Licking virus latency with licorice

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http://www.jci.org/cgi/content/full/115/3/591

J. Clin. Invest. 115:591-593 (2005). doi:10.1172/JCI200524507.

2005 by the American Society for Clinical Investigation

Commentary

Licking latency with licorice

Jeffrey I. Cohen

 

Medical Virology Section, Laboratory of Clinical Infectious Diseases, NIH,

Bethesda, Maryland, USA.

 

Address correspondence to: Jeffrey I. Cohen, Medical Virology Section,

Laboratory of Clinical Infectious Diseases, Building 10, Room 11N228, NIH,

10 Center Drive, MSC 1888, Bethesda, Maryland 20892, USA. Phone: (301)

496-5265; Fax: (301) 496-7383; E-mail: jcohen.

 

Abstract

 

Numerous viruses cause latent infections in humans, and reactivation often

results in pain and suffering. While vaccines for several of these viruses

are available or currently being studied in clinical trials, and antiviral

therapies have been successful in preventing or treating active infection,

therapy to eradicate latent infection has lagged behind. A new study

reported in this issue of the JCI shows that treatment of cells latently

infected with Kaposi sarcoma–associated herpesvirus (KSHV) with

glycyrrhizic acid, a component of licorice, reduces synthesis of a viral

latency protein and induces apoptosis of infected cells. This finding

suggests a novel way to interrupt latency.

 

See the related article beginning on page 642

 

Licorice, derived from the root of Glycyrrhiza glabra, has been used for

more than 4 millennia as a flavoring agent in foods, beverages, and tobacco

(1). Licorice is also used as an alternative medicine for the treatment of

gastric and duodenal ulcers, sore throat, bronchitis, cough, arthritis,

adrenal insufficiency, and allergic diseases. The licorice root contains

numerous compounds, including glycyrrhizic acid (GA). It is estimated that

in the United States, 3.3 mg of GA is consumed per person daily. GA

inhibits the replication of several viruses in vitro including

herpesviruses, HIV, and the SARS coronavirus. When taken orally, GA is

hydrolyzed to glycyrrhetic acid by bacteria in the gastrointestinal tract

before GA can be absorbed. Therefore, in Asia, where GA is used for the

treatment of chronic hepatitis B or C infection, the drug is infused

intravenously to achieve the appropriate serum levels.

 

In this issue of the JCI, Curreli et al. (2) show that GA induces apoptosis

of primary effusion lymphoma (PEL) cells that are transformed by Kaposi

sarcoma–associated herpesvirus (KSHV). KSHV is the etiologic agent of

Kaposi sarcoma, and the virus is present in lesions from patients with

multicentric Castleman disease and PEL. The latter presents as a malignant

effusion located in the pleural, peritoneal, or pericardial space; tumor

cells can also infiltrate the adjacent tissues. The virus is latent in PEL

cells, which express a very limited set of viral proteins. The median

survival time after diagnosis for patients with PEL is 6 to 12 months with

chemotherapy and radiation therapy; thus, newer approaches to therapy are

needed.

 

Curreli et al. (2) found that GA down-regulates synthesis of the KSHV

latency-associated nuclear antigen 1 (LANA-1) (Figure 1). LANA-1 is

expressed in all KSHV-infected cells, including PEL cells. This protein

allows the viral genome to be maintained as an episome in latently infected

cells. LANA-1 binds to p53, inhibiting p53-mediated apoptosis, and

interacts with the retinoblastoma tumor–suppressor protein (Rb), which may

prevent Rb-mediated cell cycle arrest. Curreli et al. found that

downregulation of LANA-1 by GA was associated with an alteration in the

mitochondrial membrane potential with translocation of apoptosis-inducing

factor to the nucleus, DNA fragmentation, and apoptosis (2). In addition,

cells treated with GA showed higher levels of phosphorylated (active) p53,

which resulted in cell cycle arrest at the G1 checkpoint. GA upregulated

expression of the KSHV cyclin protein (v-cyclin) but did not affect

expression of the viral FLICE-inhibitory protein (vFLIP). V-cyclin binds to

and activates cyclin-dependent kinase 6, which results in phosphorylation

and inactivation of p53 and Rb. The increased level of v-cyclin in PEL

cells treated with GA might also contribute to cell death, since

overproduction of the protein has been reported to induce apoptosis.

 

 

 

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Figure 1

Mechanisms of action of KSHV latency proteins expressed in PEL cells and

points of attack by inhibitor compounds. cdk6, cyclin-dependent kinase 6;

FADD, Fas-associated death domain–containing protein; siRNA, small

interfering RNA.

 

 

Additional molecular approaches to killing latent KSHV–infected PEL cells

 

Other approaches have been considered for the treatment of PEL, based on

KSHV gene expression in these tumors. Tumor cells from patients with PEL

produce several KSHV latency proteins, including LANA-1, v-cyclin, vFLIP,

and viral IL-6 (vIL-6) (Figure 1) (3-5). vFLIP inhibits Fas-induced

apoptosis and is responsible for most of the activation of NF-{kappa}B in

PEL cells. Prevention of vFLIP production in PEL cells by transfection with

small interfering RNA inhibited NF-{kappa}B activity and induced apoptosis

of the cells (6, 7). In addition, treatment of PEL cells with a small

molecule that inhibits NF-{kappa}B (Bay 11-7082) induced apoptosis of these

cells (8).

 

PEL cells synthesize vIL-6 in vivo, and PEL cells secrete human IL-6 and

IL-10 and vIL-6 in vitro. vIL-6 functions as an autocrine growth factor for

PEL cells in vitro, and the protein interacts with the signal transduction

protein gp130 but not with the human IL-6 receptor (IL-6R). The interaction

of vIL-6 with gp130 leads to phosphorylation of STAT3, activation of the

JAK-STAT pathway, and cell proliferation. Inhibition of STAT3 signaling by

transfection of PEL cells with a dominant negative form of STAT3, or

treatment of cells with a drug (AG490) that inhibits JAK2, resulted in

decreased production of survivin and apoptosis of the cells (9). Other

approaches have been used to inhibit the activity of IL-6. Antibody against

gp130 or soluble IL-6R inhibited PEL cell growth in vitro (10). In another

study, antibody against IL-6 inhibited growth of PEL cell tumors in SCID

mice (11). Antibody against human IL-10, and to a lesser extent antibody

against vIL-6, inhibited growth of PEL cells in vitro (12). vIL-6 induces

expression of VEGF, which promotes angiogenesis. PEL cells secrete high

levels of VEGF and have VEGF receptors on their surface (13).

Administration of neutralizing antibody against VEGF prevented development

of effusion lymphomas in irradiated SCID/beige mice injected

intraperitoneally with PEL cells.

 

Targeted attack on latency of another human herpesvirus: Epstein-Barr virus

 

Several approaches have been used to interrupt latency of Epstein-Barr

virus (EBV), the other human {gamma}-herpesvirus, which like KSHV infects

and transforms B cells. Hydroxyurea eliminated EBV episomes from

EBV-positive Burkitt lymphoma cells and reduced their malignant phenotype

in an animal model (14). Low-dose hydroxyurea reduced the tumor size of

EBV-positive central nervous system lymphomas in 2 patients with HIV (15).

 

Treatment of B cells latently infected with EBV with arginine butyrate (16)

or gemcitabine (17) followed by ganciclovir induced viral replication and

expression of the viral thymidine kinase, resulting in phosphorylation of

ganciclovir and cell death in vitro. Infusions of EBV-specific cytotoxic T

cells have been used successfully to treat patients with EBV

lymphoproliferative disease (18) or EBV-positive Hodgkin disease (19), both

of which are due to tumor cells latently infected with virus. Such

approaches might also be tried for the treatment of patients with

KSHV-positive PEL.

 

Future considerations for GA therapy

 

While GA is effective for killing PEL cells in vitro, there are several

caveats for the treatment of patients with PEL with GA. First, since GA is

rapidly hydrolyzed to glycyrrhetic acid in the gastrointestinal tract,

glycyrrhetic acid would need to be shown to be effective against PEL cells

in vitro or GA would need to be administered intravenously. After

intravenous administration of GA for treatment of hepatitis, serum levels

of GA have been shown to range from 40 to 100 µg/ml (20), compared with the

millimolar concentrations needed to induce apoptosis of PEL cells in vitro.

Thus, the levels of GA required for efficacy in vitro might not be

achievable in vivo. Second, Curreli et al. (2) found that the effects of GA

on downregulating LANA expression in PEL cells were reversible for up to

3–4 days of treatment; thus, continuous and/or prolonged courses of therapy

with GA might be needed. Third, therapeutic levels of GA might be toxic for

normal cells and tissues. Curreli et al. found that the ED50 of GA for PEL

cells was 2–3 mM; however, levels of 5–6 mM were toxic for uninfected

cells. Thus the therapeutic index for the treatment of PEL is likely to be

low. Finally, it is not clear whether the effects of GA on PEL cell lines

in vitro would also occur in tumors in vivo. Since intraperitoneal

inoculation of immunodeficient mice with PEL cells results in malignant

ascites, intravenous infusions of GA could be tested for efficacy in this

model.

 

While a compound present in licorice may seem like an unlikely candidate

for the treatment of virus-associated cancers, it is important to remember

that other traditional drugs have proved highly effective for some

infectious diseases. Extracts of the wormwood plant, which is a traditional

Chinese medication for treatment of febrile illnesses, contain artemisinin;

derivatives of this compound have become first-line treatments for

drug-resistant malaria. Thus, derivatives of GA or other traditional

medicines might be used in the future for treating human diseases caused by

latent virus infections.

 

Footnotes

 

Nonstandard abbreviations used: EBV, Epstein-Barr virus; GA, glycyrrhizic

acid; KSHV, Kaposi sarcoma–associated herpesvirus; IL-6R, IL-6 receptor;

LANA-1, latency-associated nuclear antigen 1; PEL, primary effusion

lymphoma; Rb, retinoblastoma tumor–suppressor protein; v-cyclin, KSHV

cyclin protein; vFLIP, viral FLICE-inhibitory protein; vIL-6, viral IL-6.

 

Conflict of interest: The author has declared that no conflict of interest

exists.

 

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