1. Introduction
Wound healing is the process of restoring the structure and function of damaged tissues through the homeostasis, inflammatory, proliferative, and remodeling phases [
1]. These processes are orchestrated by the interactions between multiple cells, growth factors, cytokines, etc. [
2]. The failure of this delicate process results in chronic wound or abnormal scar formation [
3]. With a growing global market and medical needs, numerous wound care products have been developed [
4]. However, traditional therapies, such as dressing or bandages protecting wounds and antibiotics to prevent infection, have limitations [
5]. Recently developed therapies using growth factors are also limited in practical use because of their high cost, low delivery rates, and poor efficacies [
6,
7]. Considering the limitations of current wound-healing agents, alternative agents are needed to overcome the hurdle of current therapies.
Over the past several decades, significant improvements have been made in understanding the relationships between signaling pathways and the wound-healing process [
8]. Among them, the Wnt/β-catenin signaling pathway plays a major role in the wound-healing process [
9,
10]. The Wnt/β-catenin signaling pathway determines the fate and proliferation of progenitor cells during embryonic development and regulates tissue homeostasis during the postnatal period [
11,
12]. Moreover, the Wnt/β-catenin signaling pathway plays an important role in the regulation of adult stem cells and tissue regeneration during wound healing [
8]. Therefore, the Wnt/β-catenin signaling pathway may be an ideal target for the development of wound-healing agents [
13,
14].
Plant extracts, including herbal medicinal agents, are relatively safe natural agents that have often been used as therapeutics. Herbal medicines are reported to provide a moist environment, lower infection, and prevent scar formation in wound management [
15]. Additionally, plant extracts cause less irritation, sensitization, and toxicity to the wound area [
16].
E. daniellii, a member of the Rutaceae family, has been used as a herbal medicine for the treatment of dermatitis, headache, and gastric inflammation [
17]. It is also known that
E. daniellii extract stimulates osteoblast differentiation via the activation of the Wnt/β-catenin signaling pathway [
18]. However, the role of
E. daniellii in wound healing has not been reported.
Here, we investigated the wound-healing ability of
E. daniellii extract. Because
E. daniellii extract is known to activate Wnt/β-catenin signaling, we first tested its role in the migration of human keratinocytes and dermal fibroblasts to investigate its role in wound healing. We also detected wound-healing marker changes in human keratinocytes and dermal fibroblasts to confirm the potential mechanism. Furthermore, we tested the in vivo wound-healing effect of
E. daniellii extract, including neo-epidermis formation, using a a mouse acute wound model [
9]. Finally, we tested the wound-healing effects of components of
E. daniellii extract by using identical in vitro and in vivo systems.
2. Materials and Methods
2.1. Preparation of E. daniellii Extract
E. daniellii was obtained from the Korea Plant Extract Bank of the Korea Research Institute of Bioscience and Biotechnology (Daejeon, Korea). The plant was collected from Boryeong-si, Chungcheongnam-do, KOREA in 2007. A voucher specimen (KRIB 0014366) was stored in the herbarium of the Korea Research Institute of Bioscience and Biotechnology. The E. daniellii plant (170 g), dried in the shade and powdered, was added to 1 L of methyl alcohol 99.9% (HPLC grade) and was extracted through 30 cycles (40 KHz, 1500 W, 15 min ultrasonication-120 min standing per cycle) at room temperature using an ultrasonic extractor (SDN-900H, SD-ULTRASONIC CO., LTD, Seoul, Korea). After filtration (Qualitative Filter No.100, HYUNDAI MICRO Co., LTD, Ansung, Korea) and drying under reduced pressure, E. daniellii extract (15.91 g) was obtained.
2.2. Components of E. daniellii Extract
The components of E. daniellii, hesperidin, vitexin, limonin, myo-inositol, and uracil were purchased from Sigma-Aldrich (St. Louis, MO, USA) and were dissolved in dimethyl sulfoxide (DMSO; Sigma Aldrich) before use.
2.3. Cell Culture and In Vitro Wound-Healing Assay
HaCaT keratinocytes and human dermal fibroblasts were cultured in Dulbecco’s modified Eagle’s medium (DMEM; Gibco, Grand Island, NY, USA) containing 10% (v/v) heat-inactivated fetal bovine serum (FBS; Gibco), 100 mg/mL of penicillin (Gibco), and 100 mg/mL of streptomycin (Gibco), at 37 °C in a humidified incubator with 5% (v/v) CO2. For the in vitro wound-healing assay, HaCaT keratinocytes or human dermal fibroblasts were seeded in 12-well plates in DMEM containing 10% FBS in triplicate at a density of 4 × 105 cells/well. After a 24 h attachment period, the monolayers were scratched with a sterile pipette tip and were incubated with a medium containing 5% FBS with or without E. daniellii extract (1 or 5 μg/mL), individual E. daniellii components (5 μM), or VPA (100 μM). After 24 h, the cells were harvested, washed once with cold phosphate-buffered saline (PBS, pH 7.4), fixed in 4% paraformaldehyde (PFA) for 15 min at room temperature, and stained with 2% (w/v) crystal violet. The wound-closure area was measured using the NIS-Elements imaging software (Nikon, Tokyo, Japan) (n = 3).
2.4. Transwell Migration Assay
Transwell migration assays were performed in matrix-coated transwell plates (8 μm pore size; Corning Life Sciences, Lowell, MA, USA), as described in our previous study [
19]. Filters were coated with bovine serum albumin (BSA) (100 μg/mL), vitrogen, and fibronectin (10 μg/mL) in PBS for 1 h at 37 °C. HaCaT keratinocytes and human dermal fibroblasts were seeded onto the filters at a density of 5 × 10
4 cells/well, and different doses of
E. daniellii extract (1 or 5 μg/mL), individual components (5 μM), or VPA (100 μM) were added to the upper and lower compartments prior to seeding the cells. After 24 h of incubation, the cells in the inner chamber were removed, and cells on the outer surface were fixed with 4% paraformaldehyde (PFA) and were stained with 2% (
w/
v) crystal violet. Migrating cells were visualized using a bright-field optical microscope (Nikon TE-200U), and the migrated areas were measured using NIS-Elements imaging software (Nikon, Tokyo, Japan) (
n = 3).
2.5. Luciferase Assay
HEK293-TOP cells were seeded into 96-well plates at a density of 2.5 × 104 cells/well and were incubated in a medium containing 10% FBS for 24 h. The cells were incubated for 24 h with or without 1, 5, 10, 25, 50 μg/mL of E. daniellii extract or 1, 5, 10, 25, 50 μM of each of its components (Sigma-Aldrich). The total cell lysates were extracted with 25 μL of 1× reporter lysis buffer (Promega, Madison, WI, USA) per well, and luciferase activity was measured using a microplate luminometer (BMG Labtech, Offenburg, Germany).
2.6. Western Blot Analysis
Cells and tissues were ground and lysed in a RIPA buffer (150 mM NaCl, 10 mM Tris, pH 7.2, 0.1% SDS, 1.0% Triton X-100, 1% sodium deoxycholate, and 5 mM EDTA). The samples were separated on 10–12% SDS polyacrylamide gels and were transferred onto PROTRAN nitrocellulose membranes (Schleicher and Schuell Co., New York, NY, USA). After blocking with PBS containing 5% non-fat dry skim milk and 0.07% (vol/vol) Tween 20, the membranes were incubated with antibodies against β-catenin (1:1000; Santa Cruz Biotechnology, Inc., Dallas, TX, USA), α-SMA (1:1000; Abcam, Cambridge, UK), keratin 14 (1:1000; Covance, Durham, NC, USA), collagen I (1:1000; Abcam), PCNA (1:500; Santa Cruz Biotechnology), α-tubulin (1:5000; Oncogene Research Products, San Diego, CA, USA), or Erk (1:5000; Cell Signaling Technology, Danvers, MA, USA) at 4 °C overnight. The samples were then incubated with a horseradish peroxidase-conjugated antirabbit (1:5000; Bio-Rad Laboratories, Hercules, CA, USA) or antimouse IgG secondary antibody (1:5000; Cell Signaling Technology). The protein bands were visualized using an enhanced chemiluminescence kit (Amersham Bioscience, Piscataway, NJ, USA) and a luminescent image analyzer, LAS-4000 (Fujifilm, Tokyo, Japan).
2.7. β-Catenin Knockdown by Small Interfering RNA Transfection
The human β-catenin small interfering RNA (siRNA) target sequences were 5′-GAAACGGCTTTCAGTTGAG-3′ and 5′-AAACTACTGTGGACCACAAGC-3′. β-catenin siRNA was transfected into HaCaT keratinocytes or human dermal fibroblasts using the Lipofectamine Plus reagent (Invitrogen, Carlsbad, CA, USA) at a final concentration of 100 nM. The cells were scratched and incubated with or without E. daniellii extract (1 or 5 μg/mL), individual components (1 or 5 μM), or VPA (100 μM). β-catenin knockdown was confirmed by immunoblotting analysis, as described earlier.
2.8. Immunocytochemistry
HaCaT keratinocytes and human dermal fibroblasts were plated in 12-well culture plates. The cells were washed once in PBS, fixed with 4% PFA in PBS for 15 min at room temperature, and permeabilized in 0.1% Triton X-100 for 30 min at room temperature. After blocking with 5% BSA for 30 min at room temperature, the cells were incubated with primary antibodies against β-catenin (1:100; BD Transduction Laboratories, Lexington, KY, USA), phalloidin (1:200; Molecular Probes, Eugene, OR, USA), or collagen I (1:100; Abcam) overnight at 4 °C. The cells were washed with PBS and incubated with Alexa Fluor 488-conjugated or Alexa Fluor 555-conjugated IgG secondary antibody (1:400; Molecular Probes, Eugene, OR, USA) for 1 h at room temperature, counterstained with 4′,6-diamidino-2-phenylindole (DAPI; 1:5000; Boehringer Mannheim, Mannheim, Germany), and examined under a confocal microscope (LSM510 META; Carl Zeiss, Gottingen, Germany).
2.9. Animals and In Vivo Wound-Healing Assay
Seven-week-old male C3H mice were purchased from Orient Bio Co. (Seongnam-si, Korea) and were allowed to adapt to their new environment for one week. The procedures were reviewed and approved by the Institutional Animal Care and Use Committee (IACUC) of the Yonsei Laboratory Animal Research Center (IACUC-A-201609-407-01, IACUC-A-201705-197-01). The animals were maintained under a 12 h light/12 h darkness cycle at 22–25 °C under conventional conditions and were fed with a standard rodent chow diet and water. To determine the therapeutic potential of the E. daniellii extract and hesperidin on wound healing, 8-week-old C3H mice were anesthetized, the dorsal hair was removed using hair clippers, the skin was cleaned with 70% ethanol, and full-thickness 1.0 cm2 dorsal wounds were made on the backs of the mice. E. daniellii extract (1 or 5 mg/mL), hesperidin (1 or 5 mM), valproic acid (VPA: 500 mM, for positive control), or epidermal growth factor (EGF: 100 μM, for positive control) was topically applied daily until wound closure (n = 6). The wound sizes were measured every other day, under the assumption that the wound depths in each animal were almost constant. Wounded skin tissue samples were harvested and evaluated using immunohistochemistry (IHC) analysis.
2.10. Immunohistochemical Analysis
The tissues were fixed with 4% PFA and paraffin-embedded tissues were sectioned into 4 μm thickness. The slides were deparaffinized in xylene and were rehydrated using a graded dose of alcohol. For antigen retrieval, the slides were autoclaved in 110 mM sodium citrate buffer. Sections were pre-incubated in PBS and then blocked by using PBS containing 5% BSA and 1% goat serum for 30 min at room temperature. Tissue sections were incubated overnight at 4 °C with primary antibodies against β-catenin (1:100; BD Transduction Laboratories), proliferating cell nuclear antigen (1:500; PCNA, Santa Cruz Biotechnology), keratin 14 (1:500; Covance), or collagen I (1:100; Abcam). The sections were rinsed with PBS, incubated with an IgG secondary antibody conjugated to Alexa Fluor 488 or Alexa Fluor 555 (1:400; Molecular Probes) for 1 h at room temperature, and counterstained with DAPI (1:5000; Boehringer Mannheim). Fluorescent signals were visualized using a LSM510 META confocal microscope (Carl Zeiss). For hematoxylin and eosin (H&E) staining, the sections were stained with hematoxylin for 5 min and eosin for 1 min. The slides were then dehydrated using a graded dose of alcohol series, cleared in xylene, and mounted in Permount (Fisher Scientific, Waltham, MA, USA). H&E-stained tissue sections were visualized using a bright-field optical microscope (Nikon TE-200U).
2.11. CellTiter-Glo Luminescent Cell Viability Assay
HaCaT keratinocytes and human dermal fibroblasts were plated at a density of 1 × 105 cells/well in a 24-well plate. The cells were then treated with DMSO, a gradient dose of E. daniellii extract, or hesperidin for 24 h. Cell viability was assessed using the CellTiter-Glo mixture, as recommended by the supplier. Adenosine triphosphate (ATP) was quantified spectrophotometrically at 560 nm using a microplate luminometer (BMG Labtech, Ortenberg, Germany).
2.12. High-performance liquid chromatography Analysis
The composition of vitexin, uracil and heperidin in E. daniellii was calculated based on UV absorbance (254 nm) peak area obtained through high-performance liquid chromatography (HPLC). HPLC analysis was performed on Shimadzu HPLC instrument (Shimadzu, Kyoto, Japan) using Agilent ZORBAX Eclipse Plus C18 column (95Å, 4.6 × 150 mm, 5 μm), mobile phase; acetonitrile and water, both containing 0.05% formic acid.
2.13. Statistical Analysis
Statistical analyses were performed using unpaired two-tailed Student’s t-test. Asterisks indicate statistically significant differences, with one asterisk indicating p < 0.05 and two asterisks indicating p < 0.005.
4. Conclusions
With the expansion of the medical and cosmetic markets, there is a growing need for new agents to overcome the limitations of current wound-healing agents [
4]. Current wound-healing agents, such as antibiotic-based agents, can result in cellular and organ toxicity [
5], and growth-factor-based agents have a short half life, high cost, limited effects on re-epithelialization, and poor absorption rate [
23]. Stem cell therapy is attractive as a regenerative therapy but has limitations in administration, quality control, cost, and safety issues [
24]. Targeting the signaling pathways involved in tissue regeneration has emerged as a new approach for the development of wound-healing agents [
8].
The transforming growth factor beta (TGF-β), Notch, Hedgehog, and Wnt/β-catenin pathways are known to be involved in tissue regeneration and are required for wound healing and skin development [
25]. Especially, the Wnt/β-catenin pathway is a major signaling pathway for stem cell activation and induces multiple factors involved in wound healing such as Collagen-1, WISP1, Keratin-14, EGFR, and VEGF [
8]. Therefore, the Wnt/β-catenin pathway is considered an attractive target for the development of wound-healing agents.
In this study, we investigated the effects of E. daniellii extract on wound healing using both in vitro and in vivo analyses. The E. daniellii extract enhanced cell migration without causing significant cellular cytotoxicity, with an increase in the levels of stress fibers and myo-fibroblast differentiation markers in human keratinocytes and dermal fibroblasts. Treatment with E. daniellii extract, the hallmark factors for wound healing, including the deposition of Collagen-1 and inductions of keratin 14 and α-SMA, was increased in a murine wound-healing model with the acceleration of the proliferation and re-epithelialization of the wound tissues.
Hesperidin, a flavanone glycoside, was identified as a specific active component of the E. daniellii extract and enhanced wound healing in both in vitro and in vivo system. We also confirmed that the wound-healing effects of both E. daniellii extract and hesperidin were mediated via the Wnt/β-catenin signaling pathway, as shown by the siRNA-mediated β-catenin knockdown. Moreover, the E. daniellii extract did not show any significant cellular toxicity.
Our data showed that E. daniellii extract and its active component, hesperidin, promote human keratinocyte and dermal fibroblast migration, and accelerate re-epithelialization in a mouse wound model via the activation of the Wnt/β-catenin signaling pathway. Since plant materials are considered to be suitable candidates for therapeutics, our findings suggest that E. daniellii extract and its active component hesperidin could have the potential to be used as bio-agents for wound healing.