Bronchopulmonary dysplasia (BPD) remains a major cause of morbidity and mortality in premature infants, and despite many advances, its pathophysiology remains incompletely understood. Exposure of the premature lung to hyperoxia is commonly implicated in its pathogenesis. However, the exact link between hyperoxia and BPD, particularly its role in the generation of myofibroblasts, the signature cell-type for lung fibrosis, is undetermined. There is increasing evidence that lipid interstitial fibroblasts play an important role in injury-repair mechanisms in various organ systems. This study demonstrates that exposure to hyperoxia augments the transdifferentiation of pulmonary lipofibroblasts to myofibroblasts. Fetal rat lung fibroblasts (FRLF) from embryonic (e) (term = e22) 18 and e21 gestation were studied. After initial culture in minimum essential medium (MEM) and 10% fetal bovine serum (FBS) in 21% O2 / 5% CO2 at 37 degrees C, FRLF were maintained in MEM and 10%FBS at 37 degrees C under control (21% O2 / 5% CO2) and under experimental conditions (24-hour exposure to 95% O2 /5% CO2) at passage (P) 1 and 5. At each passage, cells were allowed to attach to 100 cm2 culture dishes and grow in 21% O2 before being subjected to the experimental conditions. Passage 1 and 5 cells were analyzed for the expression of well-characterized lipogenic and myogenic markers based on semiquantitative competitive RT-PCR (for parathyroid hormone-related protein receptor [PTHrPR]), adipose differentiation related protein (ADRP), and alpha smooth muscle actin (alphaSMA), triglyceride uptake, and leptin assay. Serial passage and maintenance of cells in 21% O2 resulted in a significant decrease in the expression of the lipogenic markers from P1 to P5, spontaneously. This decrease was greater for e18 than for e21 FRLF. However, exposing cells to 95% O2 augmented the loss of the lipogenic markers and gain of the myogenic marker from P1 to P5 in comparison to cells maintained in 21% O2. These changes were also greater for e18 vs e21 lipofibroblasts. These changes in mRNA expression were accompanied by decreased triglyceride uptake and leptin secretion on exposure to hyperoxia. These results suggest that exposure to hyperoxia (95% O2) augments the transdifferentiation of pulmonary lipofibroblasts to myofibroblasts.