Abstract： Objective:To investigate the role of p21 in pulmonary fibrosis in mice after Acute Respiratory Distress Syndrome (ARDS).Methods:This was a laboratory study.A total of 36 SPF C57BL/6J mice were randomly divided into Saline control group, LPS-0 h group, LPS-6 h group, LPS-24 h group, LPS-48 h group, and LPS-72 h group, with 6 in each group.HE staining was used to observe the degree of fibrosis, Masson staining was used to evaluate the distribution of fibrous hyperplasia, immunohistochemical method was used to observe the expression of α-SMA, type I collagen, and p21, and Western blot analysis was used to detect the expression of α-SMA and p21 protein.After the mice were treated with 10 mg/kg LPS intratracheal instillation for 24 hours, the lung fibroblasts were extracted.The cells were transfected with Lipofectamine 2000 transfection reagent to knock down p21 expression.The mouse ARDS model was prepared; the lung fibroblasts were extracted and divided into control group (si-control group), si-control+ LPS group (si-control+ LPS-24 h group), LPS group (LPS-24 h group), and si-p21+ LPS group (siRNA-p21+ LPS-24 h group). Western blot analysis was used to detect the expression of α-SMA, p21, p-IκBα, and p-p38 protein in lung fibroblasts.Results:Compared with the Saline control group, LPS-0 h group, and LPS-6 h group, LPS-24 h group showed proliferated fibrous tissue of the airway wall under HE staining, deposited collagen under Masson staining, increased expression of α-SMA, type I collagen, and p21 by immunohistochemistry, and increased expression of p21 protein by Western blot, with the difference statistically significant (all P<0.05). With the extension of time, the expression of p21 protein by immunohistochemistry and Western blot of mouse lung tissue increased gradually, but the expression in LPS-48 h group and LPS-72 h group showed no significant difference compared with LPS-24 h group ( P>0.05). Western blot showed that, compared with the si-control group, the expression of α-SMA protein in the si-control+ LPS group and LPS group both increased ([1.10±0.12] vs [1.11±0.06] vs [0.23±0.02]), the expression of p21 increased ([0.65±0.05] vs [0.72±0.03] vs [0.16±0.00]), the expression of p-IκBα increased ([1.19±0.06] vs [1.07±0.17] vs [0.21±0.02]), and the expression of p-p38 increased ([1.18±0.05] vs [1.20±0.08] vs [0.08±0.01]), with statistical significance (all P<0.05). Western blot showed that the expressions of α-SMA, p21, p-IκBα, and p-p38 protein were all lower in the si-p21+ LPS group than those in the LPS group and in the si-control+ LPS group (α-SMA: [0.27±0.01] vs [1.11±0.06] vs [1.10±1.12], p21: [0.23±0.01] vs [0.72±0.03] vs [0.65±0.05], p-IκBα: [0.05±0.03] vs [1.07±0.17] vs [1.19±0.06], p-p38: [0.36±0.02] vs [1.20±0.08] vs [1.18±0.05]), with the difference statistically significant (all P<0.05). Conclusions:The expression of p21 increases in the early stage of pulmonary fibrosis after ARDS.Inhibiting p21 expression in pulmonary fibroblasts can decrease the deposition of ECM and inhibit the activation of p38 and IκBα.