Beer is a fermented aqueous drink based on starch and flavored with hops. Hops provide the characteristic bitterness of the beer and contribute to its aroma and foam stability.
The hop plant (Humulus lupulus L.) is a dioecious plant, meaning that there are separate male and female hop plants. Only the female produces the flowers, or, more specifically, the strobiles that are used for brewing or for potential medicinal purposes.
The term "hop' refers to the plant, and the term "hops' refers to the flower cones (""hop cones'' or ""hops'') of the hop plant. However, both terms are commonly used interchangeably. The flowers in a female hop plant are arranged in characteristic clusters on stems. The flower cluster is called an inflorescence. The female inflorescences are rich in polyphenolic compounds and acyl phloroglucides that are widely used to preserve beer and give beer its characteristic flavor and aroma.
The hop cones are green, are built like pine cones and vary in size. At the base of the leaves (bracts, also called scales) of the cones are collections of small, yellow spheres called the lupulin glands. The lupulin glands are little sacs of bitter and aromatic acids and oils. In the lupulin glands can be found the alpha-acids and their derivatives, including the iso-alpha-acids and the rho-iso-alpha acids, the beta acids, xanthohumol and 8-PN, among several other compounds. The iso-alpha-acids, including isohumulone, mainly account for the bitter taste of beer. The oils in the hops are mainly responsible for the aroma of beer.
In addition to their roles in beer, there has been great interest in studying the hop phytochemicals for possible medicinal uses. Two of the phytochemicals found in hops that have been receiving much of the attention are the flavonoid, 8-PN, for its phytoestrogenic activity, and the prenylated chalcone, xanthohumol (see Xanthohumol), for its possible broad-spectrum chemopreventive and anticancer activities. Recently, the iso-alpha acids have also been receiving attention for their possible anti-inflammatory activities.
8-PN is a prenylated flavonoid found in the lupulin glands of the female inflorescence. It is secreted as part of the hop resin (lupulin) by glandular trichomes found on the adaxial surfaces of cone bracts.
The estrogenic effects of hops have been recognized for a long time. The frequent menstrual disturbances observed in female hop-pickers during the early days of hop cone harvesting, when hops were picked by hand, suggested a potential hormonal activity of the hops. An old folk legend held that female hop-pickers, who normally lived a distance from hop gardens, regularly began to menstruate two days after arriving to pick hops. In Germany, brewery sludge baths containing about 30% hops extracts (""hop baths'') were traditionally used for the treatment of gynecological disorders. Hop baths and hops extracts were reported to be effective in reducing hot flashes in menopausal women. In 1953, Koch and Heim, in a short communication, reported that hops contained the equivalent of 20 to 300 micrograms of the female hormone 17beta-estradiol. This was the first scientific confirmation that hops possess estrogenic activity. In 1999, 8-PN, the major estrogenic substance in hops, was isolated and characterized. It has been found to be the most potent plant-derived estrogen (phytoestrogen) to date.
8-PN's empirical formula is C20H20O5, and its molecular weight is 340.37. It is described chemically as (2S)-5,7-dihydroxy-2-(4-hydroxyphenyl)-8-(3-methylbut-2-enyl)chroman-4-one. It is also known as prenylnaringenin. Its abbreviation is 8-PN. 8-PN exists as the enantiomers 2S(–)8-PN and 2R(+)8-PN, and as a racemic mixture containing both enantiomers. An enantiomer is one of two stereoisomers that are nonsuperimposable, complete mirror images of each other. The 2S(–)8-PN enantiomer has slightly more potent estrogenic activity than the 2R(+)8-PN enantiomer. Both enantiomers are found in hops. 8-PN is classified as a prenylated flavonoid, a prenylflavonoid, a prenylated flavanone, a prenylflavanone, and as a polyphenol. It belongs to the flavanone subclass of flavonoids. In addition to hops, 8-PN is also found in Anaxagorea luzonensis A. Gray, a tree indigenous to Thailand, the extracts of which are used in traditional Thai medicine for the treatment of a number of diseases. In fact, 8-PN was first identified as a phytoestrogen in the heartwood of the tree. The concentration of 8-PN in most beers is below 30 micrograms per liter. Some beers, particularly strong ales, have been reported to contain concentrations of up to 250 micrograms (0.24 milligrams) or higher per liter. Beer is the major dietary source of 8-PN.
8-PN is represented by the following chemical structure.
Actions & Pharmacology
8-prenylnaringenin (8-PN) has estrogenic activity. It may also have anticancer and antiangiogenic activities.
Mechanism of Action
Antiangiogenesio activity: 8-PN was shown to inhibit angiogenesis induced by basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF) or the synergistic effect of the two cytokines in combination. The effect was seen with bovine microvascular endothelial cells derived from the adrenal cortex and with endothelial cells from the bovine thoracic aorta. Given this finding, 8-PN could have therapeutic applications for diseases in which angiogenesis is an important component—for example, certain cancers. The exact mechanism of the antiangiogenic action of 8-PN is unclear.
Anticancer activity: 8-PN was found to inhibit estrogen receptor alpha mediated cell growth and induce apoptosis in MCF-7 human breast cancer cells.
8-PN showed a biphasic action on cell growth. At concentrations below 10 micromolar (microM), 8-PCN showed estrogenic properties, increasing cell growth in estrogen receptor-positive MCF-7 human breast cancer cells, while high concentrations of 8-PN inhibited proliferation and induced apoptosis in hormone responsive cells. The exact mechanism of this action is unclear.
Treatment of the prostate cancer cell lines PC-3 and DU145 with 8-PN was demonstrated to induce a caspase-independent form of cell death. Caspases are essential in cells for apoptosis or programmed cell death. The mechanism of action of this anticancer effect is not clear. However, vacuoles were noted in some of the cells, suggesting the induction of autophagy and consequent cell death. Autophagy is a type of programmed cell death, which is characterized by the accumulation of autophage vacuoles and the absence of caspase activation. Autophagy is a cell survival mechanism during nutrient starvation that supplies vital components until conditions improve. Autophagy has also been found to be a novel tumor suppression mechanism.
Estrogenic activity: Studies with racemic 8-PN and its enantiomers 2S(–)8-PN and 2R(+)8-PN have revealed that 8-PN is the first ERalpha (estrogen receptor alpha) selective phytoestrogen with a two- to threefold higher affinity for the ERalpha than for the ERbeta (estrogen receptor beta). 8-PN is the most potent plant-derived ERalpha agonist to date. It is 10-fold more potent than coumestrol and 100-fold more potent than genistein, but 70 times weaker than 17beta-estradiol.
Studies have revealed a greater than 3.6-fold higher estrogenic activity of 8-PN at ERalpha than at ERbeta. This is in contrast to other phytoestrogens, including genistein and coumestrol, which are primarily ERbeta agonists. 8-PN is a pure estrogen receptor agonist in vitro. Its estrogenic activity is comparable to that of estrone. Estrone is one of the three major estrogens and is much less potent than 17beta-estradiol, the primary estrogen. Although 8-PN demonstrated only 70 times weaker ERalpha agonist activity than 17beta-estradiol in an in vitro or test tube study, an in vivo study showed the estrogenic activity of 8-PN in reproductive tissue to be about 20,000-fold weaker when compared to that of beta-estradiol. The 2S(–)8-PN enantiomer shows moderately higher estrogenic activity in vitro and in vivo than the 2R(+)8-PN enantiomer.
Studies on the in vivo pharmacological profile of 8-PN were performed using three different techniques: the short-term uterus growth test in juvenile rats, the 4-week bone mineral density (BMD) model in adult ovariectomized rats and the ERE-luc (estrogen response elements luciferase) transgenic mouse model. The ERE-luc transgenic mice carry a luciferase gene that is driven by two estrogen response elements. The model appears to report reliably on the activity of liganded and unliganded estrogen receptors induced by endogenous and exogenous stimuli.
Data obtained in the BMD study in adult ovariectomized rats demonstrated that 8-PN completely protected the rats from bone loss induced by ovariectomy without stimulating the uterus and the endometrium to the extent seen for a 17beta-estradiol dose that was necessary for the same bone-protective effect. It was estimated that 8-PN had a 10-fold higher sensitivity for bone tissue compared with 17beta-estradiol. Whereas 17beta-estradiol maximally stimulated uterus growth and epithelial height, the effect of 8-PN on uterus growth and epithelial height was marginal and not different from that found in ovariectomized control animals. 8-PN was also found to have a pronounced bone tissue-specific estrogenic effect in the transgenic ERE-luc mouse model.
Given the above findings, 8-PN can be termed a natural selective estrogen receptor modulator, or SERM. A substance that demonstrates low stimulatory effects on the uterus at bone tissue-effective doses makes 8-PN a serious candidate for HRT (hormone replacement therapy). However, before it can be recommended for HRT, large scale human clinical trials, including long-term safety studies, are clearly needed and clearly warranted.
At menopause, the ovaries stop releasing eggs and a severe decline in serum 17beta-estradiol levels occurs. Low estradiol levels typically result in symptoms such as hot flashes (vasomotor symptoms), vulvar and vaginal atrophy and dryness, mood disturbances and loss of libido. It also can result in degenerative processes such as osteoporosis and cardiovascular disease. This situation once prompted many women to start HRT in order to treat and prevent these problems. Since estrogens alone stimulate endometrial proliferation, which may result in cancer, estrogens were given in combination with progestins. In 2002, a report from the Women's Health Initiative (WHI) study published in the Journal of the American Medical Association (JAMA) cast doubt on the beneficial effects of HRT. Results from the WHI study showed a higher incidence in cardiovascular disease complications, such as heart attacks and strokes, and a higher incidence of breast cancers in those postmenopausal women who were using HRT in the form of conjugated estrogens plus medroxyprogesterone. The report rekindled an interest in alternatives to classical HRT, such as plant estrogens (phytoestrogens). 8-PN is the most potent phytoestrogen known, but not much is known about its safety. The following study examined morphologic changes induced by oral long-term treatment with 8-PN in the uterus, vagina and mammary gland of castrated rats.
Rats were ovariectomized and fed for three months with soy-free chow containing either 17beta-estradiol or 8-PN, both given in two doses (a low dose and a high dose). The uteri of the rats treated with both 17beta-estradiol doses and the high 8-PN dose were found to have increased weight and showed histologic estrogen-induced features. High-dose 8-PN induced epithelial polyploidy formation unique to this group. Compared to the atrophic controls, both 17beta-estradiol doses and the high 8-PN dose induced hyperplastic epithelia in the vagina. The high doses of both 17beta-estradiol and 8PN caused secretion in the mammary gland, whereas proliferation and progesterone receptor expression were stimulated by both 17beta estradiol doses and the high 8-PN dose. 8-PN, at least at a high dose, did not come out with a clean bill of health from this study.
Further animal toxicity and human toxicity studies are needed and warranted.
A human pharmacokinetic (PK) study of 8-PN was performed in healthy postmenopausal women. The study was performed using a randomized, double-blind, placebo-controlled, dose-escalation design with three groups of eight women. In each group, six subjects received 8-PN and two subjects received placebo. 8-PN was given orally in doses of 50, 250 or 750 milligrams (mg).
All treatments were well tolerated. Serum concentrations of 8-PN demonstrated rapid enteral absorption of the prenylflavonoid, and secondary peaks were suggestive of a pronounced presystemic elimination with subsequent marked enterohepatic recirculation. Comparatively little phase 1 metabolism was recorded. Conjugation to beta-glucuronides or sulfates seemed to constitute the main metabolic pathway. Approximately 30% of the dose was recovered in the urine and feces, either as unchanged or as conjugated metabolites, over the 48-hour observation period. 8-PN elimination seemed to be a combination of hepatic conjugation, hepatic elimination and renal excretion of conjugates. Secondary peaks hindered the calculation of the disposition half-life of 8-PN. However, the rapid decrease in serum concentration after the peaks suggested a short elimination phase with a half-life of one to three hours. The 750 mg dose decreased serum luteinizing hormone (LH) concentrations. The decrease in serum LH concentrations found after the highest dose demonstrated the ability of 8-PN to cross the blood-brain barrier and interact with the hypothalamo-pituitary axis, which is regarded as a prerequisite for the successful treatment of menopausal symptoms. Follicle-stimulating hormone (FSH) serum concentrations were not significantly influenced by 8-PN at any of the doses tested.
Further human studies on the PK of 8-PN in humans are warranted, as is investigation of 8-PN and its biological effects following multiple doses.
The PK of 8-PN in rats and dogs is characterized by a low oral bioavailability caused by extensive presystemic elimination after complete enteral absorption. 8-PN is almost completely excreted via the bile in unchanged form and in urine in the form of conjugates.
In vitro studies of intestinal permeability and hepatic and intestinal metabolism of 8-PN have been performed using monolayers of the human intestinal epithelial cancer line Caco-2 (a cell model of intestinal absorption and intestinal metabolism) and human hepatocytes (a cell model of hepatic metabolism). 8-PN showed good intestinal absorption via passive diffusion. Both glucuronide and sulfate conjugates of 8-PN were detected in the Caco-2 cell incubations. The 4′-O-glucuronide was the predominant Caco-2 metabolite, followed by the 7-O-sulfate and the 4′-O-sulfate. Both phase 1 and phase 2 metabolites of 8-PN were formed by human hepatocytes. The 7-O-glucuronide was the most abundant hepatocyte metabolite, and no sulfate conjugates were detected.
Indications & Usage
The flavonoid 8-prenylnaringenin (8-PN) found in hops is the most potent known phytoestrogen and thus has been proposed as an alternative to hormone therapy for the treatment of perimenopausal symptoms. The safety and efficacy of its use in menopausal and post-menopausal women has not been established. Claims that it is an effective ""breast enhancer'' are unsubstantiated. It may have some cardioprotective and anticancer potential.
There are no reports of overdosage.
The optimal dosage of 8-PN for any health condition is unknown.
A number of 8-PN-containing dietary supplements are available which are marketed for breast enhancement. Because of the lack of long-term safety studies, these supplements are not recommended for use.
Beer is the major dietary source of 8-PN. The concentration of 8-PN in most beers is below 30 micrograms per liter. Some beers, particularly strong ales, have been reported to contain concentrations of up to 250 micrograms (0.24 milligrams) or higher per liter. Beer itself contains isoxanthohumol, a mild phytoestrogen which can be metabolized to 8-PN via the intestinal microflora. This could increase the exposure to 8-PN from beer by up to 4 mg per liter.
LiteratureBöttner M, Christoffel J, Wuttke W. Effects of long-term treatment with 8-PN and oral estradiol on the GH-IGF-1 axis and lipid metabolism in rats. J Endocrinol. 2008;198:395-401.Bowe J, Li XF, Kinsey-Jones J, et al. The hop phytoestrogen, 8-PN, reverses the ovariectomy-induced rise in skin temperature in an animal model of menopausal hot flushes. J Endocrinol. 2006;191(2):399-405.Brunelli E, Minassi A, Appendino G, et al. 8-PN, inhibits estrogen receptor-alpha mediated cell growth and induces apoptosis in MCF-7 breast cancer cells. J Steroid Biochem Mol Biol. 2007;107(3-5):140-148.Christoffel J, Rimoldi G, Wuttke W. Effects of 8-PN on the hypothalamo-pituitary-uterine axis in rats after 3-month treatment. J Endocrinol. 2006;188(3):397-405.Delmulle L, Berghe TV, Keukeleire DD, et al. Treatment of PC-3 and DU145 prostate cancer cells by prenylflavonoids from hop (Humulus lupulus L.) induces a caspase-independent form of cell death. Phytother Res. 2008;22(2):197-203.Diel P, Thomae RB, Caldarelli A, et al. Regulation of gene expression by 8-PN in uterus and liver of Wistar rats. Planta Med. 2004;70(1):39-44.Milligan S, Kalita J, Pocock V, et al. Oestrogenic activity of the hop phyto-oestrogen, 8-PN. Reproduction. 2002;123(2):235-242.Milligan SR, Kalita JC, Pocock V, et al. The endocrine activities of 8-PN and related hop (Humulus lupulus L.) flavonoids. J Clin Endocrinol Metab. 2000;85(12):4912-4915.Nikolic D, Li Y, Chadwick LR, et al. In vitro studies of intestinal permeability and hepatic and intestinal metabolism of 8-PN, a potent phytoestrogen from hops (Humulus lupulus L.). Pharm Res. 2006;23(5):864-872.Nikolic D, Li Y, Chadwick LR, et al. Metabolism of 8-PN, a potent phytoestrogen from hops (Humulus lupulus), by human liver microsomes. Drug Metab Dispos. 2004;32(2):272-279.Pepper MS, Hazel SJ, Hümpel M, et al. 8-PN, a novel phytoestrogen, inhibits angiogenesis in vitro and in vivo. J Cell Physiol. 2004;199(1):98-107.Rad M, Hümpel M, Schaefer O, et al. Pharmacokinetics and systemic endocrine effects of the phyto-oestrogen 8-PN after single oral doses to postmenopausal women. Br J Clin Pharmacol. 2006;62(3):288-296.Rimoldi G, Christoffel J, Wuttke W. Morphologic changes induced by oral long-term treatment with 8-PN in the uterus, vagina, and mammary gland of castrated rats. Menopause. 2006;13(4):669-677.Roelens F, Heldring N, Dhooge W, et al. Subtle side-chain modifications of the hop phytoestrogen 8-PN result in distinct agonist/antagonist activity profiles for estrogen receptors alpha and beta. J Med Chem. 2006;49(25):7357-7365.Rong H, Boterberg T, Maubach J, et al. 8-PN, the phytoestrogen in hops and beer, upregulates the function of the E-cadherin/catenin complex in human mammary carcinoma cells. Eur J Cell Biol. 2001;80(9):580-585.Schaefer O, Hümpel M, Fritzemeier KH, et al. 8-PN is a potent ERalpha selective phytoestrogen present in hops and beer. J Steroid Biochem Mol Biol. 2003;84(2-3):359-360.Sehmisch S, Hammer F, Christoffel J, et al. Comparison of the phytohormones genistein, resveratrol and 8-PN as agents for preventing osteoporosis. Planta Med. 2008;74(8):794-801.Zierau O, Hauswald S, Schwab P, et al. Two major metabolites of 8-PN are estrogenic in vitro. J Steroid Biochem Mol Biol. 2004;92(1-2):107-110.
Research & Summary
Because there is some evidence that standard estrogen/progestin therapy for the treatment and prevention of perimenopausal symptoms (such as ""hot flushes'') may place some women at higher risk of some cancers, heart attack and stroke, ""natural'' alternative treatments have focused on such phytoestrogenic substances as 8-PN. Both in vitro and in vivo studies leave no doubt that 8-PN is a potent estrogen receptor agonist in estrogen-responsive cells. Its estrogenic activity exceeds that of other phytoestrogenic agents such as genistein (see Genistein) and daidzein (see Daidzein). When given in drinking water to ovariectomized mice, the substance induced estrogenic stimulation of the vaginal epithelium with doses that were calculated to be 500-fold greater than that present in any beer. Another group of researchers reported that 8-PN is 100 times more potent than genistein. And though considerably weaker than the principal endogenous mammalian estrogen—estradiol—it is still potent enough that one group of authors concluded that it has ""remarkable'' potential for the treatment of conditions characterized by estrogen deficiency. In an animal model of menopausal hot flushes, 8-PN modulated vasomotor response in a way that lowered skin temperature, suggesting that the flavonoid might, indeed, be useful for ameliorating or preventing hot flushes.
Other experiments, however, have raised safety issues. Some data suggest that 8-PN, in doses sufficient to be effective in preventing/treating perimenopausal symptoms, would act on the hypothalamo-pituitary-uterine axis in some of the same negative ways that estrogen therapies do. Thus, the authors of one study concluded that, just as in standard HRT, progestins might have to be co-administered with 8-PN in order to reduce/prevent endometrial overstimulation. That would, of course, negate one of the putative advantages of this ""natural'' alternative therapy. In the first oral long-term treatment with 8-PN, changes were observed in the uteri, vaginas and mammary glands of ovariectomized rats that raised additional doubts about the safety of this substance.
Again, many of the same unfavorable sequelae of standard HRT were observed. In addition, what was described as a novel lesion was observed in the endometria of some of the 8-PN-dosed animals, a lesion not hitherto seen in animals treated with estradiol. Its significance is unknown. In view of this and prior findings that 8-PN can stimulate growth of human breast cancer cells, these researchers concluded that the clinical use of this substance could have adverse effects. More research is clearly needed before 8-PN can be said to be either safe or efficacious in the prevention and treatment of perimenopausal symptoms.
On the other hand, some in vitro studies have shown that 8-PN has antiproliferative effects on some human breast and colon cancer cell lines. It has also induced cell death in a prostate cancer cell line and, in still other studies, has inhibited angiogenesis, which could make it useful in treating vascularized forms of cancer and other angiogenesis-associated diseases. Whether it is potent enough, in this context, to have any impact in a clinical setting is unknown. Again, much more research is needed to determine the efficacy and safety of this substance as a potential anticancer agent.
Dosing ovariectomized rats for three months with 8-PN had effects on lipids that were said to be more favorable than those achieved with estradiol benzoate. However, the 8-PN had no effect on triglycerides or on HDL-cholesterol. The researchers discerned a role for it in the prevention of cardiovascular disease associated with estrogen deficiency. That use, however, is clearly premature, pending clinical safety and efficacy trials.
Finally, it has been claimed that 8-PN, and other constituents of hops, can enhance breast size in women. Thus, these constituents have been added to some ""breast-enhancement pills'' currently being marketed. There is no evidence that these substances are either safe or efficacious.
Contraindications, Precautions & Adverse Reactions
8-PN-containing dietary supplements are contraindicated in those hypersensitive to any component of an 8-PN-containing product.
Those who wish to use an 8-PN-containing dietary supplement for a health condition should first discuss this with his or her physician.
Pregnant females and nursing mothers should avoid the use of 8-PN-containing dietary supplements. Women with breast, uterine or ovarian cancer should avoid the use of 8-PN.