Cancer cells are characterized by a higher rate of protein turnover and greater demand for protein homeostasis compared to normal cells. In this scenario, the ubiquitin-proteasome system (UPS), which is responsible for the degradation of over 80% of cellular proteins within mammalian cells, becomes vital to cancer cells, making the UPS a critical target for the discovery of novel cancer therapeutics. This review systematically categorizes all current reported small molecule inhibitors of the various essential components of the UPS, including ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), ubiquitin ligases (E3s), the 20S proteasome catalytic core particle (20S CP) and the 19S proteasome regulatory particles (19S RP), as well as their mechanism/s of action and limitations. We also discuss the immunoproteasome which is considered as a prospective therapeutic target of the next generation of proteasome inhibitors in cancer therapies.Cancers 2020, 12, 902 2 of 34 core component of the nucleosome, which is critical for transcription and cell cycle, is recognized and linked by Lysine Residue 48 (K48) and further degraded by the proteasome [17,18]; DbpB (also named Y-box protein 1), a transcription factor, is reported to selectively recognize the Y-box promoter element. Studies showed that its terminal 105-amino-acid-long fragment is removed after a specific proteolytic cleavage by the proteasome complex [19,20]; NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) is located outside the nucleus and is reported to be involved in DNA transcription as well as cell survival [21,22]. The NF-κB p105 is the precursor of NF-κB p50. It is evident that NF-κB p105 is cleaved and selectively degraded at the C-terminus by proteasome, generating the active form of NF-κB p50 [23]. Products of UPS degradation can also be further degraded into single amino acids by aminopeptidases [24]. Aminopeptidases are the class of enzymes that catalyze the final steps in the ubiquitin-proteasome pathway by breaking down shorter peptides (<5 residues) into even smaller fragments [25]. Many, but not all, of aminopeptidases, are zinc metalloenzymes, such as leucine aminopeptidases (lAPs) and methionine aminopeptidases (metAPs) [26,27]. Studies showed that blocking the activity of the aminopeptidases by inhibitor of bestatin could generate a major accumulation of peptides which are ∼2-5 residues long [28].The 26S proteasome contains one/two 19S regulatory particles (19S RP) which mainly regulate the translocation of ubiquitinated proteins to the 20S CP and one 20S core particle (20S CP) in which proteolysis finally occurs [29,30]. In general, two main processes are associated with the process of degradation of proteins by the UPS: (1) tagging the to-be-degraded proteins by polyubiquitination (normally more than four ubiquitins), and (2) proteolytic degradation of the polyubiquitinated protein by the 26S proteasome complex [31,32]. Each step incorporates an intricate and complex spectrum of protein interaction...