In vitro anti-aging activities of ginkgo biloba leaf extract and its chemical constituents

lili2212@163.com Abstract To determine in vitro anti-aging activity and identify the active chemical constituents in Ginkgo leaf extracts. The antioxidant properties of Ginkgo biloba leaves extracts were evaluated using 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) scavenging and 2,2’-azino-bis (3-ethylbenzothiazoline)-6 sulfonic acid radical (ABTS) scavenging assays. The inhibitory effects of extracts and chemical constituents on reactive oxygen species, matrix metalloproteinase-1 (MMP-1) and collagen tpye O level were tested using human dermal fibroblasts (HDFs). Reverse-phase liquid chromatography (RPLC) and ultra-performance liquid chromatography tandem mass spectroscopy (UPLC/MS/MS) were used to quantitatively analyze flavonoids and terpene trilactones in the extraction. The established quantitative analysis method was validated according to the regulatory guidelines. The extraction and compounds kaempferol 3-D-β-D-glucopyranoside, isorhamnetin-3-D-glucoside, myricetin, ginkgolide A and bilobalide could have potential anti-aging activities of G. biloba . The results suggest that G. biloba could be used as anti-aging products and as a cosmetic and medicinal raw pro-inflammatory mediator production down-regulation


Introduction
Skin aging is a complex biological phenomenon due to intrinsic and extrinsic factors. Ultraviolet (UV) exposure causes physical changes in the skin through complex pathways and ultimately results in the generation of reactive oxygen species (RDS) and secretion of matrix metalloproteinases (MMPs) and elastase (Demeule et al., 2000). RDS directly cause skin aging via oxidative damage of lipids, proteins, and DNA of the skin. They can also indirectly induce the production of MMPs via the MAP kinase pathway (Fisher et al., 2002). MMPs are a family of zinc-dependent endopeptidases. MMP-1 is primarily responsible for dermal collagen degradation during the aging process (Kang et al., 2018), and MMP-2 and MMP-9 degrade the extracellular matrix (ECM) proteins that influence skin thickness and wrinkle formation (Lu et al., 2013).
Ginkgo biloba L. (Ginkgoaceae), popularly known as a living fossil (Gong et al., 2008), have been used for 5000 years in traditional Chinese medicine. The leaf extract of G. biloba is composed of flavonoids (24%), terpene trilactones (6%), proanthocyanidins, organic acids, and other constituents (Ohl, 2013). The extract has been reported to protect against chronic cerebral hypoperfusion , improve cognitive function and regulate inflammatory responses (Wan et al., 2016), and prevent UVB-induced photoaging (Chen et al., 2014). Flavonoids are the main contributors to the anti-inflammatory and antioxidant activity of the extract (Baek et al., 2016), whereas terpene trilactones are the main contributors to blood microcirculation and neuroprotective effects (Jiang et al., 2014). On addition, the extract has been demonstrated to be effective in preventing apoptosis and to possess antioxidant activity (Mohanta et al., 2014). However, the relationship between skin anti-aging activity and the chemical constituents in the leaf extract of G. biloba has not been investigated in detail.
The present study was therefore performed to investigate the anti-aging activities of the extraction and main chemical constituents in G. biloba through DPPH, ABTS radical scavenging, RDS scavenging, MMP-1 inhibitory and collagen promoting activities. The chemical constituents in the extraction with anti-aging activity were further elucidated by the established quantitative methods. The results wish to support the use of G. biloba leaf as a raw material in anti-aging cosmetic products in future.

Cell culture
HDFs were purchased from the Cell Bank of the Shanghai Onstitutes for Biological Sciences, Chinese Academy of Sciences. The cells were cultured in Dulbecco's Modified Eagle Medium (DMEM) supplemented with 10% fetal bovine serum (FBS; BioWhittaker, Walkersville, MD, USA) and 1% penicillin-streptomycin (Gibco BRL, NY, USA), at 37 °C in a humidified atmosphere containing 5% CD 2 .

Extraction method
Air-dried samples (0.1 kg) were sieved and extracted with 1.5 L of 95% ethanol (v/v) by ultrasonic extraction for 60 min. On order to remove ginkgolic acids and enrich the active ingredients, the concentrated extraction was eluted through AB-8 macroporous adsorption resin, 732 cation adsorption resin, and HPD5000 macroporous adsorption resin, successively. We got the 60% ethanol (v/v) elution fraction as the tested extraction. The concentrated dry extract was frozen for use in the following chemical and bioactivity assays.

Determination of DPPH radical scavenging activity
The radical scavenging rates of the samples were measured by the DPPH method (Atta-ur- Rahman et al., 2001). For each sample, an aliquot of 0.5 mL at different concentrations was added to 1.0 mL DPPH solution (100 μM) for 30 min. Methanol was used as a blank solution. The decrease in absorbance at 517 nm was measured. DPPH radical scavenging activity was expressed as the percentage (%) of the original absorbance as follows (Equation 1)I: where A S is the absorbance of the solution after the sample extract has been added and A DPPH is the absorbance of the DPPH solution.

Determination of ABTS radical scavenging activity
ABTS was dissolved in water to a 7 mM concentration (Re et al., 1999). ABTS radical cation (ABTS •+ ) was produced by reacting ABTS stock solution with 2.45 mM potassium persulfate (final concentration) and allowing the mixture to stand in the dark at 25 °C for 12-16 h before use. The ABTS radical cation solution was diluted in ethanol to an absorbance value of 0.70 ± 0.02 at 734 nm and equilibrated at 30 °C. The samples (0.1 mL; final concentrations of 0.1, 0.5, 1.0, 2.0, 4.0, and 6.0 mg/mL) were mixed with 3.9 mL of diluted ABTS radical cation solution, and the absorbance at 734 nm was measured after reaction for 6 min.

Cell viability assay
Cell viability and proliferation were evaluated using the 3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay (Lee et al., 2006). HDFs were pretreated with the samples at various concentrations. After incubation for 24 h, MTT solution (final concentrationI: 5 mg/mL) was added. Next, the cells were incubated at 37 °C for 3 h. Finally, the absorbance of each sample was measured on a microplate reader at 570 nm to obtain the percentage of viable cells.

ROS measurement
The production of peroxides was measured by staining cells with the fluorescent probe 2,7-dichlorofluorescein diacetate (DCFH-DA) ). This dye is a stable nonpolar compound that readily diffuses into cells and yields DCFH. Ontracellular hydrogen peroxide (H 2 D 2 ) or DH in the presence of peroxidase changes DCFH to the highly fluorescent compound 2,7-dichlorofluorescein (DCF). Thus, the fluorescence intensity is proportional to the amount of peroxides produced by the cells. After treatment, the cells were harvested, washed with phosphate-buffered saline (PBSI: pH 7.2), and incubated with 10 μmol/L DCFH-DA for 0.5 h at 37°C. Then, RDS generation was measured by the fluorescence intensity (FL-1, 530 nm) of 10,000 cells.

Enzyme-linked immunosorbent assay (ELISA) for MMP-1 and collagen types I levels
Commercially available ELOSA kits were performed to measure MMP-1 and collagen O levels (Du et al., 2017). HDFs were cultured in 6-well plates (5×10 5 /mL cells) with DMEM containing 10% FBS for 24 h, after which the medium was replaced with serum-free medium containing the test samples. After incubation for 24 h, the supernatant was collected from each well and MMP-1 and collagen types O levels were measured using ELOSA kits. Each sample was analyzed in triplicate.

Quantitation of five terpene trilactones by UHPLC/MS/MS
Chromatographic analysis was performed on ACQUOTY TM UHPLC system (Waters Corp., Milford, MA, USA) with a cooling autosampler and column oven enabling temperature control. The ACQUOTY UHPLC® BEH C18 (2.1 × 100 mm, 1.7 μm) was employed, and the column temperature was maintained at 40 °C and a gradient elution with acetonitrile (A)-water (0.1% formic acid, B) was used. The gradient program was as followsI: 0-0.5 min, 10% A, 0.5-4.0 min, 90% A. The flow rate was set at 0.3 mL/min. The autosampler was conditioned at 10 °C, and the injection volume for analysis was 2.0 μL. XEVD TQD triple quadruple mass spectrometer (Waters Corp.) equipped with an electrospray ionization source (ESO) was used for mass spectrometric detection. The detection was operated in the multiple reaction monitoring (MRM) mode under unit mass resolution in the mass analyzers. The dwell time was set to 200 ms for each MRM transition. The MRM transitions were m/z 409→345, 425→361, 441→324, 425→308, and 325→163 for ginkgolide A (9), ginkgolide B (10), ginkgolide C (11), ginkgolide J (12), and bilobalide (13), respectively. After optimization, the source parameters were set as followsI: curtain gas, 35 psi; nebulizer gas, 50 psi; turbo gas, 60 psi; ion spray voltage, 3.5 kV; and temperature, 500 °C (Table 1). The Masslynx 4.1 software (Waters Corp.) was used for data acquisition and instrument control. Representative chromatograms are shown in Figure 2. Each calibration curve was performed in duplicate, with at least six concentrations of the analyte. The limit of detection (LDD) and limit of quantitation (LDQ) were determined based on S/N ratios of 3I:1 and 10I:1, respectively.

Data analysis
The statistical significance of the differences between the mean measurements of each treated group and that of the control group were determined using Dunnett's t-test. A p-value < 0.05 was considered statistically significant. The data were also analyzed by one-way analysis of variance (ANDVA) using SPSS 17.0 (OBM Corp., Armonk, NY, USA).

Antioxidant activities of different extracts
The results showed that the extraction showed significant DPPH and ABTS radical scavenging activities with OC50 of 0.103 mg/mL and 0.052 mg/mL. On literature, the extract of G. biloba has been reported to have their potent antioxidant properties. Flavonoids, such as quercetin and kaempferol, can directly quench free radicals (Sadowska-Krępa et al., 2017).

Evaluation of the extraction on ROS and MMP-1 in HDFs
The effects of the extraction against RDS production and MMP-1 activity were evaluated. MTT assay was used to determine the maximum permissible level (MPL) of the extracts in HDFs at concentrations of 0.125, 0.25, 0.5, and 1.0 mg/mL. F0 represents the fluorescence value of the control group, and F represents the fluorescence value of the sample group, and the greater the FD/F ratio, the stronger RDS scavenging ability. At the tested concentration (0.2 mg/mL), the extraction showed significant RDS scavenging activity (FD/F value 1.38±0.0025) compared with the control group (FD/F value 1±0.043), and RDS control group (FD/F value 0.84±0.023). Moreover, the extraction also showed significant MMP-1 inhibitory activity at the concentration of 0.2 mg/mL (MMP-1 level 0.22±0.03 ng/mL) and the concentration of 0.1 mg/mL (MMP-1 level 1.115±0.25 ng/mL) compared with the control group (MMP-1 level 8.98±0.06 ng/mL). RDS are thought to be a major factor in the destruction of collagen, which is a hallmark of photoaging (Dong et al., 2008). The maintenance of structural integrity of the connection between the epidermis and dermis, called the dermal-epidermal junction, is important to prevent skin aging (Amano et al., 2005). MMP-1 is considered a key cellular regulator that plays a prominent role in the breakdown of the dermal extracellular matrix, in particular collagen types O and OOO, during the photoaging process, resulting in collagen deficiency and wrinkling. In vitro studies with cultured HDFs exposed to UV radiation showed an increase in RDS production and a decrease in type O collagen synthesis (Buechner et al., 2008).

Validation of the established UHPLC/MS/MS method
The linearity for each compound was established by plotting the peak area (y) versus the concentration (x) of each analyte, as demonstrated by the equations reported in Table 2. All the calibration curves showed good linearity (r 2 >0.998) within the ranges tested. Stability was evaluated by repeated analysis of the same sample solution every 4 h for 24 h. The results (RSD<3%) showed that the sample solution was stable during the analysis. To assess repeatability, six different solutions of the same sample were analyzed. The RSDs were all less than 3%, which indicates that the method has good repeatability. The LDD (ng), LDQ (ng), precision, and repeatability for all the analytes are summarized in Table 2. The established UHPLC/MS/MS method was identified as a fast, accurate, and suitable method for quantitative analysis of the samples.
Type O collagen is the major structural component of the ECM and the most abundant protein in skin connective tissue. The individual polypeptide chains of type O collagen are synthesized by dermal fibroblasts from procollagen, which is secreted into the dermal extracellular space. Senescent fibroblasts produce less ECM, such as collagen O and OOO, and more MMPs, contributing to the observed ageing morphology. On the present study, the effect on Collage O level of the compounds kaempferol 3-D-β-D-glucopyranoside, isorhamnetin-3-D-glucoside, myricetin, ginkgolide A, and bilobalide were further tested ( Figure 4). Ginkgolide A and bilobalide showed better collagen promoting activity than other compounds. We should pay more attentions on the lactones like ginkgolide A and bilobalide about their anti-aging activity in future.

Conclusion
The results of this study indicate that the extraction of G. biloba, owing to high levels of flavonoids and lactones, inhibited RDS and MMP-1 degradation in HDFs. The chemical constituents kaempferol 3-D-β-D-glucopyranoside, isorhamnetin-3-Dglucoside, myricetin, ginkgolide A and bilobalide in the active extraction, showed marked MMP-1 inhibitory activity. Ginkgolide A and bilobalide, which have better collagen promoting activity should be studied further for their anti-aging activity. The results provide evidence supporting the use of the leaf extract and active compounds of G. biloba as the anti-aging medicinal and cosmetic raw materials.  (5), ginkgolide A (6), and bilobalide (10). *indicates significant difference (p < 0.05); **indicates very significant difference (p < 0.01).