Although MK886 was originally identified as an inhibitor of 5-lipoxygenase activating protein (FLAP), recent data demonstrate that this activity does not underlie its ability to induce apoptosis [Datta, Biswal and Kehrer (1999) Biochem. J. 340, 371–375]. Since FLAP is a fatty-acid binding protein, it is conceivable that MK886 may affect other such proteins. A family of nuclear receptors that are activated by fatty acids and their metabolites, the peroxisome-proliferator-activated receptors (PPARs), have been implicated in apoptosis and may represent a target for MK886. The ability of MK886 to inhibit PPAR-α, −β and −γ activity was assessed using reporter assay systems (peroxisome-proliferator response element–luciferase). Using a transient transfection system in monkey kidney fibroblast CV-1 cells, mouse keratinocyte 308 cells and human lung adenocarcinoma A549 cells, 10–20μM MK886 inhibited Wy14,643 activation of PPARα by approximately 80%. Similar inhibition of PPARα by MK886 was observed with a stable transfection reporter system in CV-1 cells. Only minimal inhibitory effects were seen on PPARβ and PPARγ. MK886 inhibited PPARα by a non-competitive mechanism as shown by its effects on the binding of arachidonic acid to PPARα protein, and a dose–response study using a transient transfection reporter assay in COS-1 cells. An assay assessing PPAR ligand–receptor interactions showed that MK886 prevents the conformational change necessary for active-complex formation. The expression of keratin-1, a protein encoded by a PPARα-responsive gene, was reduced by MK886 in a culture of mouse primary keratinocytes, suggesting that PPAR inhibition has functional consequences in normal cells. Although Jurkat cells express all PPAR isoforms, various PPARα and PPARγ agonists were unable to prevent MK886-induced apoptosis. This is consistent with MK886 functioning as a non-competitive inhibitor of PPARα, but may also indicate that PPARα is not directly involved in MK886-induced apoptosis. Although numerous PPAR activators have been identified, the results show that MK886 can inhibit PPARα, making it the first compound identified to have such an effect.
- activating protein
- cell signalling
Abbreviations used: CARLA, coactivator-dependent receptor ligand assay; CMV, cytomegalovirus; DTT, dithiothreitol; EMEM, Eagle's minimal essential medium; FLAP, 5-lipoxygenase activating protein; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GST, glutathione S-transferase; LOX, lipoxygenase; NSAIDs, non-steroidal anti-inflammatory drugs; PPs, peroxisome proliferators; PPARs, peroxisome-proliferator-activated receptors; PPREs, PPAR-responsive elements; RXR, 9-cis retinoic acid receptor; SRC-1, steroid receptor coactivator-1.
- The Biochemical Society, London ©2001