Hop (Humulus lupulus)-derived bitter acids as multipotent bioactive compounds

Arginine Decarboxylase expression, polyamines biosynthesis and reactive oxygen species during organogenic nodule formation in hop

Hop (Humulus lupulus L.) is an economically important plant species used in beer production and as a health-promoting medicine. Hop internodes develop upon stress treatments organogenic nodules which can be used for genetic transformation and micropropagation. Polyamines are involved in plant development and stress responses. Arginine decarboxylase (ADC; EC 4·1.1·19) is a key enzyme involved in the biosynthesis of putrescine in plants. Here we show that ADC protein was increasingly expressed at early stages of hop internode culture (12h). Protein continued accumulating until organogenic nodule formation after 28 days, decreasing thereafter. The same profile was observed for ADC transcript suggesting transcriptional regulation of ADC gene expression during morphogenesis. The highest transcript and protein levels observed after 28 days of culture were accompanied by a peak in putrescine levels. Reactive oxygen species accumulate in nodular tissues probably due to stress inherent to in vitro conditions and enhanced polyamine catabolism. Conjugated polyamines increased during plantlet regeneration from nodules suggesting their involvement in plantlet formation and/or in the control of free polyamine levels. Immunogold labeling revealed that ADC is located in plastids, nucleus and cytoplasm of nodular cells. In vacuolated cells, ADC immunolabelling in plastids doubled the signal of proplastids in meristematic cells. Location of ADC in different subcellular compartments may indicate its role in metabolic pathways taking place in these compartments. Altogether these data suggest that polyamines play an important role in organogenic nodule formation and represent a progress towards understanding the role played by these growth regulators in plant morphogenesis.

Organogenic nodule formation in hop: a tool to study morphogenesis in plants with biotechnological and medicinal applications

The usage of Humulus lupulus for brewing increased the demand for high-quality plant material. Simultaneously, hop has been used in traditional medicine and recently recognized with anticancer and anti-infective properties. Tissue culture techniques have been reported for a wide range of species, and open the prospect for propagation of disease-free, genetically uniform and massive amounts of plants in vitro. Moreover, the development of large-scale culture methods using bioreactors enables the industrial production of secondary metabolites. Reliable and efficient tissue culture protocol for shoot regeneration through organogenic nodule formation was established for hop. The present review describes the histological, and biochemical changes occurring during this morphogenic process, together with an analysis of transcriptional and metabolic profiles. We also discuss the existence of common molecular factors among three different morphogenic processes: organogenic nodules and somatic embryogenesis, which strictly speaking depend exclusively on intrinsic developmental reprogramming, and legume nitrogen-fixing root nodules, which arises in response to symbiosis. The review of the key factors that participate in hop nodule organogenesis and the comparison with other morphogenic processes may have merit as a study presenting recent advances in complex molecular networks occurring during morphogenesis and together, these provide a rich framework for biotechnology applications.

Transport of hop bitter acids across intestinal Caco-2 cell monolayers

Several health-beneficial properties of hop bitter acids have been reported (inhibition of bone resorption and anticarcinogenic and anti-inflammatory activities); however, scientific data on the bioavailability of these compounds are lacking. As a first approach to study the bioavailability, the epithelial transport of hop alpha- and beta-acids across Caco-2 monolayers was investigated. Hop acids were added either to the apical or to the basolateral chamber and, at various time points, amounts transported to the receiving compartment were determined. The monolayer integrity control was performed by using marker compounds (atenolol and propranolol), transepithelial electrical resistance (TEER) measurement, and determination of the fluorescein efflux. The TEER and fluorescein efflux confirmed the preservation of the monolayer integrity. The membrane permeability of the alpha-acids (apparent permeability coefficients for apical to basolateral transport (P(appAB)) ranged from 14 x 10(-6) to 41 x 10(-6) cm/s) was determined to be substantially higher than that of the beta-acids (P(appAB) values ranging from 0.9 x 10(-6) to 2.1 x 10(-6) cm/s). Notably, the beta-acids exhibited significantly different bidirectional P(app) values with efflux ratios around 10. The involvement of carrier-mediated transport for beta-acids (active efflux pathway by P-gp, BCRP, and/or MRP-2 type efflux pumps) could be confirmed by transport experiments with specific inhibitors (verapamil and indomethacin). It appears that alpha-acids are efficiently absorbed, whereas the permeability of beta-acids is low. Limiting factors in the absorption of beta-acids could involve P-gp and MRP-2 type efflux transporters and phase II metabolism.