Fibroblast Inhibition Does Not Promote Normal Lung Repair After Hyperoxia

Severe alveolar epithelial injury disrupts the normal epithelial cell-fibroblast relationship, leading to abnormal repair and the uncontrolled growth of fibroblasts. It is postulated that selective inhibition of fibroblast growth after injury may allow epithelial regeneration to occur, reestablish fibroblast control, and reduce fibrosis. This hypothesis was studied using organ cultures after lung injury was induced in vivo by exposing mice to 90% oxygen for 6 days to produce necrosis of Type 1 epithelium. When explants of this lung tissue were cultured, proliferation of cuboidal epithelial cells was reduced, whereas fibroblast growth and hydroxyproline (HYP) levels were significantly higher than in controls. In attempts to inhibit fibrosis, explants were also cultured in the presence of a proline analog, L-azetidine carboxylic acid (LACA). At concentrations that reduced fibroblast proliferation, epithelial repair was also inhibited. The explants appeared fibrotic, and reduced HYP/DNA was due to lower DNA synthesis. Using pure cell cultures, LACA reduced fibroblast but not epithelial growth, and also abolished the usual increase in epithelial cell proliferation that control cells show when exposed to fibroblast supernatants. These results provide further evidence of interdependence between epithelial cells and fibroblasts such that agents that inhibit fibroblast growth after lung injury either directly or indirectly retard regeneration of the overlying epithelium, and so fail to produce normal repair.