Pharmacological inhibition of Polo-like kinase 1 (PLK1) by BI-2536 decreases the viability and survival of hamartin and tuberin deficient cells via induction of apoptosis and attenuation of autophagy

The mechanistic target of rapamycin complex 1 (mTORC1) increases translation, cell sizeand angiogenesis, and inhibits autophagy. mTORC1 is negatively regulated by hamartin andtuberin, the protein products of the tumor suppressors TSC1 andTSC2 that are mutated in Tuberous Sclerosis Complex (TSC) and sporadicLymphangioleiomyomatosis (LAM). Hamartin interacts with the centrosomal and mitotic kinasepolo-like kinase 1 (PLK1). Hamartin and tuberin deficient cells have abnormalities incentrosome duplication, mitotic progression, and cytokinesis, suggesting that thehamartin/tuberin heterodimer and mTORC1 signaling are involved in centrosome biology andmitosis. Here we report that PLK1 protein levels are increased in hamartin and tuberindeficient cells and LAM patient-derived specimens, and that this increase israpamycin-sensitive. Pharmacological inhibition of PLK1 by the small-molecule inhibitorBI-2536 significantly decreased the viability and clonogenic survival of hamartin andtuberin deficient cells, which was associated with increased apoptosis. BI-2536 increasedp62, LC3B-I and GFP-LC3 punctae, and inhibited HBSS-induced degradation of p62, suggestingthat PLK1 inhibition attenuates autophagy. Finally, PLK1 inhibition repressed theexpression and protein levels of key autophagy genes and proteins and the protein levelsof Bcl^-2 family members, suggesting that PLK1 regulates both autophagic andapoptotic responses. Taken together, our data point toward a previously unrecognized roleof PLK1 on the survival of cells with mTORC1 hyperactivation, and the potential use ofPLK1 inhibitors as novel therapeutics for tumors with dysregulated mTORC1 signaling,including TSC and LAM.