Although the metalloproteinase murine metalloelastase (MME) has been implicated in lung disorders such as emphysema and pulmonary fibrosis, the mechanisms regulating MME expression are unclear. Low m.w. fragments of the extracellular matrix component hyaluronan (HA) that accumulate at sites of lung inflammation are capable of inducing inflammatory gene expression in macrophages (Mphi). The purpose of this study was to examine the effect of HA fragments on the expression of MME in alveolar Mphi. The mouse alveolar Mphi cell line MH-S was stimulated with HA fragments over time, total RNA was isolated, and Northern blot analysis was performed. HA fragments induced MME mRNA in a time-dependent fashion, with maximal levels at 6 h. HA fragments also induced MME protein expression as well as enzyme activity. The induction of MME gene expression was specific for low m.w. HA fragments and dependent upon new protein synthesis; it occurred at the level of gene transcription. We also examined the effect of HA fragments on MME expression in inflammatory alveolar Mphi from bleomycin-injured rat lungs. Although normal rat alveolar Mphi did not express MME mRNA in response to HA fragments, alveolar Mphi from the bleomycin-treated rats responded to HA fragment stimulation by increasing MME mRNA levels. Furthermore, baseline and HA fragment-induced MME gene expression in alveolar Mphi from bleomycin-treated rats was inhibited by IFN-gamma. These data suggest that HA fragments may be an important mechanism for the expression of MME by Mphi in inflammatory lung disorders.
