Objective-The cyclin-dependent kinase (CDK) inhibitor flavopiridol is currently being tested in clinical trials as anticancer drug. Beyond its cell death-inducing action, we hypothesized that flavopiridol affects inflammatory processes. Therefore, we elucidated the action of flavopiridol on leukocyte-endothelial cell interaction and endothelial activation in vivo and in vitro and studied the underlying molecular mechanisms. Methods and Results-Flavopiridol suppressed concanavalin A-induced hepatitis and neutrophil infiltration into liver tissue. Flavopiridol also inhibited tumor necrosis factor-␣-induced leukocyte-endothelial cell interaction in the mouse cremaster muscle. Endothelial cells were found to be the major target of flavopiridol, which blocked the expression of endothelial cell adhesion molecules (intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and E-selectin), as well as NF-B-dependent transcription. Flavopiridol did not affect inhibitor of B (IB) kinase, the degradation and phosphorylation of IB␣, nuclear translocation of p65, or nuclear factor-B (NF-B) DNA-binding activity. By performing a cellular kinome array and a kinase activity panel, we found LIM domain kinase-1 (LIMK1), casein kinase 2, c-Jun N-terminal kinase (JNK), protein kinase C (PKC), CDK4, CDK6, CDK8, and CDK9 to be influenced by flavopiridol. Using specific inhibitors, as well as RNA interference (RNAi), we revealed that only CDK9 is responsible for the action of flavopiridol. Conclusion-Our

BACKGROUND AND PURPOSERoscovitine, a cyclin-dependent kinase (CDK) inhibitor that induces tumour cell death, is under evaluation as an anti-cancer drug. By triggering leukocyte apoptosis, roscovitine can also enhance the resolution of inflammation. Beyond death-inducing properties, we tested whether roscovitine affects leukocyte-endothelial cell interaction, a vital step in the onset of inflammation. EXPERIMENTAL APPROACHLeukocyte-endothelial cell interactions were evaluated in venules of mouse cremaster muscle, using intravital microscopy. In primary human endothelial cells, we studied the influence of roscovitine on adhesion molecules and on the nuclear factor-kB (NF-kB) pathway. A cellular kinome array, in vitro CDK profiling and RNAi methods were used to identify targets of roscovitine. KEY RESULTSIn vivo, roscovitine attenuated the tumour necrosis factor-a (TNF-a)-induced leukocyte adherence to and transmigration through, the endothelium. In vitro, roscovitine strongly inhibited TNF-a-evoked expression of endothelial adhesion molecules (E-selectin, intercellular cell adhesion molecule, vascular cell adhesion molecule). Roscovitine blocked NF-kB-dependent gene transcription, but not the NF-kB activation cascade [inhibitor of kB (IkB) kinase activity, IkB-a degradation, p65 translocation]. Using a cellular kinome array and an in vitro CDK panel, we found that roscovitine inhibited protein kinase A, ribosomal S6 kinase and CDKs 2, 5, 7 and 9. Experiments using kinase inhibitors and siRNA showed that the decreased endothelial activation was due solely to blockade of CDK5 and CDK9 by roscovitine. CONCLUSIONS AND IMPLICATIONSOur study highlights a novel mode of action for roscovitine, preventing endothelial activation and leukocyte-endothelial cell interaction by inhibition of CDK5 and 9. This might expand its usage as a promising anti-inflammatory compound. AbbreviationsCDK, cyclin-dependent kinase; ICAM, intercellular cell adhesion molecule; IKK, IkB kinase; IkB, inhibitor of kB; NF-kB, nuclear factor-kB; PKA, protein kinase A; RS6K, ribosomal S6 kinase; VCAM, vascular cell adhesion molecule