In this study, we compare the proteomic responses of three cell lines (A549, HCT116, and MCF-7) to treatment with three antitumor drugs (bortezomib, everolimus, and lonidamine), with a particular focus on the effects on the ubiquitin-proteasome system. Liquid chromatography–mass spectrometry (LC-MS) experiments for cellular samples were conducted using an Orbitrap Q Exactive HF mass spectrometer, and the data were analyzed utilizing the DirectMS1 method. Cellular samples treated with bortezomib and lonidamine served as negative and positive controls, respectively, while the samples treated with everolimus constituted the primary subject of investigation.