Invasiveness and metastatic capacity of tumor cells have been related to increased expression and activation of gelatinase-A (MMP-2). 9-octadecenoic acid (oleic acid, OA), a long-chain cis-unsaturated fatty acid, has been shown to partially inhibit the formation of lung metastatic colonies in an ex vivo model of implantation of metastatic cells into nude mice. Reduction of metastasis formation was suggested to be due to a decrease of MMP-2 activity in tumor tissue extracts. Since regulation of MMP-2 activity occurs at different levels, including gene expression, pro-enzyme activation and finally active enzyme inhibition, we here investigated the precise level of the inhibitory effect of OA on MMP-2 activity by oncogene-transformed human bronchial epithelial cells (BZR cells). OA, at the dose of 5 x 10(-5) M, was shown to inhibit by 50% MMP-2 activity released from BZR cells. Northern-blot analysis of mRNA encoding MMP-2, MT1-MMP, the physiological activator of MMP-2, and TIMP-2, the natural inhibitor of MMP-2, revealed that OA did not alter the steady-state levels of MMP-2, MT1-MMP and TIMP-2 mRNA. Also, gelatin zymography demonstrated that the extent of MMP-2 activation was not modified by OA treatment. On the contrary, OA could inhibit the fluorogenic quenching substrate (7-methoxycoumarin-4-yl)acetyl-L-Pro-Leu-Gly-Leu-[N-3-(2,4-dinitrophenyl)-L-2, 3-diaminopropionyl]-Ala-Arg-NH2 (Mca-Pro-Leu-Gly-Leu-Dpa-Ala-Arg-NH2) hydrolysis by recombinant MMP-2 with Ki = 4.3 microM. These data suggest that the beneficial influence of OA on the formation of lung metastatic colonies was independent of its influence on MMP-2 expression and/or activation, but could be attributed to inhibition of MMP-2 activity.