Structure-Based Screen Identification of a Mammalian Ste20-like Kinase 4 (MST4) Inhibitor with Therapeutic Potential for Pituitary Tumors

Pituitary tumors of the gonadotrope lineage are often large and invasive, resulting in hypopituitarism. No medical treatments are currently available. Using a combined genetic and genomic screen of individual human gonadotrope pituitary tumor samples, we recently identified the mammalian sterile-20 like kinase 4 (MST4) as a protumorigenic effector, driving increased pituitary cell proliferation and survival in response to a hypoxic microenvironment. To identify novel inhibitors of the MST4 kinase for potential future clinical use, computational-based virtual library screening was used to dock the SelleckChem kinase inhibitor library into the ATP-binding site of the MST4 crystal structure. Several inhibitor candidates were identified with the potential to bind with high affinity. Using a TR-FRET in vitro recombinant kinase assay, hesperadin, initially described as an Aurora kinase inhibitor, exhibited potent inhibition of the MST4 kinase at nanomolar concentrations. The L beta T2 gonadotrope pituitary cell hypoxic model was used to test the ability of this inhibitor to antagonize MST4 actions. Under short-term severe hypoxia (1% O-2), MST4 protection from hypoxia-induced apoptosis was abrogated in the presence of hesperadin. Similarly, under chronic hypoxia (5%), hesperadin blocked the proliferative and colony-forming actions of MST4 as well as the ability to activate specific downstream signaling and hypoxia-inducible factor-1 effectors. Together, these data identify hesperadin as the first potent, selective inhibitor of the MST4 kinase with the capacity to block pituitary tumor cell growth in a hypoxic microenvironment. (C) 2015 AACR.