All-Trans Retinoic Acid and Curcumin Exhibit Hormesis or Synergistic Anticancer Effects in U87 Glioblastoma Cells: Defining the Proteome Accompanying Synergism

Persistently poor glioblastoma (GBM) survival necessitates better elucidation of tumor drug responses. After observing that low curcumin and all-trans retinoic acid (ATRA) doses stimulated cell proliferation and counteracted each other’s high-dose antiproliferative effects in U87 GBM cells, drug influences on cell growth, migration, and death and the antiproliferative interaction proteome were further studied. Cell proliferation and migration were assessed by xCELLigence Real-Time Cell Analysis (RTCA). Cell death was defined using flow cytometry. Drug interactions were determined with CompuSyn software (version 1.0). Liquid Chromatography–Tandem Mass Spectrometry (LC-MS/MS), High-Performance Liquid Chromatography (HPLC), and SequestHT software (version 1.4) were utilized for peptide generation and identification. ATRA at high doses inhibited cell growth and migration more efficiently. Curcumin was more proliferative and antagonistic against anti-growth effects at low doses. Migration inhibition and apoptosis occurred synergistically at the highest drug doses. ATRA influenced the proteome more remarkably, reducing Transforming Growth Factor Beta Induced (TGFBI), Phosphoglycerate Dehydrogenase (PHGDH), tenascin, and Sequestosome 1 (SQSTM1). These effects were alleviated by curcumin, except for SQSTM1. Uveal Autoantigen With Coiled-Coil Domains And Ankyrin Repeats (UACA) and Sad1 And UNC84 Domain Containing 2 (SUN2) were increased by ATRA and curcumin, and to a lesser extent by the combination. Hexokinase 2 (HXK2) was increased by curcumin and the combination. Heme Oxygenase 1 (HMOX1) was depleted by the combination, but not by the single agents. SQSTM1 and HMOX1 reductions may mediate anticancer synergism, while the remaining changes may indicate ongoing hormetic pathways not reflected in cell counts.