Inhibition of Proliferative Activity of Pulmonary Fibroblasts by Tetrandrine

Tetrandrine, an herbal drug, has been employed in China to treat pulmonary fibrosis. To date, the mechanisms governing the antifibrotic action of tetrandrine are unknown. The present study employs a fibroblast mitogenic assay to determine whether tetrandrine directly inhibits the ability of fibroblasts to respond to stimulation by growth factors. The data indicate that tetrandrine blocks proliferation and the incorporation of tritiated thymidine into DNA by fibroblasts stimulated with human serum, PDGF plus plasma, FGF plus plasma, or TNF plus plasma. Since tetrandrine inhibits the response to a variety of growth factors, its action does not appear to involve the blockade of a specific stimulatory receptor. Tetrandrine is effective in inhibiting thymidine incorporation when added up to 6 hr after stimulation of quiescent cells, suggesting either that tetrandrine does not block the attainment of competence by fibroblasts or that its activity is not limited to blocking the attainment of competence by these cells. Growth factor-induced mitogenesis is also inhibited by nitrendipine, a calcium channel blocker, and by cytochalasin B, a microfilament blocker. However, tetrandrine treatment of fibroblasts neither results in the changes of morphology seen with cytochalasin B nor is limited to the early events of stimulus-response coupling. Therefore, the mechanism of action for tetrandrine is not identical to that for either cytochalasin B or nitrendipine. In summary, these results suggest that the antifibrotic action of tetrandrine may be mediated in part by direct inhibition of fibroblast proliferation normally associated with the development and progression of silicosis.