Alpelisib Inhibits PIK3CA-Mutant Breast Cancer Growth Through AKT-Dependent Bim Induction and Mcl-1 Degradation

Abstract

<jats:title>Abstract</jats:title> <jats:p><jats:bold>Background: </jats:bold><jats:italic>PIK3CA </jats:italic>mutations are common genomic alterations in estrogen receptor (ER)-positive breast cancers, currently, the development of selective PI3Kα (phosphatidylinositol 3-kinase α) inhibitors is ongoing. The mechanisms contributing to the anticancer activity of alpelisib in <jats:italic>PIK3CA</jats:italic>-mutant breast cancer cells and the mechanism of acquired resistance to alpelisib remain elusive. <jats:bold>Methods:</jats:bold> Drug-sensitive cell lines were exposed to alpelisib to establish alpelisib-resistant cell lines. Western blotting was used to assess changes in protein expression. Apoptosis was evaluated by flow cytometry. <jats:italic>In vivo </jats:italic>with mouse xenograft models and <jats:italic>in vitro</jats:italic> colony formation and MTS and assay were carried out to determine the growth inhibitory effects of the tested drugs. Protein half-lives were examined and proteasome inhibitors were used to estimate protein degradation. Gene knockdown was carried out using shRNA or siRNA. <jats:bold>Results:</jats:bold> In the present study, we report the potent induction of apoptosis by<jats:bold> </jats:bold>alpelisib in <jats:italic>PIK3CA</jats:italic>-mutant breast cancer cell lines. AKT phosphorylation suppression, AKT/Foxo3a-dependent Bim induction, and AKT/GSK-3β-dependent Mcl-1 degradation were observed. Apoptosis induced by alpelisib was attenuated by Mcl-1 (4A) overexpression or Bim suppression. Furthermore, alpelisib could not modulate Mcl-1 or Bim levels in cell lines that were resistant to alpelisib. AKT inhibitor and alpelisib combination restored the sensitivity of alpelisib-resistant cells to growth inhibition and apoptosis<jats:italic> in vitro</jats:italic> and <jats:italic>in vivo</jats:italic>. <jats:bold>Conclusions: </jats:bold>Therefore, modulation of Mcl-1 degradation and AKT-dependent Bim induction are crucial for mediating the resistance and sensitivity of <jats:italic>PIK3CA</jats:italic>-mutant breast tumor cells to alpelisib, thus making it a productive strategy for overcoming acquired resistance to alpelisib.</jats:p>