Objective To explore the effects and mechanism of rat epidermal stem cells (ESCs) that were treated with exogenous vascular endothelial growth factor (VEGF) on the healing of deep partial-thickness burn wounds in rats.
Methods ESCs were isolated and cultured from the trunk skin of a 3-month-old female Sprague-Dawley (SD) rat. The third passage of cultured cells in the logarithmic growth phase was used in experiments (1)-(3). (1) The cells were routinely cultured in keratinocytes-specified serum-free medium (K-SFM) (the same routine culture condition below). The morphology of cells cultured for 3 and 5 days was observed under the inverted optical microscope. (2) After 24 hours in routine culture, the expression of cell surface markers CD44, CD45, CD11b, and CD11c was detected by flow cytometer, with triplicate samples. (3) Four batches of cells were collected, and each batch was divided into VEGF group or blank control group according to the random number table. The cells in blank control group were routinely cultured, while the cells in VEGF group were cultured in K-SFM containing VEGF in the final mass concentration of 10 ng/mL. The protein expressions of cytokeratin 19 (CK19) and CK10 in cells cultured for 10 days were detected by Western blotting. The Nanog mRNA expression in cells cultured for 0 (immediately), 2, 4, 6, 8, and 10 day (s) was detected by real-time fluorescent quantitative reverse transcription polymerase chain reaction. The absorbance value was detected with cell counting kit-8 in cells cultured for 2, 4, 6, 8, and 10 days. The clone number of more than 50 cells was observed and counted under the optical microscope in cells cultured for 10 days, and the cell colony formation rate was calculated. Three samples at each time point was analysed. (4) Thirty-six 3-month-old SD rats (either male or female) were used for the study, and two deep partial-thickness burn wounds (10 mm in diameter) were created in each rat by pressing a 100 ℃ electric iron plate on symmetric dorsal side. According to the random number table, the injured rats were divided into VEGF+ ESCs group, ESCs alone group, and blank control group, with 12 rats and 24 wounds in each group. From 0 (immediately) to 2 day (s) after injury, 20 μL phosphate buffer solution (PBS) was injected into each wound in the three groups in one time, once a day, with the solution in VEGF+ ESCs group containing 0.8×10
6 cells/mL of ESCs treated by 10 ng/mL VEGF for 10 days, the solution in ESCs alone group containing 0.8×10
6 cells/mL of ESCs without any treatment, and the solution in blank control group being PBS only. On post first injection day (PFID) 0 (immediately), 3, 7, and 14, three rats from each group were taken respectively according to the random number table for wound healing assessment, and the wound healing rates on PFID 3, 7, and 14 were calculated. The mice at each time point were sacrificed with wound tissue harvested for histology, and the skin structure was observed by hematoxylin-eosin staining. Data were statistically analyzed with independent sample
t test, analysis of variance for factorial design, least significant difference test, and Bonferroni correction.
Results (1) By day 3 in culture, cells distributed in slowly-growing clusters. By day 5, the clusters were large and round, in which the cells mainly with large and round nuclei and little cytoplasm were observed. The above results were consistent with the morphological characteristics of ESCs. (2) The positive expression rate of CD44 was (94.3±1.2) %, and the expressions of CD45, CD11b, and CD11c were negative. The cells were confirmed as ESCs. (3) Compared with those of blank control group, the protein expression of CK19 in the cells of VEGF group was significantly increased after 10 days in culture (
t=3.756,
P<0.05), while the protein expression of CK10 was significantly decreased (
t=3.149,
P<0.05). Compared with those of blank control group, the Nanog mRNA expression in the cells cultured for 0 and 2 day (s) and absorbance values of the cells cultured for 2 and 4 day (s) were not significantly changed in VEGF group (
t=0.58, 0.77, 0.53, 3.02,
P>0.05), while the Nanog mRNA expression in the cells cultured for 4, 6, 8, and 10 days and absorbance values of the cells cultured for 6, 8, and 10 days were significantly increased in VEGF group (
t=6.34, 5.00, 5.58, 4.61, 5.65, 10.78, 15.51,
P<0.01). After 10 days in culture, the cell colony-forming rate in VEGF group was (56.4±1.3) %, significantly higher than (31.5±1.3) % of blank control group (
t=13.96,
P<0.01). (4) The burn wounds of rats in the three groups were confined to the superficial dermis of the skin on PFID 0. On PFID 3, normal skin tissue at wound margins slightly contracted in the rats of VEGF+ ESCs group, which was earlier than that in the other two groups. On PFID 7, the newly generated epidermis covered most parts of the rat wounds in VEGF+ ESCs group, and some of the epithelium crawled around the rat wounds in ESCs alone group, but no obvious epithelialization was observed in the rat wounds in blank control group. On PFID 14, the rat wounds in VEGF+ ESCs group were basically healed, while some parts of the rat wounds were unhealed in ESCs alone group, and most parts of the rat wounds were unhealed in blank control group. On PFID 3, the wound healing rates of rats in the three groups were similar (
P>0.05). On PFID 7 and 14, the wound healing rates of rats in ESCs alone group, respectively (26.0±2.0) % and (64.4±4.7) %, were obviously higher than (12.4±1.1) % and (29.1±3.3) % of blank control group (
P<0.01), all of which were obviously lower than (41.0±2.4) % and (91.3±3.5) % of VEGF+ ESCs group (
P<0.01). On PFID 3, infiltration of a large number of inflammatory cells were observed in the rat wounds in VEGF+ ESCs group, which was earlier than those in the other two groups. On PFID 7, a large number of endothelial cells were observed in the rat wounds in VEGF+ ESCs group, while proliferation of a few endothelial cells were observed in the rat wounds in ESCs alone group, and a large number of inflammatory cells infiltrated the rat wounds in blank control group. On PFID 14, the newly generated epidermal cells covered nearly all the rat wounds in VEGF+ ESCs group and most parts of the rat wounds in ESCs alone group, while a large number of endothelial cells were observed and the newly generated epidermal cells covered some parts of the rat wounds in blank control group.
Conclusions ESCs of rats treated with exogenous VEGF can promote the healing of deep partial-thickness burn wounds in rats, which may be related to VEGF′s roles in promoting the proliferation of ESCs and reducing its differentiation level, so as to maintain the potency of stem cells.
DownLoad: