The molecular basis of interstitial pulmonary fibrosis caused by antineoplastic agents

Abstract

Pulmonary toxicity is an important, proliferation-independent, undesirable aspect of anticancer chemotherapy, which can ultimately develop into lethal interstitial pulmonary fibrosis. The loss of lung function, which defines pulmonary fibrosis, is due to pathological alterations in the content or composition of extracellular matrix components such as collagen, fibronectin, glycosaminoglycans and elastin (1). Biochemical studies suggest that the alteration in matrix is due primarily to an increase in extracellular matrix deposition. The altered synthesis of matrix may reflect an expansion of a cell population (e.g. fibroblasts) (15, 20). New matrix-producing cells also may be recruited into the lungs by the chemotactic activities of polypeptides (15). I n addition, the constitutive levels of matrix synthesis by the resident cell population could be altered. There is increasing evidence that endogenous growth factors and cytokines participate in the remodeling of the interstitial regions of the lungs (20). One focus of our research has been to identify these factors and the mechanisms that regulate altered collagen and extracellular matrix deposition. We have studied the pulmonary toxicity of the DNA interactive anticancer agents, bleomycin (BLM) and cyclophosphamide (CYC). Wh en administered to animals, they cause a reproducible fibrosis that has great similarity to the final fibrotic lesion seen in humans (4, 8, 16). We and others (3, 4, 7, 8, 16) have identified inbred murine strain differences in the pulmonary responsiveness to both BLM and CYC. Thus, C57B1/6 mice are much more sensitive to BLM-induced fibrosis compared to BALB/c mice. Conversely, BALB/c mice are more sensitive to CYC-induced fibrosis than C57B1/6 mice. The differences in strain sensitivity to these fibrogenic agents may reflect differences in drug metabolizing enzymes in the lungs (3, 7). In the case of BLM, the resistant strains have higher levels of the inactivating enzyme, bleomycin hydrolase, which has recently been identified as a member of the thiol protease family (17). Th e availability of murine strains with different sensitivities to fibrogenic drugs provides an important tool for identifying essential prefibrotic factors of fibrosis. The accumulation of pulmonary collagen in C57B1/6N mice after S.C. BLM infusion is preceded by sequential increases in total pulmonary fibronectin, ~(~111 procollagen and sl,I procollagen mRNA (7), as well as increases in a,IV procollagen and a,IV procollagen mRNA levels. Other laboratories have observed similar results with fibronectin, a,111 procollagen and a,1 procollagen mRNA in rats after BLM (2, 9, 13, 14). In our studies, however. these increases are largely confined to the sensitive C57B1/6N mice and are