BACKGROUND:The balance between proteinases and antiproteinases plays an important role in tissue destruction and remodelling. In chronic obstructive pulmonary disease (COPD) and emphysema, an imbalance between matrix metalloproteinases (MMPs) and inhibitors of tissue metalloproteinase (TIMPs) has been reported. Alveolar macrophages are considered to be the main source of MMPs. We therefore have analyzed the effects of free and liposomal all trans-retinoic acid (ATRA) on the expression of MMP-9 and TIMP-1 in bronchoalveolar lavage (BAL) cells from patients with COPD and patients with other lung diseases. MATERIAL AND METHODS:BAL cells were incubated 1-3 day with either liposomal or free ATRA. Supernatants were tested for MMP-9 and TIMP-1 protein in specific ELISA systems; mRNA analysis was performed by semi-quantitative RT-PCR and by quantitative LightCycler PCR. RESULTS:We demonstrate that either liposomal or free ATRA selectively down-regulates MMP-9 and up-regulates TIMP-1. At the protein level, MMP-9 is decreased 3-fold and TIMP-1 is increased 3.5-fold compared to the base line with empty liposomes or untreated cells. The ratio of MMP-9 and its inhibitor TIMP-1, which may be crucial to the overall proteolytic potential decreased by factor 8. That this countercurrent effect of ATRA is not due to an altered protein stability but to transcriptional regulation could be demonstrated by RT-PCR. Quantitative LightCycler analysis revealed a 2.5-fold decrease of MMP-9 mRNA and a 4.5 fold increase of TIMP- 1 mRNA. CONCLUSIONS:These data suggest that ATRA treatment via its impact on the proteinase/antiproteinase ratio may become a new therapeutic strategy for patients with inflammatory destructive lung diseases.