Therapeutic potential of gossypol: An overview

Abstract

Context: Polyphenols are naturally occurring compounds found in fruits, vegetables, cereals, and beverages. Polyphenols occupy a unique place in biological science for their pharmacological properties. Gossypol is a polyphenolic compound that has attracted attention because of its biological effects.

Objective: Gossypol is reported to exhibit antifertility, antioxidant, anticancer, antivirus, antiparasitic, and antimicrobial properties and lower plasma cholesterol. These are summarized with attention to the mechanisms of activity.

Methods: This review summarizes the results of studies obtained in a comprehensive search of ScienceDirect, PubMed, Scirus, and Web of Science.

Results and conclusion: The results of these studies provide a comprehensive understanding of the biological action of gossypol and its potential for the prevention of and therapy for resistant tumors and chronic human diseases such as HIV, malaria, and psoriasis.

• Antiapoptotic
• antifertility
• antimicrobial
• antioxidant
• antiparasitic
• antitumor
• antivirus

Introduction

Gossypol [(2,2′-binaphthalene)-8,8′-dicarboxaldehyde, 1,1′,6,6′,7,7′-hexahydroxy-5,5′-diisopropyl-3,3′-dimethyl] is a lipid-soluble polyphenolic compound extracted from the cotton plant (genus Gossypium) and the tropical tree Thespesia populnea (L.) Sol. ex Corrêa, both members of the family Malvaceae (Figure 1; Jaroszewski et al., 1992a; King & de Silva, 1968). Gossypol is produced in the plant by the dimerization of two molecules of hemigossypol and is best classified as a dimeric-sesquiterpenoid. Sesquiterpenoids are a class of terpenes having three isoprene units that protect a plant from pathogens and insects (Stipanovic et al., 1986).

Figure 1. Cotton plant (left) and T. populnea plant (right).

History

Gossypol was first discovered by Longmore (1886). Marchlewski (1899) purified it via precipitation from an ether solution using acetic acid to produce gossypol acetic. These investigators tried to use gossypol as a dye, but found it to be unstable when exposed to light. The chemical was eventually named gossypol because of its origin from the genus Gossypium and its polyphenolic chemical nature.

A study of couples using crude cottonseed oil for cooking in a region of China during the 1950s showed low birth rates, with men having very low sperm counts and women experiencing amenorrhea. Subsequent studies have confirmed the antifertility effect of gossypol in mammals and humans (Coutinho, 2002). In the 1970s, a research team from the Institute of Pharmacology of the Chinese Academy of Medical Sciences began a series of experiments on 10 000 volunteers that continued for over a decade. They found that taking a daily gossypol pill provided reliable male contraception that did not affect their hormonal balance (Taylor et al., 1991).

Few side effects other than transient hypocalemia have been reported (Bi et al., 1981; Prasad & Diczfalusy, 1982; Qian & Wang, 1984); this has led to reduced scientific interest in gossypol. Later studies of experimental tumor models indicate that gossypol is a potential antitumor chemotherapeutic agent (Tso, 1984; Tuszynski & Cossu, 1984).

Chemical properties

The structure of gossypol consists of two naphthalene rings joined by a single internaphthyl bond between the 2- and 2′-carbon atoms. The presence of six phenolic hydroxyl groups and two aldehydic groups makes gossypol chemically reactive (Figure 2; James, 2006).

Figure 2. Chemical structure of gossypol.

Gossypol is a polyphenolic bissesquiterpene that has been isolated as a racemic mixture from cottonseed. Gossypol exists as (+) and (−) enantiomers because of hindered rotation around the binaphthyl bond (Figure 3; Freedman et al., 2003).

Figure 3. Structure of gossypol enantiomers.

Jaroszewski et al. (1992b) investigated the gossypol racemization energy barrier using a molecular mechanics program and found that racemization of gossypol requires inaccessibly high energy and, thus, the individual enantiomers are optically stable under normal conditions (e.g., ambient temperature and neutral pH).

Gossypol has a complicated reaction chemistry that stems from its different tautomeric forms. Adams et al. (1960) proposed three tautomeric forms – aldehyde, ketone (quinoid), and lactol (hemiacetal) – to explain some of these reactions and their properties and degradation products.

Biological properties

Antifertility/contraceptive

Gossypol is non-steroidal and does not affect hormone levels, but does inhibit sperm production and motility in male animals and humans. It acts as a contraceptive by inhibiting enzyme systems that effect energy metabolism in sperm and spermatogenic cells (Coutinho, 2002; Wang et al., 2009).

Human lactate dehydrogenase (LDH) has five isoenzymes. Under anaerobic glucose conditions, pyruvate is reduced to lactate by LDH in the presence of NADH. Numerous reports suggest that the antifertility properties of gossypol are associated specifically with the (−)-isomer. (−)-Gossypol is a non-selective competitive inhibitor of NADH binding with LDH. Yu et al. (2001) attribute its antifertility action to inhibition of mitochondrial LDH-C₄ (LDH-X), which is present only in the testes and sperm and is essential for energy production. However, the mode of action is complex and involves the inhibition of a number of essential enzyme systems, including ribonucleotide reductase (McClarty et al., 1985), malate dehydrogenase (MDH), glyceraldehyde-3-phosphate dehydrogenase (GA₃PDH) (Ikeda, 1990), and cytoplasmic phospholipase A₂ (cPLA₂). The latter enzyme plays an important role in the acrosomal reaction during sperm maturation (Dodou, 2005; Vainio et al., 1985).

Antioxidant properties

Polyphenols are secondary metabolites of plants and are generally involved in defense against ultraviolet radiation or aggression by pathogens. Like many other aromatic phenolic chemicals, gossypol is an effective and potent natural antioxidant (Laughton et al., 1989). For example, gossypol was found to protect carotene against preformed fat peroxides in vitro and also act as a carotene protecting antioxidant in vivo (Hove, 1944).

Gossypol was reported to inhibit rat liver microsomal peroxidation caused by incubation with ferric/ascorbate (IC₅₀ < 0.1 µM; Hove & Hove, 1944). In some cases, the modification of phenolic hydroxyl groups on gossypol significantly decreases the chemical antioxidative abilities of free radical scavenging activity, reducing power assay, DNA damage prevention, and demonstrating that the hydroxyl groups are critical to antioxidation (Wang et al., 2008).

Li et al. (2000) found that gossypol in the presence of Fe³⁺/ascorbate protects supercoiled plasmid DNA from damage in a dose-dependent manner. Dodou et al. (2005) postulated that the antioxidant properties of gossypol may be useful in diseases characterized by lipid oxidative damage, such as psoriasis.

Antitumor properties

Researchers have assessed the anticancer properties of gossypol against many types of cancer cell lines using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) or flow cytometry cell-viability assays. The effect on cells of (−)-gossypol at a lower concentrations was more potent in comparison with (+)-gossypol or racemic gossypol (Kitada et al., 2003; Liu et al., 2002; Oliver et al., 2004). The effects included inhibition of cytoplasmic and mitochondrial enzymes involved in energy production (Zhai et al., 2006), uncoupling oxidative phosphorylation (Ueno et al., 1988) and depletion of cellular adenosine triphosphate (Flack et al., 1993).

Gossypol was also shown to inhibit key nuclear enzymes responsible for DNA replication and repair, including DNA polymerase α (Keniry et al., 1989) and topoisomerase II, and to block DNA synthesis in HeLa cells (Rosenberg et al., 1986). Wang and Rao (1984) reported that inhibition of DNA synthesis was achieved with 10 µM gossypol by blocking the G1/S checkpoint in MCF-7 cells after 24 h of incubation.

Gossypol has shown in vitro to inhibit cell cycling by modulating regulatory proteins Rb and cyclin D₁, elevating TGF-β1 gene expression and inhibiting protein kinase C activity (Ligueros et al., 1997; Shidaifat, 1997; Teng, 1995).

Telomerase is a reverse transcriptase which helps to stabilize the length of telomeres. The absence of telomerase activity causes replication senescence and cell death. Gossypol has been shown to induce apoptosis and repress telomerase activity via transcriptional downregulation and posttranslational modification of hTERT in human leukemia cells. Transcriptional downregulation involves the inactivation of c-Myc and posttranslational modification that of Akt (Moon et al., 2008a). Gossypol also downregulates the expression of NF-kappaB-regulated gene products, including inhibitors of apoptosis such as the proteins IAP-1, IAP-2, and X-linked IAP. Moon et al. (2008b) suggested that gossypol-induced apoptosis partially involves suppression of NF-kappaB activity.

(−)-Gossypol induces complete cytochrome c release from mitochondria, increases caspase-3 and caspase-9 activity, and causes apoptotic death. Balakrishnan et al. (2008) found that (−)-gossypol acts as a BH₃ mimetic and binds to the BH3-binding domain in pro-apoptotic proteins of the Bcl₂ family, displacing pro-death partners to induce apoptosis. It enhances the antitumor activity of X-ray irradiation and chemotherapeutic agents such as docetaxel that exert antitumor activity via inhibition of the antiapoptotic protein Bcl-x_L and increasing proapoptotic Noxa and Puma (Meng et al., 2008; Xu et al., 2005).

Antivirus properties

Gossypol has been reported to possess antiviral properties against enveloped viruses, including HIV-1, HSV-2, influenza, and parainfluenza (Lin et al., 1993; Vander Jagt et al., 2000). Although the compound is significantly less potent than AZT, it lacks the serious side effect on bone marrow toxicity that is associated with AZT treatment. It is not clear whether the inhibition of HIV-1 reverse transcriptase by gossypol is the primary mechanism of action. Lin et al. (1989) have synthesized analogs of gossypol for potential antiviral activity against HIV-1.

Antiparasitic properties

Malaria is a mosquito-borne infectious disease caused by protozoan parasites of the genus Plasmodium. Four species (Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale, Plasmodium vivax) can infect and be transmitted by humans (Mendis et al., 2001). Gossypol derivatives with ethyl, propyl, or isopropyl side chains and gossylic nitrile 1,1-divalerate have shown stronger inhibition than other gossypol derivatives against the growth of P. falciparum (Razakantoanina et al., 2000; Royer et al., 1986).

Gossypol also exhibits activity against Endameba histolytica and Trypanosoma cruzi. The molecular mechanism behind gossypol antiparasitic activity could be the selective inhibition of vital and essential enzymes in the anaerobic life cycle of parasites (Gonzalez-Garza et al., 1993; Montamat et al., 1982).

Antimicrobial properties

Margalith (1967) demonstrated the antibiotic properties of gossypol against sporeformers and lactobacilli by testing its inhibitory effect on microorganisms in cottonseed meal-fed animals. The result showed that gossypol caused a fundamental change in the equilibrium of the microflora of the gastrointestinal tract.

Vadehra et al. (1985) reported that gossypol is a more potent antibacterial agent against Gram positive organisms (Streptococcus spp., Bacillus spp., Staphylococcus aureus) as opposed to Gram-negative bacteria such as Pseudomonas aeruginosa, Salmonella spp., Klebsiella pneumoniae, Shigella spp., Proteus spp., and Escherichia coli. This may originate from structural differences in the cell wall and cell membrane of Gram-positive and Gram-negative groups. For example, Gram positive bacteria have more peptidoglycan in their cell walls and lack the outer membrane found in Gram-negative organisms. This possibly influences the transport of gossypol to its target site.

Plasma cholesterol reduction properties

Cholesterol is a lipid produced by the liver and is vital for normal body function. Elevated levels of low density lipoprotein (LDL) have been linked to an increased risk of heart disease. Shandilya et al. (1982) found that gossypol administered orally at 10 mg/kg/day for 6 months to adult male Cynomolgus monkeys caused a significant decrease in total plasma cholesterol and LDL without a significant decrease in high density lipoprotein (HDL) levels. The possible mechanisms of this action can be attributed to a reduction in intestinal absorption of dietary cholesterol and a decrease in hepatic synthesis of LDL.

Conclusion

The numerous studies outlined above show that gossypol has the potential for prevention and therapy of various cancers and chronic human diseases. Gossypol is a versatile molecule with an abundance of biological properties. It has the potential for use in the development of drugs for disorders as varied as resistant tumors, HIV, malaria, and psoriasis. Further investigation on the mechanisms, the nature of the active compounds and appropriate dose levels are needed for therapeutic exploitation of gossypol.

Declaration of interest

The authors report no conflict of interest. The authors alone are responsible for the content and writing of this article.