Acyclic Triterpenoids from Alpinia katsumadai Seeds with Proprotein Convertase Subtilisin/Kexin Type 9 Expression and Secretion Inhibitory Activity

Abstract

Cholesterol is one of the primary causes of cardiovascular disease. Investigating and developing potential drugs to effectively treat hypercholesterolemia are therefore of critical importance. Proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors have been developed to lower the levels of low-density lipoprotein cholesterol in patients with hypercholesterolemia. In this study, we aimed to identify compounds that inhibit the PCSK9 mRNA expression and secretion. The bioassay-guided investigation of Alpinia katsumadai seeds utilizing a PCSK9 mRNA expression monitoring assay yielded the isolation and identification of seven new compounds. Among these were three acyclic triterpenoids (1−3), an acyclic sesquiterpenoid (5), one arylpentanoid (6), and two diarylheptanoids (7 and 8), alongside 10 known compounds. The structures of these compounds were determined using nuclear magnetic resonance (NMR) spectroscopy, vibrational circular dichroism (VCD), and electronic circular dichroism (ECD). The absolute configurations of compounds 1 and 2 were identified by comparing the calculated and experimental VCD data as the ECD method was unable to distinguish the diastereomers. All the isolated compounds were evaluated for their regulatory effects on the low-density lipoprotein receptor (LDLR) and PCSK9 mRNA expression, as well as PCSK9 secretion. Of the tested compounds, two of the acyclic triterpenoids (1 and 2) demonstrated potent effects in downregulating PCSK9 at both the mRNA and protein levels, compared with the positive control (berberine chloride). Additionally, compound 1 inhibited PCSK9 secretion to a level comparable to that of berberine chloride. This study identifies compounds that inhibit PCSK9 mRNA expression and secretion, offering significant contributions to the development of novel drugs for the effective treatment of hypercholesterolemia..