Regulating ubiquitin-proteasome system-dependent protein degradation for cancer therapy

Abstract

As an important protein degradation pathway, the ubiquitin-proteasome system plays a critical role in mediating protein-amino acid homeostasis in cancer development and serves as a potential target for cancer treatment. Although proteasome inhibition and targeted protein degradation approaches display encouraging results in preclinical studies, inadequate therapeutic efficiency as monotherapy and unfavorable adverse effects may hinder clinical application. To solve these problems, numerous nanodrug delivery platforms and stimuli-responsive prodrugs have been constructed to promote drug retention or activation in tumor tissues. These carefully-designed platforms are able to carry multiple drugs, enabling synergistic cancer treatment. However, although much effort has been dedicated to designing complicated nanoplatforms, how these encapsulated drugs interact cooperatively to induce biological effects is often neglected. Meanwhile, since most of nanoparticles are designed to directly target cancer cells, the interaction between nanoparticles and the tumor microenvironment is also often overlooked.

This thesis aimed to construct nanoparticle systems and light-responsive prodrugs to not only enhance therapeutic specificity of proteasome inhibitors or targeted protein degraders, but also improve treatment efficiency by incorporating synergistic strategies. The research also demonstrated the mechanism behind the synergistic effect in detail and depicted the crosstalk between the tumor microenvironment and nanodrugs.

The first study was to develop a synergistic nanoparticle system using proteasome inhibitors and demonstrate how the tumor microenvironment influences the effect. Solid tumors are often deprived of nutrients such as amino acids. In this study, amino acid starvation was found to activate the ubiquitin-proteasome system, rendering cancer cells more susceptible to proteasome inhibitors. A pH-responsive polymersome loaded with a macropinocytosis inhibitor and a proteasome inhibitor was developed to concurrently block protein internalization and proteasomal degradation, exhausting intracellular amino acids for cancer starvation therapy.

In the second project, CDK4/6 PROTAC was designed to specifically induce CDK4/6 degradation and G1 cell cycle arrest. The results indicated that G1-arrested cancer cells were more sensitive to photodynamic therapy and the combination induced synergistic anti-cancer effect. The mechanism study showed that this effect was mediated by mitochondrial biogenesis. Additionally, a self-assembled, carrier-free nanoparticle was created to co-deliver these two therapeutic agents, which not only enabled direct cytotoxicity in cancer cells, but also remodulated the immunosuppressive tumor microenvironment.

To avoid premature release and precisely control CDK4/6 PROTAC activation used in the second project, an opto-chemical controlled prodrug was constructed in the third study. CDK4/6 PROTAC was caged with a photoremovable group, which will only release active PROTAC after light irradiation to degrade CDK4/6 proteins. This light-controlled technology enabled spatiotemporal regulation of CDK4/6 proteins and G1 cell cycle arrest, offering a possible approach to enhance the selectivity for cancer therapy as well as a chemical tool to precisely manipulate CDK4/6 proteins for fundamental biomedical research.

In summary, both nanodrug delivery systems and light-responsive prodrug approaches presented in this thesis enable higher specificity and efficacy for cancer treatment. These platforms exploit different elements in the tumor microenvironment, not only enabling temporary elimination of cancer cells, but also contributing to long-term cancer intervention.