Current Pharmaceutical Biotechnology - Volume 8, Issue 4, 2007

Since the ancient times plants have been rich sources of medicines. This is still true even in the modernized Societies. New natural compounds potentially leading to innovative drugs are being discovered from plants. We are often astonished by how much man enjoys the benefits of the huge chemical diversity of plants not only for providing medicines but also pesticides, flavors, dyes and other industrial materials. More recently plant biotechnology, including cell culture and genetic manipulation, offers new possibilities for the development of more effective pharmaceuticals and feasible production of drugs from plants. This Special Issue aims to review the state-of-the-art for medicinal plant biotechnology. All six articles deal with plant-derived compounds which are clinically used nowadays. These chapters are contributed by the leading scientists from throughout the world. They describe the current status of biosynthesis and accumulation of pharmaceutical compounds in plants and plant cell cultures, gene identification for the production of these compounds, and future prospects of medicinal plant biotechnology. We hope this Special Issue will carry the torch for the research on medicinal plants in the future, and we look forward to breakthrough innovations in drug developments through modern plant biotechnology.

Camptothecin (CPT) and its derivatives have been received considerable attention recently. Two semi-synthetic derivatives, topotecan and irinotecan, are currently prescribed as anticancer drugs. Several more are now in clinical trial. CPT is produced in many plants belonging to unrelated orders of angiosperms. At present, CPT supplied for pharmaceutical use is extracted from the plants, Camptotheca acuminata and Nothapodytes foetida. Several efforts have been made to sustain a stable production of CPT by in vitro cell cultures of C. acuminata, N. foetida and Ophiorrhiza pumila. Recent report showed that plants are not the only sources that produce CPT. CPT was reported to be produced from the endophytic fungus isolated from the inner bark of N. foetida. The hairy root cultures of C. acuminata and O. pumila produce and secrete CPT into the medium in large quantities. These reports suggest the possibility to develop large-scale production of CPT. In addition, recent advance in the cloning and characterization of biosynthetic enzymes involved in CPT biosynthetic pathway provides valuable information for developing genetically engineered CPT-producing plants.

Many plants belonging to the Solanaceae family have been used as a source of pharmaceuticals for centuries because of their active principles, tropane and nicotine alkaloids. Tropane alkaloids, atropine, hyoscyamine and scopolamine, are among the oldest drugs in medicine. On the other hand nicotine, the addictive agent in tobacco, has only recently gained attention as a backbone for novel potential alkaloids to be used for certain neurological diseases. The biotechnological production of alkaloids utilizing plant cells as hosts would be an attractive option. However, to date very little success in this field has been gained because of the lack of understanding how these compounds are synthesized in a plant cell. Metabolic engineering attempts have already shown that when the rate-limiting steps of the biosynthetic pathway are completely known and the respective genes cloned, the exact regulation towards desired medicinal products will be possible in the near future. The new functional genomics tools, which combine transcriptome and metabolome data, will create a platform to better understand a whole system and to engineer the complex plant biosynthetic pathways. With the help of this technology, it is not only possible to produce known plant metabolites more effectively but also to make arrays of new compounds in plants and cell cultures.

Higher plants produce diverse classes of metabolites. Metabolic engineering offers tremendous potential to improve the production and quality of these chemicals. This report summarizes the possibility of using metabolic engineering in benzylisoquinoline alkaloid biosynthesis. Benzylisoquinoline alkaloids, such as morphine, sanguinarine, and berberine, are synthesized from tyrosine via reticuline in Magnoliaceae, Ranunculaceae, Berberidaceae, Papaveraceae, and many other species. The early pathway from tyrosine to reticuline is common among many plant species, whereas there is more diversity in late pathways. This review describes several strategies to improve the yield and quality of benzylisoquinoline alkaloids. First, the overexpression of a rate-limiting enzyme in an early pathway to increase the overall alkaloid yield is discussed. Second, the introduction of a new branch into the pathway has been shown to produce novel metabolites. Finally, the possibility of accumulating a pathway intermediate by the knock-down of a key step is examined. Further metabolic modification is also discussed, since the latter two modifications may lead to the production of novel compound( s) from an accumulated intermediate through metabolic activation. These metabolic changes could be further modified to increase chemical diversity through somatic variation in cell culture. Besides this direct metabolic engineering with isolated biosynthetic genes, the regulation of biosynthetic activity with transcription factors and/or with reconstruction of the entire biosynthesis will also be discussed for the next generation of metabolite production.

Paclitaxel is a potent chemotherapeutic agent approved in the treatment of a variety of cancers, and under evaluation for the treatment of Alzheimer’s and heart disease. Originally isolated from Taxus brevifolia, this highly substituted ring diterpenoid belongs to a family of plant secondary metabolites known as taxoids. Paclitaxel is currently supplied through both a semi-synthetic process and plant cell culture. Taxus spp. cell culture offers the potential to produce large amounts of paclitaxel and related taxoids, although variability in accumulation and low yields represent key limitations. Thus, intense efforts have been put forth towards understanding Taxus spp. metabolism to increase paclitaxel accumulation in cell culture. While elicitation and environmental optimization have provided some success in increasing paclitaxel accumulation in vitro, understanding metabolism of paclitaxel on the molecular level is essential for process optimization. Utilizing direct and indirect molecular techniques, a further understanding of paclitaxel biosynthesis has been gained, though knowledge into other aspects of paclitaxel global metabolism, such as regulation, transport, and degradation is lacking. Taxus spp. cell cultures are highly heterogeneous, displaying significant cell-cell variability in growth and paclitaxel accumulation. Information gathered on culture subpopulations as well as putative transcriptional bottlenecks in paclitaxel biosynthesis, coupled with successful transformation of Taxus spp. will allow for the targeted metabolic engineering of Taxus spp. or other model organisms for paclitaxel accumulation to ensure future supply of this important pharmaceutical.

Cannabinoids, consiting of alkylresorcinol and monoterpene groups, are the unique secondary metabolites that are found only in Cannabis sativa. Tetrahydrocannabinol (THC), cannabidiol (CBD) and cannabichromene (CBC) are well known cannabinoids and their pharmacological properties have been extensively studied. Recently, biosynthetic pathways of these cannabinoids have been successfully established. Several biosynthetic enzymes including geranylpyrophosphate: olivetolate geranyltransferase, tetrahydrocannabinolic acid (THCA) synthase, cannabidiolic acid (CBDA) synthase and cannabichromenic acid (CBCA) synthase have been purified from young rapidly expanding leaves of C. sativa. In addition, molecular cloning, characterization and localization of THCA synthase have been recently reported. THCA and cannabigerolic acid (CBGA), its substrate, were shown to be apoptosis-inducing agents that might play a role in plant defense. Transgenic tobacco hairy roots expressing THCA synthase can produce THCA upon feeding of CBGA. These results open the way for biotechnological production of cannabinoids in the future.

Among a large number of plant secondary metabolites, alkaloids comprise one of the most important groups due to their strong and divergent biological activities, and some are applied for clinical use. Alkaloids are often highly accumulated in particular organs of medicinal plants, which are called the ‘medicinal part’, whereas it is known that some alkaloids are translocated from source organs to such sink organs. The movement of biosynthetic intermediates from specific cells to other types of cells in tissue, and further detailed movement within the organelles in a cell is also suggested. However, little is known how alkaloids are transported across membranes and finally accumulated in specific organelles such as vacuole of the sink organ. To increase the productivity of valuable alkaloids in planta, not only biosynthetic genes of alkaloids but also genes involved in their transport will be important. Recently, the involvement of ABC transporters in the translocation of berberine alkaloid from root to rhizome was reported, while H+ antiporters were also suggested as the responsible transporters for vacuolar accumulation of the alkaloid. In this review, we describe intra-organ, intra-tissue and intra-cellular transport of the alkaloid via membrane transports. Furthermore, we discuss the possibility of increasing alkaloid production in transgenic plants by using alkaloid transporter genes.