A number of the spindle assembly factors (SAFs) which are regulated by Ran GTPase during mitosis have variety of well recognized and intensively studied (but often not well understood) connections to cancer: BRCA1, HURP, TPX2, Aurora A, TACC3, survivin, RHAMM, cdk11. Also the levels of Ran are highly increased in many human tumors and unlike in non-cancer derived cells, the growth of cancer-derived cells is inhibited by Ran RNAi. Arguably the best characterized mechanism potentially linking Ran to cancer is the RanGTP, importin beta and importin alpha1 regulating the activation of Aurora A through its binding to TPX2. RanGTP is required for the release of TPX2 from its inhibitory complex with importin alpha1-importin beta. The importin-freeTPX2 binds Aurora A, protecting it from dephosphorylation and thus supporting sustained Aurora A kinase activity towards is many mitotic targets. More than 30 Aurora A kinase inhibitors are being developed as potential cancer therapeutics and several of them entered phase I and II clinical trials. However, the deletion of Aurora A gene is lethal in embryos and the heterozygous mice develop significantly higher number of tumors and aneuploidy. Complete inhibition of the Aurora A could therefore potentially induce de novo aneuploidy and cancer. We developed fluorescence resonance energy transfer (FRET)-based sensors for the RanGTP-induced importin alpha1-importin beta dissociation. These sensors were successfully tested at the NIH Chemical Genomics Center (NCGC), Rockville, MD, as applicable in quantitative highthroughput screen (qHTS) for small molecule inhibitors of the key steps involved in the Ran-regulated importin alpha1 mitotic function. We are currently applying for funding to perform the qHTS with a library containing 300 000 compounds. In a longer term, we will work towards developing the hits from the screen into antimitotic drugs suitable for cancer treatment.
