Structurally diverse terpenoids and their DRAK2 inhibitory activities: A follow-up study on the vulnerable conifer Pseudotsuga forrestii

Glycolipid metabolism disorders (GLMDs) have been reported to be involved in dysfunctions of a few key enzymes such as ATP-citrate lyase (ACL) and death-associated protein kinase (DAPK)-related apoptosis inducing protein kinase-2 (DRAK2). Exploring new and effective small molecule ligands for these enzymes, natural products are expected to have the advantage over synthesized compounds. In our preceding work, a number of C50 terpenoids with bioactivities against ACL have been found from Pseudotsuga forrestii (a 'vulnerable' Yunnan Douglas fir) by the guidance of an LC-MS/MS-based molecular ion networking strategy. The combination of the rest portions of the EtOAc fraction from this precious plant sample did not predict any C50 compounds, but it was detected to be rich in C40/C30/C20 terpenoids and preliminarily exhibited DRAK2 inhibition. In this follow-up study, ten undescribed terpenoids (1-10) and 25 structurally related known compounds were further isolated and characterized. The new structures and their absolute configurations were determined by spectroscopic methods and/or X-ray diffraction analyses. 7,13-Friedo-lanostanes 1 and 2 feature intriguing skeletons with an unusual alpha, beta-unsaturated spirolactone unit formed between C-13 and C-23 via oxygen-bridge or with an 8 (14 -> 13)-abeo moiety. Triterpene-monoterpene hybrid 3 could be regarded as a unique tetraterpene formed by intermolecular [4 + 2] Diels-Alder cycloaddition between a rearranged lanostane (dienophile) and a beta-myrcene (diene). Methyl 15-methoxy-8,11,13-abietatrien-18-yl-succinate (9) and 3-oxo-24(E)-lanosta-8,24-dien-26-oic acid (35) were confirmed to have DRAK2 inhibitory effects with IC50 values of 6.6 and 4.1 mu M, respectively. A molecular docking study disclosed alternative H-bonds/interacting amino acid residues between compounds 9/ 35 and the DRAK2 enzyme when comparing with those of the synthesized inhibitors reported in literature. The above findings could support the important role of protecting plant species diversity in support of chemical diversity and potential sources of new therapeutic agents for the treatment of GLMDS.