Withanolides: Elucidating steroidal lactone biosynthesis in Nightshades

Abstract

Withania somnifera (Solanaceae) is well known in ayurvedic medicine as a strengthening tonic for various medical purposes. Its effects are mainly due to withanolides, a class of steroidal lactones with diverse oxidation patterns present in various nightshade plants. Pharmacological studies attributed anti-proliferative and anti-inflammatory properties to withanolides. However, obtaining medicinally active withanolides can be complicated, as complex mixtures are present in producing plants and total synthesis of withanolides is costly and time consuming. Therefore, investigation of their biosynthesis is important to enable biotechnological enhancement and to provide novel insights into plant steroid biochemistry. This work aimed to investigate withanolide biosynthesis in Physalis peruviana and Withania somnifera. Both plants were investigated for their main withanolides, as producers can accumulate a diverse array of withanolides, depending on the cultivation conditions. Here, besides several known withanolides, two yet unknown, truncated withanolides (irinan A (1) and B (2)) were isolated from P. peruviana and their structures were elucidated. As intermediates of withanolide biosynthesis were needed for enzyme assays but are neither known, nor commercially available, metabolic engineering in yeast was attempted to divert yeast ergosterol biosynthesis towards production of 24-methyldesmosterol (3), the last known precursor in withanolide biosynthesis. However, while production of the precursor 24-methylenecholesterol (4) was temporarily observed, 3 did not accumulate. Furthermore, based on the biosynthetic hypothesis, 21 cytochrome P450 (P450) and 14 dehydratase (DH) gene candidates were selected after analysis of three withanolide-producing species. Of those, 17 P450 and 6 DH candidates could be cloned and evaluated by gene silencing in W. somnifera, identifying 5 P450 and 2 DH gene candidates where silencing evoked significant decrease of the main withanolide (withaferin A, 5). Those candidates were further examined by heterologous expression experiments in the model plant Nicotiana benthamiana. Here activity on the substrate 24-methyldesmosterol was detected for one candidate (P450-7), while another exhibited activity on native cycloartenol (6) from the host plant (P450-17). Further investigation of P450-17 revealed that orthologs were present in tomato and potato, both non-producers of withanolides. In both plants P450-17 homologous genes are arranged in gene clusters, with neither the genes nor the cluster being reported before. In conclusion, this work provides insights into oxidations involved in withanolide biosynthesis and yet unknown phytosterol pathways in Solanaceae plants.