The inhibition of bromo- and extraterminal domains (BET) has shown an anti-proliferative effect in triple negative breast cancer (TNBC). In this article we explore mechanisms of resistance to BET inhibitors (BETi) in TNBC, with the aim of identifying novel ways to overcome such resistance. Two cellular models of acquired resistance to the BET inhibitor JQ1 were generated using a pulsed treatment strategy. MTT, colony formation, and cytometry assays revealed that BETi-resistant cells were particularly sensitive to PLK1 inhibition. Targeting of the latter reduced cell proliferation, especially in resistant cultures. Quantitative PCR analysis of a panel of mitotic kinases uncovered an increased expression of AURKA, TTK, and PLK1, confirmed by Western blot. Only pharmacological inhibition of PLK1 showed anti-proliferative activity on resistant cells, provoking G2/M arrest, increasing expression levels of cyclin B, pH3 and phosphorylation of Bcl-2 proteins, changes that were accompanied by induction of caspase-dependent apoptosis. JQ1-resistant cells orthotopically xenografted into the mammary fat pad of mice led to tumours that retained JQ1-resistance. Administration of the PLK1 inhibitor volasertib resulted in tumour regression. These findings open avenues to explore the future use of PLK1 inhibitors in the clinical setting of BETi-resistant patients.
This work has been supported by Instituto de Salud Carlos III (PI16/01121 and PI19/00808), ACEPAIN; Diputación de Albacete, CIBERONC and CRIS Cancer Foundation (to A. Ocaña). Ministry of Economy and Competitiveness of Spain (BFU2015-71371-R), the Instituto de Salud Carlos III through the Spanish Cancer Centers Network Program (RD12/0036/0003) and CIBERONC, the scientific foundation of the AECC and the CRIS Foundation (to A. Pandiella). The work carried out in our laboratories receive support from the European Community through the regional development funding program (FEDER). J.C. Montero is a recipient of a Miguel Servet fellowship program (CP12/03073 and CPII17/00015) and receives research support from the ISCIII (grants PI15/00684 and PI18/00796). E.M. Galan-Moya is funded by the implementation research program of the UCLM (UCLM resolution date: July 31, 2014), with a contract for accessing the Spanish System of Science, Technology and Innovation-SECTI (co-funded by the European Commission/FSE funds) and Council of Communities of Castilla-La Mancha (JCCM) (SBPLY/19/180501/000173). M. Burgos is funded by the Spanish Ministry for Science and Innovation, ref: BFU2015-69874-R and Castilla-La Mancha support grant for Biomedicine and Health Science Research ref: II-2018_11.