Taxus cell suspension culture is a sustainable technology for the industrial production of paclitaxel (Taxol®), a highly modified diterpene anti-cancer agent. The methyl jasmonate (MJ)-mediated paclitaxel biosynthetic pathway is not fully characterized, making metabolic engineering efforts difficult. Here, promoters of seven genes (TASY, T5αH, DBAT, DBBT, PAM, BAPT, and DBTNBT), encoding enzymes of the paclitaxel biosynthetic pathway were isolated and used to drive MJ-inducible expression of a GUS reporter construct in transiently transformed Taxus cells, showing that elicitation of paclitaxel production by MJ is regulated at least in part at the level of transcription. The paclitaxel biosynthetic pathway promoters contained a large number of E-box sites (CANNTG), similar to the binding sites for the key MJ-inducible transcription factor AtMYC2 from Arabidopsis thaliana. Three MJ-inducible MYC transcription factors similar to AtMYC2 (TcJAMYC1, TcJAMYC2, and TcJAMYC4) were identified in Taxus. Transcriptional regulation of paclitaxel biosynthetic pathway promoters by transient over expression of TcJAMYC transcription factors indicated a negative rather than positive regulatory role of TcJAMYCs on paclitaxel biosynthetic gene expression.

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.

BackgroundTaxol® (paclitaxel) promotes microtubule assembly and stabilization and therefore is a potent chemotherapeutic agent against wide range of cancers. Methyl jasmonate (MJ) elicited Taxus cell cultures provide a sustainable option to meet the growing market demand for paclitaxel. Despite its increasing pharmaceutical importance, the molecular genetics of paclitaxel biosynthesis is not fully elucidated. This study focuses on identification of MJ responsive transcripts in cultured Taxus cells using PCR-based suppression subtractive hybridization (SSH) to identify genes involved in global pathway control.ResultsSix separate SSH cDNA libraries of paclitaxel-accumulating Taxus cuspidata P991 cell lines were constructed at three different post-elicitation time points (6h, 18h and 5 day) to identify genes that are either induced or suppressed in response to MJ. Sequencing of 576 differentially screened clones from the SSH libraries resulted in 331 unigenes. Functional annotation and Gene Ontology (GO) analysis of up-regulated EST libraries showed enrichment of several known paclitaxel biosynthetic genes and novel transcripts that may be involved in MJ-signaling, taxane transport, or taxane degradation. Macroarray analysis of these identified genes unravelled global regulatory expression of these transcripts. Semi-quantitative RT-PCR analysis of a set of 12 candidate genes further confirmed the MJ-induced gene expression in a high paclitaxel accumulating Taxus cuspidata P93AF cell line.ConclusionsThis study elucidates the global temporal expression kinetics of MJ responsive genes in Taxus suspension cell culture. Functional characterization of the novel genes identified in this study will further enhance the understanding of paclitaxel biosynthesis, taxane transport and degradation.

In developing and developed nations, plant cell culture systems are used to supply desirable compounds in lieu of chemical synthesis or natural extraction. When plant cell culture systems are unable to meet commercial demand, metabolic engineering offers a method to increase yields. However, to benefit from metabolic engineering approaches, effective transient transformation methods are required to rapidly identify and characterize key regulatory genes before intensive, time-consuming stable transformation efforts can proceed. This paper describes a particle bombardment-mediated transient transformation system for Taxus spp. in cell culture. Optimal parameters were established for the T. cuspidata cell line P991 and the T. canadensis cell line CO93D, resulting in reliable, efficient, transient expression of the firefly luciferase gene under control of the constitutive CaMV 35S promoter. Multiple bombardments and larger gold microcarriers (1.6 vs 1.0 microm in diameter) were particularly effective in increasing luciferase activity and in reducing variation among replicates. This particle bombardment-mediated transformation system was also shown to be capable of transiently expressing the DsRed and beta-glucuronidase reporter genes under the control of the maize ubiquitin and CaMV 35S promoters, respectively. With the ability to transiently transform Taxus spp. cell cultures using a variety of promoters and reporters, characterization of genes related to paclitaxel accumulation in culture can now proceed.

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