Application of PROTACs in Hematological Malignancies--Review / 中国实验血液学杂志

Proteolysis-targeting chimeras (PROTACs) are heterobifunctional small molecules by utilizing the ubiquitin proteasome system (UPS) to degrade proteins of interest. PROTACs have exhibited unprecedented efficacy and specificity in degrading various oncogenic proteins because of their unique mechanism of action, ability to target "undruggable" and mutant proteins. A series of PROTACs have been developed to degrade multiple key protein targets for the treatment of hematologic malignancy. Notably, PROTACs that target BCL-XL, IRAK4, STAT3 and BTK have entered clinical trials. The known PROTACs that have the potential to be used to treat various hematological malignancies are systematically summarized in this review.

Advances in the preclinical and clinical research of proteolysis targeting chimera / 生物工程学报

Proteolysis targeting chimera (PROTAC) refers to heterobifunctional small molecules that can simultaneously bind an E3 ubiquitin ligase and a target protein, enabling specific degradation of the target protein with the aid of the ubiquitin proteasome system. At present, most PROTAC drugs are in the clinical trial stage, and the ligands are mainly non-covalent compounds. PROTAC drugs have the advantage of overcoming drug resistance and degrading "undruggable" target proteins, but non-covalent ligands could lead to the hook effect that undermines drug efficacy. With its own advantages, covalent ligands can avoid the occurrence of this phenomenon, which is of great help to the development of PROTAC. This review summarizes the progress in preclinical and clinical research and application of PROTAC molecules targeting three different classes of protein targets, including intranuclear, transmembrane, and cytosolic proteins. We also offer perspective discussions to provide research ideas and references for the future development of PROTAC.

NEDD4-1 deficiency impairs satellite cell function during skeletal muscle regeneration

BACKGROUND: Satellite cells are tissue-specific stem cells primarily responsible for the regenerative capacity of skeletal muscle. Satellite cell function and maintenance are regulated by extrinsic and intrinsic mechanisms, including the ubiquitin-proteasome system, which is key for maintaining protein homeostasis. In this context, it has been shown that ubiquitin-ligase NEDD4-1 targets the transcription factor PAX7 for proteasome-dependent degradation, promoting muscle differentiation in vitro. Nonetheless, whether NEDD4-1 is required for satellite cell function in regenerating muscle remains to be determined. RESULTS: Using conditional gene ablation, we show that NEDD4-1 loss, specifically in the satellite cell population, impairs muscle regeneration resulting in a significant reduction of whole-muscle size. At the cellular level, NEDD4-1-null muscle progenitors exhibit a significant decrease in the ability to proliferate and differentiate, contributing to the formation of myofibers with reduced diameter. CONCLUSIONS: These results indicate that NEDD4-1 expression is critical for proper muscle regeneration in vivo and suggest that it may control satellite cell function at multiple levels.

Advances of targeted protein degradation technology and its applications in diseases therapy / 生物工程学报

Targeted protein degradation (TPD) technology facilitates specific and efficient degradation of disease-related proteins through hijacking the two major protein degradation systems in mammalian cells: ubiquitin-proteasome system and lysosome pathway. Compared with traditional small molecule-inhibitors, TPD-based drugs exhibit the characteristics of a broader target spectrum. Compared with techniques interfere with protein expression on the gene and mRNA level, TPD-based drugs are target-specific, efficaciously rapid, and not constrained by post-translational modification of proteins. In the past 20 years, various TPD-based technologies have been developed. Most excitingly, two TPD-based therapeutic drugs have been approved by FDA for phase Ⅰ clinical trials in 2019. Despite of the early stage characteristics and various obstructions of the TPD technology, it could serve as a powerful tool for the development of novel drugs. This review summarizes the advances of different degradation systems based on TPD technologies and their applications in disease therapy. Moreover, the advantages and challenges of various technologies were discussed systematically, with the aim to provide theoretical guidance for further application of TPD technologies in scientific research and drug development.

Research progress on proteasome subunits in regulating occurrence and development of hepatocellular carcinoma / 浙江大学学报·医学版

Proteasome is the eukaryotic organelle responsible for degradation of short-lived proteins and involved in maintaining cellular protein homeostasis. It has been reported that during the occurrence and development of hepatocellular carcinoma (HCC), the regulatory particle subunits of proteasome regulate a series of tumor-related proteins, and proliferation, survival-associated signaling molecules, including PTEN gene, P53, Bcl-2, Bcl-2 interacting mediator of cell death (Bim), cyclin-dependent kinase 4(CDK4), transforming growth factor β receptor (TGFBR), E2F1, growth factor receptor-bound protein 2 (GRB2) . Meanwhile, these subunits regulate some tumor-associated pathway protein, such as signal transducer and activator of transcription 3 (STAT3) and protein kinase B (AKT), inducing their malfunction to promote the occurrence, proliferation, invasion and metastasis of HCC. The core particle subunits are more to perform the degradation of HCC-related proteins, so inhibitors targeting the core particle show a good anti-tumor effect. This review summarizes the current research progress on the regulation and mechanism of proteasome subunits in promoting the occurrence and development .

Reduced semen quality in patients with testicular cancer seminoma is associated with alterations in the expression of sperm proteins / 亚洲男科学杂志(英文版)

Testicular cancer seminoma is one of the most common types of cancer among men of reproductive age. Patients with this condition usually present reduced semen quality, even before initiating cancer therapy. However, the underlying mechanisms by which testicular cancer seminoma affects male fertility are largely unknown. The aim of this study was to investigate alterations in the sperm proteome of men with seminoma undergoing sperm banking before starting cancer therapy, in comparison to healthy proven fertile men (control group). A routine semen analysis was conducted before cryopreservation of the samples (n = 15 per group). Men with seminoma showed a decrease in sperm motility (P = 0.019), total motile count (P = 0.001), concentration (P = 0.003), and total sperm count (P = 0.001). Quantitative proteomic analysis identified 393 differentially expressed proteins between the study groups. Ten proteins involved in spermatogenesis, sperm function, binding of sperm to the oocyte, and fertilization were selected for validation by western blot. We confirmed the underexpression of heat shock-related 70 kDa protein 2 (P = 0.041), ubiquinol-cytochrome C reductase core protein 2 (P = 0.026), and testis-specific sodium/potassium-transporting ATPase subunit alpha-4 (P = 0.016), as well as the overexpression of angiotensin I converting enzyme (P = 0.005) in the seminoma group. The altered expression levels of these proteins are associated with spermatogenesis dysfunction, reduced sperm kinematics and motility, failure in capacitation and fertilization. The findings of this study may explain the decrease in the fertilizing ability of men with seminoma before starting cancer therapy.

Genes related to maintenance of autophagy and successful aging / Genes relacionados à manutenção da autofagia e envelhecimento bem-sucedido

ABSTRACT Considering aging as a phenomenon in which there is a decline in essential processes for cell survival, we investigated the autophagic and proteasome pathways in three different groups: young, older and oldest old male adults. The expression profile of autophagic pathway-related genes was carried out in peripheral blood, and the proteasome quantification was performed in plasma. No significant changes were found in plasma proteasome concentrations or in correlations between proteasome concentrations and ages. However, some autophagy- and/or apoptosis-related genes were differentially expressed. In addition, the network and enrichment analysis showed an interaction between four of the five differentially expressed genes and an association of these genes with the transcriptional process. Considering that the oldest old individuals maintained both the expression of genes linked to the autophagic machinery, and the proteasome levels, when compared with the older group, we concluded that these factors could be considered crucial for successful aging.

RESUMO Considerando o envelhecimento como um fenômeno em que há um declínio nos processos essenciais a sobrevivência celular, investigamos as vias autofágica e proteassômica em três grupos: jovens, idosos e longevos. O perfil de expressão dos genes relacionados à via autofágica foi analisado em sangue periférico, e a quantificação do proteassoma realizada em plasma. Não foram encontradas alterações significativas nas concentrações plasmáticas de proteassoma ou na correlação entre as concentrações de proteassoma e as idades. No entanto, alguns genes relacionados a autofagia e / ou apoptose foram expressos diferencialmente. Além disso, as análises de rede e de enriquecimento mostraram uma interação entre quatro dos cinco genes diferencialmente expressos e a associação desses ao processo transcricional. Considerando que os indivíduos longevos mantiveram tanto a expressão de genes ligados à maquinaria autofágica, quanto os níveis de proteassoma quando comparados aos idosos, concluímos que esses fatores poderiam ser considerados cruciais para o envelhecimento bem-sucedido.

Subversion of immunoproteasome subunit expression in dengue virus serotype 2-infected HepG2 cells

Abstract: INTRODUCTION: Infection with all serotypes of dengue virus (DV) results in augmented antigen presentation by MHC class I molecules. However, the upregulation of immunoproteasome subunits only results from infection with two serotypes. This study aims to elucidate changes in the expression of immunoproteasome subunits resulting from infection with DV, particularly DV serotype 2 (DV2). METHODS: HepG2 cells were grown in various culture milieu. Total cellular RNA and proteins were extracted and quantified. RESULTS: Results demonstrated sequestration of immunoproteasome subunits LMP2 and LMP7 in DV2-infected cells. CONCLUSIONS: This study provides insights into the mechanisms underlying immune evasion by DV.

Six-week anaerobic training improves proteolytic profile of diabetic rats

Objective To evaluate the effect of six-week anaerobic training on the mRNA expression of genes related to proteolysis Ubb (Ubiquitin), E2-14kDa, Trim63 (MuRF1 protein) and Nfkb1 in the skeletal muscle of diabetic rats.Materials and methods Four groups were established: DE (DiabetesExercised), DS (Diabetes Sedentary), CE (Control Exercised) and CS (Control Sedentary). The training consisted of 3 sets of 12 jumps in the liquid mean with load equivalent to 50% of BW for 6 weeks. Euthanasia occurred under ip anesthesia, and blood, adipose tissue and skeletal muscles were collected. Gene expression was quantified by RT–PCR in the gastrocnemius muscle. ANOVA one-way was used for comparison among groups, with post-hoc (Tukey) when necessary, considering p < 0.05.Results We observed reduction in the body weight and adipose tissue in the diabetic groups. The muscle mass was reduced in DS, which could be reversed by training (DE). Although DS and DE have presented similar body weight, the training protocol in DE promoted reduction in the adipose tissue, and increase of muscle mass. Anaerobic training was efficient to reduce glycaemia only in the diabetic animals until 6 hours after the end of training. The Trim63 gene expression was increased in DS; decreased Ubb gene level was observed in trained rats (CE and DE) compared to sedentary (CS and DS), and DE presented the lowest level of E2-14kDa gene expression.Conclusion Six-week anaerobic training promoted muscle mass gain, improved glycemic control, and exerted inhibitory effect on the proteolysis of gastrocnemius muscle of diabetic rats.

Beta catenin is degraded by both caspase-3 and proteasomal activity during resveratrol-induced apoptosis in HeLa cells in a GSK3β-independent manner.

Increased activity of β-catenin, an important transcriptional activator for survival and proliferation-associated genes has been linked with many cancers. We examined whether β-catenin is a target of resveratrol and whether its degradation contributes to the pro-apoptotic effects of resveratrol. HeLa cells were exposed to 60 µM resveratrol for 48 h. Apoptosis was confirmed by measurement of annexin V externalization, caspase-3 activation and DNA fragmentation. Induction of apoptosis was observed as early as 12 h, when both caspase-3 activation and PARP (poly ADP ribose polymerase) cleavage occurred. Nuclear β-catenin levels remained unchanged for 48 h during resveratrol exposure. However, extranuclear cell lysate β-catenin underwent a decrease at a late stage of apoptosis namely at 36-48 h. Alterations in the phosphorylation status of Akt/GSK3β were not observed during resveratrol-induced apoptosis. Furthermore, inhibition of GSK3β activity which is largely responsible for β-catenin degradation failed to influence β-catenin stability. However, inhibition of caspase-3 activity prevented the decline in β-catenin levels at 36-48 h of resveratrol exposure. Lactacystin, a proteosomal inhibitor also prevented the degradation of β-catenin by resveratrol. In conclusion, resveratrol induced apoptosis in HeLa cells in an Akt/GSK3β-independent manner and down-regulated β-catenin levels during apoptosis through action of caspase-3 and proteasomal degradation, independent of GSK3β-mediated phosphorylation.

HIF-1alpha Upregulation due to Depletion of the Free Ubiquitin Pool

Hypoxia-inducible factor 1alpha (HIF-1alpha), which transactivates a variety of hypoxia-induced genes, is rapidly degraded under nomoxia through the hydroxylation-ubiquitination-proteasome pathway. In this study, we addressed how HIF-1alpha is stabilized by proteasome inhibitors. The ubiquitin pool was rapidly reduced after proteasome inhibition, followed by the accumulation of non-ubiquitinated HIF-1alpha. The poly-ubiquitination of HIF-1alpha was resumed by restoration of free ubiquitin, which suggests that the HIF-1alpha stabilization under proteasome inhibition is attributed to depletion of the free ubiquitin pool. Ni2+ and Zn2+ also stabilized HIF-1alpha with depletion of the free ubiquitin pool and these effects of metal ions were attenuated by restoration of free ubiquitin. Ni2+ and Zn2+ may disturb the recycling of free ubiquitin, as MG132 does. Based on these results, the state of the ubiquitin pool seems to be another critical factor determining the cellular level of HIF-1alpha.

Polyubiquitin chain-dependent protein degradation in TRIM30 cytoplasmic bodies

Viral infection induces numerous tripartite motif (TRIM) proteins to control antiviral immune signaling and viral replication. Particularly, SPRY-containing TRIM proteins are found only in vertebrates and they control target protein degradation by their RING-finger and SPRY domains, and proper cytoplasmic localization. To understand TRIM30 function, we analyzed its localization pattern and putative roles of its RING-finger and SPRY domains. We found that TRIM30 is located in actin-mediated cytoplasmic bodies and produces colocalized ubiquitin chains in SPRY domain- and RING-finger domain-dependent ways that are degraded by autophagy and the proteasome. These results suggest a TRIM protein-dependent degradation mechanism by cytoplasmic body formation with actin networks.

Phospholipase D2 promotes degradation of hypoxia-inducible factor-1alpha independent of lipase activity

Hypoxia-inducible factor-1alpha (HIF-1alpha) is a key transcriptional mediator that coordinates the expression of various genes involved in tumorigenesis in response to changes in oxygen tension. The stability of HIF-1alpha protein is determined by oxygen-dependent prolyl hydroxylation, which is required for binding of the von Hippel-Lindau protein (VHL), the recognition component of an E3 ubiquitin ligase that targets HIF-1alpha for ubiquitination and degradation. Here, we demonstrate that PLD2 protein itself interacts with HIF-1alpha, prolyl hydroxylase (PHD) and VHL to promote degradation of HIF-1alpha via the proteasomal pathway independent of lipase activity. PLD2 increases PHD2-mediated hydroxylation of HIF-1alpha by increasing the interaction of HIF-1alpha with PHD2. Moreover, PLD2 promotes VHL-dependent HIF-1alpha degradation by accelerating the association between VHL and HIF-1alpha. The interaction of the pleckstrin homology domain of PLD2 with HIF-1alpha also promoted degradation of HIF-1alpha and decreased expression of its target genes. These results indicate that PLD2 negatively regulates the stability of HIF-1alpha through the dynamic assembly of HIF-1alpha, PHD2 and VHL.

Degradation of misfolded proteins in neurodegenerative diseases: therapeutic targets and strategies

Mammalian cells remove misfolded proteins using various proteolytic systems, including the ubiquitin (Ub)-proteasome system (UPS), chaperone mediated autophagy (CMA) and macroautophagy. The majority of misfolded proteins are degraded by the UPS, in which Ub-conjugated substrates are deubiquitinated, unfolded and cleaved into small peptides when passing through the narrow chamber of the proteasome. The substrates that expose a specific degradation signal, the KFERQ sequence motif, can be delivered to and degraded in lysosomes via the CMA. Aggregation-prone substrates resistant to both the UPS and the CMA can be degraded by macroautophagy, in which cargoes are segregated into autophagosomes before degradation by lysosomal hydrolases. Although most misfolded and aggregated proteins in the human proteome can be degraded by cellular protein quality control, some native and mutant proteins prone to aggregation into beta-sheet-enriched oligomers are resistant to all known proteolytic pathways and can thus grow into inclusion bodies or extracellular plaques. The accumulation of protease-resistant misfolded and aggregated proteins is a common mechanism underlying protein misfolding disorders, including neurodegenerative diseases such as Huntington's disease (HD), Alzheimer's disease (AD), Parkinson's disease (PD), prion diseases and Amyotrophic Lateral Sclerosis (ALS). In this review, we provide an overview of the proteolytic pathways in neurons, with an emphasis on the UPS, CMA and macroautophagy, and discuss the role of protein quality control in the degradation of pathogenic proteins in neurodegenerative diseases. Additionally, we examine existing putative therapeutic strategies to efficiently remove cytotoxic proteins from degenerating neurons.

Silencing of KIF14 interferes with cell cycle progression and cytokinesis by blocking the p27(Kip1) ubiquitination pathway in hepatocellular carcinoma

Although it has been suggested that kinesin family member 14 (KIF14) has oncogenic potential in various cancers, including hepatocellular carcinoma (HCC), the molecular mechanism of this potential remains unknown. We aimed to elucidate the role of KIF14 in hepatocarcinogenesis by knocking down KIF14 in HCC cells that overexpressed KIF14. After KIF14 knockdown, changes in tumor cell growth, cell cycle and cytokinesis were examined. We also examined cell cycle regulatory molecules and upstream Skp1/Cul1/F-box (SCF) complex molecules. Knockdown of KIF14 resulted in suppression of cell proliferation and failure of cytokinesis, whereas KIF14 overexpression increased cell proliferation. In KIF14-silenced cells, the levels of cyclins E1, D1 and B1 were profoundly decreased compared with control cells. Of the cyclin-dependent kinase inhibitors, the p27Kip1 protein level specifically increased after KIF14 knockdown. The increase in p27Kip1 was not due to elevation of its mRNA level, but was due to inhibition of the proteasome-dependent degradation pathway. To explore the pathway upstream of this event, we measured the levels of SCF complex molecules, including Skp1, Skp2, Cul1, Roc1 and Cks1. The levels of Skp2 and its cofactor Cks1 decreased in the KIF14 knockdown cells where p27Kip1 accumulated. Overexpression of Skp2 in the KIF14 knockdown cells attenuated the failure of cytokinesis. On the basis of these results, we postulate that KIF14 knockdown downregulates the expression of Skp2 and Cks1, which target p27Kip1 for degradation by the 26S proteasome, leading to accumulation of p27Kip1. The downregulation of Skp2 and Cks1 also resulted in cytokinesis failure, which may inhibit tumor growth. To the best of our knowledge, this is the first report that has identified the molecular target and oncogenic effect of KIF14 in HCC.

Necessidades proteicas, morbidade e mortalidade no paciente grave: fundamentos e atualidades / Protein requirements, morbidity and mortality in critically ill patients: fundamentals and applications

Evidências recentes sugerem que o balanço proteico negativo secundário à doença grave se associa ao aumento de morbidade. A perda da proteína corporal total é inevitável nesse cenário, mesmo com uma abordagem nutricional agressiva, e resulta, principalmente, do catabolismo da fibra muscular esquelética. O principal mecanismo bioquímico e metabólico envolvido nesse processo é o sistema ubiquitina-proteassoma, que, paradoxalmente, consome a adenosina trifosfatocomo fonte energética e motriz. É possível que a neutralidade do balanço proteico nessas instâncias clínicas, seja tão importante na melhora dos desfechos quanto atingir a meta calórica estimada ou medida pela calorimetria indireta. Estudos recentes apontam a utilização de concentrações mais elevadas de proteínas na terapia nutricional do paciente grave como importante para um impacto positivo na mortalidade. A proposta deste trabalho foi revisar alguns princípios da terapia nutricional relativos ao metabolismo proteico, sinalizar para as principais assertivas das diretrizes das sociedades especializadas e comentar estudos recentes, que abordam a questão em tela, sob a visão crítica da experiência clínica dos autores.

Recent evidence suggests that a negative protein balance secondary to severe disease is associated with increased morbidity. A loss of total body protein is inevitable in this scenario, even with an aggressive nutritional approach, primarily due to the catabolism of skeletal muscle fibers. The ubiquitin-proteasome system is the primary metabolic and biochemical mechanism involved in this process; paradoxically, this system consumes adenosine triphosphate as its energy source. It is possible that a neutral protein balance in these clinical situations is important for improving outcomes and achieving the caloric goals estimated or measured by indirect calorimetry. Recent studies have suggested that the use of higher protein concentrations in nutritional therapy for critically ill patients may help to reduce mortality. The purpose of this study was to review some of the nutrition therapy principles related to protein metabolism, evaluate the main assertions of the guidelines of specialty societies and review the recent studies that address these issues using critical insights from the authors' clinical experience.

REGγ is a strong candidate for the regulation of cell cycle, proliferation and the invasion by poorly differentiated thyroid carcinoma cells

REGγ is a proteasome activator that facilitates the degradation of small peptides. Abnormally high expression of REGγ has been observed in thyroid carcinomas. The purpose of the present study was to explore the role of REGγ in poorly differentiated thyroid carcinoma (PDTC). For this purpose, small interfering RNA (siRNA) was introduced to down-regulate the level of REGγ in the PDTC cell line SW579. Down-regulation of REGγ at the mRNA and protein levels was confirmed by RT-PCR and Western blot analyses. FACS analysis revealed cell cycle arrest at the G1/S transition, the MTT assay showed inhibition of cell proliferation, and the Transwell assay showed restricted cell invasion. Furthermore, the expression of the p21 protein was increased, the expression of proliferating cell nuclear antigen (PCNA) protein decreased, and the expression of the p27 protein was unchanged as shown by Western blot analyses. REGγ plays a critical role in the cell cycle, proliferation and invasion of SW579 cells. The alteration of p21 and PCNA proteins related to the down-regulation of REGγ suggests that p21 and PCNA participate in the process of REGγ regulation of cell cycle progression and cell proliferation. Thus, targeting REGγ has a therapeutic potential in the management of PDTC patients.

Comparative proteomics analysis of chronic atrophic gastritis: changes of protein expression in chronic atrophic gastritis without Helicobacter pylori infection

Chronic atrophic gastritis (CAG) is a very common gastritis and one of the major precursor lesions of gastric cancer, one of the most common cancers worldwide. The molecular mechanism underlying CAG is unclear, but its elucidation is essential for the prevention and early detection of gastric cancer and appropriate intervention. A combination of two-dimensional gel electrophoresis and mass spectrometry was used in the present study to analyze the differentially expressed proteins. Samples from 21 patients (9 females and 12 males; mean age: 61.8 years) were used. We identified 18 differentially expressed proteins in CAG compared with matched normal mucosa. Eight proteins were up-regulated and 10 down-regulated in CAG when compared with the same amounts of proteins in individually matched normal gastric mucosa. Two novel proteins, proteasome activator subunit 1 (PSME1), which was down-regulated in CAG, and ribosomal protein S12 (RPS12), which was up-regulated in CAG, were further investigated. Their expression was validated by Western blot and RT-PCR in 15 CAG samples matched with normal mucosa. The expression level of RPS12 was significantly higher in CAG than in matched normal gastric mucosa (P < 0.05). In contrast, the expression level of PSME1 in CAG was significantly lower than in matched normal gastric mucosa (P < 0.05). This study clearly demonstrated that there are some changes in protein expression between CAG and normal mucosa. In these changes, down-regulation of PSME1 and up-regulation of RPS12 could be involved in the development of CAG. Thus, the differentially expressed proteins might play important roles in CAG as functional molecules.

Intracellular protein degradation: from a vague idea through the lysosome and the ubiquitin-proteasome system and onto human diseases and drug targeting / La degradación intracelular de proteínas: Desde una vaga idea, a través del lisosoma y el sistema ubiquitina-proteosoma a las enfermedades humanas y el blanco de las drogas

Between the 1950s and 1980s, scientists were focusing mostly on how the genetic code is transcribed to RNA and translated to proteins, but how proteins are degraded has remained a neglected research area. With the discovery of the lysosome by Christian de Duve it was assumed that cellular proteins are degraded within this organelle. Yet, several independent lines of experimental evidence strongly suggested that intracellular proteolysis is largely non-lysosomal, but the mechanisms involved remained obscure. The discovery of the ubiquitin-proteasome system resolved the enigma. We now recognize that degradation of intracellular proteins is involved in regulation of a broad array of cellular processes, such as cell cycle and division, regulation of transcription factors, and assurance of the cellular quality control. Not surprisingly, aberrations in the system have been implicated in the pathogenesis of human disease, such as malignancies and neurodegenerative disorders, which led subsequently to an increasing effort to develop mechanism-based drugs.

Entre los años 1950 y 1980 los científicos focalizaron sus estudios sobre la forma en que el código genético es transcripto al ARN y traducido a las proteínas, dejando de lado la forma en que éstas se degradan. Con el descubrimiento de los lisosomas por Christian de Duve se asumió que las proteínas se degradaban en el interior de esa organela. Sin embargo, varias líneas de trabajo independientes sugerían fuertemente que la proteólisis intracelular era en su mayor parte no lisosómica, aunque se desconocían sus mecanismos. El descubrimiento del sistema ubiquitina-proteosoma resolvió el enigma. Ahora sabemos que la degradación intracelular de proteínas participa en la regulación de un amplio espectro de procesos celulares como la división y el ciclo celular, la regulación de los factores de transcripción y el control de la calidad celular. No es sorpresa entonces que las aberraciones del sistema estén relacionadas con la patogénesis de enfermedades humanas como tumores y desórdenes neurodegenerativos, lo que llevó luego a un esfuerzo para desarrollar drogas basadas en este mecanismo.