Discovery of Flavonoids from Scutellaria baicalensis with Inhibitory Activity Against PCSK 9 Expression: Isolation, Synthesis and Their Biological Evaluation

Nine flavonoids were isolated and identified from a chloroform-soluble fraction of the roots of Scutellaria baicalensis through a bioactivity-guided fractionation using a proprotein convertase subtilisin/kexin type 9 (PCSK9) monitoring assay in HepG2 cells. All structures were established by interpreting the corresponding spectroscopic data and comparing measured values from those in the literature. All compounds were assessed for their ability to inhibit PCSK9 mRNA expression; compounds 1 (3,7,2′-trihydroxy-5-methoxy-flavanone) and 4 (skullcapflavone II) were found to suppress PCSK9 mRNA via SREBP-1. Furthermore, compound 1 was found to increase low-density lipoprotein receptor protein expression. Also, synthesis of compound 1 as a racemic mixture form (1a) was completed for the first time. Natural compound 1 and synthetic racemic 1a were evaluated for their inhibitory activities against PCSK9 mRNA expression and the results confirmed the stereochemistry of 1 was important.


Introduction
Cardiovascular disease (CVD) is a class of well-recorded diseases that lead to prominent adult mortality worldwide. High cholesterol level in the plasma has been identified as a major cause of CVD [1]. Cholesterol is an essential substance to cell membranes, but excess levels of it, either in biosynthesis, uptake, or storage, is tremendously associated to heart-related diseases. Over the past 20 years, statin therapy has been the standard treatment for successful cholesterol reduction [2]. However, some patients with familial hypercholesterolemia (FH) fail to reach the desired cholesterol concentration using common statin therapy [3,4] even at maximal statin dose. Low density lipoprotein receptors (LDL-R) are expressed on the surface of hepatocytes and bind to LDL particles [5,6]. This LDL-R/LDL complex is primarily internalized into the cell through clathrin-coated vesicles, after which the complex dissociates and LDL-C gets degraded into lipids and amino acids. After this process, LDL-R moves back to the cell surface, suggesting that the recycling of LDL-R seems to be significant in lowering LDL-C levels in the plasma [7,8]. In patients with FH, this recycling system is impaired by high concentrations of proprotein convertase subtilisin/kexin type 9 (PCSK9). PCSK9 is known to bind LDL-R and prevent its recycling, which decreases LDL-R expression on cell surfaces and consequently results in high levels of LDL-C in the plasma, leading to hypercholesterolemia [9,10]. Therefore, PCSK9 inhibition would increase LDL-R expression, which in turn decreases cholesterol levels in the plasma. Thus far, two antibody drugs have been approved as PCSK9 inhibitors, while no small molecules have ever reached at the clinical trials [11,12]. Moreover, only a few natural products in either extracts or individual chemicals have been explored for PCSK9 regulation [13][14][15][16][17][18].
Scutellaria baicalensis belongs to the Lamiaceae family [19] and is native to the soil of Asian countries. The plant has been used as a food additive and for traditional medicine [20]; its roots have been used for the treatment of allergies, inflammation [21,22], fever, dysentery [23], pneumonia, influenza [24], diarrhea [25], and have potential anticancer activities [26,27]. Phytochemical investigations of S. baicalensis roots have disclosed flavonoids, phenylethanoids and sterols as the main chemical constituents [28].

Inhibitory Activity against PCSK9 mRNA Expression of Root Extract from S. baicalensis
In the preliminary screening assay to assess medicinal plant extracts for their inhibitory activity against PCSK9 mRNA expression using HepG2 cells, a methanolic extract of S. baicalensis was found to inhibit PCSK9 mRNA expression. Thus, subsequent partitioning with organic solvents (hexane, chloroform, butyl alcohol and water-soluble extracts) was conducted and the resultant solvent-soluble extracts were tested again using the same bioassay ( Figure 1). A chloroform-soluble extract demonstrated inhibitory activities on PCSK9 mRNA expression. Therefore, the chloroform-soluble fraction was further investigated to discover potential molecules in the extract that are responsible for the inhibitory effects on PCSK9 expression.

Stereochemistry Determination of Compound 1
The stereochemistry of C-2 and C-3 in 1 was determined to be in a trans orientation based on the observation that the vicinal coupling constant between H-2 and H-3 was 11.2 Hz, consistent with what is known in the literature regarding compound 2 [39,40]. Furthermore, circular dichroism (CD) spectroscopy of 1 revealed a positive Cotton effect at 328 nm and a negative Cotton effect at 299 nm, suggesting that the configurations of C-2 and C-3 were 2R and 3R, respectively [41]. The structure of 1 was thus characterized as (2R,3R), 3,7,2′-trihydroxy-5-methoxyflavanone.

Inhibitory Activity against PCSK9 mRNA Expression of Isolates from S. baicalensis
The isolated 1-6, 8 and 9 from this study were evaluated for their PCSK9 mRNA expression in HepG2 cells. Compound 7 was excluded for cytotoxicity ( Figure 3A). Only 1 and 4 (skullcapflavone II) exhibited inhibitory activity at 20 µM (84.4% and 42.4%, respectively) ( Figure 3B). In addition, some compounds from the same plant that have been previously isolated in our laboratory [26] were tested in the same assay at 20 µM in order to assess structure-activity relationship ( Figure S15). None of them were active in this assay system. From the structures 1 (active) and 6 (inactive), it was inferred that a hydroxyl group at C-3 position might provide the different activities. When the structure of skullcapflavone II (4) was compared with the similar structures of molsoflavone and alnetin in Figure  S15, no conclusive result was reached due to the limited number of structures.
Out of the active compounds, compound 1 was further tested for its PCSK9 and LDL-R protein expression and it was found that compound 1 was able to inhibit PCSK9 and increase LDL-R protein expression, respectively ( Figure 3C). As mentioned earlier, PCSK9 facilitates LDL-R degradation and prevents LDL-R recycling. Taking into consideration this function of PCSK9, compound 1 may potentially lower cholesterol levels by decreasing PCSK9 expression and concomitantly increasing LDL-R expression.

Stereochemistry Determination of Compound 1
The stereochemistry of C-2 and C-3 in 1 was determined to be in a trans orientation based on the observation that the vicinal coupling constant between H-2 and H-3 was 11.2 Hz, consistent with what is known in the literature regarding compound 2 [39,40]. Furthermore, circular dichroism (CD) spectroscopy of 1 revealed a positive Cotton effect at 328 nm and a negative Cotton effect at 299 nm, suggesting that the configurations of C-2 and C-3 were 2R and 3R, respectively [41]. The structure of 1 was thus characterized as (2R,3R), 3,7,2 -trihydroxy-5-methoxyflavanone.

Inhibitory Activity against PCSK9 mRNA Expression of Isolates from S. baicalensis
The isolated 1-6, 8 and 9 from this study were evaluated for their PCSK9 mRNA expression in HepG2 cells. Compound 7 was excluded for cytotoxicity ( Figure 3A). Only 1 and 4 (skullcapflavone II) exhibited inhibitory activity at 20 µM (84.4% and 42.4%, respectively) ( Figure 3B). In addition, some compounds from the same plant that have been previously isolated in our laboratory [26] were tested in the same assay at 20 µM in order to assess structure-activity relationship (Supplementary Materials Figure S15). None of them were active in this assay system. From the structures 1 (active) and 6 (inactive), it was inferred that a hydroxyl group at C-3 position might provide the different activities. When the structure of skullcapflavone II (4) was compared with the similar structures of molsoflavone and alnetin in Supplementary Materials Figure S15, no conclusive result was reached due to the limited number of structures.
Out of the active compounds, compound 1 was further tested for its PCSK9 and LDL-R protein expression and it was found that compound 1 was able to inhibit PCSK9 and increase LDL-R protein expression, respectively ( Figure 3C). As mentioned earlier, PCSK9 facilitates LDL-R degradation and prevents LDL-R recycling. Taking into consideration this function of PCSK9, compound 1 may potentially lower cholesterol levels by decreasing PCSK9 expression and concomitantly increasing LDL-R expression.

Comparison of Inhibitory Activity against PCSK9 mRNA Expression between Compound 1 and 1a
Due to its activity, we selected compound 1 for synthesis and designed the synthetic scheme shown in Scheme 1. Our approach to synthesize racemic 1a began with di-MOM protected 1-(2-hydroxy-4,6-bis(methoxymethyl)phenyl)ethanone (11), which was readily prepared from the commercially available 2′,4′,6′-trihydroxyacetophenone (10) by treatment with MOMCl and ethyldiisopropylamine [42]. Compound 11 was converted to chalcone 14 through methylation using dimethyl sulfate in acetone, followed by Claisen-Schmidt aldol condensation with 2-(methoxymethyl)benzaldehyde (13) in the presence of aqueous base [43,44]. Epoxidation of compound 14 with alkaline hydrogen peroxide at room temperature produced 15 in 89% yield [45]. Treatment of the epoxide 15 employing various acidic conditions for the purpose of instantaneous MOM deprotection and intramolecular 6-endo opening of epoxide afforded flavanol 1a, however, the yield of the product was limited to 3-5%, with a variety of undesired products [44,46]. Finally, treatment of 15 with 12% conc HCl in methanol produced compound 1a as a racemic mixture in 11% yield as shown in Scheme 1 [47].

Comparison of Inhibitory Activity against PCSK9 mRNA Expression between Compound 1 and 1a
Due to its activity, we selected compound 1 for synthesis and designed the synthetic scheme shown in Scheme 1. Our approach to synthesize racemic 1a began with di-MOM protected 1-(2-hydroxy-4,6-bis(methoxymethyl)phenyl)ethanone (11), which was readily prepared from the commercially available 2 ,4 ,6 -trihydroxyacetophenone (10) by treatment with MOMCl and ethyldiisopropylamine [42]. Compound 11 was converted to chalcone 14 through methylation using dimethyl sulfate in acetone, followed by Claisen-Schmidt aldol condensation with 2-(methoxymethyl)benzaldehyde (13) in the presence of aqueous base [43,44]. Epoxidation of compound 14 with alkaline hydrogen peroxide at room temperature produced 15 in 89% yield [45]. Treatment of the epoxide 15 employing various acidic conditions for the purpose of instantaneous MOM deprotection and intramolecular 6-endo opening of epoxide afforded flavanol 1a, however, the yield of the product was limited to 3-5%, with a variety of undesired products [44,46]. Finally, treatment of 15 with 12% conc HCl in methanol produced compound 1a as a racemic mixture in 11% yield as shown in Scheme 1 [47]. To compare bioactivity of natural 1 and racemic 1a, these two compounds were tested in HepG2 cells for their inhibitory activities against PCSK9 mRNA expression. As expected (Figure 4), natural 1 seemed to be more potent than racemic 1a because racemic 1a are composed of two isomers. From this result, PCSK9 mRNA expression might be modulated by 1 with specific configurations.   To compare bioactivity of natural 1 and racemic 1a, these two compounds were tested in HepG2 cells for their inhibitory activities against PCSK9 mRNA expression. As expected (Figure 4), natural 1 seemed to be more potent than racemic 1a because racemic 1a are composed of two isomers. From this result, PCSK9 mRNA expression might be modulated by 1 with specific configurations. To compare bioactivity of natural 1 and racemic 1a, these two compounds were tested in HepG2 cells for their inhibitory activities against PCSK9 mRNA expression. As expected (Figure 4), natural 1 seemed to be more potent than racemic 1a because racemic 1a are composed of two isomers. From this result, PCSK9 mRNA expression might be modulated by 1 with specific configurations.   Further analysis demonstrated that downregulation of SREBP-1 mRNA was detected in compounds 1 and 4, suggesting inhibition of PCSK9 mRNA expression was mediated by SREBP-1 as reported in the literature [48]. Thus, it merits enantioselective synthesis of (2R,3R) compound 1 and its derivatives for further chemical modifications and in vivo studies.

General Information
Nuclear magnetic resonance (NMR) spectra were obtained using a Varian 400 spectrometer (Varian, Palo Alto, CA, USA) 400 MHz spectrometer operated at 400 MHz for 1 H-NMR and at 100 MHz for 13 C-NMR. High-resolution mass spectra data were measured on a Xevo G2 Q-TOF mass spectrometer (Waters, Milford, MA, USA). Fourier Transform Infrared (FT-IR) recorded on a Nicolet™ iS™ 5 FT-IR spectrometer (ThermoFisher Scientific, Madison, WI, USA) were used. Ultraviolet visible spectroscopy was performed using a DU 730 UV/Vis spectrophotometer (Beckman Coulter GmbH, North Rhine-Westphalia, Germany). Optical rotation and CD data are also given in the Supplementary file. Semi-preparative high performance liquid chromatography (HPLC) was performed on a system equipped with a Gilson 321 pump and Gilson 172 Diode Array Detector (Gilson, Madison, WI, USA) using YMC-pack Ph (250 × 20 mm) and YMC-pack Ph (250 × 10 mm) HPLC columns (YMC, Kyoto, Japan). Water was purified using a Milli-Q system (Waters Corporation, Milford, MA, USA). Column chromatography on C-18 RP silica gel (Cosmosil, Kyoto, Japan) and Sephadex LH-20 (GE Healthcare, Uppsala, Sweden) was conducted, and TLC analysis on silica gel 60 F 254 plates (Merck, Darmstadt, Germany) was done. The spots were visualized by spraying with 10% aqueous H 2 SO 4 .

Cell Culture and Chemical Reagents
The HepG2 human hepatocellular liver cell line was obtained from the Korea Research Institute of Bioscience and Biotechnology (Daejeon, Korea) and grown in Eagle's Minimum Essential Medium (EMEM) containing 10% fetal bovine serum and 100 U/mL penicillin/streptomycin sulfate. Cells were incubated in a humidified 5% CO 2 atmosphere at 37 • C. EMEM, penicillin, and streptomycin were purchased from Hyclone (Logan, UT, USA). Bovine serum albumin was purchased from Sigma-Aldrich (St. Louis, MO, USA). Antibodies against PCSK9, LDL-R, and β-actin were purchased from Abcam, Inc. (Cambridge, MA, USA). PCSK9, LDL-R, SREBP1, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) oligonucleotide primers were purchased from Bioneer Corp. (Daejeon, Korea). The solvents for extraction and isolation (methanol, ethyl acetate, n-butyl alcohol, chloroform, n-hexane, etc.) were purchased from SK Chemical (Seoul, Korea). The solvents for HPLC-grade acetonitrile (MeCN) and methanol were also purchased from SK Chemical. The solvent for NMR (CD 3 OD) was obtained from Cambridge Isotope Laboratories, Inc. (Andover, MA, USA).