerosis in animal models and is associated with cardiovascular risks in clinical studies. Here, we investigate the impact of targeted inhibition of the first step in TMAO generation, commensal microbial TMA production, on diet-induced atherosclerosis. A structural analog of choline, 3,3-dimethyl-1-butanol (DMB), is shown to non-lethally inhibit TMA formation from cultured microbes, to inhibit distinct microbial TMA lyases, and to both inhibit TMA production from physiologic polymicrobial cultures (e.g., intestinal contents, human feces) and reduce TMAO levels in mice fed a high-choline or L-carnitine diet. DMB inhibited choline diet-enhanced endogenous macrophage foam cell formation and atherosclerotic lesion development in apolipoprotein e(-/-) mice without alterations in circulating cholesterol levels. The present studies suggest that targeting gut microbial production of TMA specifically and non-lethal microbial inhibitors in general may serve as a potential therapeutic approach for the treatment of cardiometabolic diseases.

The cardioprotective protein apolipoprotein A1 promotes potent anti-tumorigenic effects.

Here, we show that apolipoprotein A1 (apoA1), the major protein component of high density lipoprotein (HDL), through both innate and adaptive immune processes, potently suppresses tumor growth and metastasis in multiple animal tumor models, including the aggressive B16F10L murine malignant melanoma model. Mice expressing the human apoA1 transgene (A1Tg) exhibited increased infiltration of CD11b(+) F4/80(+) macrophages with M1, anti-tumor phenotype, reduced tumor burden and metastasis, and enhanced survival. In contrast, apoA1-deficient (A1KO) mice showed markedly heightened tumor growth and reduced survival. Injection of human apoA1 into A1KO mice inoculated with tumor cells remarkably reduced both tumor growth and metastasis, enhanced survival, and promoted regression of both tumor and metastasis burden when administered following palpable tumor formation and metastasis development. Studies with apolipoprotein A2 revealed the anti-cancer therapeutic effect was specific to apoA1. In vitro studies ruled out substantial direct suppressive effects by apoA1 or HDL on tumor cells. Animal models defective in different aspects of immunity revealed both innate and adaptive arms of immunity contribute to complete apoA1 anti-tumor activity. This study reveals a potent immunomodulatory role for apoA1 in the tumor microenvironment, altering tumor-associated macrophages from a pro-tumor M2 to an anti-tumor M1 phenotype. Use of apoA1 to redirect in vivo elicited tumor-infiltrating macrophages toward tumor rejection may hold benefit as a potential cancer therapeutic.

Determinants of cytokine induction by small interfering RNA in human peripheral blood mononuclear cells.

Synthetic small interfering RNAs (siRNAs) can trigger a strong innate immune response in mammalian cells. This nonspecific side effect may hinder the application of siRNAs as tools in gene silencing. Chemically synthesized siRNAs, including traditional 19-mers with 2-nt 3' overhangs, longer duplexes with blunt or 3' overhangs, and asymmetric duplexes with a blunt end and a 2-nt 3' overhang, can evoke strong dose-dependent interferon-alpha (IFN-alpha) and tumor necrosis factor-alpha (TNF-alpha) release in human peripheral blood mononuclear cells (PBMCs). This response is independent of retinoic acid-inducible gene I but may involve endosomal toll-like receptors (TLRs). The immunostimulatory effect of the siRNAs is directly related to either or both of the strands of the duplex in a sequence-dependent manner. However, although some single-stranded RNAs and siRNAs potently evoked both IFN-alpha and TNF-alpha induction, these responses were not always coupled. In accordance with this, specific chemical modifications differentially altered cytokine production, suggesting recruitment of different TLRs in a sequence-dependent manner.

A structural basis for discriminating between self and nonself double-stranded RNAs in mammalian cells.

Nonspecific effects triggered by small interfering RNAs (siRNAs) complicate the use of RNA interference (RNAi) to specifically downregulate gene expression. To uncover the basis of these nonspecific activities, we analyzed the effect of chemically synthesized siRNAs on mammalian double-stranded RNA (dsRNA)-activated signaling pathways. siRNAs ranging from 21 to 27 nucleotides (nt) in length activated the interferon system when they lacked 2-nt 3' overhangs, a characteristic of Dicer products. We show that the recognition of siRNAs is mediated by the RNA helicase RIG-I and that the presence of 3' overhangs impairs its ability to unwind the dsRNA substrate and activate downstream signaling to the transcription factor IRF-3. These results suggest a structural basis for discrimination between microRNAs that are endogenous Dicer products, and nonself dsRNAs such as by-products of viral replication. These findings will enable the rational design of siRNAs that avoid nonspecific effects or, alternatively, that induce bystander effects to potentially increase the efficacy of siRNA-based treatments of viral infections or cancer.

Poly(I-C)-induced Toll-like receptor 3 (TLR3)-mediated activation of NFkappa B and MAP kinase is through an interleukin-1 receptor-associated kinase (IRAK)-independent pathway employing the signaling components TLR3-TRAF6-TAK1-TAB2-PKR .

Recent studies show that a member of the interleukin-1 (IL-1)/Toll receptor superfamily, Toll-like receptor 3 (TLR3), recognizes double-stranded RNA (dsRNA). Because of the similarity in their cytoplasmic domains, IL-1/Toll receptors share signaling components that associate with the IL-1 receptor, including IL-1 receptor-associated kinase (IRAK), MyD88, and TRAF6. However, we find that, in response to dsRNA, TLR3 can mediate the activation of both NFkappaB and mitogen-activated protein (MAP) kinases in IL-1-unresponsive mutant cell lines, including IRAK-deficient I1A and I3A cells, which are defective in a component that is downstream of IL-1R but upstream of IRAK. These results clearly indicate that TLR3 does not simply share the signaling components employed by the IL-1 receptor. Through biochemical analyses we have identified an IRAK-independent TLR3-mediated pathway. Upon binding of dsRNA to TLR3, TRAF6, TAK1, and TAB2 are recruited to the receptor to form a complex, which then translocates to the cytosol where TAK1 is phosphorylated and activated. The dsRNA-dependent protein kinase (PKR) is also detected in this signal-induced TAK1 complex. Kinase inactive mutants of TAK1 (TAK1DN) and PKR (PKRDN) inhibit poly(dI.dC)-induced TLR3-mediated NFkappaB activation, suggesting that both of these kinases play important roles in this pathway.