REGULATION OF LUNG FIBROBLAST ALPHA-SMOOTH MUSCLE ACTIN EXPRESSION, CONTRACTILE PHENOTYPE, AND APOPTOSIS BY IL-1-BETA

IL-1 is important in regulating lung inflammation and potentially fibr osis as well. To clarify the role of this cytokine vis-a-vis changes i n lung fibroblast phenotype in pulmonary fibrosis, the effects of IL-1 beta on isolated lung fibroblasts were examined. Rat lung fibroblasts were treated with increasing doses of IL-1 beta and examined for effe cts on cell number, cu-smooth muscle actin expression, apoptosis, nitr ic oxide (NO) production, and contractility in collagen gels, The resu lts show that IL-1 beta caused dose-dependent down-regulation of alpha -smooth muscle actin protein and mRNA expression. The kinetics of mRNA inhibition was rapid and preceded the effects on protein expression. This IL-1 beta-induced decrease in actin expression was associated wit h inhibition of contractility evaluated using fibroblast-populated col lagen gels. Since IL-1 beta inhibition of actin expression was accompa nied by reduction in cell number, the effect on apoptosis was examined . Significant increase in the number of apoptotic nuclei and DNA fragm entation was observed upon IL-1 beta treatment, with a dose-response c urve that mirrored that for the decline in actin-positive cells. More than one-half of the apoptotic cells were actin positive at high IL-1 beta doses, suggesting that the actin-expressing cells may be more sus ceptible to IL-1 beta-induced apoptosis. IL-1 beta also induced NO pro duction in these cells, which was inhibited by NG-monomethyl-L-arginin e. Similarly, IL-1 beta-induced apoptosis and inhibition of actin expr ession were inhibited by this arginine analogue. Hence, induction of a poptosis by IL-1 beta via NO production may be an important mechanism for regulating lung fibroblast cu-smooth muscle actin expression, and consequently its contractile phenotype as well.