Lung fibroblasts from animals breathing 100% oxygen produce growth factors for alveolar type II cells

Type II cells were isolated from rats with a purity of 80-95% with less than 4% macrophages. These cells, after plating for approximately 16 h, were cultured with 50% RPMI 1640 and 50% (vol/vol) conditioned medium obtained from confluent hamster lung fibroblasts, together with 0.1% fetal calf serum (FCS). Conditioned media were obtained from either fibroblasts derived from normal hamsters breathing room air [normoxic-conditioned medium (NCM)] or from hamsters exposed for 4 days to 100% O2 [hyperoxic-conditioned medium (HCM)]. Controls consisted of 100% minimal essential medium (MEM) containing 0.1% FCS. Over a 96-h culture period, NCM stabilized cell populations but was unable to induce proliferation. In contrast, at low cell densities, HCM could cause a two- to threefold increase in type II cell number within 24-48 h after introduction. This effect could not be demonstrated at high cell densities. When tested with FCS concentrations ranging from 0 to 10%, maximum effects were obtained using 0.1-0.2% FCS. We conclude that lung fibroblasts from oxidant-injured hamsters produce growth factors that can stimulate at least one mitotic division in cultured type II cells, which are plated at low density. These factors are absent, or present in much lower concentration, in lung fibroblasts from normal animals.