Effects of silica on lung collagen.

A single intratracheal injection of 50 mg crystalline silica (quartz) into rats produces silicosis. This animal model may be used to study collagen metabolism during the early, middle, and late phases of lung injury, corresponding respectively to the stages of lung injury, development of discrete granulomas, and development of mature silicotic nodules. The early phase is characterized by a rapid increase in the rate of synthesis of lung collagen (within one week of instillation) and increased deposition of excess lung collagen (significant increases within two weeks of instillation). Later phases are characterized by a continuing increase in deposition of excess lung collagen for at least one year after instillation. Silica-induced fibrosis is unique among all the animal models (and in most human fibrotic diseases) thus far examined, in that the excess collagen deposited in the lung contains normal ratios of the two major collagen types of the lung: types I and III. This collagen is nonetheless biochemically different from normal lung collagen. There are reproducible and characteristic differences in the intermolecular cross-links of the collagen in lungs from rats injected with silica. Within one month of silica instillation (the earliest time point examined thus far), an increased hydroxylysine content of collagen can be appreciated. The reducible dysfunctional cross-links are also more likely to be derived from hydroxylysine (i.e. the ratio of dihydroxylated to monohydroxylated cross-links increases). Within four months of silica instillation (and increasingly thereafter), increased amounts of the mature trifunctional cross-link hydroxypyridinium (derived from three residues of hydroxylysine) can also be appreciated, seemingly paralleling the evolution of mature silicotic nodules in these lungs. These changes in cross-linking of lung collagen seem to be common to all the animal models of pulmonary fibrosis examined, and are also consistent with changes occurring in human fibrotic lungs. Preliminary observations suggest that the locus of cross-linking remains the same: hydroxylysine replaces lysine in the primary structure of a specific collagen alpha chain to form the altered cross-links. Thus, there may be molecular markers for the collagen of fibrosis in diseased lungs.