Induction of telomerase activity in fibroblasts from bleomycin-injured lungs

Bleomycin-induced lung injury causes increased fibroblast numbers in the lu ng and pulmonary fibrosis. Studies of fibroblasts isolated from such injure d lungs have revealed evidence of increased intrinsic proliferative capacit y, but the mechanism is unknown. Telomerase catalyzes the addition of telom eric DNA repeats onto chromosomal ends, which is associated with increased cellular life span or immortality. To examine whether telomerase might play a role in regulating fibroblast proliferative capacity in pulmonary fibros is, lung fibroblasts were isolated from rats treated with endotracheal inje ctions of phosphate-buffered saline or bleomycin. At selected time points, the rats were killed and lung fibroblasts isolated. The isolated cells and lung tissue were then used in experiments for measurement of telomerase act ivity. The results show undetectable telomerase activity in fibroblasts iso lated from control uninjured lungs, or in the control lung tissue extracts. Similar results were obtained in cells and lung tissue from Days 1, 3, and 28 bleomycin-injured lungs. However, significant telomerase activity was d etected in fibroblasts and tissue extracts isolated from Days 7, 14, and 21 bleomycin-treated rat lungs, with maximal activity observed in the Day 14 samples. Analysis of the isolated cells for telomerase messenger RNA or rev erse transcriptase expression, combined with alpha-smooth-muscle actin expr ession by immunohistochemistry, revealed that telomerase expression localiz ed primarily to nonmyofibroblasts. These findings suggest that in addition to elevated growth factor expression, the injured lung fibroblast populatio n may contain cells with increased life span, which could contribute to the observed overall increase in lung fibroblast numbers.