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Published in final edited form as: Virology. 2013 Nov 27;449:96–103. doi: 10.1016/j.virol.2013.11.003

Inhibition of HIV by Legalon-SIL is independent of its effect on cellular metabolism

Jan McClure 1, Daciana H Margineantu 2, Ian R Sweet 3, Stephen J Polyak 1,4,#
PMCID: PMC3909448  NIHMSID: NIHMS545495  PMID: 24418542

Abstract

In this report, we further characterized the effects of silibinin (SbN), derived from milk thistle extract, and Legalon-SIL (SIL), a water-soluble derivative of SbN, on T cell metabolism and HIV infection. We assessed the effects of SbN and SIL on peripheral blood mononuclear cells (PBMC) and CEM-T4 cells in terms of cellular growth, ATP content, metabolism, and HIV infection. SIL and SbN caused a rapid and reversible (upon removal) decrease in cellular ATP levels, which was associated with suppression of mitochondrial respiration and glycolysis. SbN, but not SIL inhibited glucose uptake. Exposure of T cells to SIL (but not SbN or metabolic inhibitors) during virus adsorption blocked HIV infection. Thus, both SbN and SIL rapidly perturb T cell metabolism in vitro, which may account for its anti-inflammatory and anti-proliferative effects that arise with prolonged exposure of cells. However, the metabolic effects are not involved in SIL’s unique ability to block HIV entry.

Keywords: silymarin, silibinin, T cell, HIV, metabolism

INTRODUCTION

Natural products offer wide chemical diversities that are touted in complementary and alternative medicinal practices. Moreover, many pharmaceutical and research drugs are derived from natural products (Newman and Cragg, 2012). Research on natural products often focuses on their effects on human cells, and how these effects translate into anti-infective, anti-proliferative, and anti-inflammatory properties of the compounds.

There are approximately 34 million people currently living with HIV-1 infection, and nearly 30 million people have died of AIDS-related causes since the beginning of the epidemic (UNAIDS, 2011, 2012). However, even with effective virus control by antiretroviral therapy (ART), many HIV+ subjects suffer from chronic inflammation, which is associated with various inflammatory diseases and both AIDS-defining and non-AIDS-defining cancers (Cutrell and Bedimo, 2013; Dubrow et al., 2012). Thus, there is a need to develop anti-inflammatory therapies for HIV+ subjects.

Many natural products have been shown to have anti-inflammatory properties (Asres et al., 2005). One such product, silymarin (SM), derived from the seeds of the milk thistle plant (Silybum marianum (L.) Gaertn.), has been consumed orally for thousands of years since Pedanius Dioscorides first described the plant in Materia Medica (circa 50 AD), and is one of the 10 most popular natural products consumed by western society (Polyak et al., 2013a). Many HIV+ patients consume SM with the belief that it helps protect the liver against damage from certain antiretroviral drugs and HIV-induced inflammation: (http://www.aidsinfonet.org/fact_sheets/view/735#WHY_DO_PEOPLE_WITH_HIV_USE_SILYMARIN). The major component of SM is known as silibinin (SbN), which is a diastereomeric mixture of two flavonolignans called silybin A and silybin B. Both SM and SbN block hepatitis C (HCV) infection in vitro (Polyak et al., 2013a; Polyak et al., 2010; Polyak et al., 2007; Polyak et al., 2013b; Wagoner et al., 2011; Wagoner et al., 2010). An intravenous formulation of SbN, where silybin A and silybin B have been succinated (Supplemental Figure 1) is known as Legalon-SIL (SIL), and reduces circulating viral loads in HCV-infected patients (Beinhardt S et al., 2012; Beinhardt et al., 2011; Ferenci et al., 2008; Neumann et al., 2010). We have recently shown that SIL inhibits human immunodeficiency virus-1 (HIV-1) infection coincident with dose-dependent reductions in T-cell activation and proliferation. (McClure et al., 2012). In the current study, we further characterized the effects of SIL and SbN on T cell metabolism and HIV infection.

RESULTS

SIL causes rapid reductions in intracellular ATP levels prior to any observable cytostatic effects

We recently showed that SIL inhibits T cell activation and proliferation coincident with inhibition of HIV infection (McClure et al., 2012). SIL was shown to slow the proliferation of T cells without inducing cell death. In order to gain more insight into the cytostatic effects of SIL, we first performed a kinetic experiment that included early time points. We compared the effect of SIL on cell number and viability (by direct cell counting with trypan Blue and by measuring intracellular ATP levels.)

As shown in Figure 1A, SIL caused dose-dependent inhibition of CEM T cell growth after 24 hours exposure of cells to SIL. However, no observable effect on cell number was observed when cells were incubated with SIL for 15 minutes, 1 hour, or 4 hours. In direct contrast, SIL caused significant dose-dependent inhibition of intracellular ATP levels at all time points analyzed, even at the earliest time analyzed (15 minutes; Figure 1B; p<0.05). The increase in ATP levels over time reflects cell proliferation. The data indicate that SIL causes rapid, dose-dependent suppression of intracellular ATP levels prior to any observable effects on cell growth.

Figure 1. SIL causes rapid, early inhibition of intracellular ATP levels.

Figure 1

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CEM T cells were incubated at the indicated concentrations of SIL and cells were either counted by trypan blue exclusion (A) or intracellular ATP levels were measured by ATPlite assay (B) at the indicated time points. As compared to untreated cells, all doses of SIL resulted in significant suppression of cellular ATP levels at all time points (p<0.05).

Inhibition of intracellular ATP levels and cell growth requires continual exposure to SIL and SbN and is rapidly reversible upon removal of the mixtures

Figure 2A shows that both SIL and SbN cause rapid, dose-dependent decreases in ATP levels in both PBMC and CEM T cells within 10 minutes of addition. SIL appeared to cause a more rapid and pronounced decline in ATP levels compared to SbN. Moreover, intracellular ATP levels rapidly returned to normal upon removal of SIL (Figure 2B) and SbN (Figure 2D), which also correlated with a restoration of cell growth when the mixtures were removed (Figures 2C, E). As previously shown (Wagoner et al., 2011), when cells were exposed to the mixtures for 24-72 hours, SbN was more toxic to cells than similar doses of SIL. In summary, pulse treatment of T cells with either SIL or SbN does not result in a durable effect on T cell growth kinetics or intracellular ATP levels. Thus, these mixtures must be continually present in culture in order to inhibit cellular ATP levels and cell growth.

Figure 2. SbN and SIL cause rapid and reversible decreases in cellular ATP levels.

Figure 2

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A, PBMC or CEM T cells were incubated with the indicated concentrations of SIL or SbN and 10 minutes later, cellular ATP levels were measured. Panels B through E, CEM T cells were exposed to the indicated doses of SIL (panels B and C) or SbN (panels D and E) for 30 minutes. Cells were either maintained in the same medium (“SIL/SbN full exposure”), or washed and resuspended in fresh medium containing SIL or SbN (“SIL 30 min → wash →fresh SIL/SbN”), or in fresh medium without SIL or SbN (“SIL 30 min → wash →no SIL/SbN”). Cells were immediately assayed for ATP content (panels B and D) or incubated for an additional 72 hours before cells counts were measured by trypan blue exclusion (panels C and E).

SbN and SIL rapidly inhibit mitochondrial respiration and glycolysis in T cells

Since both SbN and SIL caused such a rapid reduction in cellular ATP levels, we posited that the mixtures alter T cell metabolism, similar to previous studies in rodent livers (Colturato et al., 2012; Detaille et al., 2008; Guigas et al., 2007). Hence, biochemical studies were first performed with SIL on T cell metabolism. Oxygen consumption rate (OCR, a measure of mitochondrial respiration) and acidification of the media arising from extrusion of lactic acid from cells during glycolysis (extracellular acidification rate, ECAR) were measured using a Seahorse XF24-3 analyzer. Within minutes of addition of the mixture, SIL reduced both OCR (Figure 3A) and ECAR (Figure 3B), suggesting that SIL blocks mitochondrial respiration and glycolysis.

Figure 3. SIL inhibits T cell metabolism.

Figure 3

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CEM cells were cultured in a Seahorse XF24-3 analyzer in medium containing unbuffered DMEM, 25 mM glucose, 2 mM glutamine, and 1 mM pyuvate. Cells were allowed to equilibrate for 25 minutes before SIL (1mM) was injected into the culture chambers (indicated by the arrow). Panel A depicts the oxygen consumption rate (OCR) over time, while panel B depicts the extracellular acidification rate (ECAR).

We then examined the effects of repeated dosing of SIL and SbN on mitochondrial respiration and glycolysis. As can be seen in Figure 4A, repeated injections of 125 μM of SIL or SbN produced similar effects on OCR, which appeared first as a rapid decline that slightly recovered, but was persistently suppressed with repeated dosing. Repeated SIL and SbN dosing led to sustained suppression of the ECAR (Figure 4B).

Figure 4. SbN and SIL suppress mitochondrial respiration and glycolysis in T cells.

Figure 4

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CEM T cells were cultured in a Seahorse XF24-3 analyzer and allowed to equilibrate for 30 minutes prior to injection of 125 μM of SIL or SbN at the times indicated by the arrows. A, oxygen consumption rate (OCR). B, extracellular acidification rate (ECAR), presented as percent change relative to baseline.

The results demonstrate for first time that both the natural mixture (SbN) and the chemically modified mixture SIL suppress metabolism of human T cells, consistent with previous data showing that silibinin suppresses hepatic glucose metabolism by modulating glycolysis, oxidative phosphorylation and gluconeogenesis (Colturato et al., 2012; Detaille et al., 2008; Guigas et al., 2007). The acute effect of SIL and SbN on both glycolysis and oxidative phosphorylation suggests that these compounds inhibit glucose utilization upstream of mitochondria, either by interfering with glucose uptake or with cytosolic glucose metabolism. Moreover, the OCR trend following SIL and SbN administration indicate that cells only partially engage in the oxidation of alternative cellular energy substrates and compensate inefficiently for the rapid decline in metabolic rate. Thus, we next determined whether glucose could rescue the inhibitory effects of SIL or SbN on OCR and ECAR. Cells were first equilibrated in medium with or without 125μM SIL or SbN. Figure 5A shows following the first injection of 0.5 mM glucose, SIL and SbN treated cells had a consistently higher OCR as compared to control cells (Figure 5A). The reduction in OCR in response to glucose supplementation is consistent with previous studies showing that some types of cancer cells decrease their OCR after glucose addition: a phenotype known as the Crabtree effect (Crabtree, 1929). Addition of the ATP synthase inhibitor oligomycin, a mitochondrial phosphorylation inhibitor that abolishes ADP-stimulated respiration in intact mitochondria, suppressed the remaining OCR. Finally, injection of the uncoupling agent, carbonyl cyanide m-chlorophenyl hydrazone (CCCP), which dissipates the mitochondrial proton gradient, stimulated respiration (OCR) in control and SIL treated cells by approximately 35%. In contrast, SbN treated cells did not increase OCR. The data suggest that SIL and SbN do not inhibit respiration through effects on mitochondrial enzymes but through the availability of glucose metabolites that fuel respiration.

Figure 5. Further characterization of SbN and SIL inhibition of T cell metabolism.

Figure 5

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CEM T cells were plated in unbuffered DMEM, 2mM glutamine, and no glucose and allowed to equilibrate for 25 minutes in the presence or absence (control) of 125μM SbN or SIL before injections (indicated by arrows) with 0.5mM glucose (GLUC), 2.5 mM glucose (GLUC), 5μM oligomycin (OLIGO), and 500 nM CCCP. A, OCR, B, ECAR, presented as percent change relative to baseline.

Figure 5B shows that glucose supplementation led to increased lactic acid production in control cells. SIL and SbN treated cells did not increase ECAR in response to two injections of glucose. Addition of oligomycin to control cells led to an initial increase in ECAR, suggesting that cells upregulated glycolysis to compensate for the respiratory defect. CCCP addition suppressed ECAR. In contrast, lactic acid production in SIL or SbN treated cells could not be rescued by glucose, nor by addition of phosphorylation inhibitor (oligomycin) or uncoupling agent (CCCP). Cumulatively, the data suggest that SbN and SIL inhibit the glycolytic pathway.

SbN but not SIL inhibits glucose uptake in T cells

To further characterize the metabolic effects of SbN and SIL, we next determined their effect on glucose uptake in T cells. As shown in Figure 6, positive controls glucose and cytochalasin B both inhibited radiolabeled glucose update. In addition, SbN but not SIL inhibited glucose uptake, consistent with a previous study where SbN inhibited glucose uptake in Chinese hamster ovary (CHO) cells (Zhan et al., 2011). Since SIL and SbN both reduced OCR and ECAR (Figures 4 and 5), the data suggest that these changes in OCR and ECAR were not mediated by reduction in glucose uptake.

Figure 6. SbN but not SIL blocks glucose uptake into T cells.

Figure 6

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CEM cells were exposed to tritiated 3H-2-deoxyglucose in the presence of 20 mM glucose, an inhibitor of glucose uptake, cytochalasin B (CytB), 333μM or 1mM SbN, 1.25 mM SIL, or DMSO (the solvent control for SbN).

SbN and SIL inhibition of intracellular ATP is associated with inhibition of glycolysis but not glucose uptake

We then asked whether inhibition of glycolysis and/or glucose uptake could mimic SbN’s and SIL’s rapid and reversible effects on cellular ATP levels shown in Figure 2. We therefore compared cellular ATP levels in CEM cells treated with known inhibitors of glycolysis and glucose uptake. As shown in Figure 7, only 2-deoxyglucose (2-DG), an inhibitor of glycolysis that inhibits hexokinase, was capable of inducing rapid inhibition of cellular ATP levels similar to that of SIL and SbN. Glucose uptake inhibitors STF-31, Fasentin, and cytochalasin B had no effect on cellular ATP levels. The results suggested that glycolysis inhibition is associated with rapid suppression of cellular ATP levels.

Figure 7. SbN and SIL suppression of cellular ATP is associated with inhibition of glycolysis but not glucose transport.

Figure 7

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CEM cells were incubated with the indicated concentrations of glucose uptake inhibitors (STF-31, Fasentin, and cytochalasin B (CytB)), the glycolysis inhibitor 2-deoxyglucose (2-DG) and SIL or SbN for 15 minutes or 60 minutes, in the presence of media containing or not containing glucose. ATP levels were then measured. Doses are as follows: SIL 1mM, SbN 1mM/1%DMSO, 2-DG 50mM, CytB 10uM/.05%DMSO, Fasentin 100uM/0.1%DMSO, STF-31 10uM/.01%DMSO, DMSO 1%.

SIL and SbN inhibition of HIV infection is independent of metabolic inhibition

Because SIL inhibits HIV infection (McClure et al., 2012), and SIL and SbN inhibit T cell metabolism (as shown above), we wanted to know whether the metabolic effects of these mixtures are involved in their anti-HIV effects. Thus, we compared the effect of adding SIL, SbN, and metabolic inhibitors during the initial stages of HIV-1 infection of CEM T cells. Because both SIL and SbN can inhibit cell growth when cells are continually exposed to the mixtures for 24 hours or greater (Figure 2 and (McClure et al., 2012; Morishima et al., 2010; Wagoner et al., 2011)), and because SbN (>100uM) and 2-DG treatment are toxic to cells within 24 hours, we developed a quantitative infectious center (QIC) assay to evaluate the effects of transient exposure of cells to drugs during the early phases of HIV infection (i.e. within the first 0.5-2 hours) before the cytostatic and cytotoxic effects are observable.

As shown in Figure 8 at high dose, both SIL and to a lesser extent, SbN, inhibited HIV infection in the QIC assay in both CEM T cells and PBMCs. However, 2-DG and oxamate (an inhibitor of lactic acid dehydrogenase) had no effect on HIV infection. The data indicate that the early and rapid effects of SIL and SbN on T cell metabolism do not appear to be involved in the suppression of early HIV infection. However, because high doses of SIL and SbN were used in this experiment, we next exploited the QIC assay to examine timing and dose-dependency of SIL and SbN addition to cells during the early stages of HIV infection.

Figure 8. High dose SIL and SbN but not glycolysis inhibitors block HIV infection.

Figure 8

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PBMC and CEM T cells were exposed to HIV-LAI at an MOI of 0.07 in the presence of SIL (1mM), SbN (1mM), 2-deoxyglucose (2-DG; 50mM), oxamate (Ox; 50 μM), or DMSO (D; 1%, solvent control for SbN) for 2.5 hours. Cells were washed to remove drug and non-adsorbed virus then serially diluted in 96 well plates and co-cultured with 25,000/well non-infected PBMC or CEM T cells. Cultures were maintained for 13 days before HIV-1 p24 antigen was quantified by ELISA. Compared to media control, SIL caused significant suppression of HIV infection in both CEM T cells and PBMCs (p<0.0001, denoted by the asterisk * in the figure). Although less potent than SIL, SbN also suppressed HIV infection relative to DMSO solvent control in both CEM T cells and PBMCs (p<0.02, denoted by the double asterisk **).

SIL inhibits HIV entry into T cells

We compared the effect adding and removing SIL or SbN to CEM T cells before virus exposure or during virus adsorption. As a control, we exposed cells to the mixtures for the full duration of the experiment. Figures 9A and 9B shows the expected result with the positive control conditions: continuous exposure of T cells to SIL or SbN inhibited HIV infection. In contrast, pretreating cells with SIL or SbN for 1 hour, followed by drug removal prior to virus infection had no effect on HIV infection. This result is consistent with the rapid recovery of cell growth and ATP levels shown above. Intriguingly, SIL (Figure 9A) but not SbN (Figure 9B) treatment during the 1-hour virus adsorption period caused inhibition of HIV infection measured as p24 antigen production in culture supernatants. When virus replication was also measured in the QIC assay on the same samples, the same effect was observed. That is, SIL, but not SbN caused dose-dependent inhibition of HIV infection when added only during the 1-hour virus adsorption period (Figure 9C). These effects were not unique to the virus isolate (LAI) and cell line (CEM) used, as SIL potently inhibited many other CXCR4 and CCR5 co-receptor utilizing HIV isolates in PHA-activated PBMCs, assessed in our QIC assay (Figure 10). The data indicate that SIL but not SbN blocks HIV entry into T cells.

Figure 9. SIL not SbN inhibits the early stages of HIV infection.

Figure 9

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CEM T cells were pretreated for 1 hour with the indicated doses of SIL or SbN, followed by drug removal and virus adsorption (“SIL or SbN on Cell Pre-Rx”). Cells were also exposed to SIL or SbN during virus adsorption only (“SIL or SbN during Adsorption”), or for the duration of the experiment (“SIL or SbN Continuous Rx”). Cells were infected with HIV lai at m.o.i. of 0.02 at 37°C for 1 hour. Residual inoculum and drugs were removed and cells cultured in media with or without fresh drug (bulk cultures, panels A and B) or titered and co-cultured with fresh CEM for QIC assay (panel C). P24 antigen was measured by ELISA at 7 days post-infection in panels A (for SIL) and B (for SbN). Panel C shows the QIC results for both SIL and SbN.

Figure 10. SIL inhibits entry of multiple HIV isolates into PBMCs.

Figure 10

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PBMC were stimulated with PHA and were exposed to HIV-1 isolates at an MOI of 0.05-0.07 in the presence or absence of 500 μM SIL for 30-60 minutes at 37°C. Cells were washed to remove drug and non-adsorbed virus then serially diluted in 96 well plates and co-cultured with 25,000/well non-infected PBMC in the QIC assay. Cultures were maintained for 7-10 days before HIV-1 p24 antigen was quantified by ELISA. RF-A and RF-B represent the same viral isolate tested in 2 separate assays. Compared to the media control, SIL caused significant suppression of all HIV isolates in PBMCs (p<0.001).

DISCUSSION

Cumulatively, the data presented in the current report indicate that SbN and SIL induce a rapid metabolic changes in cells through modulation of glucose metabolism, yet this phenotype does not involve blockade of glucose transport since only SbN but not SIL inhibits glucose transport. Moreover, the rapid and reversible effects of SbN and SIL on T cell metabolism (decline of ATP, OCR, ECAR, glucose uptake (for SbN)) do not appear to be involved in the early blockade of HIV binding and entry that appears to be unique to SIL. We propose that the structural differences between SIL and SbN account for unique effects of SIL on HIV entry, while the metabolic effects of SIL and SbN may be involved in the anti-inflammatory and anti-proliferative effects that arise in cells upon prolonged exposure of cells to these mixtures.

We showed that SIL and SbN induce rapid, reversible effects on T cell metabolism and that SIL inhibits HIV early in the virus infection cycle. To further address the early effects of SIL and SbN on HIV infection, prior to the effects on cell population size (SIL) or cell toxicity (SbN), we developed a novel quantitative infectious center assay (QIC) which measures the number of target cells infected during a brief virus and drug exposure. SIL but not SbN inhibited HIV infection potently within a 1 hour virus adsorption phase at 37°C. The data indicate that SIL has a unique effect in blocking virus entry. Since other drugs that inhibit cellular metabolism by blocking glucose uptake or glycolysis did not inhibit HIV infection in the QIC assay, the early effects of SIL and SbN on T cell metabolism are not involved in SIL’s blockade of virus entry. Thus, we conclude that a major anti-HIV effect of SIL likely involves the blocking of virus entry. With prolonged exposure of cells to these mixtures, the metabolic or activation state of the target cell becomes compromised, which can also attenuate HIV replication as previously shown (McClure et al., 2012). Importantly, SIL is not directly virucidal to HIV-1 (Supplemental Figure 2), further supporting the concept that SIL is somehow preventing virus from entering target cells. An alternative explanation could be that SbN binds and inhibits the activity of additional metabolic enzymes with a broader effect while SIL has a more specific effect. In support of this, the acute effects of SbN and SIL on reduction of cellular ATP are fairly similar (Figure 2A; although SIL causes faster suppression of ATP levels than SbN; see below), but the long-term effect (>24h) of SbN is more dramatic, both in terms of cell number and OCR, than SIL. Further studies are required to define exactly how SIL attenuates virus entry, and the cellular targets of SbN versus SIL.

The apparent lack of reserve metabolic capacity following CCCP treatment (Figure 5A) may be the result of two factors. The first factor may be pyruvate, which is an important direct substrate for the TCA cycle and its exclusion from medium decreases the spare respiratory capacity (Diers et al., 2012). Since the cells were plated in XF24 DMEM supplemented with glutamine but not pyruvate at the start of this experiment, the lack of a reserve respiratory capacity (i.e. the return of OCR to higher than baseline levels) may be simply due to the omission of pyruvate in the experiment. Second, relative to the pre-Oligomycin state, CCCP stimulated respiration (OCR) in control and SIL treated cells by approximately 35% (Figure 5A). In contrast, SbN treated cells did not increase OCR. Thus, the second explanation why CCCP did not induce OCR is that SbN treated cells may be unable to access and/or utilize glucose. This potential explanation is supported by the observation that SbN but not SIL blocked glucose uptake (Figure 6). These differences between SIL and SbN might be involved in SbN induced cytoxicity, while SIL is relatively non-toxic.

Since both SIL and SbN have been shown to block HCV entry during fusion of viral and endosomal membranes (Wagoner et al., 2011; Wagoner et al., 2010), we were surprised that SIL but not SbN inhibited HIV entry. Moreover, SbN has been recently shown to block clathrin-mediated endocytosis to inhibit HCV entry (Blaising et al., 2013). Since HIV enters cells by fusion of viral and plasma membranes (Gibson and Arts, 2012) as opposed to viral membrane fusion with cellular endosomal membranes as occurs during HCV entry, it is possible that the unique role of SIL on HIV entry is dependent on the structure of SIL and its interaction with the cell surface. However, since HIV may also use clathrin to enter and traffic within cells (Permanyer et al., 2010; von Kleist et al., 2011), it remains unclear at present why only very high doses of SbN block HIV entry (Figure 8). Moreover, since SIL decreases cellular ATP levels faster than SbN (Figure 2A), while SbN is more toxic than SIL (Figure 2E), SIL may enter and exit cells via transporters faster than SbN. Due to its polar structure, SbN may enter cells via passive diffusion, and be retained in cells, possibly accounting for its higher toxicity compared to SIL. Again, we suspect the molecular differences between SIL and SbN account for their differential interactions with cells in terms of virus entry, glucose transport, and metabolism.

Our data on inhibition of T cell mitochondrial respiration and glycolysis by SIL and SbN are consistent with previous studies showing that silibinin modulates metabolism in rats (Colturato et al., 2012; Detaille et al., 2008; Guigas et al., 2007), and that silibinin induces energy stress in colorectal carcinoma cells (Raina et al., 2013). It is possible that the inhibition of glycolysis observed in this study is due to an inhibition of glucose-6-phosphate kinase (Guigas et al., 2007) or pyruvate kinase (Detaille et al., 2008). Moreover, our results in T cells also corroborate previous studies showing that silibinin inhibits glucose transport across the plasma membrane in 3T3-L1 adipocytes by perturbing the glucose transporter GLUT4 (Zhan et al., 2011). Moreover, the HIV Nef protein inhibits GLUT4 mediated glucose transport (Cheney et al., 2011). We find it intriguing that SbN but not SIL inhibited glucose uptake, yet SIL but not SbN inhibited HIV entry. Given that IL-7 enhances GLUT1 mediated glucose uptake to facilitate HIV replication (Loisel-Meyer et al., 2012), it can be surmised that SIL’s inhibition of HIV likely does not involve blockade of glucose uptake. These observations further underscore the conclusion that the metabolic effects of SIL and SbN are not involved in SIL’s early blockade of HIV entry. Thus, we hypothesize that the metabolic effects of silibinin are mechanistically involved in the anti-inflammatory functions of this class of natural products. In this regard, we have found that the parent extract, silymarin, regulates the expression of key cellular genes and signal transduction pathways that are involved in energy metabolism (E.S. Lovelace and S.J. Polyak, manuscript in preparation). Furthermore, SM also suppresses inflammation and oxidative stress (Polyak et al., 2013a), all of which are elevated in chronic immune activation (CIA) associated with HIV-1 infection, even in individuals taking combination antiretroviral therapy (ART). While ART effectively controls HIV replication, it does not fully control CIA. Thus, the antiviral, anticancer (Agarwal et al., 2006), anti-inflammatory, and antioxidant properties of SM may represent a novel approach to combating virus-induced chronic inflammation such as observed for patients with chronic hepatitis C, HIV, and HIV-HCV co-infection. Nonetheless, it must be emphasized that this in-vitro study does not support (nor refute) any benefit to HIV-positive individuals by taking silymarin-containing supplements.

MATERIALS AND METHODS

Chemicals

SbN was purchased from Sigma (St. Louis, MO), and was solubilized in DMSO (Sigma). SIL was kindly provided by Ralf-Torsten Pohl (Rottapharm/Madaus, Germany) and solubilized in PBS. The composition and structures of the mixtures is shown in Supplemental Figure 1. Glucose, 2-deoxyglucose, oligomycin, carbonyl cyanide m-chlorophenyl hydrazone (CCCP), cytochalasin B, and fasentin were also purchased from Sigma. STF-31 was purchased from Calbichem (EMD Millipore, Billerica, MA).

Cells

Peripheral blood mononuclear cells (PBMC) were isolated from leukapheresis units from ten healthy HIV-1 seronegative donors under written informed consent according to University of Washington Institutional Review Board (UW IRB) regulations. For the current studies, our work on these archived cryopreserved PBMC samples was deemed by the UW IRB to not meet the federal regulatory definition of “human subject research”, based on the Code of Federal Regulations (CFR), Title 45, Public Welfare Department Of Health And Human Services, Part 46, Protection Of Human Subjects (45 CFR 46.102(f)). PBMC cryopreserved in liquid nitrogen were thawed, stimulated with 1.5 mg/ml PHA in IMDM media supplemented with 10% FBS, and cultured for three days. The CEM-T4 cell line (Foley et al., 1965) was obtained through the AIDS Reagent Program. CEM cells were cultured in IMDM media supplemented with 10% FBS and maintained in log phase by sub-culturing every 3-4 days.

Viruses

The viruses (and co-receptor usage) that were used included HIV-1 LAI (X4) and HIV-1 BAL (R5), V9D6 (R5), RF (X4/R5), JFRL (R5), UG92029 (X4), and UG92031 (R5), and were obtained through the AIDS Reagent Program. Viral stocks were generated in PHA-stimulated PBMC cultures. LAI and BAL represent prototype clade B CXCR4 and CCR5 co-receptor-using virus isolates, respectively. For PBMC-based assays, the infectious titers of the virus stocks were determined by end-point dilution in PHA-stimulated PBMC as described (McClure et al., 2012). The TCID (tissue culture infectious dose) was derived using the Reed-Meunch statistical method (Reed and Muench, 1938).

Infection of PBMCs and CEM Cells

For HIV-1 replication studies, PBMC were thawed, stimulated with 1.5 mg/ml PHA in IMDM media supplemented with 10% FBS, and cultured for three days. PHA-blasts were washed and resuspended in IMDM media containing 10% FCS and 10 IU/ml recombinant interleukin 2 (IL-2; PeproTek, Rocky Hill, NJ). Cells were plated and concurrently exposed to SIL or SbN and virus. Residual virus inoculum was washed out after 24 hours and cells fed with fresh medium containing IL-2 and SIL. Since CEM cells do not require activation to support HIV infection, the cells were directly infected with virus in the presence or absence of SIL or SbN. Culture supernatants were monitored for virus every 3-4 days post-infection. HIV-1 production by PBMCs and CEM cells was measured by determining the p24 levels in culture supernatants using an in-house double-antibody sandwich ELISA (McClure et al., 2012). A standard curve of absorbance at 450nm was generated for each assay plate using a p24 standard obtained from the AIDS Vaccine Program at the National Cancer Institute (NCI)-Frederick Cancer Research and Development Center (Frederick, MD).

HIV Quantitative Infectious Center Assay (QIC)

The Quantitative Infectious Center Assay (QIC) represents an adaptation of the virus endpoint dilution assay. The QIC assay was developed to investigate anti-HIV activity by test compounds during virus adsorption/early infection and prior to cell toxicity. Briefly, virus (HIV-1 LAI or HIV-1 BAL), cells (PBMC PHA-blasts or CEM), and test compounds were incubated for 0.5 – 2 hours at 37°C (MOI=0.02-0.05), followed by a thorough washout of drug and inoculum. Quadruplicate aliquots of the virus-exposed cells were serially diluted and plated into 96 well plates containing uninfected feeder target cells (PBMC PHA-blasts or CEM). The cells were cultured for 5-10 days, after which the wells were scored for the presence of viral replication using an HIV p24 antigen capture assay. The infected cell titer (IC50 log10) was determined using the Reed-Meunch statistical method. The actual virus strains, MOI, incubation times and temperatures are specified in each figure legend.

ATP assay

Cells were plated in 96 well plates and were exposed to SIL, SbN or drugs for varying times. Cellular ATP levels were assessed using the ATPlite kit (Perkin Elmer).

Metabolic Assessment of Cells

To assess the effects of SIL and SbN on mitochondrial respiration and glycolysis, we used the Seahorse XF24-3 instrument (Seahorse Biosciences). Cells generate ATP from glucose metabolism via coupled oxidative phosphorylation in mitochondria (respiration) or via incomplete oxidation to lactic acid in the cytosol (aerobic glycolysis). The XF24-3 Analyzer allows for investigation of cellular bioenergetic metabolism via measuring the cellular oxygen consumption rate (OCR) (mitochondrial respiration) and the extracellular acidification rate (ECAR) which is a surrogate for glycolysis. For each assay, CEM cells were suspended in Seahorse Assay Medium - DMEM (unbuffered) with 25mM glucose, 2mM glutamine and plated into XF24 well plates coated with Cell-Tak (BD Biosciences) at 4 × 105 or 5 × 105 cells per well in 100 μl. The plates were spun for 5 minutes at 300 g and then placed at 37°C for 30 minutes to allow the cells to attach to the plate surface. The volume per well was increased to 700 μl with assay medium and the plates were loaded in the instrument for measurements after cartridge calibration. The instrument automatically injected test compounds. The details are provided in each figure legend.

Glucose Uptake Assay

As an estimate of glucose uptake the rate of 3H-2-deoxyglucose (Perkin-Elmer) accumulation was measured as previously described (Sweet et al., 2004). CEM cells were first pre-incubated in KRB containing 0 mM glucose for 30 min in a 5% CO2 incubator at 37 °C. Subsequently, cells (50 μL) were transferred into 12-75 test tubes containing 140 μl of 0 mM-glucose KRB (with the indicated additions), incubated for 5 min, and then 10 μl of 3H-2-deoxyglucose (0.2 μCi) was added by use of a repeater pipette for a total incubation volume of 200 μL. At about 29 min after addition of 3H-2-deoxyglucose, the cell suspension was transferred to 0.4-ml centrifuge tubes containing 100 μl of an oil mixture, and then the free and the cell-associated radioactivity were separated at exactly 30 min by spinning the cells through the oil layer, freezing in liquid nitrogen and counting the cell pellet.

Statistics

Data were analyzed by Student’s T tests and p values of less that 0.05 were considered significant.

Supplementary Material

01

Highlights.

  • Silibinin (SbN) and Legalon-SIL (SIL) are cytoprotective natural product mixtures

  • SbN and SIL reduce T cell oxidative phosphorylation and glycolysis in vitro

  • SIL but not SbN blocks entry of multiple HIV isolates into T cells in vitro

  • SIL’s suppression of HIV appears independent of its effects on T cell metabolism

  • Metabolic effects of SIL and SbN may be relevant in inflammatory diseases

ACKNOWLEDGEMENTS

This work was partially supported by the University of Washington (UW) Center for AIDS Research (CFAR), a National Institutes of Health (NIH) funded program (P30 AI027757) which is supported by the following NIH Institutes and Centers (National Institute of Allergy and Infectious Disease, National Cancer Institute, National Institute of Mental Health, National Institute on Drug Abuse, National Institute of Child Health & Human Development, National Heart, Lung, and Blood Institute, National Institute on Aging), and by the National Center for Research Resources and the National Center for Advancing Translational Sciences, National Institutes of Health, through Grant UL1RR025014. This work was also supported in part by the DRC Cell Functional Analysis Core (National Institutes of Health Grant DK17047, to IRS).

Footnotes

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