OBJECTIVETo investigate the effects of mouse adipose-derived stem cell conditioned medium (ADSC-CM) on apoptosis of keratinocytes (human epithelial cell line HaCaT) induced by thermal injury in vitro.

METHODS(1) Adipose-derived stem cells (ADSCs) from inguinal adipose tissue of 5 healthy BALB/c mice were isolated, cultured, and purified by collagenase digestion in vitro. The 3rd passage of cells were collected for morphologic observation, detection of expressions of surface markers CD31, CD34, CD45, CD90, and CD105 with flow cytometer, and identification of adipogenic and osteogenic differentiation. (2) HaCaT cells were incubated in water at 51.5 °C for 35 seconds to reproduce thermal injury model, and then the apoptosis rate was detected immediately after injury by flow cytometer. (3) Thermally injured HaCaT cells were divided into routine culture group (RC, cultured with DMEM containing 10% FBS), serum-free group (cultured with serum-free DMEM), 50%ADSC-CM group (cultured with DMEM containing 50%ADSC-CM), and 100%ADSC-CM group (cultured with 100%ADSC-CM) according to the random number table. After 24 hours, apoptosis of HaCaT cells was observed by acridine orange-ethidium bromide (AO-EB) staining; apoptotic rate was determined by flow cytometer; the mRNA and protein levels of Bcl-2 and caspase-3 were respectively determined by real-time fluorescent quantitative RT-PCR technique and Western blotting (protein level was denoted as gray value); the cell cycles were determined by flow cytometer. All above experiments were repeated for 3 times. Data were processed with one-way analysis of variance and LSD- t test.

RESULTS(1) The 3rd passage of cells proliferated well showing fusiform shape similar to fibroblasts. The positive expression rates of CD31, CD34, and CD45 were less than 10.0%, while those of CD90 and CD105 were above 90.0%. The cells could differentiate into adipocytes and osteoblasts. They were identified as ADSCs. (2) Immediately after injury, apoptotic rate of HaCaT cell was (9.8 ± 0.4)%. (3) The number of apoptotic cells was significantly higher in serum-free group than in the other three groups with AO-EB staining. The apoptotic rate of serum-free group [(8.1 ± 1.2)%] was significantly higher than that of 50%ADSC-CM group [(6.0 ± 0.8)%], group RC [(4.6 ± 0.8)%], or 100%ADSC-CM group [(3.1 ± 0.4)%], with t values respectively 3.02, 4.96, 6.60, P values below 0.01. There was no statistically significant difference in apoptotic rate between group RC and 100% ADSC-CM group (t = 1.50, P > 0.05), while statistically significant difference was found between 100% ADSC-CM group and 50%ADSC-CM group (t = 10.21, P < 0.01). (4) The mRNA level of Bcl-2 of serum-free group (0.34 ± 0.08) was significantly lower than that of group RC, 50%ADSC-CM group, and 100%ADSC-CM group (0.98 ± 0.04, 0.77 ± 0.05, 1.06 ± 0.04, with t values respectively 12.87, 8.07, 14.11, P values below 0.01). Compared with that of 100%ADSC-CM group, the mRNA level of Bcl-2 of group RC was slightly decreased (t = 0.08, P > 0.05) and that of 50%ADSC-CM group was significantly decreased (t = 8.08, P < 0.01). (5) The mRNA level of caspase-3 of serum-free group (1.15 ± 0.05) was obviously higher than that of 50%ADSC-CM group (0.72 ± 0.11), group RC (0.41 ± 0.03), or 100%ADSC-CM group (0.38 ± 0.11), with t values respectively 6.93, 13.97, 22.79, P values below 0.01. Compared with 100%ADSC-CM group, the mRNA level of caspase-3 was slightly increased in group RC (t = 0.05, P > 0.05) and significantly increased in 50%ADSC-CM group (t = 4.77, P < 0.01). (6) The protein level of Bcl-2 was significantly lower in serum-free group (0.93 ± 0.04) than in group RC, 50%ADSC-CM group, and 100%ADSC-CM group (1.74 ± 0.06, 1.32 ± 0.05, 1.90 ± 0.04, with t values respectively 20.45, 11.15, 31.38, P values below 0.01). Compared with that of 100%ADSC-CM group, the protein level of Bcl-2 of group RC was slightly decreased (t = 1.33, P > 0.05), but that of 50%ADSC-CM group was obviously decreased (t = 17.30, P < 0.01). (7) The protein level of caspase-3 was obviously higher in serum-free group (0.63 ± 0.08) than in 50%ADSC-CM group, group RC, and 100%ADSC-CM group (0.46 ± 0.03, 0.29 ± 0.08, 0.21 ± 0.03, with t values respectively 3.28, 5.05, 8.46, P values below 0.01). Compared with that of 100%ADSC-CM group, the protein level of caspase-3 of group RC was slightly increased (t = 0.08, P > 0.05), but that of 50%ADSC-CM group was significantly increased (t = 3.52, P < 0.05). (8) Compared with that of serum-free group, the percentage of cells in G2/M phase of each of the other 3 groups was significantly decreased (with t values respectively 6.88, 4.08, 7.28, P < 0.05 or P < 0.01). Compared with that in serum-free group, the percentage of cells in S phase was significantly increased in group RC and 100% ADSC-CM group (with t values respectively 2.67 and 2.40, P values below 0.05). There was no statistically significant difference in the percentage of cells in G0/G1 phase among all groups (F = 0.70, P > 0.05).

CONCLUSIONS100% xenogeneic ADSC-CM can suppress apoptosis of HaCaT cells induced by thermal injury through regulating the expression of Bcl-2 and caspase-3, and accelerate cell cycle progression by ameliorating the retardation of cell growth in G2/M phase, and all these effects may give rise to some potential in the treatment of burn wounds at early stage.