Key words
Humulus lupulus L. - Cannabaceae - menopause - 8-prenylnaringenin - xanthohumol - isoxanthohumol
- safety
Hop Secondary Metabolites
Hop Secondary Metabolites
Humulus lupulus L. (hop) (Cannabaceae) is well-known for the usage of female
inflorescences such as bitterness, preservative, and a flavoring agent in beer
brewing, but at the same time it has a long history as a medicinal herb. Hop baths
have been recommended to alleviate gynecological disorders with reference to the
frequently observed menstrual disorders in female hop-pickers suggesting an
estrogenic mode of action [1]. Its importance as a
traditional medicinal herb has also been recognized by the European Medicines Agency
EMEA that has published a monograph about its use and safety [2]. Among terpenes and bitter acids, a number of prenylflavanoids
secreted by lupulin glands have been identified in the strobiles. In 1999, the
investigation of an estrogenically active fraction containing as the main compound
xanthohumol (XH; 0.1–1 % in dry hops) but also isoxanthohumol (IX),
6-prenylnaringenin (6-PN), and 8-prenylnaringenin (8-PN; 100 mg/kg dry hops) ([Fig. 1]) led to the discovery of 8-PN as the major
active constituent, turning out to be the most potent phytoestrogen known to date
[1], [3], [4], [5], [6], [7], [8]. Prior to this, 8-PN or 8-isopentenylnaringenin, as it
was originally called by its discoverers [9], had
already been isolated from the Thai tree Anaxagorea luzonensis A. Gray
(Annonaceae) [9]. In addition, 8-PN is a constituent in
Sophora tomentosa L. (Fabaceae) and in Marshallia grandiflora
Beadle & F. E. Boynt (Asteraceae) [10], [11]. In contrast to most other known phytoestrogens,
8-PN exhibits a twofold higher preference for estrogen receptor α
(ERα) than for ERβ
[6].
Fig. 1 Prenylflavanones contained in hops and hop extracts.
The majority of the chalcones found in H. lupulus easily isomerize to their
corresponding flavanones as they possess free 2′-OH groups. The conversion of
xanthohumol leads solely to isoxanthumol, whereas desmethylxanthohumol (DMX)
isomerizes to 8-prenylnaringenin or 6-prenylnaringenin, thus DMX is the major source
of most known hop flavonoids [12], [13]. The conversion of XH can be induced by thermal
treatment or by an increased pH value, hence IX is the most abundant prenylflavanone
found in beer [14], [15].
The metabolic activation of the weakly estrogenic IX to the strong ER agonist 8-PN
by the gut flora increases estrogenic potency of hops preparations [16], [17], [18], [19]. The achieved
levels of 8-PN and other metabolites show high interindividual variability
justifying the separation into poor, moderate, and high producers [17], [19].
Experimental Data
Effectiveness of hop flavanones in preventing osteoporosis
One of the most prominent physiological effects of the hormonal decline
associated with menopause is the loss of bone mass ultimately leading to
osteoporosis. It has been shown that in addition to hop-derived rho iso-alpha
acids [20], [21], all
major hop flavanones 8-PN, 6-PN, XH, and IXH possess E2-like effects on
osteogenic differentiation in vitro
[22].
Concerning the in vivo situation, one of the first publications on 8-PN
elucidated its effect on bone mineral density (BMD) of ovariectomized (ovx) rats
following 14 days of treatment. It showed that a dose of 30 mg/kg body
weight/day starting at the second day after ovx resulted in an even higher BMD
than that observed in sham-operated untreated animals, while untreated ovx
animals had reduced BMD. Hümpel et al. demonstrated a dose-dependent inhibition
of ovx-induced bone loss in rats following s. c. treatment with 18 mg/kg 8-PN
over a period of 28 days [23]. Since hop contains
about equal amounts of both 2S(−) and 2R(+) 8-PN enantiomers, the second part of
this study compared effects of the racemic mixture to those of 2S(−)-8-PN alone.
No significant difference was observed indicating that both enantiomers are
nearly equally potent at preventing menopause-induced bone loss. More recent
work also suggests improved biomechanical properties due to increased bmp when
8-PN is taken up orally as a food content [24].
Interestingly, in this study the effect of 8-PN on these parameters was much
larger than that of genistein or resveratrol.
Alleviation of hot flashes by 8-PN
Vasomotoric climacteric complaints like hot flashes and sweating are thought to
be caused by a small increase in core body temperature in conjunction with a
narrowing of the thermoneutral zone that describes the range of the body
temperature in which no shivering or sweating occurs [25]. These processes are regulated by the thermoregulatory nucleus in
the anterior hypothalamus. The only common experimental model to investigate the
potency of a drug to alleviate hot flashes is the measurement of differences in
tail skin temperature (TST) of ovx rats. While intact rats display a marked
decrease of TST during the active phase, this response is lost in ovx animals.
This can be reverted by treatment with either estrogens or the
α-adrenoceptor agonist clonidine which is widely used for the treatment
of hot flashes in patients for whom hormone replacement therapy is
contraindicated, suggesting that both irregularities in thermoregulation are
triggered by a similar mechanism [26]. Using this
in vivo model, 8-PN has been shown to restore normal TST after 5 days
of treatment with the relatively low dose of 400 µg/kg per day s. c. as well as
after oral treatment with 7.5 mg/animal/day, indicating that 8-PN may indeed be
able to alleviate climacteric hot flashes [27].
Influence on sexual motivation
Although difficult to differentiate from general age-related effects, sexual
motivation seems to decrease due to menopause [28].
One possibility to test sexual motivation in animals is the so-called partner
preference test [29]. According to Di Viesta et
al., oral treatment of female rats with hop extract dose-dependently decreases
the number and cumulative time of visits to a female stimulus animal although
the number and time of visits to a male stimulus animal is increased slightly.
In addition, perceptive behavior was increased dose-dependently in general, but
lordosis was unaffected [30]. This hints at the
potency of this extract to improve at least some aspects of sexual motivation.
On the other hand pure hop secondary metabolites have not been tested so
far.
Clinical Data
Clinical data are essential to assess the effectiveness as well as the safety of hop
extracts and 8-PN for the treatment of menopausal discomforts.
One small clinical trial assessed pharmacokinetics, endocrine effects, and
tolerability of a single dose of 50, 250, or 750 mg 8-PN in healthy postmenopausal
women. It demonstrated that 8-PN is quickly and completely resorbed in the human
intestine and that relevant plasma levels are achieved. This was inferred from a
decrease of LH levels, an effect assumed to be necessary for the successful
treatment of menopausal symptoms [31].
For the first prospective clinical study assessing the effectiveness of a hop extract
against climacteric complaints, 67 women between 45 and 60 years of age were treated
for 12 weeks with capsules containing either placebo or hop extract standardized to
a dose of 100 µg/day or 250 µg/day 8-PN, identical to that contained in the food
supplement MenoHop. Results show that hop extract may be somewhat effective in
treating menopausal discomforts especially against hot flushes, but no clear
dose-response correlation could be demonstrated [32].
In a subsequent crossover pilot study by the same group, the effectiveness of hop
extract as a drug to relieve menopausal complaints was tested in more detail. And
although the number of participants was very low, the outcome also showed a slight
improvement of menopause-related symptoms [33].
Interestingly, both hop extract and placebo treatment led to an improvement after
eight weeks of treatment, but the subsequent treatment swap led to a further
improvement only in the active-treatment-after-placebo group, while the women in the
placebo-after-active-treatment group experienced a slight regression of all outcome
measures.
Another study assessed the efficacy and safety of vaginal application of a gel
containing hop extract among others in postmenopausal women with urogenital atrophy.
Unfortunately, this study was noncontrolled, making it hard to draw solid
conclusions [34].
Taken together, evidence for the effectiveness of hop extracts from clinical studies
is still very weak. The number of participants in these studies was very low. In
addition, the outcome was inevitably based on questionnaire data. These are
subjective, and in the case of the Kupperman index, outdated, incomplete and include
an arbitrary weighting of the queried symptoms [35]. No
clinical information is available on the effects of hop extracts on factors
associated with osteoporosis even though the effects of 8-PN on bone are among the
more pronounced in cell culture as well as in animal experiments, and there have
been no trials conducted so far using pure 8-PN instead of hop extract.
In contrast to classical hormone therapy, for which there is a plethora of
prospective and retrospective studies with very large patient numbers [36], [37], [38], [39], [40], [41], there is very
little information on the safety of hop extracts or 8-PN treatment. It is known that
8-PN is an ERα agonist [42], and proliferative,
cancer-promoting effects of estrogens are predominantly mediated by this ER subtype.
Uterotrophic assays demonstrate only a mild effect of 8-PN on the uterus wet weight
[7], [8], but evidence
for long-term safety of hop extracts and 8-PN in particular is very weak except for
a 90-day feeding experiment with ovariectomized rats where 8-PN affected uterine wet
weight as well as uterine and mammary histology similar to E2, albeit much weaker
[43]. In addition, in silico analysis of hop
extract constituents identified a number of substances that are potentially
hepatotoxic [44].
Conclusion
It has been shown that the soy constituent genistein, which is by far the most
thoroughly investigated phytoestrogen, is not a good substitute for classical HT,
since its effectiveness is dependent on lifelong exposure, but the studies reviewed
here indicate that hop extracts may be more promising. While the estrogenicity of
the main constituent 8-PN is well established [1], [3], [4], [5], [8], [45], the in vitro and in vivo experimental
data on the effectiveness of hop extracts on climacteric symptoms is so far limited
to osteoporosis, hot flashes, and to some degree sexual motivation. There is no high
quality clinical data on the effectiveness and safety of hop extracts or 8-PN. An
even less characterized alternative to 8-PN may be the structurally very similar
flavanone 6-(1,1dimethylallyl)naringenin which has been isolated from the African
tree Monotes engleri. While this compound is also a potent ER agonist, it
does not possess selectivity for ERα (unpublished data) and appears not to be
uterotrophic [8]. This has to be seen as an advantage
with regard to safety as tumor promotion should be a smaller concern with this
substance.
Acknowledgements
This work has been supported by the Deutsche Forschungsgemeinschaft DFG KR
3768/2–1.
