Lipid-Lowering Activities of Cucurbitacins Isolated from Trichosanthes cucumeroides and Their Synthetic Derivatives

In the ongoing efforts to discover natural cholesterol-lowering compounds, dihydrocucurbitacin B, isolated from Trichosanthes cucumeroides roots, was found to promote LDL uptake by upregulating LDLR protein in a PCSK9-dependent process. In this study, an in-depth investigation of T. cucumeroides roots afforded 27 cucurbitacins (1–27), including seven new cucurbitacins (1–7), and their structures were elucidated by spectroscopic data analyses. In order to gain insight into their structure–activity relationship, cucurbitacin derivatives (B1–11 and DB1–11) were synthesized. Evaluation of lipid-lowering activities of these cucurbitacins by an LDL uptake assay in HepG2 cells revealed that most of the compounds improved the LDL uptake rate, among which hexanorisocucurbitacin D (6) and isocucurbitacin D (21) exhibited the highest activities (rates of 2.53 and 2.47, respectively), which were comparable to that of the positive control, nagilactone B (rate of 2.07). According to a mechanistic study by Western blot analysis, compounds 6 and 21 dose-dependently increased LDLR protein levels and reduced PCSK9 protein levels, representing promising new lipid-lowering drug candidates.